I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up, grit
my teeth, and push into a spinning piece of aluminum to make my desired
profile.

Is this a sensible thing to consider? Is there some other way (short of
CNC machining) to easily duplicate cavities in the ends of a bunch of
aluminum tubes?

--
http://www.wescottdesign.com

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up, grit
my teeth, and push into a spinning piece of aluminum to make my desired
profile.

Is this a sensible thing to consider? Is there some other way (short of
CNC machining) to easily duplicate cavities in the ends of a bunch of
aluminum tubes?

Hobbing?


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up, grit
my teeth, and push into a spinning piece of aluminum to make my desired
profile.

Is this a sensible thing to consider? Is there some other way (short of
CNC machining) to easily duplicate cavities in the ends of a bunch of
aluminum tubes?

Hobbing?

P.S. Not the gear-making process, this one:

http://tinyurl.com/hobbing



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up, grit
my teeth, and push into a spinning piece of aluminum to make my desired
profile.

Is this a sensible thing to consider? Is there some other way (short
of CNC machining) to easily duplicate cavities in the ends of a bunch
of aluminum tubes?

Hobbing?

P.S. Not the gear-making process, this one:

http://tinyurl.com/hobbing

Well, the cavity is cylindrically symmetrical, so I'm not sure that
"hobbing" is the right term.

It's probably been a screw machine operation since 1920, or at least some
sort of tracing operation.

Sigh -- I'm always behind the curve.

--
http://www.wescottdesign.com

"Tim Wescott" wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up, grit
my teeth, and push into a spinning piece of aluminum to make my desired
profile.

Is this a sensible thing to consider? Is there some other way (short
of CNC machining) to easily duplicate cavities in the ends of a bunch
of aluminum tubes?

Hobbing?

P.S. Not the gear-making process, this one:

http://tinyurl.com/hobbing

Well, the cavity is cylindrically symmetrical, so I'm not sure that
"hobbing" is the right term.

It's probably been a screw machine operation since 1920, or at least some
sort of tracing operation.

Sigh -- I'm always behind the curve.

I must be dull tonight, but I can't visualize what you're doing. Are you
talking about some kind of forming in the axis of a spinning tube, or
turning the outside diameter to a profile?

--
Ed Huntress

On Sat, 9 Apr 2011 00:32:29 -0400, "Ed Huntress"
wrote:


"Tim Wescott" wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up, grit
my teeth, and push into a spinning piece of aluminum to make my desired
profile.

Is this a sensible thing to consider? Is there some other way (short
of CNC machining) to easily duplicate cavities in the ends of a bunch
of aluminum tubes?

Hobbing?

P.S. Not the gear-making process, this one:

http://tinyurl.com/hobbing

Well, the cavity is cylindrically symmetrical, so I'm not sure that
"hobbing" is the right term.

It's probably been a screw machine operation since 1920, or at least some
sort of tracing operation.

Sigh -- I'm always behind the curve.

I must be dull tonight, but I can't visualize what you're doing. Are you
talking about some kind of forming in the axis of a spinning tube, or
turning the outside diameter to a profile?

Like ED, I'm not picturing your cut. That said, form tools up to 3/8
wide are no trouble on a lathe. For example, I cut a V groove in a
pulley for a V belt all the time.

This assumes your lathe is rigid enough for the cut.

Karl

Tim Wescott wrote:

I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up, grit
my teeth, and push into a spinning piece of aluminum to make my desired
profile.

Is this a sensible thing to consider? Is there some other way (short of
CNC machining) to easily duplicate cavities in the ends of a bunch of
aluminum tubes?

--
http://www.wescottdesign.com

Are you referring to a lathe "form tool"? If so that's pretty standard
stuff.

On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim Wescott" wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up,
grit my teeth, and push into a spinning piece of aluminum to make my
desired profile.

Is this a sensible thing to consider? Is there some other way (short
of CNC machining) to easily duplicate cavities in the ends of a bunch
of aluminum tubes?

Hobbing?

P.S. Not the gear-making process, this one:

http://tinyurl.com/hobbing

Well, the cavity is cylindrically symmetrical, so I'm not sure that
"hobbing" is the right term.

It's probably been a screw machine operation since 1920, or at least
some sort of tracing operation.

Sigh -- I'm always behind the curve.

I must be dull tonight, but I can't visualize what you're doing. Are you
talking about some kind of forming in the axis of a spinning tube, or
turning the outside diameter to a profile?


Let me know if you can see this, it should explain:

http://www.hippocketaeronautics.com/...rum/index.php?
action=dlattach;topic=5869.0;attach=53766;image

I'm not so much interested in the flange around the outside as the wheel
profile on the inside.

Per Pete and Karl I guess I'm looking for a form tool, but one that gets
applied in the axial rather than the radial direction. I'm not so
interested in cutting the mating flanges -- that's both easy enough and
persnickety enough that I should do it by hand. It's getting the tire
and hub shapes cut repeatably that interests me.

So I guess the real question is: should I be able to do this with a form
tool, assuming enough rigidity in my lathe?

--
http://www.wescottdesign.com

"Tim Wescott" wrote in message
...
On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim Wescott" wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up,
grit my teeth, and push into a spinning piece of aluminum to make my
desired profile.

Is this a sensible thing to consider? Is there some other way (short
of CNC machining) to easily duplicate cavities in the ends of a bunch
of aluminum tubes?

Hobbing?

P.S. Not the gear-making process, this one:

http://tinyurl.com/hobbing

Well, the cavity is cylindrically symmetrical, so I'm not sure that
"hobbing" is the right term.

It's probably been a screw machine operation since 1920, or at least
some sort of tracing operation.

Sigh -- I'm always behind the curve.

I must be dull tonight, but I can't visualize what you're doing. Are you
talking about some kind of forming in the axis of a spinning tube, or
turning the outside diameter to a profile?


Let me know if you can see this, it should explain:

http://www.hippocketaeronautics.com/...rum/index.php?
action=dlattach;topic=5869.0;attach=53766;image

Sorry, that isn't working for me. Or I don't know how to get there.

I'll wait and see what others come up with. It sounds like you want to do
some kind of form-tool operation, in which case, as someone mentioned, the
key is the length of contact for the cutting edge. I've seen some really
long ones -- the whole length of a chess piece, for example -- performed on
South Bend 10" lathes with complete success.

But I may be on the wrong track. I'll wait.

--
Ed Huntress


I'm not so much interested in the flange around the outside as the wheel
profile on the inside.

Per Pete and Karl I guess I'm looking for a form tool, but one that gets
applied in the axial rather than the radial direction. I'm not so
interested in cutting the mating flanges -- that's both easy enough and
persnickety enough that I should do it by hand. It's getting the tire
and hub shapes cut repeatably that interests me.

So I guess the real question is: should I be able to do this with a form
tool, assuming enough rigidity in my lathe?

--
http://www.wescottdesign.com

On Sat, 09 Apr 2011 12:41:01 -0400, Ed Huntress wrote:

"Tim Wescott" wrote in message
...
On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim Wescott" wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up,
grit my teeth, and push into a spinning piece of aluminum to make
my desired profile.

Is this a sensible thing to consider? Is there some other way
(short of CNC machining) to easily duplicate cavities in the ends
of a bunch of aluminum tubes?

Hobbing?

P.S. Not the gear-making process, this one:

http://tinyurl.com/hobbing

Well, the cavity is cylindrically symmetrical, so I'm not sure that
"hobbing" is the right term.

It's probably been a screw machine operation since 1920, or at least
some sort of tracing operation.

Sigh -- I'm always behind the curve.

I must be dull tonight, but I can't visualize what you're doing. Are
you talking about some kind of forming in the axis of a spinning tube,
or turning the outside diameter to a profile?


Let me know if you can see this, it should explain:

http://www.hippocketaeronautics.com/...rum/index.php?
action=dlattach;topic=5869.0;attach=53766;image

Sorry, that isn't working for me. Or I don't know how to get there.

I'll wait and see what others come up with. It sounds like you want to
do some kind of form-tool operation, in which case, as someone
mentioned, the key is the length of contact for the cutting edge. I've
seen some really long ones -- the whole length of a chess piece, for
example -- performed on South Bend 10" lathes with complete success.

But I may be on the wrong track. I'll wait.

Is this better?

http://www.wescottdesign.com/temp/wheel.jpg

It's a Smithy lathe, and a form tool is what I was thinking about, except
that instead of applying it radially it'd be applied axially -- which
means that the ratio of cutting speeds from inside to outside would be
huge.

I actually sorta-kinda used a forming tool for the tire profile, then cut
the hub by hand -- I just want something that'll do a more consistent job
of it.

--
http://www.wescottdesign.com

On 4/9/2011 12:28 PM, Tim Wescott wrote:

So I guess the real question is: should I be able to do this with a form
tool, assuming enough rigidity in my lathe?


My Dad used to do the inverse of this when making the wheels for wooden
toy cars. He ground a form cutter with a tire like profile, mounted it
on the arm of a fly cutter in a drill press, cut the profile on one side
of the workpiece, flipped the workpiece, cut the profile on the opposite
side, then finished cutting the wheel out with a hole saw. I see no
reason why you couldn't profile the end of a .75" bar with a form tool.

