Q: We have a quality issue with a supplier where at least one hole on a formed part has what appears to be a bulge on the punch side of the material. Material is high tensile/low alloy steel, 0.38 in. thick, 50-KSI yield. Is there an industry standard tolerance for the height of this bulge? A flatness legend for a stamped/drilled plate? As a manufacturer, we should have called it a flatness, but we didn’t. We want to change the impression but don’t want to “over-tolerate” a characteristic.
A: There is no industry standard for bulging that may or may not occur around a drilled hole. In fact, the bulge you speak of can occur to some degree around any piercing, not just a round piercing. But there are several ways to control and treat this very common problem.
In my experience, flatness specs stated on components are usually application driven. We’ve come across callouts that require absolutely no measurable raised material (bulge) in or around certain features. If we can’t achieve this through tool design alone, we’ve resorted to creating a slight recess about 0.001 to 0.002 inches deep around the element. It’s easy on base material thicker than 0.010″, but it gets tricky on material 0.005″ or less.
This strike can be made after cropping the item, but sometimes we have forged a recess and then cropped through the recessed area. If there is a bulge, as long as it is below the top surface of the base material plane, it is fine.
There are two things to consider when designing the tool. First, there is optimum clearance between the punch and the die which will minimize bulging while drilling. As you drill through the material, if the clearance is too tight, the cutting pressure will be higher than normal and the material will be pushed outward and away from the trim punch as it enters and passes through the material base, causing a bulge. As the punch retracts out of the material, some of this bulge will relax and produce a hole that is slightly smaller than the actual size of the trim punch.
If the clearance is too large, as the trim punch penetrates and passes through the base material, the base material will stretch down into the die opening. Then, at the slug’s fracture point, the base material snaps back into place. This minimizes the bulge but may produce a hole slightly larger than the size of the punch.
Second, for progressive dies with a top-mounted spring-loaded stripper design, there is a common precision tool construction style called the three-plate design. The three plates are the punch holder, stripper and die punch. The most obvious purpose of the stripper is to remove base material from all punches and forming tools as the press moves up after completing work at the bottom dead center of the stroke. The stripper also clamps the progressive tape after the tool pilots locate it precisely on the progression.
Often the pressure on the progressive tape is so great due to the drilling, cutting and forming that you will see a slight deformation in the pilot holes as they try to hold the tape in place. If you do, then your stripper isn’t doing her job. The stripper springs must be strong enough to prevent any movement of the progressive stripper. Most importantly, the spring preload should be set so that when the stripper comes into contact with the progressive band, there is sufficient pressure to prevent movement. Otherwise, you will see dimensional instability in your parts.
The stripper covers the entire surface of the progressive strip, but there is a lot of surface that no work is being done on. So here’s a little tip: grind the face of the stripper, 0.001 to 0.002 inch, where no work is being done. This concentrates all the spring pressure where you do the work, giving you two to three times the clamping pressure where it’s needed without changing the springs.
