Finish on sheet metal punches - grinding/polishing worn punches

Have some 0.280" punches in C frames that I have been using in a manual press for a while - perhaps 500 - 1000 parts. This is a low use application Am having trouble with material (0.030 - 0.040 aluminum) hanging up on the punches and lube helps some but doesn't eliminate the issue. They originally had very stiff springs that required as much or more pressure than my punching and I had trouble getting enough pressure for several punches at a time so replaced the springs with ones of about half the force. I looked at the worst punch and die under magnification and was surprised to see the punch had small scratches around the tip and going up the body like it was always like that and not fully polished - very small but still there. The edge of the punch is rounded a bit in a few spots which may be wear from prior use (got them used). I can grind a bit off the length to renew the edge but would also like to polish them up to reduce sticking. Any recommendations on how to proceed with either of these operations? I am hoping I can polish the diameter without increasing the clearance significantly. I have access to some tool grinding equipment but not sure exactly what equipment is available to me - will check later this week. Or I could spin them with a lathe and polish them with very fine emery paper or polishing compound.

I have some slightly larger punches that use a different mounting, different manufacturer and the punches are highly polished. No trouble with those dry and a urethane tube to push the part off.

Hi, with any form of punching I always use a fixed stripper plate to prevent the part rising up and hanging onto the punch.
Ian..

Urethane may not have enough force to press the part off the punch once the punch has gone through the material.........the stripper plate is a fixed item attached to the bottom part of the die and is made such that the thickness of the material passes beneath it and prevents the punch pulling the material up with it.......if you haven't done any press tool work you won't understand it.
Ian.

I would polish them in the lathe with fine emory. To sharpen, grinding a bit off of the end is correct. Grind the die also. Both punch & die should have a sharp edge.

Thanks Jim! What grit would you suggest for emery? I pulled the worst one for hanging up and it has some roughness but hard to see without magnification. Been meaning to pull the one giving the least trouble and compare the finish. Surprisingly the dies look much better but may take just a little off while I am at it.

Regarding a stripper plate I appreciate the suggestion and understand the concept just fine but the punches are like this (B series):

https://www.unipunch.com/How-The-Tooling-Works


The press ram can't pull up the punch so a stripper plate will never work with this type of setup.

Depending on the starting condition, I might start with some 120 and finish with some 400. Backing the emory up with a wood or metal dowel would be the best way to do it. Be careful that you don't taper the tool when polishing. I have a whole bunch of various shaped and grit stones for polishing, but emory will do in a pinch. A tip: When punching only go deep enough to punch through the material + another 0.020 or so.

Quote:

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Originally Posted by Jim27

Thanks Jim! What grit would you suggest for emery? I pulled the worst one for hanging up and it has some roughness but hard to see without magnification. Been meaning to pull the one giving the least trouble and compare the finish. Surprisingly the dies look much better but may take just a little off while I am at it.

Regarding a stripper plate I appreciate the suggestion and understand the concept just fine but the punches are like this (B series):

https://www.unipunch.com/How-The-Tooling-Works


The press ram can't pull up the punch so a stripper plate will never work with this type of setup.

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This system already has a stripper built in, for it to work correctly the punch has to retract inside the stripper, the press you are using may not going up high enough after each operation leaving the punch still outside the stripper

I decided to start with finer grit and see how it went. Didn't want to remove any more than needed. Took some 600 grit backed with a small block of wood and put the punch in a drill press. It cleaned up quickly. Then I sharpened by grinding about 0.005 to 0.020 off the tip depending on the condition of each. They work great now with no sticking! Now that I know I will do this again when it becomes a problem. Need to install some stops to limit how far the punches go.