Kevin Gallimore

On Apr 8, 10:38*pm, Tim Wescott wrote:
I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up, grit
my teeth, and push into a spinning piece of aluminum to make my desired
profile.

Is this a sensible thing to consider? *Is there some other way (short of
CNC machining) to easily duplicate cavities in the ends of a bunch of
aluminum tubes?

--http://www.wescottdesign.com

A 3/8" wide tool shouldn't be too much for aluminum, if your lathe is
solid. I make forming tools to cut round or vee belt grooves in
pulleys somewhat narrow and wiggle them in so only the end or one side
takes a chip at any instant, reducing the forces and chatter.

jsw

"Tim Wescott" wrote in message
...
On Sat, 09 Apr 2011 12:41:01 -0400, Ed Huntress wrote:

"Tim Wescott" wrote in message
...
On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim Wescott" wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up,
grit my teeth, and push into a spinning piece of aluminum to make
my desired profile.

Is this a sensible thing to consider? Is there some other way
(short of CNC machining) to easily duplicate cavities in the ends
of a bunch of aluminum tubes?

Hobbing?

P.S. Not the gear-making process, this one:

http://tinyurl.com/hobbing

Well, the cavity is cylindrically symmetrical, so I'm not sure that
"hobbing" is the right term.

It's probably been a screw machine operation since 1920, or at least
some sort of tracing operation.

Sigh -- I'm always behind the curve.

I must be dull tonight, but I can't visualize what you're doing. Are
you talking about some kind of forming in the axis of a spinning tube,
or turning the outside diameter to a profile?


Let me know if you can see this, it should explain:

http://www.hippocketaeronautics.com/...rum/index.php?
action=dlattach;topic=5869.0;attach=53766;image

Sorry, that isn't working for me. Or I don't know how to get there.

I'll wait and see what others come up with. It sounds like you want to
do some kind of form-tool operation, in which case, as someone
mentioned, the key is the length of contact for the cutting edge. I've
seen some really long ones -- the whole length of a chess piece, for
example -- performed on South Bend 10" lathes with complete success.

But I may be on the wrong track. I'll wait.

Is this better?

http://www.wescottdesign.com/temp/wheel.jpg

Aha. Yes, that does it.


It's a Smithy lathe, and a form tool is what I was thinking about, except
that instead of applying it radially it'd be applied axially -- which
means that the ratio of cutting speeds from inside to outside would be
huge.

I actually sorta-kinda used a forming tool for the tire profile, then cut
the hub by hand -- I just want something that'll do a more consistent job
of it.

--
http://www.wescottdesign.com

Ok. I get it.

Form tools are tricky in a couple of ways. There are basically two types:
conventional tools, with a positive rake throughout, and skiving tools,
which have zero rake. The former type lead to some very tricky problems of
geometry, which would be made much worse by approaching the work axially.
They're often held in a special dovetail toolholder.

So, to be practical, the thing to try probably is a skiving tool -- a form
tool with no top rake. That allows you to use a flat top on the tool that
you just cut back for the form, and then cut front clearance and, if
necessary, some local side clearance -- either left or right, or both on one
tool. This is still a tricky operation, as you can imagine by thinking about
what you're trying to accomplish. It's usually done on optical profile
grinders or wirecut EDMs.

However, I've made little skiving tools, on the order of yours, just by
grinding away with a Dremel. I had lots more spare time in those days.

The difference in cutting speed across the face would be a problem in steel,
but your molds (compression molds?) look like they're aluminum. The only
problem there is possible chatter or roughness near the center, made worse
by the zero top rake. My experience with facing tells me it could be
perfectly Ok. Just keep the tool sharp and lube it with something to enhance
surface finish near the center.

I'd run the tool pretty fast, because you can get away with a lot using HSS
in aluminum, which should help your finish near the center.

Form tools usually require some experimenting, anyway. Just try it and see
what happens.

(Hubbing, sometimes called "hobbing" and described above, is likely to
present more problems than form-cutting, IMO.)

--
Ed Huntress

Why not just get or make a master pattern and then make an epoxy mold
from that?

-jim

Tim Wescott wrote:

On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim Wescott" wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up,
grit my teeth, and push into a spinning piece of aluminum to make my
desired profile.

Is this a sensible thing to consider? Is there some other way (short
of CNC machining) to easily duplicate cavities in the ends of a bunch
of aluminum tubes?

Hobbing?

P.S. Not the gear-making process, this one:

http://tinyurl.com/hobbing

Well, the cavity is cylindrically symmetrical, so I'm not sure that
"hobbing" is the right term.

It's probably been a screw machine operation since 1920, or at least
some sort of tracing operation.

Sigh -- I'm always behind the curve.

I must be dull tonight, but I can't visualize what you're doing. Are you
talking about some kind of forming in the axis of a spinning tube, or
turning the outside diameter to a profile?

Let me know if you can see this, it should explain:

http://www.hippocketaeronautics.com/...rum/index.php?
action=dlattach;topic=5869.0;attach=53766;image

I'm not so much interested in the flange around the outside as the wheel
profile on the inside.

Per Pete and Karl I guess I'm looking for a form tool, but one that gets
applied in the axial rather than the radial direction. I'm not so
interested in cutting the mating flanges -- that's both easy enough and
persnickety enough that I should do it by hand. It's getting the tire
and hub shapes cut repeatably that interests me.

So I guess the real question is: should I be able to do this with a form
tool, assuming enough rigidity in my lathe?

--
http://www.wescottdesign.com

On 04/09/2011 10:52 AM, jim wrote:
(top posting fixed)
Tim Wescott wrote:

On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

snip
Why not just get or make a master pattern and then make an epoxy mold
from that?

-jim


Because the process is to put a flat piece of foam in the mold, clamp
it, then chuck it in an oven to heat. The foam* softens and expands to
meet the edges of the mold. Then you take it out of the oven and let it
cool, and voile! A part (or a mess -- it's best to enter any sort of
casting process with your eyes open).

So I like the idea of using something that's nicely heat conductive. I
could look around for a heat conductive epoxy (JB Weld may work, even --
I understand that it's metal filled). But it's hard to beat aluminum
for heat conductivity and easy machining (well, brass maybe). And it
looks purty.

I'm inventing the process, at least as applies to 3-D molding. I may be
the 90000th person to do this, but _I_ haven't heard of anyone doing
this. I've heard of folks molding Depron sheets around forms, by
binding the sheet to the form with Ace bandages, then heating. It's
what inspired me to try this -- but this is something that I'm cooking
up** out of my own head as I go.

* The foam is Depron, which is a close-cell polystyrene foam sold as
insulation in Europe, but used for model airplanes the world over.

** Pun not intended, but welcomed with open arms.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

On 04/09/2011 10:20 AM, Ed Huntress wrote:
"Tim wrote in message
...
On Sat, 09 Apr 2011 12:41:01 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen up,
grit my teeth, and push into a spinning piece of aluminum to make
my desired profile.

Is this a sensible thing to consider? Is there some other way
(short of CNC machining) to easily duplicate cavities in the ends
of a bunch of aluminum tubes?

Hobbing?

P.S. Not the gear-making process, this one:

http://tinyurl.com/hobbing

Well, the cavity is cylindrically symmetrical, so I'm not sure that
"hobbing" is the right term.

It's probably been a screw machine operation since 1920, or at least
some sort of tracing operation.

Sigh -- I'm always behind the curve.

I must be dull tonight, but I can't visualize what you're doing. Are
you talking about some kind of forming in the axis of a spinning tube,
or turning the outside diameter to a profile?


Let me know if you can see this, it should explain:

http://www.hippocketaeronautics.com/...rum/index.php?
action=dlattach;topic=5869.0;attach=53766;image

Sorry, that isn't working for me. Or I don't know how to get there.

I'll wait and see what others come up with. It sounds like you want to
do some kind of form-tool operation, in which case, as someone
mentioned, the key is the length of contact for the cutting edge. I've
seen some really long ones -- the whole length of a chess piece, for
example -- performed on South Bend 10" lathes with complete success.

But I may be on the wrong track. I'll wait.

Is this better?

http://www.wescottdesign.com/temp/wheel.jpg

Aha. Yes, that does it.


It's a Smithy lathe, and a form tool is what I was thinking about, except
that instead of applying it radially it'd be applied axially -- which
means that the ratio of cutting speeds from inside to outside would be
huge.

I actually sorta-kinda used a forming tool for the tire profile, then cut
the hub by hand -- I just want something that'll do a more consistent job
of it.

--
http://www.wescottdesign.com

Ok. I get it.

Form tools are tricky in a couple of ways. There are basically two types:
conventional tools, with a positive rake throughout, and skiving tools,
which have zero rake. The former type lead to some very tricky problems of
geometry, which would be made much worse by approaching the work axially.
They're often held in a special dovetail toolholder.

So, to be practical, the thing to try probably is a skiving tool -- a form
tool with no top rake. That allows you to use a flat top on the tool that
you just cut back for the form, and then cut front clearance and, if
necessary, some local side clearance -- either left or right, or both on one
tool. This is still a tricky operation, as you can imagine by thinking about
what you're trying to accomplish. It's usually done on optical profile
grinders or wirecut EDMs.

However, I've made little skiving tools, on the order of yours, just by
grinding away with a Dremel. I had lots more spare time in those days.

