What Constitutes “Normal” Sheet Metal?
Shear strength: 25,000-75,000 psi (0.172-0.157 kN/mm2)
Normal sheet metal will provide the most trouble-free operation and longest tool life. Material that is not in the normal range but still within the capacity of the punch press may require special tools, high lubrication, multiple hits and/or other procedures to produce a satisfactory job. Call Mate customer service for suggestions.
Combating Material Warpage
If you’re punching a large number of holes in a sheet and the sheet does not stay flat, it could be caused by the cumulative effect of punching. Each time a hole is punched, material surrounding the hole is stretched downward, placing the top of the sheet in tension. The downward movement causes a corresponding compression at the bottom of the sheet. For a few holes, the effect is insignificant, but as the number of holes increases the tension and compression can multiply to the point where the sheet deforms.
One way to counteract this effect is to punch every other hole first and then come back and punch the remaining holes. This places the same amount of force on the sheet, but it disrupts tension/compression accumulation that occurs when punching operations follow one another in close succession and in the same direction. It also allows the first set of holes to absorb some of the distorting effect of the second set.