"Tim Wescott" wrote in message
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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