Ask the Stamping Expert: What’s the industry-standard tolerance for pierced-plate flatness?

Q: We have a quality issue with a supplier in which at least one hole on a formed piece has what appears to be a bulge on the punch side of the material. The material is high-strength/low-alloy steel, 0.38 in. thick, 50-KSI yield. Is there an industry-standard tolerance for the height of that bulge? A flatness callout for stamped/pierced plate? As the manufacturer, we should have called out a flatness, but we did not. We want to change the print but don’t want to “overtolerance” a feature.

A: There is no industry standard for the bulging that may or may not occur around a pierced hole. In fact, the bulging that you speak to may occur to some extent around any piercing, not just a round pierce. But there are a few ways you can control and deal with this very common issue.

In my experience, the flatness specifications spelled out on components usually are application-driven. We have come across callouts that require absolutely no measurable raised material (bulging) in or around certain features. If we can’t achieve this solely through tool design, we have resorted to coining a slight recess—about 0.001 to 0.002 in. deep—around the feature. This is easy on base material thicker than 0.010 in., but it gets tricky on material 0.005 in. or thinner.

This coining can be done after trimming the feature, but at times we have coined a recess and then trimmed through the recessed area. If there’s a bulge, as long as it’s below the top surface of the base material plane, it’s OK.

There are two things to consider during tool design. First, there is an optimal punch-to-die clearance that will minimize bulging during piercing. As you pierce the material, if the clearance is too tight, the cutting pressure will be higher than normal, and material will be pushed outward and away from the trim punch as it enters and passes through the base material, causing bulging. As the punch retracts out of the material, some of this bulging will relax and yield a hole slightly smaller than the actual trim punch size.

If the clearance is too large, as the trim punch enters and passes through the base material, the base material will stretch downward into the die opening. Then at the point of slug fracture, the base material snaps back. This minimizes bulging but may yield a hole that is slightly bigger than the punch size.

Second—for progressive dies with a top-mounted, spring-loaded spring stripper design—is a common precision tool build style called the three-plate design. The three plates are the punch holder, stripper, and die chase. The most obvious purpose of the stripper is to strip the base material off all the punches and forming tools as the press moves upward after completing the work at bottom dead center of the stroke. The stripper also clamps the progressive strip after the tool pilots have accurately located it on progression.

Often the pressure on the progressive strip is so great because of piercing, trimming, and forming that you will see slight deformation in the pilot holes as they try to keep the strip on location. If you do, then your stripper is not doing its job. The stripper springs must be strong enough to prevent any movement of the progressive strip. More important, the preload on the springs must be set so that when the stripper contacts the progressive strip, there is sufficient pressure to prevent any movement. If not, you will see dimensional instability in your parts.

The stripper covers the entire progressive strip surface area, but there’s a lot of surface area in which no work is being done. So here’s a little trick: Grind away the face of the stripper, 0.001 to 0.002 in., where no work is being done. This concentrates all the spring pressure where you are doing the work, giving you two to three times the clamping pressure where it is needed without changing the springs.