When Galling Occurs on Punch Tips
Galling is an adhesion to the punch tip of metal being punched, caused by pressure and heat.
The best technique for removing galling is to rub it off with a fine stone (STO29911). The rubbing should be done parallel to the direction of the punching motion. This will polish the surface which contacts the material, decreasing the chance of any future galling. Do not sandblast, belt sand or use other harsh abrasive methods. These create a coarse surface finish to which material adheres more easily to the tool.
If galling (build up on punch flank) is a problem, order punches with 2° total back taper. Examine a Mate punch closely and you’ll find that the punch tip is largest at the cutting edge. It’s because we normally build in 1/4° total back taper (1/8° per side). This minute change in size facilitates stripping: material is much less likely to adhere to the punch. Grind life is not affected. The reduction in diameter is so small that the punch remains within normal tolerances for both hole size and die clearance throughout its life. Most likely you don’t even notice the back taper when examining your punches. But it’s there, working to make your punching more trouble free. If galling is especially troublesome, order a combination of 2° total back taper (1° per side) on the punch and 20% to 30% clearance for the die.