Shearing Machine – Types, Principle, Working, Operations

Shear Mechanism : 

Types of Shearing Machine 

• Squaring shears are usually used for trimming and cutting sheet, strip, or plate to a specific size (Fig.). These shears (also called resquaring or guillotine shears) are available in a wide range of sizes and designs. Some types of lines also permit slitting when the workpiece moves for shearing.
• The sheet, strip, or plate is held firmly by hold-down devices while the upper knife moves down past the lower knife. Most sheet or plate is sheared by setting the upper knife at an angle (Fig. ).
• The position of one of the knives can be adjusted to maintain optimal clearance between the knives. Squaring shears can be actuated mechanically, hydraulically, or pneumatically.

Mechanical Shearing Machine : 

• The power train of a mechanical shear consists of a motor, the flywheel, a worm shaft that is gear driven by a flywheel, a clutch that connects the worm gear drive to the driven shaft, and a ram actuated by the driven shaft through eccentrics and connecting links.
• Under most operating conditions, a mechanical shear can deliver more strokes per minute (spm) than a hydraulic shear. Some mechanical shears cycle as fast as 100 spam.
• Another advantage of the mechanical shear is that, because of the energy stored in the flywheel, a smaller motor can be used for intermittent shearing.

Hydraulic Shearing Machine : 

• Hydraulic shears are actuated by a motor-driven pump that forces oil into a cylinder against a piston; the movement of the piston energizes the ram holding the upper knife.
• A hydraulic shear can make longer strokes than a mechanical shear. Hydraulic shears are designed with a fixed load capacity.

Pneumatic Shearing Machine : 

• Pneumatic shears are used almost exclusively for shearing thin metal (seldom thicker than 1.50 mm, or 0.060 in.) in relatively short pieces (seldom longer than 1.5 m, or 5 ft).
• Activation of air cylinders makes the shear blade beam move to make a cut. Shop compressed air or a freestanding air compressor is used to provide power to air cylinders.

Alligator shearing Machine : 

• Alligator shears have a shearing action similar to that of a pair of scissors. The lower knife is stationary, and the upper knife, held securely in an arm, moves in an arc around a fulcrum pin.
• This type of machine is most widely used for shearing bars and bar sections and for preparing scrap.

Rotary Shearing Machine 

• Rotary shearing, or circle shearing (not to be confused with slitting), is a process for cutting sheet and plate in a straight line or in contours utilizing two revolving, tapered circular cutters.
• The shearing of workpieces into circular blanks requires the use of a holding fixture that permits rotation of the workpiece to generate the desired circle. For straight-line cutting in a rotary shear, a straight-edge fixture is used, mounted in the throat of the machine behind the cutter heads.

When the metal is inserted into hydraulic machines, it is secured by clamps so it does not shift under high pressure. To ensure that the cut is smooth and even for a 90 degree cut, a squaring arm or back gauge must be used. Minute burrs may be formed by the edge of the cut metal; these must be removed by grinding. If you use hydraulic shears, be careful of the little marks left by the cutting blades and holding clamps while working. You will need to remove them or otherwise account for them

Principle Of Shearing:

To find the shear force for a cut we can go back to the basic mechanics of materials (with one adjustment factor).

F = 0.7twUTS

where,

F = force needed to shear

t = thickness of sheet

w = width of sheet

UTS = Ultimate Shear Strength of material

Shearing Operations : 

• The most common shearing operations are punching-where the sheared slug is scrap or may be used for some other purpose-and blanking-where the slug is the part to be used and the rest is scrap.
• Shearing may also be done between a punch and die, as shown in Fig. The shearing operations make which use of a die, include punching, blanking, piercing, notching, trimming, and nibbling.

Punching/Blanking

Punching or blanking is a process in which the punch removes a portion of material from the larger piece or a strip of sheet metal. If the small removed piece is discarded, the operation is called punching, whereas if the small removed piece is the useful part and the rest is scrap, the operation is called blanking.

Piercing:

It is a process by which a hole is cut (or torn) in metal. It is different from punching in that piercing does not generate a slug. Instead, the metal is pushed back to form a jagged flange on the backside of the hole.

A pierced hole looks somewhat like a bullet hole in a sheet of metal.

Trimming:

When parts are produced by die casting or drop forging, a small amount of extra metal gets spread out at the parting plane. This extra metal, called flash, is cut – off before the part is used, by an operation called trimming. The operation is very similar to blanking and the dies used are also similar to blanking dies. The presses used for trimming have, however, a relatively larger table.

Notching:

It is an operation in which a specified small amount of metal is cut from a blank. It is different from punching in the sense that in notching cutting line of the slug formed must touch one edge of the blank or strip. A notch can be made in any shape. The purpose of notching is generally to release metal for fitting up.

Nibbling:

Nibbling is a variation of notching, with overlapping notches being cut into the metal. The operation may be resorted to produce any desired shape, for example flanges, collars, etc.

Perforating :

Perforating is an operation is which several uniformly spaced holes are punched in a sheet of metal. The holes may be of any size or shape. They usually cover the entire sheet of metal.

Process Parameter of Shearing Machine : 

• The shape of the punch and die
• The speed of punching
• Lubrication
• The clearance, c, between the punch and the die.

The clearance, c, between the punch and the die.

The clearance is a major factor in determining the shape and the quality of the sheared edge. As the clearance increases, the zone of deformation (Fig. ) becomes larger and the sheared edge becomes rougher. The sheet tends to be pulled into the clearance region, and the perimeter or edges of the sheared zone become rougher. Unless such edges are acceptable as produced, secondary operations may be
required to make them smoother (which will increase the production cost).

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