Punches and dies

[jscarmozza]

Should dies be slightly undercut? I made a punch and die that produces the part I want but the part won't release from the die without a lot of prying, which ruins the part. If I make a new die with undercut sides, will that allow the part to be sheared but remain loose in the die?

[WesHowe]

I never tried to do it yet myself, but I read that you use an "angle of clearance", which is basically a taper, of "1 to 2 degrees". Obviously, the taper gets larger at the bottom of the die, to provide clearance for the slug to fall through.

I don't have a reference handy, but I have seen end mills that have a taper built into them, I presume to make it easier to machine dies.

From Audel's "Machinists and Tool Makers Handy Book" by Frank D. Graham, Copyright 1941, 1942.

[Rich_Carlstedt]

In simple terms, YES
There are many factors to consider, like die depth and ductility of the material be cut.
I assume it is a 'Blanking" die, in that you wish to cut out a form (Blank) and not also form the material.
When blanking and " forming" the material you must evaluate the spring-back of the material as well as it may expand in the die cavity .
Die clearance is important. If the clearance ( difference in Punch OD and die ID ) is say 5 % of the material thickness ( thats .0005 for a material .010" thick) and is called "fine" blanking. Such work has a very low burr and high cutting forces are required but die clearance angles are smaller
A 1 Degree clearance angle is a fair amount. Please know that 1 degree of relief is equal to .017" per inch of thickness of the die. So if your part OD is 2.000 which then becomes the die opening or ID, and if the die was one inch deep, the die clearance of 1 degree means the ID at the die bottom is 2.034"
If you punch the blank through the die , it works out pretty good.
If you expect the part to come up back through the cutting region, then you need a stripper operation, or die cushion. Basically you put a spring loaded center in your die hole and when the punch presses the material through the die, and the punch starts to retract, the spring cushion pushes the part back up through the die. Now this will increase the force to perforate because you must also overcome the spring force.

Hope this helps
Rich

[Rick]

For the home shop a taper relief may not be the easiest way to produce clearance in the die. I have seen some dies that just have the die opening relieved a bit from underneath and leave about an 1/8" down from the cutting surface of the opening at size. The punch then pushes the blank down thru this "at size" area and into the relieved area. As Rich said above there are many factors to consider. The way I mentioned I would not do for a high volume tool but for just a few parts it will work fine.

[jscarmozza]

I attached a photo of the punch, die and part; it's a 1" scale tie plate for my home RR track. I made a lot of them but it was with a three part operation, cut the stock to length, punch the fastener hole then punch the track flange tab, I figured I could make a punch and die to do everything in one operation...it almost works. I thought I had everything pretty well covered with the alignment dowels, springs to lift the punch and the part extractor which is made up of 4 dowel pins that ride on a wedge under the die to push the part out of the die when the punching operation is done. What usually happens is the part sticks to the round hole punch on the upper punch and in the tab recess in the lower die, requiring some fancy screwdriver work to pry the part out. Now that you see what I'm doing, what do you think is wrong?

[NP317]

Off topic:
"Punches and dies."
Reminds me of "Eats shoots and leaves."
All about punctuation.

~RN

[earlgo]

You probably won't want to hear this but in industry this would be a 2 station progressive die.
*One station to punch the finger tab and hole(s) in a cut to width strip. This would permit ejector pins to eject the tab from the cavity without the stress of having to eject the whole part from a cavity that also punched the periphery.
*The second to shear off the part to width.
Maybe this suggestion will inspire a solution that would work for you.
--earlgo

[jscarmozza]

Thanks to all for the advice, I'm going to have to rethink the tool; maybe do it in two steps. In any case, a redesign is in order.