Die Science: Solving punch and die chipping problems

Stampers and die builders are cutting and forming steels with higher strengths than ever, which continue to increase each year. Twenty-five years ago, if a metal had a tensile strength of 60,000 pounds per square inch (PSI), it was considered “high strength” and difficult to cut and form. Today it’s not uncommon for stampers to form metals with tensile strengths of more than 174,000 PSI.

These higher-strength metals require greater forces and energy to form and cut. Subsequently, cutting high-strength metals increases the shock loading that occurs on the cutting sections and components. Excessive shock loading of punches and dies often causes the cutting edges to chip prematurely or even crack.

The Role of the Press

Statistically speaking, the root cause of punch chipping is the press type. Certain types of presses, even when new or in perfect operating condition, are not suited for high-tonnage precision applications. Gap-frame and C-frame presses are two typical examples. Although these types of presses are very popular and widely used, they often have high deflection rates when subjected to forces nearing their capacity. In other words, when the tonnage on the ram increases, the amount that the ram deflects with respect to the bolster goes up, which causes poor pierce punch and cutting section alignment.

Most gap-frame presses are rated for deflection per inch of throat depth. For example, if you have a gap-frame press with a 24-inch-deep bed rated at 100 tons and you calculate ram-to-bolster deflection when 100 tons is applied to the press, the ram could be out of parallel to the bolster as much as 0.048 in. This calculation uses a deflection rate of 0.002 in. per inch of throat depth. Most older gap-frame presses are rated at 0.0015- to 0.002-in. deflection per inch of throat depth.

Press vibration also is a problem with gap presses, especially when using solid-carbide punches. Although carbide is extremely wear-resistant, its ability to absorb shock and vibration is very poor.

Whenever possible, use a straight-side or box-frame press. These presses generally deflect 12 times less than a gap-frame press. The guidance system on a straight-side press typically is more precise and rigid than on a gap-frame press as well.

Using a servo-drive press also can reduce punch chipping because, unlike a conventional crank-drive press, a servo-driven press can slow the ram down at the point the cutting punches contact the sheet material, which reduces the shock loading of the cutting sections. And unlike a conventional crank-drive press, a servo press typically has full energy available at this contact point, making it an ideal metal cutting and forming machine.

Remember that all of the work performed in a press is best suited to take place directly under the ram and ideally within the boundaries of the ram gibbing. Avoid having dies extend beyond the ram and bolster surface, as this will most likely cause the ram to tip and severely deflect, especially if cutting or forming is taking place in the overhanging area.

And here’s a tip about tipping: If you are unsure if your press ram is tipping, bouncing, jumping, or deflecting, take a slow-motion video of the press cycling and watch it carefully.

Tool Steel Selection

When punch chipping occurs, choose a tool steel with high-impact toughness. The most popular conventional tool steel with impressive impact toughness is S-7, but while this type of tool steel will help reduce chipping, it will likely require frequent sharpening.

Certain grades of powdered metallurgy tool steels are very well suited for applications requiring high impact toughness. They offer not only the required impact toughness, but also abrasive and adhesive wear resistance. Keep in mind, though, that powdered metallurgy tool steels can be a bit pricey.

Post-EDM Stress Relieving

The process of wire electrical discharge machining (EDM) also can cause steel damage that can result in premature punch breakdown. At a microscopic level, wire burning looks like a bolt of lightning coming off the wire and striking the tool steel section. This discharge creates a molten crater of steel lava to be discharged out of the tool steel section.

Essentially, this process melts air-hardened tool steel while it is submerged in water. The quick quenching process makes the steel very brittle at the wire-cut surface. Using a low-amperage skin cut can help reduce the tool steel damage. For intricate wire-burned punches, the wire-burned section should be stress-relieved after EDM. This tempering process—heating the tool steel section in a furnace to about 50 degrees F below its final tempering temperature—restores the toughness of the cutting edge while maintaining the necessary hardness.

Punch Preparation

Edge Prep. Take the time to stone a very small radius (about 0.002 in.) on the edge of your cutting punches to help prevent the edge from chipping or breaking off. Simply removing the grinding burr with a soft piece of brass can help.

Coating. Tool steel coatings such as titanium and carbide help reduce friction and wear. Coating a cutting punch can increase its life as much as 500 percent. However, keep in mind that coating a punch does nothing to increase its toughness; it improves wear resistance only.

Sharpening. Very simply, punches can break because of a poor sharpening method. Using the improper grinding wheel and failing to keep the pierce punch cool during the grinding process can result in micro stress cracking or heat checking. To prevent problems, avoid burning punches and use the proper grinding wheel. A coarse-grit, soft-bond wheel usually is best for premium tool steel punches.