The difference in cutting speed across the face would be a problem in steel,
but your molds (compression molds?) look like they're aluminum. The only
problem there is possible chatter or roughness near the center, made worse
by the zero top rake. My experience with facing tells me it could be
perfectly Ok. Just keep the tool sharp and lube it with something to enhance
surface finish near the center.

I'd run the tool pretty fast, because you can get away with a lot using HSS
in aluminum, which should help your finish near the center.

Form tools usually require some experimenting, anyway. Just try it and see
what happens.

(Hubbing, sometimes called "hobbing" and described above, is likely to
present more problems than form-cutting, IMO.)


Ed:

Thank you -- this is just the sort of discussion I was looking for. You
forgot to mention that "rake" applies both to the face and the outside
edges (I learned that already, as I made a partial tool for the tire
portion of this, and had to put rake into it already).

I'll give this a whirl, and see how I do. I think I'm going to attempt
a conventional tool, as I'm going to have to think backwards and
sideways just to get zero rake around the corners, I may as well think a
bit skewed as well to get positive rake.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

"Tim Wescott" wrote in message
...
On 04/09/2011 10:20 AM, Ed Huntress wrote:
"Tim wrote in message
...
On Sat, 09 Apr 2011 12:41:01 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter
model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen
up,
grit my teeth, and push into a spinning piece of aluminum to make
my desired profile.

Is this a sensible thing to consider? Is there some other way
(short of CNC machining) to easily duplicate cavities in the ends
of a bunch of aluminum tubes?

Hobbing?

P.S. Not the gear-making process, this one:

http://tinyurl.com/hobbing

Well, the cavity is cylindrically symmetrical, so I'm not sure that
"hobbing" is the right term.

It's probably been a screw machine operation since 1920, or at least
some sort of tracing operation.

Sigh -- I'm always behind the curve.

I must be dull tonight, but I can't visualize what you're doing. Are
you talking about some kind of forming in the axis of a spinning
tube,
or turning the outside diameter to a profile?


Let me know if you can see this, it should explain:

http://www.hippocketaeronautics.com/...rum/index.php?
action=dlattach;topic=5869.0;attach=53766;image

Sorry, that isn't working for me. Or I don't know how to get there.

I'll wait and see what others come up with. It sounds like you want to
do some kind of form-tool operation, in which case, as someone
mentioned, the key is the length of contact for the cutting edge. I've
seen some really long ones -- the whole length of a chess piece, for
example -- performed on South Bend 10" lathes with complete success.

But I may be on the wrong track. I'll wait.

Is this better?

http://www.wescottdesign.com/temp/wheel.jpg

Aha. Yes, that does it.


It's a Smithy lathe, and a form tool is what I was thinking about,
except
that instead of applying it radially it'd be applied axially -- which
means that the ratio of cutting speeds from inside to outside would be
huge.

I actually sorta-kinda used a forming tool for the tire profile, then
cut
the hub by hand -- I just want something that'll do a more consistent
job
of it.

--
http://www.wescottdesign.com

Ok. I get it.

Form tools are tricky in a couple of ways. There are basically two types:
conventional tools, with a positive rake throughout, and skiving tools,
which have zero rake. The former type lead to some very tricky problems
of
geometry, which would be made much worse by approaching the work axially.
They're often held in a special dovetail toolholder.

So, to be practical, the thing to try probably is a skiving tool -- a
form
tool with no top rake. That allows you to use a flat top on the tool that
you just cut back for the form, and then cut front clearance and, if
necessary, some local side clearance -- either left or right, or both on
one
tool. This is still a tricky operation, as you can imagine by thinking
about
what you're trying to accomplish. It's usually done on optical profile
grinders or wirecut EDMs.

However, I've made little skiving tools, on the order of yours, just by
grinding away with a Dremel. I had lots more spare time in those days.

The difference in cutting speed across the face would be a problem in
steel,
but your molds (compression molds?) look like they're aluminum. The only
problem there is possible chatter or roughness near the center, made
worse
by the zero top rake. My experience with facing tells me it could be
perfectly Ok. Just keep the tool sharp and lube it with something to
enhance
surface finish near the center.

I'd run the tool pretty fast, because you can get away with a lot using
HSS
in aluminum, which should help your finish near the center.

Form tools usually require some experimenting, anyway. Just try it and
see
what happens.

(Hubbing, sometimes called "hobbing" and described above, is likely to
present more problems than form-cutting, IMO.)


Ed:

Thank you -- this is just the sort of discussion I was looking for. You
forgot to mention that "rake" applies both to the face and the outside
edges (I learned that already, as I made a partial tool for the tire
portion of this, and had to put rake into it already).

Well, if you're plunging the tool straight in, the rake (if any) is on the
top of the tool, and the relief, for clearance, is on the front and,
theoretically, both sides. But real-world skiving tools typically have no
side clearance. The side clearance appears automatically as a result of
front clearance and angles in the shape you're cutting.

But sometimes not. It depends on the geometry of the cut. You can think
through it. Watch for decreasing clearance on the outside of the tool, due
to the radius of the workpiece.


I'll give this a whirl, and see how I do. I think I'm going to attempt a
conventional tool, as I'm going to have to think backwards and sideways
just to get zero rake around the corners, I may as well think a bit skewed
as well to get positive rake.

Good luck. You can project the finished workpiece shape onto the top of the
tool, with rake, by using a CAD program.

Watch out for the effectively changing center height as you plunge the tool,
if you choose to use top rake. It's like patting your head while rubbing
your stomach. g

--
Ed Huntress


--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

On 04/09/2011 02:19 PM, Ed Huntress wrote:
"Tim wrote in message
...
On 04/09/2011 10:20 AM, Ed Huntress wrote:
"Tim wrote in message
...
On Sat, 09 Apr 2011 12:41:01 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter
model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen
up,
grit my teeth, and push into a spinning piece of aluminum to make
my desired profile.

Is this a sensible thing to consider? Is there some other way
(short of CNC machining) to easily duplicate cavities in the ends
of a bunch of aluminum tubes?

Hobbing?

P.S. Not the gear-making process, this one:

http://tinyurl.com/hobbing

Well, the cavity is cylindrically symmetrical, so I'm not sure that
"hobbing" is the right term.

It's probably been a screw machine operation since 1920, or at least
some sort of tracing operation.

Sigh -- I'm always behind the curve.

I must be dull tonight, but I can't visualize what you're doing. Are
you talking about some kind of forming in the axis of a spinning
tube,
or turning the outside diameter to a profile?


Let me know if you can see this, it should explain:

http://www.hippocketaeronautics.com/...rum/index.php?
action=dlattach;topic=5869.0;attach=53766;image

Sorry, that isn't working for me. Or I don't know how to get there.

I'll wait and see what others come up with. It sounds like you want to
do some kind of form-tool operation, in which case, as someone
mentioned, the key is the length of contact for the cutting edge. I've
seen some really long ones -- the whole length of a chess piece, for
example -- performed on South Bend 10" lathes with complete success.

But I may be on the wrong track. I'll wait.

Is this better?

http://www.wescottdesign.com/temp/wheel.jpg

Aha. Yes, that does it.


It's a Smithy lathe, and a form tool is what I was thinking about,
except
that instead of applying it radially it'd be applied axially -- which
means that the ratio of cutting speeds from inside to outside would be
huge.

I actually sorta-kinda used a forming tool for the tire profile, then
cut
the hub by hand -- I just want something that'll do a more consistent
job
of it.

--
http://www.wescottdesign.com

Ok. I get it.

Form tools are tricky in a couple of ways. There are basically two types:
conventional tools, with a positive rake throughout, and skiving tools,
which have zero rake. The former type lead to some very tricky problems
of
geometry, which would be made much worse by approaching the work axially.
They're often held in a special dovetail toolholder.

So, to be practical, the thing to try probably is a skiving tool -- a
form
tool with no top rake. That allows you to use a flat top on the tool that
you just cut back for the form, and then cut front clearance and, if
necessary, some local side clearance -- either left or right, or both on
one
tool. This is still a tricky operation, as you can imagine by thinking
about
what you're trying to accomplish. It's usually done on optical profile
grinders or wirecut EDMs.

However, I've made little skiving tools, on the order of yours, just by
grinding away with a Dremel. I had lots more spare time in those days.

The difference in cutting speed across the face would be a problem in
steel,
but your molds (compression molds?) look like they're aluminum. The only
problem there is possible chatter or roughness near the center, made
worse
by the zero top rake. My experience with facing tells me it could be
perfectly Ok. Just keep the tool sharp and lube it with something to
enhance
surface finish near the center.

I'd run the tool pretty fast, because you can get away with a lot using
HSS
in aluminum, which should help your finish near the center.

Form tools usually require some experimenting, anyway. Just try it and
see
what happens.

(Hubbing, sometimes called "hobbing" and described above, is likely to
present more problems than form-cutting, IMO.)


Ed:

Thank you -- this is just the sort of discussion I was looking for. You
forgot to mention that "rake" applies both to the face and the outside
edges (I learned that already, as I made a partial tool for the tire
portion of this, and had to put rake into it already).

Well, if you're plunging the tool straight in, the rake (if any) is on the
top of the tool, and the relief, for clearance, is on the front and,
theoretically, both sides. But real-world skiving tools typically have no
side clearance. The side clearance appears automatically as a result of
front clearance and angles in the shape you're cutting.

