SECOND IN A THREE-PART CUTTING SERIES

February 2011 - Cutting with shears consists of dividing a workpiece by using a two-edged tool that enables two cutting edges to cross each other in their movement. The edges work opposite each other. Some cutting tools use only one edge to penetrate material, such as a chisel.

The wedge-shaped cutting edges of shears—the top blade and bottom blade—slide past each other during the shearing process. The material becomes notched at both edges of the cut, and the pressure of the cutting wedges cold-hardens the material grain structure.

The resistance of the material increases while cutting with shears, as a result of which the blades can penetrate only up to a certain depth. When users raise the pressure further, the tensile strength of the material becomes insufficient. The grain structure breaks down, and the two ruptured surfaces are forced apart. The ruptured surfaces caused by using shears are irregular. The outer zones are smooth, and the inner zones are rough. (See Fig. 1)

The upper and lower blades of a shear have cutting wedges. The wedge angle typically is between 75 degrees and 85 degrees. An applied clearance angle of about 2 degrees has to be applied so the cutting wedges do not rub against the shearing surface of the workpiece and damage it.

A blade play of 1/10 to 1/20 of the material thickness ensures the blades do not damage each other. If the play is too large, the material is squeezed between the cutting wedges, damaging the shears and giving rise to a burr on the workpiece. (See Fig. 2)

Lever effect of the blades
The slanting surfaces of the blades, which act against each other and cross while cutting, cause the workpiece to rotate. When using hand shears, holding the sheet down prevents this rotation. For this reason, shearing machines include suppressors that press down on the sheets being cut to counter the turning moments that are generally very high.

Shearing equipment cuts a workpiece in one abrupt motion. Guillotine shears cut along the entire edge, and the to-be-cut material has the tendency to move up. Applying counter pressure by hand or a mechanical stop keeps the material in position. The shearing strength of a material is the resistance to being cut by a shearing tool. The cutting force necessary for the process of shearing is generated by a lever effect. (See Fig. 3)

The rotary moment of M2 = F2 x L2 at the cutting edge is the cutting force times the distance to the fulcrum. An equilibrium exists when the two moments are equal and opposite each other.

Blade rake anglez
When an operator pushes a metal sheet too far toward the fulcrum, the metal has a tendency to slide out from between the cutting blades. The cause of this is the forward thrust resulting from the application of cutting force. (See Fig. 4)

Both forces can be combined in a parallelogram of forces. The workpiece slides out when the resulting force is greater than the frictional force between the workpiece and the cutting edges.

An opening angle of about 15 degrees is the most favorable. If the angle is greater, the workpiece slides out of the shears. If it is smaller, the cutting cross-section and, therefore, the cutting force required, becomes too large. FFJ

Udo O.J. Huff is an independent consultant with project experience in machine building, welding engineering, training and development. He holds Master of Education and Bachelor of Science in Technology degrees from Bowling Green State University. Questions or comments? E-mail uhuff@sbcglobal.net.