Sheet Metal Fabrication - Punching - Preventing Galling

The combination of pressure and heat in nature can create metamorphic rocks.  In sheet metal fabrication punching, the combination can create a condition called "Galling" where metal bits can stick to the tooling.

To stop it, one must  make sure you understand a basic rule of engineering: Never have two pieces of the same grade of material rubbing against each other. Just like a cam shaft in a car engine is made of different material than the valves and the lifters, so too should the punch tooling be made of a material different from the work piece.

D2 used to be a common material for punch tooling (though it’s not as common these days), but the last thing you wanted was to punch a D2 tool into stainless steel. If you did, galling probably resulted, because both stainless steel and D2 contain chromium.

 
Slots cut into the punch face help break the vacuum created between it and the work piece to help prevent slug pulling. Ejector pins mitigate slug pulling as well, but if they fail, the pins can be pulled out to create a ported punch, which itself can help reduce the problem of metal bits sticking to the tooling.

Modern technology has produced tool coatings to help reduce heat in the punching process; therefore, one can reduce the chance of galling. Some coatings work better for certain material grades and specific applications, so be sure to communicate with your tooling supplier.

When punching a round hole, the hole naturally collapses slightly after the punch tip penetrates the sheet. A straight punch can rub against the work piece material throughout the stroke. This contact/rubbing creates more friction and heat, increasing the chances for galling. Back-taper can help here. If the punch has some back-taper, it makes only brief contact with hole sides after punching through.

You might think tapering would work with the die as well, but this isn’t necessarily the case. When a die has a negative taper—a die with a smaller opening at the top than at the bottom—material tends to become trapped at the top of the die. This again creates friction, heat, and more chance for galling. In this case, you may want to consider alternative die geometries (such as the negative-positive geometry described previously) that allow room for debris to evacuate the die cavity.

Die clearance (that is, the space between the outside of the punch and inside of the die) plays a big role in galling prevention. Tooling manufacturers publish die clearance charts, of course, and these can be a good starting point. But the best way to figure the right die clearance is to test a variety of die clearances with the material you’re working with and then determine which works best.

The optimal die clearance can depend on the speed of the punch. Though it may seem a little counter intuitive, a slower punch tends to require slightly more die clearance—just a few percentage points larger.

Older mechanical punch presses have long strokes of travel, with punches penetrating the material at high speed. Modern machines attain higher hit rates and greater productivity not by increasing the speed of the punch itself, but by using shorter stroke lengths so that the punch doesn’t travel as far. It penetrates the material and then rises up so that the tip is just barely over the top of the sheet. The machine performs more hits in less time, but the punch contacts the material at a slower speed. 

Yet another way to reduce the chance of galling is to ensure punches don’t overheat. Punches can heat up significantly after punching hundreds of holes in quick succession. In these cases, you may consider doubling up or even tripling up your tooling. You can punch a series of holes with one tool, then switch stations to another identical tool. This gives the other tool a chance to cool down.