Metal Shearing

One of the most basic and oldest metal working operation is shearing (also - machine metal shearing). Metal Shearing, or Machine Metal Shearing, is defined as the mechanical cutting of large sheets of metal into smaller pieces of predetermined sizes. A metal shearing operation that completes an entire perimeter is known as blanking, with the resulting work piece being called a blank.

Most metal shearing operations are accomplished by the action of two blades, one fixed and one moving vertically, meeting progressively from one side of the metal material to the other much like ordinary hand shears. The angular alignment of the shear blades is called the rake. Also to be considered is the blade or knife clearance to each other. Both rake and clearance are a function of the type and thickness of the material to be cut.

The “slip-plane” on the shear is the final cracking from both the top and bottom of the work after the descending upper blade partially cuts through the work. This upper blade is usually inclined in relation to the bottom blade, 1/2 to 2-1/2 degrees. This concentrates cutting pressure exactly at the juncture of the shear blades and assures a cut exactly parallel to the blades. The slight offset also helps clean material from between the blades.

Shearing is also done on a “shearing die” mounted in a stamping press, however most metal shearing is accomplished with a machine designed especially for the operation and is called a “shear.”

The typical shear consists of:

• a fixed bed to which one blade is attached

• a vertically moving crosshead which mounts on the upper blade

• a series of hold-down pins or feet which hold the material in place while

the cutting occurs

• a gauging system, either front, back, or squaring arm, to produce specific work piece sizes

Metal Shears may be operated manually, mechanically, hydraulically, or pneumatically. They can also be classified by their design. “Gap” and “gapless” metal shears are defined by their side frames and the maximum size metal sheet they can handle.

“Right angle” shears have two shear blades set at a 90 degree angle to each other and will cut simultaneously in two directions.

“CNC” shears are programmable to cut various sizes by automatically feeding material into the blades.

“Ironworkers” are designed to cut angle and bar stock and to perform punching operations. The sharpness of the knives or blades critically determine the edge quality of the cut and the accurate size of the work piece. Dull or improperly gapped or positioned blades will create in the cut piece, either:

• a camber or deviation from a straight edge on the drop side of the shear

• a bow which is the tendency of the sheared part to arch in the center

• a twist which is the angular distortion of the part from end to end

Another common shearing operation is known as “slitting.” This operation begins with a master coil of a given width. Material from the master coil is fed through a series of rotary knives set to produce a group of more narrow stock widths for subsequent processing.