Ask the Stamping Expert: How can I prevent slivers in blanking operations?

Q: We are making shallow drawn cuts from 0.007-inch-thick aluminum base material with a thin polymer coating. The cups are assorted sizes from 0.375 to 0.75 in. and equal depths. We pierce the blank and use a Ballantine to carry it in the strip web through the draw operations, and finally we pinch-trim it through the die. Our problem is that slivers generated in blanking accumulate in the tool and mark the drawn cups. What can we do?

A: For cutting aluminum in Ballantining for drawing in a progressive die, C2 carbide grades are preferable. They are very hard and keep the sharp edge very well.

For punching, the carbide grade depends on the cross section of the punch. The thicker the cross section, the harder the carbide should be. To mitigate breakage of whole punches, sometimes a hybrid punch with a small carbide tip silver-brazed in place is preferable.

When using carbide for machining processes, the tip must be radiused and mirror-polished. For punching in a progressive die, the face and the walls of the punch also must be mirror-polished; otherwise, as speed increases, microwelds of aluminum will adhere to the walls of the punch in areas of microscopic surface imperfections, creating slivers as they pass through the strip.

If you’re cutting punches with wire EDM, use caution. The process pulls out the cobalt binder used in the manufacture of carbide, causing a work-affected zone on the surface. While multiple skim cuts can nearly eliminate the work-affected zone, grinding still yields the best results for very fine and critical applications.

If the grinding lines are parallel to the front of the punch, the punch will act as a fine file and generate slivers. Grind all punches in the same axis as the punching motion whenever possible.

Sometimes slivers are a function of the clearance between the punch and die. Extra clearance will result in less friction between the blank and the walls of the die as you push the blank back up into the strip.

Another way to avoid small slivers is to use a negative angle in all punches. The negative angle is measured in ten-thousandths of an inch. On a 2-in.-long punch, the punch size at the back end may be smaller by 0.0006 to 0.0008 in.; the cutting end of the punch would be larger by the same amount.

To lap the sides of a round punch without getting a barrel-like shape, try a copper disk 0.25 to 0.375 in. thick after grinding the punch sides to 0.02-degree negative taper.

When polishing, you might assume that the sides are parallel, but it’s best to inspect them under a microscope and measure them with a fine precision micrometer. Any punch with a smaller front than back might create small filings.

For pesky slivers caused by the slug breaking through the material, you could inject very light lubricant into the bottom side of the strip and on top of the die to disperse the slivers and prevent marking of the manufactured parts. The press will need to be modified to reclaim the oil by passing it through a filtration system before it can be recirculated.

Many times when a punch is reworked, not all of the worn-out surface is removed. When servicing your punches, inspect the edge under a stereomicroscope at 200-to-1 minimum to see the remaining microscopic negative cutting edge. This will show you how much material to remove to prevent chip generation.