I know how to calculate the force to punch the hole, but how do I know if that force will exceed the limits of the punch tip? Is there some sort of chart available?
Answer: I suggest you contact your punch manufacturer for help. There are many variables that can influence punch-tip breakage and your punch manufacturer will be a valuable resource for quickly finding the root cause of your failure(s).
Still, it is important to understand the science behind the problems we experience in the press shop. Therefore, we’ll discuss the calculations you inquired about and how you can use these results as selection criteria for your punches.
Fp = (L) (t) (st)
L = pd
Obtaining accurate shear stress data (st) can be quite difficult. As a result, approximations are commonly used. The approximate shear strength for mild steel is 70 to 80 percent of the ultimate tensile strength of the steel.
Shear strength will vary among different alloys but also can vary significantly within the same material type. Shear strengths for copper, for example, have been reported to be between 50 and 90 percent of their ultimate tensile strength, depending on the alloy.
Regardless of material type or alloy, if dull edges and large cutting clearances are used in the die, the shear strength begins to approach the nominal tensile strength of the material because the fracture mode becomes closer to pure tension rather than pure shear. For this reason, many tooling engineers will play it safe by using nominal tensile strength values in their calculations. However, this may be a costly safety net since the final punch often will be over-designed.
Ft = Fp/p (1/2d)
Ft = Fp/p (1/2d – 1/2d1)2
Taking this engineering approach to punch selection eliminates guesswork and maximizes punch-tip strength economically. MF
Related Enterprise Zones: Tool & Die
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