But sometimes not. It depends on the geometry of the cut. You can think
through it. Watch for decreasing clearance on the outside of the tool, due
to the radius of the workpiece.


I'll give this a whirl, and see how I do. I think I'm going to attempt a
conventional tool, as I'm going to have to think backwards and sideways
just to get zero rake around the corners, I may as well think a bit skewed
as well to get positive rake.

Good luck. You can project the finished workpiece shape onto the top of the
tool, with rake, by using a CAD program.

Watch out for the effectively changing center height as you plunge the tool,
if you choose to use top rake. It's like patting your head while rubbing
your stomach.g

Whoops -- I was seeing "rake" and thinking "clearance". Skiving is
probably the right way to go, unless I _really_ want to get busy with
the dremel tool.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

"Tim Wescott" wrote in message
...
On 04/09/2011 02:19 PM, Ed Huntress wrote:
"Tim wrote in message
...
On 04/09/2011 10:20 AM, Ed Huntress wrote:
"Tim wrote in message
...
On Sat, 09 Apr 2011 12:41:01 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter
model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen
up,
grit my teeth, and push into a spinning piece of aluminum to
make
my desired profile.

Is this a sensible thing to consider? Is there some other way
(short of CNC machining) to easily duplicate cavities in the
ends
of a bunch of aluminum tubes?

Hobbing?

P.S. Not the gear-making process, this one:

http://tinyurl.com/hobbing

Well, the cavity is cylindrically symmetrical, so I'm not sure
that
"hobbing" is the right term.

It's probably been a screw machine operation since 1920, or at
least
some sort of tracing operation.

Sigh -- I'm always behind the curve.

I must be dull tonight, but I can't visualize what you're doing.
Are
you talking about some kind of forming in the axis of a spinning
tube,
or turning the outside diameter to a profile?


Let me know if you can see this, it should explain:

http://www.hippocketaeronautics.com/...rum/index.php?
action=dlattach;topic=5869.0;attach=53766;image

Sorry, that isn't working for me. Or I don't know how to get there.

I'll wait and see what others come up with. It sounds like you want
to
do some kind of form-tool operation, in which case, as someone
mentioned, the key is the length of contact for the cutting edge.
I've
seen some really long ones -- the whole length of a chess piece, for
example -- performed on South Bend 10" lathes with complete success.

But I may be on the wrong track. I'll wait.

Is this better?

http://www.wescottdesign.com/temp/wheel.jpg

Aha. Yes, that does it.


It's a Smithy lathe, and a form tool is what I was thinking about,
except
that instead of applying it radially it'd be applied axially -- which
means that the ratio of cutting speeds from inside to outside would be
huge.

I actually sorta-kinda used a forming tool for the tire profile, then
cut
the hub by hand -- I just want something that'll do a more consistent
job
of it.

--
http://www.wescottdesign.com

Ok. I get it.

Form tools are tricky in a couple of ways. There are basically two
types:
conventional tools, with a positive rake throughout, and skiving tools,
which have zero rake. The former type lead to some very tricky problems
of
geometry, which would be made much worse by approaching the work
axially.
They're often held in a special dovetail toolholder.

So, to be practical, the thing to try probably is a skiving tool -- a
form
tool with no top rake. That allows you to use a flat top on the tool
that
you just cut back for the form, and then cut front clearance and, if
necessary, some local side clearance -- either left or right, or both
on
one
tool. This is still a tricky operation, as you can imagine by thinking
about
what you're trying to accomplish. It's usually done on optical profile
grinders or wirecut EDMs.

However, I've made little skiving tools, on the order of yours, just by
grinding away with a Dremel. I had lots more spare time in those days.

The difference in cutting speed across the face would be a problem in
steel,
but your molds (compression molds?) look like they're aluminum. The
only
problem there is possible chatter or roughness near the center, made
worse
by the zero top rake. My experience with facing tells me it could be
perfectly Ok. Just keep the tool sharp and lube it with something to
enhance
surface finish near the center.

I'd run the tool pretty fast, because you can get away with a lot using
HSS
in aluminum, which should help your finish near the center.

Form tools usually require some experimenting, anyway. Just try it and
see
what happens.

(Hubbing, sometimes called "hobbing" and described above, is likely to
present more problems than form-cutting, IMO.)


Ed:

Thank you -- this is just the sort of discussion I was looking for. You
forgot to mention that "rake" applies both to the face and the outside
edges (I learned that already, as I made a partial tool for the tire
portion of this, and had to put rake into it already).

Well, if you're plunging the tool straight in, the rake (if any) is on
the
top of the tool, and the relief, for clearance, is on the front and,
theoretically, both sides. But real-world skiving tools typically have no
side clearance. The side clearance appears automatically as a result of
front clearance and angles in the shape you're cutting.

But sometimes not. It depends on the geometry of the cut. You can think
through it. Watch for decreasing clearance on the outside of the tool,
due
to the radius of the workpiece.


I'll give this a whirl, and see how I do. I think I'm going to attempt
a
conventional tool, as I'm going to have to think backwards and sideways
just to get zero rake around the corners, I may as well think a bit
skewed
as well to get positive rake.

Good luck. You can project the finished workpiece shape onto the top of
the
tool, with rake, by using a CAD program.

Watch out for the effectively changing center height as you plunge the
tool,
if you choose to use top rake. It's like patting your head while rubbing
your stomach.g

Whoops -- I was seeing "rake" and thinking "clearance". Skiving is
probably the right way to go, unless I _really_ want to get busy with the
dremel tool.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Yes, keep it as simple as you can. Don't forget that the workpiece radius
will take care of clearance on the inside-facing features of the tool, but
you'll need extra clearance on the outside-facing features, because of the
radius of the workpiece.

Making these tools is almost a lost art, although dovetail form tools with
top rake are still common in production. With CAD and a CNC wirecut EDM or
CNC/optical profile grinder, the whole process of making them in simpler in
commercial toolmaking. But you can make perfectly good skiving tools by
hand.

--
Ed Huntress

"Tim Wescott" wrote in message
...
On 04/09/2011 02:19 PM, Ed Huntress wrote:
"Tim wrote in message
...
On 04/09/2011 10:20 AM, Ed Huntress wrote:
"Tim wrote in message
...
On Sat, 09 Apr 2011 12:41:01 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter
model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just sharpen
up,
grit my teeth, and push into a spinning piece of aluminum to
make
my desired profile.

Is this a sensible thing to consider? Is there some other way
(short of CNC machining) to easily duplicate cavities in the
ends
of a bunch of aluminum tubes?

Since you got me started on this, g here are two other things to consider.

First, on regular skiving tools, which plunge in from the side of the work,
it's common when cutting steel to set the top of the tool 0.002" or so above
the centerline to get some effective top rake, or, more commonly, 0.002"
*below* the centerline, to avoid the need, in shallow cuts, for front
clearance. When wire EDM first came on the market, a lot of tools were made
this way, because you couldn't tilt the wire on those early ones. They just
left them square on the ends, with no clearance.

This probably means little when cutting aluminum and plunging from the end
is no place to try it, anyway. But if you like working with these tools and
try them from the side, particularly on steel, keep it in mind.

The other point, about your workpiece material: It looks like you're
machining wrought barstock, probably 6061 or 2024. I don't know the figures
for 6061 but in 2024, the thermal conductivity in the T3 or T4 state is
roughly half that of the same material in the annealed state.

I'm sure you won't be annealing the material just from heating the foam, but
if you have in mind to anneal the tool first, to get better conductivity,
keep in mind that the material will revert to the tempered state in a couple
of hours if you don't give it a full anneal.

Neither of things probably matter, but I hate to leave details hanging
around, just in case. d8-)

--
Ed Huntress

On Sat, 09 Apr 2011 18:27:27 -0400, Ed Huntress wrote:

"Tim Wescott" wrote in message
...
On 04/09/2011 02:19 PM, Ed Huntress wrote:
"Tim wrote in message
...
On 04/09/2011 10:20 AM, Ed Huntress wrote:
"Tim wrote in message
...
On Sat, 09 Apr 2011 12:41:01 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter
model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just
sharpen up,
grit my teeth, and push into a spinning piece of aluminum to
make
my desired profile.

Is this a sensible thing to consider? Is there some other
way (short of CNC machining) to easily duplicate cavities in
the ends
of a bunch of aluminum tubes?

Since you got me started on this, g here are two other things to
consider.

The other point, about your workpiece material: It looks like you're
machining wrought barstock, probably 6061 or 2024. I don't know the
figures for 6061 but in 2024, the thermal conductivity in the T3 or T4
state is roughly half that of the same material in the annealed state.

I'm sure you won't be annealing the material just from heating the foam,
but if you have in mind to anneal the tool first, to get better
conductivity, keep in mind that the material will revert to the tempered
state in a couple of hours if you don't give it a full anneal.

Neither of things probably matter, but I hate to leave details hanging
around, just in case. d8-)

It is, indeed, 6061, heat treated to some three or four-digit code with a
"T6" at the beginning. So I'm assuming that for my purposes it's T6.

