Metal Shears

Is there any formula for calculating slitting &shearing clearance relating to metal thickness ?

I have problem in slitting blades they always broken whenever I change setting .

A punch (or moving blade) is used to push the workpiece against the die (or fixed blade), which is fixed. Usually the clearance between the two is 5 to 10% of the thickness of the material, but dependent on the material. Clearance is defined as the separation between the blades, measured at the point where the cutting action takes place and perpendicular to the direction of blade movement. It affects the finish of the cut (burr) and the machine’s power consumption. This causes the material to experience highly localized shear stresses between the punch and die. The material will then fail when the punch as moved 15 to 60% the thickness of the material, because the shear stresses are greater than the shear strength of the material and the remainder of the material is torn. Two distinct sections can be seen on a sheared workpiece, the first part being plastic deformation and the second being fractured. Because of normal inhomogeneities in materials and inconsistencies in clearance between the punch and die, the shearing action does not occur in a uniform manner. The fracture will begin at the weakest point and progress to the next weakest point until the entire workpiece has been sheared; this is what causes the rough edge. The rough edge can be reduced if the workpiece is clamped from the top with a die cushion. Above a certain pressure the fracture zone can be completely eliminated

Knife Clearance. Horizontal knife clearance is the most significant of these variables. In the “old days,” the clearance was almost always set to 10 percent of the metal thickness. For example, a 0.008-inch clearance was used to slit 0.080-in.-thick metal. There are a few metal thicknesses and types on which the 10 percent rule happens to work; however, 10 percent is rarely the correct clearance.

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