"DoN. Nichols" wrote in message
...
On 2010-07-08, Bob La Londe wrote:
I need to figure out a way to punch out shapes from .025 aluminum sheet.
I've got the little taig mill of course, but I'm thinking it will be
incredibley slow, and if the project works I'll want to punch a lot of
them.
The shape is roughly triangular with an outside shear length of
approximately 6 inches.
I also considered the idea of making some kind of punch that I could use
in
my press, but I haven't any idea how to start or what material to use for
the punch itself. Also, I am thinking something as slow as a hydraulic
press is going to tear and distort the metal rather than punch it out.
Hmm ... for distortion -- make the punch nose flat, and the die
angled so it starts cutting near the middle of one side and at the point
opposite (assuming that the triangle is equilateral) so the distortion
is in the waste stock instead of the workpiece. (This probably means
that you want to shear to blanks not too much oversized to start with.)
Hmm ... this sounds like a job for a flywheel punch press. But
with a 6" perimeter, my little 1-1/4 ton one would not have a chance.
O.K. A bit of calculation, with a full perimeter of 6", a
thickness of 0.025" and 6061-T6 aluminum at 30,000 PSI shear strength
gives 2.25 tons using the formula:
Tons=Pi*D*S*T/2000
and replacing the Pi*D with your 6" perimeter.
Here is a 7-1/2 ton one on eBay if you are near New Jersey:
320556364771
And a 10 ton one:
360275737690
And a 5 ton one:
360268548345
Search for "punch press"
These essentially have a flywheel spinning around a shaft with
an eccentric which drives the punch holder a short distance. There is a
dog clutch which is tripped by some mechanism or other (some are air,
other mechanical) after which there is a quick up-and-down stroke and
then the clutch releases and pauses for the next trip.
Note that these strokes are quick and thus quite dangerous to
the user. Probably best if you set it up so both hands are required to
trip it.
I
made a couple of custom punches out of some stainless rod stock for
another
project a while back and even for paper they only lasted a very short
time.
Which stainless? They are not what I would choose for a punch
anway. And paper has clay in it, so it is rather abrasive compared to
aluminum.
I would use a high carbon steel (and did -- D2 air hardening --
to make some punches to cut out circles). Machine it to near the
desired shape, harden and temper it, and then grind it to final
dimensions.
And make sure that there are guide pins to keep the punch and
die aligned.
And add a stripper to pull the workpiece off of the punch on the
retract stroke.
Going back to the idea of using the mill. What do you think is the most
aggressive cut and cutter I could use? The spindle RPM maxes at 10,000
and
my max IPM is about 30 for rapids, but with any force for feeding is 20.
I
have a couple details on the cut that could double as holding tabs and be
cut as a seperate operation.
Part of the problem is that you need a way to pull the chips out
as the workpiece is cut. Since you are cutting through, put some
sacrificial material below it. Use a shop vac with a reduced nozzle to
pull air quickly past the cutter.
Perhaps have a toggle clamp which you can swing into place after
the cutter passes so you don't need the tabs.
But if you want quick -- you are really talking about a punch
press job here.
Or perhaps a waterjet cutter with a stack of plates to perhaps
1" thick or so (depending on precision needed) so you make a lot of them
in one machine pass. (This means contracting it out, of course.)
FWIW -- the 1 ton punch press (or is it 1-1/4 Ton?) that I have
I can just barely carry from one horizontal surface to another not too
far away. Anything bigger -- expect to use something to help lift it.
Good Luck,
DoN.
--
Email: | Voice (all times): (703) 938-4564
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--- Black Holes are where God is dividing by zero ---
FWIW, the recommendations to make punch and die sets for this job seem a
little cavalier. The punch/die clearance will be rather small and difficult
to keep consistent with a manual mill, unless one is very good and has the
right tools.
Tawwwwwm mentioned O1, which would do it easily for a few hundred parts, but
heating a die of that size with a torch, as he recommends, sounds like a
disaster waiting to happen. O1 is forgiving but this is an inside shape, fer
chrissakes. I wouldn't try it without a decent heat-treating furnace.
D2 is overkill. It would work fine, but you're making the job more difficult
than it has to be.
If I had to do it (and I wouldn't -- I'd have it wirecut; I think the
clearance is enough that you can do a one-shot punch/die set from a single
piece of steel) I'd make it from a free-machining grade made for case
hardening and have it case hardened by a heat treating service. The cost is
modest and it beats the heck out of doing a high-wire act with a torch.
As for curving the die set (cutting "shear" into the die, if the saved piece
is the inside piece of the punching) to reduce forces, it's not a bad idea
if you really have to. But if you can't develop 2.5 tons of force with your
press, you have another problem. A press lighter than that is likely to
strain, trying to keep the die centered in the punch. You'd need guide pins.
Building a punch and die with guide pins and bushings is not for tyros. And
don't forget to file some clearance into the die -- or make it thin, and
make a die shoe with a slightly larger hole.
This is not a trivial job. If the stock was 0.006", it *could* be trivial.
--
Ed Huntress