I'm mostly thinking of heat conductivity in competition with plastic or
wood. Since the current process involves clamping up one mold with a
pair of vice grips and tossing the assembly in the oven for half an hour,
I can't claim that speed is of the essence. Before I start worrying
about the thermal conductivity of the aluminum I should think about
better ways to conduct heat _to_ the aluminum, like fins and a fan in the
oven, or immersing the mold in boiling water, instead of putting it in a
still-air oven.

I'm still tickled that I can make a wheel for a 7-gram rubber power plane
that barely registers on my 100mg-per-step scale -- it'll take me a while
to get past that.

--
http://www.wescottdesign.com

"Tim Wescott" wrote in message
...
On Sat, 09 Apr 2011 18:27:27 -0400, Ed Huntress wrote:

"Tim Wescott" wrote in message
...
On 04/09/2011 02:19 PM, Ed Huntress wrote:
"Tim wrote in message
...
On 04/09/2011 10:20 AM, Ed Huntress wrote:
"Tim wrote in message
...
On Sat, 09 Apr 2011 12:41:01 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim Wescott
wrote:

I want to make a bunch of identical molds, for 3/4" diameter
model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just
sharpen up,
grit my teeth, and push into a spinning piece of aluminum to
make
my desired profile.

Is this a sensible thing to consider? Is there some other
way (short of CNC machining) to easily duplicate cavities in
the ends
of a bunch of aluminum tubes?

Since you got me started on this, g here are two other things to
consider.

The other point, about your workpiece material: It looks like you're
machining wrought barstock, probably 6061 or 2024. I don't know the
figures for 6061 but in 2024, the thermal conductivity in the T3 or T4
state is roughly half that of the same material in the annealed state.

I'm sure you won't be annealing the material just from heating the foam,
but if you have in mind to anneal the tool first, to get better
conductivity, keep in mind that the material will revert to the tempered
state in a couple of hours if you don't give it a full anneal.

Neither of things probably matter, but I hate to leave details hanging
around, just in case. d8-)

It is, indeed, 6061, heat treated to some three or four-digit code with a
"T6" at the beginning. So I'm assuming that for my purposes it's T6.

Yes. The other codes have meaning, but they aren't relevant here.


I'm mostly thinking of heat conductivity in competition with plastic or
wood. Since the current process involves clamping up one mold with a
pair of vice grips and tossing the assembly in the oven for half an hour,
I can't claim that speed is of the essence. Before I start worrying
about the thermal conductivity of the aluminum I should think about
better ways to conduct heat _to_ the aluminum, like fins and a fan in the
oven, or immersing the mold in boiling water, instead of putting it in a
still-air oven.

How hot does it have to be?


I'm still tickled that I can make a wheel for a 7-gram rubber power plane
that barely registers on my 100mg-per-step scale -- it'll take me a while
to get past that.

That's very cool. Do you know who Bob Hatschek is? (Or have you heard of the
Hatschek Hook?) He held some free-flight glider records at one time, and he
got me interested in indoor rubber. I made some with film-covered wings
about 20 years ago but it was only a passing interest.

The Hatschek Hook, which was a towline release hook for competition gliders,
and which Bob sold worldwide, is the only product I ever knew of that was
made exclusively on a Unimat. g

Bob was an editor at _American Machinist_ in its glory days.

--
Ed Huntress


--
http://www.wescottdesign.com

On Sat, 09 Apr 2011 22:39:06 -0400, Ed Huntress wrote:

"Tim Wescott" wrote in message
...
On Sat, 09 Apr 2011 18:27:27 -0400, Ed Huntress wrote:

"Tim Wescott" wrote in message
...
On 04/09/2011 02:19 PM, Ed Huntress wrote:
"Tim wrote in message
...
On 04/09/2011 10:20 AM, Ed Huntress wrote:
"Tim wrote in message
...
On Sat, 09 Apr 2011 12:41:01 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim
Wescott wrote:

I want to make a bunch of identical molds, for 3/4"
diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just
sharpen up,
grit my teeth, and push into a spinning piece of aluminum
to make
my desired profile.

Is this a sensible thing to consider? Is there some other
way (short of CNC machining) to easily duplicate cavities
in the ends
of a bunch of aluminum tubes?

Since you got me started on this, g here are two other things to
consider.

The other point, about your workpiece material: It looks like you're
machining wrought barstock, probably 6061 or 2024. I don't know the
figures for 6061 but in 2024, the thermal conductivity in the T3 or T4
state is roughly half that of the same material in the annealed state.

I'm sure you won't be annealing the material just from heating the
foam, but if you have in mind to anneal the tool first, to get better
conductivity, keep in mind that the material will revert to the
tempered state in a couple of hours if you don't give it a full
anneal.

Neither of things probably matter, but I hate to leave details hanging
around, just in case. d8-)

It is, indeed, 6061, heat treated to some three or four-digit code with
a "T6" at the beginning. So I'm assuming that for my purposes it's T6.

Yes. The other codes have meaning, but they aren't relevant here.


I'm mostly thinking of heat conductivity in competition with plastic or
wood. Since the current process involves clamping up one mold with a
pair of vice grips and tossing the assembly in the oven for half an
hour, I can't claim that speed is of the essence. Before I start
worrying about the thermal conductivity of the aluminum I should think
about better ways to conduct heat _to_ the aluminum, like fins and a
fan in the oven, or immersing the mold in boiling water, instead of
putting it in a still-air oven.

How hot does it have to be?

Damned if I know. I _do_ know that if I bake it for 30 minutes at 150
degrees it forms, but doesn't seem to do so as well. If I bake it for 30
minutes at 200, it works better. I should try 250 -- I expect I'll melt
the plastic, but that'll be a learning experience, right?

--
http://www.wescottdesign.com

"Tim Wescott" wrote in message
...
On Sat, 09 Apr 2011 22:39:06 -0400, Ed Huntress wrote:

"Tim Wescott" wrote in message
...
On Sat, 09 Apr 2011 18:27:27 -0400, Ed Huntress wrote:

"Tim Wescott" wrote in message
...
On 04/09/2011 02:19 PM, Ed Huntress wrote:
"Tim wrote in message
...
On 04/09/2011 10:20 AM, Ed Huntress wrote:
"Tim wrote in message
...
On Sat, 09 Apr 2011 12:41:01 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Sat, 09 Apr 2011 00:32:29 -0400, Ed Huntress wrote:

"Tim wrote in message
...
On Fri, 08 Apr 2011 23:16:17 -0400, Spehro Pefhany wrote:

On Fri, 08 Apr 2011 23:08:55 -0400, Spehro Pefhany
wrote:

On Fri, 08 Apr 2011 21:38:25 -0500, the renowned Tim
Wescott wrote:

I want to make a bunch of identical molds, for 3/4"
diameter model
airplane wheels.

I'm envisioning a tool that's 3/8" across, that I just
sharpen up,
grit my teeth, and push into a spinning piece of aluminum
to make
my desired profile.

Is this a sensible thing to consider? Is there some other
way (short of CNC machining) to easily duplicate cavities
in the ends
of a bunch of aluminum tubes?

Since you got me started on this, g here are two other things to
consider.

The other point, about your workpiece material: It looks like you're
machining wrought barstock, probably 6061 or 2024. I don't know the
figures for 6061 but in 2024, the thermal conductivity in the T3 or T4
state is roughly half that of the same material in the annealed state.

I'm sure you won't be annealing the material just from heating the
foam, but if you have in mind to anneal the tool first, to get better
conductivity, keep in mind that the material will revert to the
tempered state in a couple of hours if you don't give it a full
anneal.

Neither of things probably matter, but I hate to leave details hanging
around, just in case. d8-)

It is, indeed, 6061, heat treated to some three or four-digit code with
a "T6" at the beginning. So I'm assuming that for my purposes it's T6.

Yes. The other codes have meaning, but they aren't relevant here.


I'm mostly thinking of heat conductivity in competition with plastic or
wood. Since the current process involves clamping up one mold with a
pair of vice grips and tossing the assembly in the oven for half an
hour, I can't claim that speed is of the essence. Before I start
worrying about the thermal conductivity of the aluminum I should think
about better ways to conduct heat _to_ the aluminum, like fins and a
fan in the oven, or immersing the mold in boiling water, instead of
putting it in a still-air oven.

How hot does it have to be?

Damned if I know. I _do_ know that if I bake it for 30 minutes at 150
degrees it forms, but doesn't seem to do so as well. If I bake it for 30
minutes at 200, it works better. I should try 250 -- I expect I'll melt
the plastic, but that'll be a learning experience, right?

Yeah, it sounds like it's the right time for some experiments. And if the
right temperature turns out to be 212 F, you may have a couple of problems
solved at once.

--
Ed Huntress


--
http://www.wescottdesign.com

On Sat, 09 Apr 2011 13:47:57 -0700, Tim Wescott
wrote:

SNIP
Because the process is to put a flat piece of foam in the mold, clamp
it, then chuck it in an oven to heat. The foam* softens and expands to
meet the edges of the mold. Then you take it out of the oven and let it
cool, and voile! A part (or a mess -- it's best to enter any sort of
casting process with your eyes open).
SNIP

I've never done it, but it reads like what you want to do would be
achieved with blow molding. You could even put "tread" on the tire
then.

Take care. Good Luck.

Brian Lawson,
Bothwell, Ontario.

