Ask the Stamping Expert: Reducing press tonnage by staggering clustered punch length

A: If all eight punches have the same cutting surface area and you are using 43 tons to pierce the material, then simple mathematics equals 5.37 tons per punch. In reality, however, most progressive dies will have spring pads, lifters, or stripper springs that get compressed during the stamping process, so your press tonnage will have to be adjusted to include them. A good way to determine the exact press tonnage required is to cycle the tool in the press with no material and measure the tonnage.

In blanking, the tonnage usage of the press builds rapidly as the punches enter the raw material. At the point the raw material yields, it fractures; the slug can fall with minor resistance, and tonnage drops rapidly. This is called the point of snap-through. Assume pierced material snap-through will occur at 40 percent punch penetration into the raw material. Since 12-ga. steel is 0.105 in. thick, 40 percent punch penetration would equal 0.042-in. punch penetration into the base material. So at 0.042-in. penetration, the force to pierce through the steel is nearly complete, minus minimal dragging of the punch as it continues to travel through the raw material, pushing the blank into the die. So if four punches are 0.042 in. shorter than the four others, your tonnage is theoretically cut in half.

A good way to confirm the fracture point is to measure the shear and break on the side walls of the blank or slug. The shear is shiny and square to the die side of the blank. The break is the top surface of the blank. It is rough and slightly tapered from the shear to the top surface of the raw material.

It’s important to know what the fracture point is, as several factors can influence it. For example, adjusting the punch-to-die clearance can change the fracture point from 25 to 75 percent. This is done when the percentage of shear must be greater than standard practices would otherwise yield. In some blanking applications, the shear length requirement is defined relative to the blank thickness. The tighter the punch-to-die clearance, the greater the shear length.

The base material specifications also can affect the fracture point. In general, the harder the base material temper, the shorter the shear length. This is because harder material tends to snap through at lower punch penetration.

Also remember that as the punch and dies get dull, tonnage requirements will increase.

So, for your application, start with 43 tons divided by eight punches, or about 5.4 tons per pierce. Try setting three punches at a baseline height for piercing. Then set three punches at 0.042 in. longer than baseline and two at 0.042 in. shorter than baseline. With this setup, you will have a maximum tonnage usage of 16.2 at any given point. Adding in the drag forces and springs using a maximum of 2 tons will keep you in a safety factor of about 18 tons.

If the tooling and press stroke allow it, just to be safe, try using a 0.075-in. offset in punch lengths rather than 0.042 in. Again, remember this measurement will change as the tooling gets dull. In addition, set up the two punches at the shortest length to minimize the snap-through of the ram slamming into your tooling down stops. Try not to use more than 80 percent of the press’s rated tonnage. If you need to, go to a four-length stagger matrix of 2- by 2- by 2- by 2-in. lengths.