Shearing is a general name for most sheet metal cutting, but in a specific sense, designates a cut in a straight line across a strip, sheet or bar. This procedure leaves a clean edge on the piece of the metal that is sheared or cut. Shearing machines are used to cut or shear metal sheets in many ways. The particular method chosen depends on several factors such as the size and shape of the parts required and the numbers needed.The moving cutting member of a shearing machine may be actuated by:
(a) Hand lever in bench shearing machines
(b) Foot treadle in treadle guillotines
(c) Electric motor or hydraulic system in power guillotines
Shearing Machine Principle
There is a wide variety of machines for shearing sheet metal from the basic elementary snips to portable and static power machines. Whether the machine is operated by hand, foot or power, in each case the basic principle of metal cutting is the shearing action of a moving blade in relation to a fixed blade.
The standard type of bench shear and all guillotines are used for straight-line cutting. The basic principle of these machines is that one blade is fixed (bottom blade) and moving blade (inclined to the fixed blade) is brought down to meet the fixed blade as shown in image below.
If the cutting members of the guillotine or shearing machine are arranged parallel to each other, the area under shear would be the cross-section of the material to be cut, i.e. length x thickness.
The top cutting member of a shearing machine is always inclined to the bottom member to give a shearing angle of approximately 5. Top Cutting Blade Inclined image below shows that with this arrangement of blades, the area under shear is greatly reduced and consequently the force required to shear the material is considerably reduced.
The shear blades are provided with a ‘rake angle’ of approximately 87 and there must be a clearancebetween the cutting edges of the blades to assist in the cutting action. A common rule is that the clearance should not exceed 10% of the thickness to be cut and must be varied to suit the particular material, e.g. mild steel (10%), brass (4%) and aluminium (3%).