The shearing action can be explained in the following four stages:
(a) Stage 1. As the top cutting member is moved downwards and brought to bear on the metal with continuing pressure, the top and bottom surfaces of the metal are deformed.
(b) Stage 2. As the pressure increases, the internal fibres of the metal are subject to deformation. This is plastic deformation prior toshearing.
(c) Stage 3. After a certain amount of plastic deformation, the cutting members begin to penetrate. The uncut metal ‘work hardens’ at the edges.
(d) Stage 4. Fractures begin to run into the work hardened metal from the point of contact of the cutting members. When these fractures meet, the cutting members penetrate the whole of the metal thickness.
With all shearing machines, a sufficient force must be applied to the moving blade to overcome the shear strength of the material and cause it to shear along the line of action.
Effect of Clearance
Correct clearance is essential for obtaining good shearing effect. The effects of clearance are as follows:
(a) Excessive clearance causes a burr to form on the underside of the sheet.
(b) With no clearance, overstrain is caused and the edge of the sheet becomes flattened on the underside.
(c) With the correct clearance, optimum shearing results are obtained.