Shearing Operation Fundamental
A shear force is applied that will cut off part of a sheet. The cut off ‘blank’ becomes the workpiece. Shearing, also known as die cutting, is a process that 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 bladesorm of sheet metal or plates, however, rods can also be sheared. Shearing-type operations include blanking, piercing, roll slitting, and trimming.
- When you apply a high-pressure tool through a metal plate and remove part of the metal, the process is called shearing. Shear machining devices include punching machines, which make small discs, and blanking machines, which produce washers and similar objects.
- SHEARING is a method for cutting a material piece into smaller pieces using a shear knife to force the material past an opposition shear knife in a progression from.
- Shearing is widely used to divide large, flat stock such as sheet, strip, and plate. Shearing of sheet, strip, and plate is broadly classified according to the type of blade (knife or cutter) used as either straight or rotary.
- Straight-knife shearing is used for squaring and cutting flat stock to the required shape and size. It is usually used for producing square and rectangular shapes, although triangles and other straight-sided shapes are also sheared with straight knives.
The shearing process characteristics include:
- Its ability to make straight-line cuts on flat sheet stock
- Metal placement between an upper and lower shear blades
- Its trademark production of burred and slightly deformed metal edges
- Its ability to cut relatively small lengths of material at any time since the shearing blades can be mounted at an angle to reduce the necessary shearing force required.
The shearing process involves cutting sheet metal, as well as plates, bars, and tubing of various cross-section, into individual pieces by subjecting it to shear stress, typically using a punch and a die, similar to the action of a paper punch. The punch and die may be of any shape, such as circular or straight blades, similar to a pair of scissors.
Important variables in the shearing process are:
- The punch force.
- The speed of the punch
- The edge condition of the sheet.
- The punch and die materials.
- The punch-die clearance.
Shear Mechanism :
Types of Shearing Machine
will subsequently be formed or drawn.
- 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.
Working Of Hydraulic Shearing Machine:
Principle Of Shearing:
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 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.
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.
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.
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 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 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
- 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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