December 12, 2006
Numerous factors contribute to the premature failure or breakage of piercing and cutting punches. Two common factors are operator error and incorrect die cutting clearance.
Editor's Note: This is one article in a three-part series. Part I discusses typical causes of premature failure. Part II covers the roles of cutting shear and retainers. Part III discusses how tool steel selection, press deflection, and heat from processes such as EDM and grinding affect the likelihood of punch breakage.
With the development of new, high-strength materials comes a dramatically increased failure rate for cutting punches. It's no secret that higher-strength materials require considerably more force to cut than the more traditional, softer grades of steel. Excessive force, deflection, or shock can cause cutting punches to break.
A broken punch, if carried through an automated system such as a transfer or progressive die, can result in severe die damage, not to mention a great deal of downtime and frustration. Numerous factors contribute to the premature failure or breakage of piercing and cutting punches. Two of the most common contributors are operator error and incorrect cutting clearance.
Common operator mistakes include over- or underfeeding a progressive die, double metal, poor strip-starting location, and incorrect shut height.
You can address this problem in two ways:
Unfortunately, most die-building and stamping shops have an across-the-board cutting clearance that they use for all of their cutting applications. Most of the time it is about 10 percent of the metal's thickness per side. If the shop is stamping mostly aluminum, it might be about 8 percent per side; with stainless steel, 12 percent per side.
Although these percentages are good rules of thumb, they are certainly not always the best clearance for every cutting application. Using the wrong clearance, especially one that is insufficient, can cause excessive force to be applied to the punch. If the force is too great, it might cause the punch to deflect and break.
For example, let's say that you want to pierce a 0.5-inch hole in a piece of 0.125-in.-thick 304 stainless steel. Using the rule of thumb, this would calculate to 0.015 in. per side for a total difference of 0.030 in. between the punch size and the matrix size. This is an acceptable engineered cutting clearance.
Now let's change one of the factors, so that you're piercing a 0.100-in. hole in 0.125-in.-thick 304 stainless steel. The only parameter that changed is the pierce hole diameter. According to the rule of thumb, this would still calculate to 0.015 in. per side. If you use this cutting clearance, you will prematurely wear out and possibly break your punches. The ideal cutting clearance for this application is about 20 percent of the metal's thickness per side, or double the rule of thumb.
Many stampers are concerned that such a large clearance will cause cutting burrs. However, the tendency for producing a large cutting burr is actually reduced. If the cut has a small radial feature, a very high compressive load will be applied in that area. As the punch gets smaller with respect to the material's thickness, the radius of the punch also gets smaller.
So, wherever there is a small radial feature in a cut, the cutting clearance must be increased to reduce the compression. If the distance between the punch and the matrix is insufficient, the metal may be dragged or smeared down the side wall of the hole, causing a high-compressive burr (see Figure 1). Reducing the compression will dramatically reduce the cutting force needed, as well as reduce the unnecessary loading and deflection of the punch (see Figures 2a and 2b).
Using the proper cutting clearance also will help to reduce the stripping force necessary to strip the punch from the material. Your punch manufacturer can help you determine the ideal cutting clearance for your particular application. Certain suppliers will even conduct a special clearance test for you.
Using the correct clearance will not only help you reduce punch breakage, but it also will help reduce the normal wear that takes place on the edges and sides of the cutting punch. In addition, keep in mind that the fundamental rule regarding compression also applies to oddly shaped wire-burned punches and cutting sections.
Until next time ... best of luck!
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