Die Science: Dealing with an “AFib” die in stamping operations

FIGURE 1. The pilots must extend beyond the surface of the pressure pad so that they have fully engaged into the strip or part before holding pressure is applied. Typically, the pilots should engage into the material about one to two times the material's thickness before the pad makes contact.

I haven’t been feeling the best in the past few weeks, and recently I had an episode where I felt my heart was going to pound out of my chest. I felt quite nauseated and not up to par overall. After enough persistent nagging from my wife (out of love), I finally gave in and did the sensible thing—I made an appointment with my doctor.

After performing an electrocardiogram, the doctor diagnosed me as having AFib, or atrial fibrillation. That means the chambers in my heart were out of time with each other; they weren't synchronized properly. As a result, I wasn't receiving the necessary blood flow or the oxygen to make me feel good. My doctor described it as a four-cylinder car running on only three cylinders. I was zigging when I should have been zagging. I definitely needed a tuneup. My heart needed to be timed out, and each of the four chambers needed to be synchronized.

Oddly enough, as I sat there in the doctor's office, it reminded me of how important it is that stamping dies be properly timed. I know that sounds a bit strange, but I live my life constantly exposed to dies and sheet metal stampings, so to me it’s natural to default to the world in which I spend so much time. In any case, this article is not intended to talk about my health but rather the importance of timing as it occurs in a die.

Poor Pilot Timing

First, let me clarify that a pilot has just one function: locate and properly register the part. This basic function applies to all dies that use pilots.

Usually associated with progressive dies, pilots typically are cylinder-shaped pins mounted to the upper die that locate and register each one of the parts and strip in their respective stations. They often are mounted to the upper die shoe and, in many cases, to a precision guided stripper. Mounting them in a guided stripper typically makes them more rigid and accurate.

In any case, the pilots must extend beyond the surface of the pressure pad so that they have fully engaged into the strip or part before any holding pressure is applied (see Figure 1). Pilots should engage into the material one to two times the material’s thickness before the pad makes contact.

If they are below the surface of the pressure pad, the pressure pad will be holding the part down tight to the die while the pilot pin is attempting to locate it. This typically will cause the pilot pin to bend or the hole in which it enters to elongate or distort. On the other hand, excessive pilot entry before pressure pad contact may result in the parts sticking to the pilots on the upper half of the die and the strip returning with the stroke of the press. This problem can be fixed by limiting pilot entry and putting small ejector pins around each pilot in the die. Keep in mind that the pilot ejector pins do not need to be high pressure, as their function is only to push the part or strip off from the pilot.

Poor Timing of Scrap Cutters

Scrap cutters are custom-designed to cut the perimeter scrap of a large part into smaller, more manageable pieces. They must be timed to cut the scrap into smaller sections after the perimeter of the part has been completely trimmed. As the name implies, scrap cutters are intended to cut the scrap, not to trim the part. If they start to cut the metal before the outer perimeter of the part is trimmed away, they likely will cause distortion or surface defects in the finished panel. If they are too low or shaped incorrectly, they will not cut the scrap.

The timing of scrap cutters is sometimes difficult, because they have to be shaped and timed to allow the part to nest completely on the lower die and scrap cutters without distorting the part shape when the pressure pad pushes the part to the power cutting punch. For this reason, they often are contoured and shaped to the shape of the scrap.

Poor Pad or Stripper Timing

Pressure pads (or strippers) are pressure-loaded plates, either flat or contoured, that hold or strip material during the cutting and forming process. If the pressure pads do not hold the material or part tightly enough to the die, the part or strip could move or distort while being formed or cut. The holding pressure usually is generated by coil springs or nitrogen springs.

The pads must be shaped accurately with respect to the part and timed so that the part is located and secured properly before cutting or forming begins. If the pad sits below a forming or cutting surface, part cutting and forming will happen before the part has been secured.

Timing is everything, both in hearts and stamping dies. Don’t let your dies go into AFib! Locate first, secure second, and perform work third. By the way, I’m seeing a cardiologist next week. Hopefully, he will get my heart timed out.