"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