Shearing, also known as die cutting, is a process which cuts stock without the formation of chips or the use of burning or melting. In strict technical terms, the process of “shearing” involves the use of straight cutting blamestorm of sheet metal or plates; however rods can also be sheared. Shearing-type operations include: blanking, piercing, roll slitting, and trimming.
Sheet metal cutting is a major classification for many different press working operations. Cutting operations involve the separation of the metal of the sheet in certain areas. This separation is caused by shearing forces acting on the metal through the edges of the punch and die. Press working, a term referencing sheet metal operations in general, involves the working of a sheet between two die. In press working, the upper die is called a punch. Sheet and plate generally refers to rolled metal with a high surface area to volume ratio. The difference is that sheet metal is under 1/4 inch (6mm) in thickness, while plate metal is thicker. Most of the sheet metal cutting processes discussed can be performed on both sheet and plate metal, although for many sheet metal operations difficulties will arise with increasing plate thickness. Usually “sheet” and “sheet metal” is also referencing plate.
Types of Shear
A bench shear, also known as a lever shear, is a bench mounted shear with a compound mechanism to increase the mechanical advantage. It is usually used for cutting rough shapes out of medium sized pieces of sheet metal, but cannot do delicate work. For the small shear, it mostly designed for a wide field of applications. Light weight and easy efficient operation, yet very sturdy in construction. The cutting blades fitted are carefully and accurately ground to give easy, clean quick cuts, and free of burrs. These special features help the operators save a great deal of their energy. But some shearing machines can cut sheet bar and flat bar up to 10mm.
For larger shears the moving blade may be set on an angle or “rocked” in order to shear the material progressively from one side to the other; this angle is referred to as the shear angle. This decrease the amount of force required, but increases in the stroke. A 5 degree shear angle decreases the force by about 20%. The amount of energy used is still same. The moving blade may also be inclined 0.5 to 2.5°, this angle is called the rake angle, to keep the material from becoming wedged between the blades, however it compromises the squareness of the edge.
A power shear is electrically or pneumatically powered hand tool designed to blank large pieces of sheet metal. They are designed to cut straight lines and relatively large radius curves. They are advantageous over a band saw because there is not a size limit. Large versions can cut sheet metal up to 12 gauge.
A throat less shear is a cutting tool used to make complex straight and curved cuts in sheet metal. The throatless shear takes its name from the fact that the metal can be freely moved around the cutting blade allowing great flexibility in shapes that can be cut.
Cutoff and Parting of Sheet Metal
Cutoffs and partings are important basic sheet metal cutting processes performed in manufacturing industry. Cutoffs need not be straight; rather they may be over several lines and/or curves. Partings are similar, in that a discrete part is cut from a sheet or strip of metal along a desired geometric path. The difference between a cutoff and a parting is that a cutoff can be nestled perfectly on the sheet metal, due to its geometry. With cutoffs, the cutting of sheet metal can be done over one path at a time and there is practically no waste of material. With partings, the shape cannot be nestled precisely. Partings involve cutting the sheet metal along two paths simultaneously. Partings waste a certain amount of material that can be significant.
Clearance during a Cutting Process
Clearance is an important factor in the design of a sheet metal cutting process. As discussed earlier, cutting clearance is the straight line, lateral distance between the edge of the punch and the edge of the die hole. The way the metal separates, hence the cut, will be largely determined by the clearance. Optimization of cutting clearances, dependent upon specific process factors, will provide a quality cut. When clearances are correctly employed, the mechanics of sheet metal cutting should occur as described in the preceding section.
Improper values could compromise the cutting process. More clearance than required causes the sheet metal to get forced between the cutting edges. Fracture occurs incorrectly and the resulting edge is usually not desirable.
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This is a demonstrational and educational video about blade(hand) shearing produced to improve technique in this method of shearing.
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