On 04/10/2011 03:19 PM, Brian Lawson wrote:
On Sat, 09 Apr 2011 13:47:57 -0700, Tim
wrote:

SNIP
Because the process is to put a flat piece of foam in the mold, clamp
it, then chuck it in an oven to heat. The foam* softens and expands to
meet the edges of the mold. Then you take it out of the oven and let it
cool, and voile! A part (or a mess -- it's best to enter any sort of
casting process with your eyes open).
SNIP

I've never done it, but it reads like what you want to do would be
achieved with blow molding. You could even put "tread" on the tire
then.

Interesting thought. Probably beyond the sophistication I was aiming
for (mostly hand tools, with bits & bobs turned on the lathe).

And I think blow molding is for styrofoam beads, which isn't what Depron
is. I'm not sure that I've seen styrofoam beads that are small enough
for a wheel that's 3/16" thick and 3/4" diameter.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

On Mon, 11 Apr 2011 09:53:05 -0700, Tim Wescott
wrote:

On 04/10/2011 03:19 PM, Brian Lawson wrote:
On Sat, 09 Apr 2011 13:47:57 -0700, Tim
wrote:

SNIP
Because the process is to put a flat piece of foam in the mold, clamp
it, then chuck it in an oven to heat. The foam* softens and expands to
meet the edges of the mold. Then you take it out of the oven and let it
cool, and voile! A part (or a mess -- it's best to enter any sort of
casting process with your eyes open).
SNIP

I've never done it, but it reads like what you want to do would be
achieved with blow molding. You could even put "tread" on the tire
then.

Interesting thought. Probably beyond the sophistication I was aiming
for (mostly hand tools, with bits & bobs turned on the lathe).

And I think blow molding is for styrofoam beads, which isn't what Depron
is. I'm not sure that I've seen styrofoam beads that are small enough
for a wheel that's 3/16" thick and 3/4" diameter.

The only blow molding I'm familiar with is for extremely high
production of containers like milk bottles, shampoo bottles, etc. A
parison is extruded, clamped in the mold, then the parison is blown
against the mold with air. Usually a bunch of molds are arranged on a
wheel or other contraption, and open and close around the parisons
automatically. Spits out containers at a high rate. There may be a
lab analog of this process for testing, don't know.

Foam sheet is turned into cups in thermoforming machines. Large
plastic sheets are shaped into various objects via vacuum forming.
There are hobbyist versions of that process. Other processes include
rotomolding (large tanks), injection molding (solid parts), injection
blow molding, injection stretch blow molding, etc.

Pete Keillor

Pete Keillor

On Mon, 11 Apr 2011 09:53:05 -0700, Tim Wescott
wrote:

On 04/10/2011 03:19 PM, Brian Lawson wrote:
On Sat, 09 Apr 2011 13:47:57 -0700, Tim
wrote:

SNIP
Because the process is to put a flat piece of foam in the mold, clamp
it, then chuck it in an oven to heat. The foam* softens and expands to
meet the edges of the mold. Then you take it out of the oven and let it
cool, and voile! A part (or a mess -- it's best to enter any sort of
casting process with your eyes open).
SNIP

I've never done it, but it reads like what you want to do would be
achieved with blow molding. You could even put "tread" on the tire
then.

Interesting thought. Probably beyond the sophistication I was aiming
for (mostly hand tools, with bits & bobs turned on the lathe).

And I think blow molding is for styrofoam beads, which isn't what Depron
is. I'm not sure that I've seen styrofoam beads that are small enough
for a wheel that's 3/16" thick and 3/4" diameter.

Blow molding is what is used to make plastic bottles. Basically a
thick plastic goober* is extruded vertically (the plastic has to be of
a type that has a viscosity high enough that it doesn't drool off the
nozzle). The mold clamps around the parison (sealing off the bottom
end) and then it's inflated with compressed air through the top. After
it cools a bit, a nice sharp knife called a moil** slices off a bit at
the top (usually the top of the neck where the closure goes), the mold
opens and the bottle falls out. The tab at the bottom is stripped off
automatically or manually.

I don't know how you'd do this without a rather complex machine- but
the molds are relatively easy because the pressures are very low by
molding standards.


* It's called a "parison", a term borrowed from a French term
"paraison", relating to glass blowing.

** not joking, that's what it's called-- at least it's not spelled
"mohel".

On 04/11/2011 10:14 AM, Spehro Pefhany wrote:
On Mon, 11 Apr 2011 09:53:05 -0700, Tim
wrote:

On 04/10/2011 03:19 PM, Brian Lawson wrote:
On Sat, 09 Apr 2011 13:47:57 -0700, Tim
wrote:

SNIP
Because the process is to put a flat piece of foam in the mold, clamp
it, then chuck it in an oven to heat. The foam* softens and expands to
meet the edges of the mold. Then you take it out of the oven and let it
cool, and voile! A part (or a mess -- it's best to enter any sort of
casting process with your eyes open).
SNIP

I've never done it, but it reads like what you want to do would be
achieved with blow molding. You could even put "tread" on the tire
then.

Interesting thought. Probably beyond the sophistication I was aiming
for (mostly hand tools, with bits& bobs turned on the lathe).

And I think blow molding is for styrofoam beads, which isn't what Depron
is. I'm not sure that I've seen styrofoam beads that are small enough
for a wheel that's 3/16" thick and 3/4" diameter.

Blow molding is what is used to make plastic bottles. Basically a
thick plastic goober* is extruded vertically (the plastic has to be of
a type that has a viscosity high enough that it doesn't drool off the
nozzle). The mold clamps around the parison (sealing off the bottom
end) and then it's inflated with compressed air through the top. After
it cools a bit, a nice sharp knife called a moil** slices off a bit at
the top (usually the top of the neck where the closure goes), the mold
opens and the bottle falls out. The tab at the bottom is stripped off
automatically or manually.

I don't know how you'd do this without a rather complex machine- but
the molds are relatively easy because the pressures are very low by
molding standards.


* It's called a "parison", a term borrowed from a French term
"paraison", relating to glass blowing.

** not joking, that's what it's called-- at least it's not spelled
"mohel".

Ah -- that wouldn't work, and once again I'm confused in my terminology.
The application is very weight-sensitive: indoor rubber power
competition rules call out a seven gram minimum weight for this sort of
plane*, and that minimum is _very_ hard to build down to. I've got a
pair of wheels that look injection molded, but have about the same wall
width as most of the blow molded things I've seen. They weigh in at
600mg for the pair, while my wheels weigh in at less than 100mg each.

So what do you call molding with styrofoam beads?

* AFAIK, just about anything with wheels and a minimum weight calls out
7 grams -- 14 grams for outdoor.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

On Mon, 11 Apr 2011 11:26:15 -0700, Tim Wescott
wrote:

So what do you call molding with styrofoam beads?

EPS (Expanded Poly Styrene) molding.

I looked into setting up a line once, and it was too much hassle-
required a boiler to heat the beads etc., but I think this process
might actually be possible on a table top using your aluminum molds.
Worth looking into. Alternatively maybe some kind of reaction molding
(RIM) that leaves the wheels full of bubbles but with a tough skin.
Getting the materials in sensible quantities for you could be a
challenge.

* AFAIK, just about anything with wheels and a minimum weight calls out
7 grams -- 14 grams for outdoor.

"Tim Wescott" wrote in message
...
On 04/11/2011 10:14 AM, Spehro Pefhany wrote:
On Mon, 11 Apr 2011 09:53:05 -0700, Tim
wrote:

On 04/10/2011 03:19 PM, Brian Lawson wrote:
On Sat, 09 Apr 2011 13:47:57 -0700, Tim
wrote:

SNIP
Because the process is to put a flat piece of foam in the mold, clamp
it, then chuck it in an oven to heat. The foam* softens and expands
to
meet the edges of the mold. Then you take it out of the oven and let
it
cool, and voile! A part (or a mess -- it's best to enter any sort of
casting process with your eyes open).
SNIP

I've never done it, but it reads like what you want to do would be
achieved with blow molding. You could even put "tread" on the tire
then.

Interesting thought. Probably beyond the sophistication I was aiming
for (mostly hand tools, with bits& bobs turned on the lathe).

And I think blow molding is for styrofoam beads, which isn't what Depron
is. I'm not sure that I've seen styrofoam beads that are small enough
for a wheel that's 3/16" thick and 3/4" diameter.

Blow molding is what is used to make plastic bottles. Basically a
thick plastic goober* is extruded vertically (the plastic has to be of
a type that has a viscosity high enough that it doesn't drool off the
nozzle). The mold clamps around the parison (sealing off the bottom
end) and then it's inflated with compressed air through the top. After
it cools a bit, a nice sharp knife called a moil** slices off a bit at
the top (usually the top of the neck where the closure goes), the mold
opens and the bottle falls out. The tab at the bottom is stripped off
automatically or manually.

I don't know how you'd do this without a rather complex machine- but
the molds are relatively easy because the pressures are very low by
molding standards.


* It's called a "parison", a term borrowed from a French term
"paraison", relating to glass blowing.

