Ask the Stamping Expert: Can I pierce material in a progressive die in the opposite direction?

Q: Do you have much experience with piercing material in the opposite direction than usual in a progressive die? In other words, the slug goes through the punch shoe.

A: Yes. This is very doable and not uncommon. I assume you need to do this because the piercing direction is a part-functional requirement. In other words, the shear and break sides of the cut opening that result during stamping are spelled out, and they are in the opposite direction from other part features or the outer perimeter. Sometimes this is not explicitly specified, and there might be just one feature that has the shear and break requirement.

If the design engineer determines that a part can be stamped only in an upward direction, the feature must be pierced up. Many times we are forced to stamp a part in a counterintuitive manner, but we engineer creative solutions, such as using nitrogen springs for piercing before dead bottom of the stroke or bending up instead of wiping down on a form so we do not need to pierce up. Piercing up is not terribly difficult, but it is more complicated and requires careful and exact maintenance.

There are two ways to accomplish opposite piercing directions on one part:

1. Use a compound die.The actual outer-perimeter shape-cutting die block is mounted on top of the die shoe, and the mating punch of the same shape (offset for clearance) is mounted on the bottom. The punch can have multiple holes in it to act as a die to accept punches mounted on the top shoe inside the cutting die block.

A pressure pad is put in the top die so that as the die closes, the perimeter punch on the bottom blanks the part into the top cutting die block (as the pressure pad backs up) at the same time as the punches on top pierce through the material and leave their slugs in the perimeter punch on the bottom. The slugs fall through the bottom die shoe and into a scrap bucket.

As the die opens, the pressure pad pushes the part off the inner punches and ejects it from the perimeter cutting die block. As the part falls down, it is blown out of the press or picked up and removed manually. This is a slow process, and hydraulic springs are available that will eject only when signaled to do so.

This compound blanking method allows time to bring in and position a robot to accept the part for removal. It also allows part features to be sized and positioned as accurately as the tool build.

2. Build a standard progressive die. The pierce punch is mounted on the bottom, surrounded by a pressure pad. The pressure pad is level with the punch cutting edge to protect and allow material to feed over it. As the die closes, the pad is pushed down, exposing the punch. The travel of the pad and subsequent die strip should be limited to the material thickness plus the depth you wish to push the slug into the mating top component.

The top component is a fixed punch (we will call it a D-punch) with a hold in it acting as a die cavity. Fixed rigidly to the top punch holder, it has a hole or shape in it with land. The backup plate and die shoe are engineered with a progressively larger tapered hole so the slugs have nowhere to catch.

The D-punch should have a step backburned into it so the slug is pushed through land and taper and is free on all sides. Airflow must be good to suck out the slug on every stroke. If the slug is left in the D-punch and the hole is essentially blocked, there will be no airflow and therefore no suction. Slugs will build up in the D-punch, eventually getting so packed that they crack the tooling. Slug sensors can help ensure slugs are removed after every hit or every other hit. While pass-through vacuums have the greatest suction power and smallest footprint, some air-assist should be built into the tool as close as possible to the D-punch.