Ask the Stamping Expert: How can I reduce blanking tonnage required?

A: While I can’t speak for all mechanical press manufacturers, I can tell you that our rule of thumb with straight-side presses is never go over 80 percent of the press’s rated capacity. On a C-frame press, I would not go over 60 to 70 percent. Even with the tie bars in the front of the C frame preloaded, the flexing of the press frame can result in an inconsistent hit and increases the part’s dimensional variation in production.

If you are pushing that limit, you can mitigate the issue somewhat by installing a counterbalance. This is a pneumatic cylinder mounted to the top of the press to apply and balance the force on the ram throughout the press cycle. Without this counterbalance, there is literally no force on the ram until you do the work at the bottom of the stroke. This massive force at a single point in the cycle causes the frame to open and spring back when the blanking is done, which causes vibration, and vibration causes 75 percent of tooling wear. Imagine looking at the press from the side without the counterbalance. In slow motion the frame would look like the yellow character in the game Pac-Man!

Many of us in the stamping industry have experienced the need for higher tonnage for some applications, but we don’t want to buy more presses if we can avoid it. Here is an example of how to reduce the tonnage required for a particular stamping application.

Punching 12-gauge stainless steel with eight rectangular punches would require 43 tons using the following formula:

Perimeter x Material thickness x Shear strength (in tons) = Required tonnage

Since the eight punches have equal cutting surfaces, the tonnage per punch is 5.37. Most progressive dies have spring pads, lifters, or stripper springs that get compressed during the stamping process, so tooling tonnage requirements must 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.

The percentage of press tonnage capacity needed for blanking 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, which usually occurs at 40 percent punch penetration into the raw material. Since 12-ga. steel is 0.105 inch thick, 40 percent punch penetration is 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, tonnage is theoretically cut in half.

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. 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 punches and dies get dull, tonnage requirements will increase.

So, for this example, start with 43 tons divided by eight punches, or about 5.4 tons per punch. 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 the drag forces and springs, using a maximum of 2 tons will keep you in a safety factor of about 18 tons.

Remember, set up the two punches at the shortest length to minimize the snap-through of the ram slamming into your tooling down-stops.