** not joking, that's what it's called-- at least it's not spelled
"mohel".

Ah -- that wouldn't work, and once again I'm confused in my terminology.
The application is very weight-sensitive: indoor rubber power competition
rules call out a seven gram minimum weight for this sort of plane*, and
that minimum is _very_ hard to build down to. I've got a pair of wheels
that look injection molded, but have about the same wall width as most of
the blow molded things I've seen. They weigh in at 600mg for the pair,
while my wheels weigh in at less than 100mg each.


As others have said, blow molding is a process carried out with solid resins
in a semi-liquid state. I've never heard of anything that small being blow
molded, nor have I heard of doing it with styrene beads, but technology
marches on and I mostly don't.


So what do you call molding with styrofoam beads?


Molding polystyrene beads. g (Styrofoam is a trademark name for EPF --
expanded polystyrene foam, and it's close-cell material. Molding the beads
produces a porous material.)

Molding polystyrene beads is done by filling a mold with them and then
injecting steam or hot air. I don't think you want to go there.

--
Ed Huntress

* AFAIK, just about anything with wheels and a minimum weight calls out 7
grams -- 14 grams for outdoor.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Ed Huntress wrote:
"Tim Wescott" wrote in message
...
On 04/11/2011 10:14 AM, Spehro Pefhany wrote:
On Mon, 11 Apr 2011 09:53:05 -0700, Tim
wrote:

On 04/10/2011 03:19 PM, Brian Lawson wrote:
On Sat, 09 Apr 2011 13:47:57 -0700, Tim
wrote:

SNIP
Because the process is to put a flat piece of foam in the mold,
clamp it, then chuck it in an oven to heat. The foam* softens
and expands to
meet the edges of the mold. Then you take it out of the oven
and let it
cool, and voile! A part (or a mess -- it's best to enter any
sort of casting process with your eyes open).
SNIP

I've never done it, but it reads like what you want to do would be
achieved with blow molding. You could even put "tread" on the
tire then.

Interesting thought. Probably beyond the sophistication I was
aiming for (mostly hand tools, with bits& bobs turned on the
lathe). And I think blow molding is for styrofoam beads, which isn't
what
Depron is. I'm not sure that I've seen styrofoam beads that are
small enough for a wheel that's 3/16" thick and 3/4" diameter.

Blow molding is what is used to make plastic bottles. Basically a
thick plastic goober* is extruded vertically (the plastic has to be
of a type that has a viscosity high enough that it doesn't drool
off the nozzle). The mold clamps around the parison (sealing off
the bottom end) and then it's inflated with compressed air through
the top. After it cools a bit, a nice sharp knife called a moil**
slices off a bit at the top (usually the top of the neck where the
closure goes), the mold opens and the bottle falls out. The tab at
the bottom is stripped off automatically or manually.

I don't know how you'd do this without a rather complex machine- but
the molds are relatively easy because the pressures are very low by
molding standards.


* It's called a "parison", a term borrowed from a French term
"paraison", relating to glass blowing.

** not joking, that's what it's called-- at least it's not spelled
"mohel".

Ah -- that wouldn't work, and once again I'm confused in my
terminology. The application is very weight-sensitive: indoor rubber
power competition rules call out a seven gram minimum weight for
this sort of plane*, and that minimum is _very_ hard to build down
to. I've got a pair of wheels that look injection molded, but have
about the same wall width as most of the blow molded things I've
seen. They weigh in at 600mg for the pair, while my wheels weigh in
at less than 100mg each.


As others have said, blow molding is a process carried out with solid
resins in a semi-liquid state. I've never heard of anything that
small being blow molded, nor have I heard of doing it with styrene
beads, but technology marches on and I mostly don't.


So what do you call molding with styrofoam beads?


Molding polystyrene beads. g (Styrofoam is a trademark name for EPF
-- expanded polystyrene foam, and it's close-cell material. Molding
the beads produces a porous material.)

Molding polystyrene beads is done by filling a mold with them and then
injecting steam or hot air. I don't think you want to go there.


* AFAIK, just about anything with wheels and a minimum weight calls
out 7 grams -- 14 grams for outdoor.

--

Tim Wescott

what about rotational molding? that's usually done with powder, and the
wheels would be hollow when done.

"chaniarts" wrote in message
...
Ed Huntress wrote:
"Tim Wescott" wrote in message
...
On 04/11/2011 10:14 AM, Spehro Pefhany wrote:
On Mon, 11 Apr 2011 09:53:05 -0700, Tim
wrote:

On 04/10/2011 03:19 PM, Brian Lawson wrote:
On Sat, 09 Apr 2011 13:47:57 -0700, Tim
wrote:

SNIP
Because the process is to put a flat piece of foam in the mold,
clamp it, then chuck it in an oven to heat. The foam* softens
and expands to
meet the edges of the mold. Then you take it out of the oven
and let it
cool, and voile! A part (or a mess -- it's best to enter any
sort of casting process with your eyes open).
SNIP

I've never done it, but it reads like what you want to do would be
achieved with blow molding. You could even put "tread" on the
tire then.

Interesting thought. Probably beyond the sophistication I was
aiming for (mostly hand tools, with bits& bobs turned on the
lathe). And I think blow molding is for styrofoam beads, which isn't
what
Depron is. I'm not sure that I've seen styrofoam beads that are
small enough for a wheel that's 3/16" thick and 3/4" diameter.

Blow molding is what is used to make plastic bottles. Basically a
thick plastic goober* is extruded vertically (the plastic has to be
of a type that has a viscosity high enough that it doesn't drool
off the nozzle). The mold clamps around the parison (sealing off
the bottom end) and then it's inflated with compressed air through
the top. After it cools a bit, a nice sharp knife called a moil**
slices off a bit at the top (usually the top of the neck where the
closure goes), the mold opens and the bottle falls out. The tab at
the bottom is stripped off automatically or manually.

I don't know how you'd do this without a rather complex machine- but
the molds are relatively easy because the pressures are very low by
molding standards.


* It's called a "parison", a term borrowed from a French term
"paraison", relating to glass blowing.

** not joking, that's what it's called-- at least it's not spelled
"mohel".

Ah -- that wouldn't work, and once again I'm confused in my
terminology. The application is very weight-sensitive: indoor rubber
power competition rules call out a seven gram minimum weight for
this sort of plane*, and that minimum is _very_ hard to build down
to. I've got a pair of wheels that look injection molded, but have
about the same wall width as most of the blow molded things I've
seen. They weigh in at 600mg for the pair, while my wheels weigh in
at less than 100mg each.


As others have said, blow molding is a process carried out with solid
resins in a semi-liquid state. I've never heard of anything that
small being blow molded, nor have I heard of doing it with styrene
beads, but technology marches on and I mostly don't.


So what do you call molding with styrofoam beads?


Molding polystyrene beads. g (Styrofoam is a trademark name for EPF
-- expanded polystyrene foam, and it's close-cell material. Molding
the beads produces a porous material.)

Molding polystyrene beads is done by filling a mold with them and then
injecting steam or hot air. I don't think you want to go there.


* AFAIK, just about anything with wheels and a minimum weight calls
out 7 grams -- 14 grams for outdoor.

--

Tim Wescott

what about rotational molding? that's usually done with powder, and the
wheels would be hollow when done.

That's for things like industrial drums and plastic kayaks. I don't think
you'd have much luck in such small sizes.

If it were me, the first thing I'd try is pressing the wheels out of
Styrofoam (closed-cell) or beadboard (open-cell) with hot mold halves. It
doesn't take a lot of heat to make polystyrene pliable. You'd have maximum
density at the wheel tread, and a bit of skin on the outside. You probably
could get away with a low-density polystyrene foam.

--
Ed Huntress

"Ed Huntress" wrote in message
...

"chaniarts" wrote in message
...
Ed Huntress wrote:
"Tim Wescott" wrote in message
...
On 04/11/2011 10:14 AM, Spehro Pefhany wrote:
On Mon, 11 Apr 2011 09:53:05 -0700, Tim
wrote:

On 04/10/2011 03:19 PM, Brian Lawson wrote:
On Sat, 09 Apr 2011 13:47:57 -0700, Tim
wrote:

SNIP
Because the process is to put a flat piece of foam in the mold,
clamp it, then chuck it in an oven to heat. The foam* softens
and expands to
meet the edges of the mold. Then you take it out of the oven
and let it
cool, and voile! A part (or a mess -- it's best to enter any
sort of casting process with your eyes open).
SNIP

I've never done it, but it reads like what you want to do would be
achieved with blow molding. You could even put "tread" on the
tire then.

Interesting thought. Probably beyond the sophistication I was
aiming for (mostly hand tools, with bits& bobs turned on the
lathe). And I think blow molding is for styrofoam beads, which isn't
what
Depron is. I'm not sure that I've seen styrofoam beads that are
small enough for a wheel that's 3/16" thick and 3/4" diameter.

Blow molding is what is used to make plastic bottles. Basically a
thick plastic goober* is extruded vertically (the plastic has to be
of a type that has a viscosity high enough that it doesn't drool
off the nozzle). The mold clamps around the parison (sealing off
the bottom end) and then it's inflated with compressed air through
the top. After it cools a bit, a nice sharp knife called a moil**
slices off a bit at the top (usually the top of the neck where the
closure goes), the mold opens and the bottle falls out. The tab at
the bottom is stripped off automatically or manually.

