I want to make a bunch of identical molds, for 3/4" diameter model
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
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
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.
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.
Let me know if you can see this, it should explain:
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?
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.
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
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
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.
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.
Aha. Yes, that does it.
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
(Hubbing, sometimes called "hobbing" and described above, is likely to
present more problems than form-cutting, IMO.)
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
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.
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.
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.
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
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
Since you got me started on this, 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-)