I don't know how you'd do this without a rather complex machine- but
the molds are relatively easy because the pressures are very low by
molding standards.


* It's called a "parison", a term borrowed from a French term
"paraison", relating to glass blowing.

** not joking, that's what it's called-- at least it's not spelled
"mohel".

Ah -- that wouldn't work, and once again I'm confused in my
terminology. The application is very weight-sensitive: indoor rubber
power competition rules call out a seven gram minimum weight for
this sort of plane*, and that minimum is _very_ hard to build down
to. I've got a pair of wheels that look injection molded, but have
about the same wall width as most of the blow molded things I've
seen. They weigh in at 600mg for the pair, while my wheels weigh in
at less than 100mg each.


As others have said, blow molding is a process carried out with solid
resins in a semi-liquid state. I've never heard of anything that
small being blow molded, nor have I heard of doing it with styrene
beads, but technology marches on and I mostly don't.


So what do you call molding with styrofoam beads?


Molding polystyrene beads. g (Styrofoam is a trademark name for EPF
-- expanded polystyrene foam, and it's close-cell material. Molding
the beads produces a porous material.)

Molding polystyrene beads is done by filling a mold with them and then
injecting steam or hot air. I don't think you want to go there.


* AFAIK, just about anything with wheels and a minimum weight calls
out 7 grams -- 14 grams for outdoor.

--

Tim Wescott

what about rotational molding? that's usually done with powder, and the
wheels would be hollow when done.

That's for things like industrial drums and plastic kayaks. I don't think
you'd have much luck in such small sizes.

If it were me, the first thing I'd try is pressing the wheels out of
Styrofoam (closed-cell) or beadboard (open-cell) with hot mold halves. It
doesn't take a lot of heat to make polystyrene pliable. You'd have maximum
density at the wheel tread, and a bit of skin on the outside. You probably
could get away with a low-density polystyrene foam.

Correction, to avoid confusion: Beadboard, and other products made from
compacted polystyrene beads (such as coffee cups), are *not* open-cell. The
finished product usually is somewhat porous, but the beads themselves are
closed-cell.

Styrofoam is extruded, close-cell foam, and it's not made from compacted
beads.

Also BTW: you might need some mold release on those mold halves if you're
going to trying heating them and pressing the wheels out of sheet. PVA
(polyvinyl alcohol, available at boat-repair supply places and craft shops)
might be the right thing. It's used as the primary release agent for
polyester resin, which is loaded with styrene monomer. But considering that
the pressed sheet won't be liquefied, vaseline petroleum jelly

--
Ed Huntress

On 4/11/2011 2:40 PM, Ed Huntress wrote:

Molding polystyrene beads.g (Styrofoam is a trademark name for EPF --
expanded polystyrene foam,

You may be looking for the word "Extruded".

Kevin Gallimore

"axolotl" wrote in message
...
On 4/11/2011 2:40 PM, Ed Huntress wrote:

Molding polystyrene beads.g (Styrofoam is a trademark name for EPF --
expanded polystyrene foam,

You may be looking for the word "Extruded".

Kevin Gallimore

Styrofoam is indeed extruded, but the generic definition for EPF is
"expanded." Extruded foam was expanded in the process of making it.

You'll see both definitions for EPF in the industry literature.

--
Ed Huntress

On 04/11/2011 11:49 AM, Ed Huntress wrote:
wrote in message
...
Ed Huntress wrote:
"Tim wrote in message
...
On 04/11/2011 10:14 AM, Spehro Pefhany wrote:
On Mon, 11 Apr 2011 09:53:05 -0700, Tim
wrote:

On 04/10/2011 03:19 PM, Brian Lawson wrote:
On Sat, 09 Apr 2011 13:47:57 -0700, Tim
wrote:

SNIP
Because the process is to put a flat piece of foam in the mold,
clamp it, then chuck it in an oven to heat. The foam* softens
and expands to
meet the edges of the mold. Then you take it out of the oven
and let it
cool, and voile! A part (or a mess -- it's best to enter any
sort of casting process with your eyes open).
SNIP

I've never done it, but it reads like what you want to do would be
achieved with blow molding. You could even put "tread" on the
tire then.

Interesting thought. Probably beyond the sophistication I was
aiming for (mostly hand tools, with bits& bobs turned on the
lathe). And I think blow molding is for styrofoam beads, which isn't
what
Depron is. I'm not sure that I've seen styrofoam beads that are
small enough for a wheel that's 3/16" thick and 3/4" diameter.

Blow molding is what is used to make plastic bottles. Basically a
thick plastic goober* is extruded vertically (the plastic has to be
of a type that has a viscosity high enough that it doesn't drool
off the nozzle). The mold clamps around the parison (sealing off
the bottom end) and then it's inflated with compressed air through
the top. After it cools a bit, a nice sharp knife called a moil**
slices off a bit at the top (usually the top of the neck where the
closure goes), the mold opens and the bottle falls out. The tab at
the bottom is stripped off automatically or manually.

I don't know how you'd do this without a rather complex machine- but
the molds are relatively easy because the pressures are very low by
molding standards.


* It's called a "parison", a term borrowed from a French term
"paraison", relating to glass blowing.

** not joking, that's what it's called-- at least it's not spelled
"mohel".

Ah -- that wouldn't work, and once again I'm confused in my
terminology. The application is very weight-sensitive: indoor rubber
power competition rules call out a seven gram minimum weight for
this sort of plane*, and that minimum is _very_ hard to build down
to. I've got a pair of wheels that look injection molded, but have
about the same wall width as most of the blow molded things I've
seen. They weigh in at 600mg for the pair, while my wheels weigh in
at less than 100mg each.


As others have said, blow molding is a process carried out with solid
resins in a semi-liquid state. I've never heard of anything that
small being blow molded, nor have I heard of doing it with styrene
beads, but technology marches on and I mostly don't.


So what do you call molding with styrofoam beads?


Molding polystyrene beads.g (Styrofoam is a trademark name for EPF
-- expanded polystyrene foam, and it's close-cell material. Molding
the beads produces a porous material.)

Molding polystyrene beads is done by filling a mold with them and then
injecting steam or hot air. I don't think you want to go there.


* AFAIK, just about anything with wheels and a minimum weight calls
out 7 grams -- 14 grams for outdoor.

--

Tim Wescott

what about rotational molding? that's usually done with powder, and the
wheels would be hollow when done.

That's for things like industrial drums and plastic kayaks. I don't think
you'd have much luck in such small sizes.

If it were me, the first thing I'd try is pressing the wheels out of
Styrofoam (closed-cell) or beadboard (open-cell) with hot mold halves. It
doesn't take a lot of heat to make polystyrene pliable. You'd have maximum
density at the wheel tread, and a bit of skin on the outside. You probably
could get away with a low-density polystyrene foam.

Right now I'm cold-pressing, and then heating. I'm not sure if I want
to go to the trouble of hot-pressing -- but, maybe it's the way to go.
Hmm. More complications...

(And there's no sign of the product sticking to the mold).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

On Mon, 11 Apr 2011 11:26:15 -0700, Tim Wescott
wrote:

big snip
Ah -- that wouldn't work, and once again I'm confused in my terminology.
The application is very weight-sensitive: indoor rubber power
competition rules call out a seven gram minimum weight for this sort of
plane*, and that minimum is _very_ hard to build down to. I've got a
pair of wheels that look injection molded, but have about the same wall
width as most of the blow molded things I've seen. They weigh in at
600mg for the pair, while my wheels weigh in at less than 100mg each.

So what do you call molding with styrofoam beads?

* AFAIK, just about anything with wheels and a minimum weight calls out
7 grams -- 14 grams for outdoor.

I think vacuum forming might give you something useful for relatively
light weight. Heat up a sheet of something formable clamped in a
frame via IR (think toaster oven), pull it out, place it over two male
half molds, suck down. Cut out halves and glue together when cool.

On the other hand, it probably won't be as light as some rims bent up
and glued out of graphite prepreg tape, with graphite spokes and
graphite tube hub (maybe cure in the same toaster oven). If your
target complete model weight is 7 grams, anything molded will be
pretty heavy by comparison.

Pete Keillor

Spehro Pefhany wrote:

I don't know how you'd do this without a rather complex machine- but
the molds are relatively easy because the pressures are very low by
molding standards.

It might be "complex," but it doesn't necessarily have to be "big" -
Many years ago, there was an amusement park in Excelsior, MN, with a
way kewl wood coaster, fun house, arcade, and all that other fun amusement
park stuff that's illegal today because it's too dangerous for our oh-so-
fragile little darlings. One of the things in the arcade was a box no
bigger than a jukebox, that for a quarter would blow-mold a little plastic
unicorn shelf knick-knack right before your very eyes. It would close the
mold, shoot the goo, run cooling water through the mold, open the mold, and
scrape the little unicorn off the platen down into the pick-up bin.

I'd think the only "complex" part would be figuring out the right
temperatures and pressures for the molding process itself - the rest
is just sequencing. :-)

I once made a process sequencer with a stack of garolite cams that I cut
on the rotary cable, and a bank of industrial-grade microswitches. :-)

Cheers!
Rich

Tim Wescott wrote:

So what do you call molding with styrofoam beads?

I dunno, but do they have that in an aerosol form, like that expanding
urethane stuff?

Good Luck!
Rich