Shearing Machine – Types, Principle, Working, Operations

Shearing Operation Fundamental

Shearing :

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.

Shearing: Definition 

  • 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.
Direct tool ordinary machining is NOT the same as shear machining. With standard tool machining, a single or multiple point tool can be employed to take out a piece of metal from the metal sheet and/or block. This process required repeated pounding of the metal until the required dimensions are achieved. With shear machining, the tool’s cutting edge removes the metal from the plate. While this happens, maximum pressure is applied. The tool, however, only touches the metal one time.

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.
  • Lubrication.
Both mechanical and hydraulic machines can perform shear machining. Hydraulic shears cut and score sheet metal quickly and accurately. They work well for factories that do a lot of metal fabrication. Also, hydraulic shears are best if the operation requires intense pressure. They don’t require a lot of maintenance, will operate continuously, and are fast and quiet. Hydraulic shear machines also take up less space than mechanical shear machines while applying the same amount of pressure.

Shear Mechanism : 

The shear cutting or punching action results from a closing motion of two sharp, closely adjoined edges on material placed between them. The material is stressed in shear to the point of fracture while going through three phases
1. Deformation:
As the cutting edges begin to close on the material, deformation occurs on both sides of the material next to the cut edge.
2. Penetration:
The cutting edges cut or penetrate the material, causing fracture lines.
3. Fracture:
The point where the upper and lower fracture lines meet. At this point the work is done, but in punching, the punch must continue to move through the material to clear the slug.
shear mechanism phases
shear mechanism phases

Types of Shearing Machine 

1. Straight-Knife Shearing
In straight-knife shearing, the flat workpiece is placed between a stationary lower knife and a movable upper knife. As the upper knife is forced downward, it cuts the metal into two parts. Straight-knife shearing is the most economical method of cutting straight-sided blanks from flat sheet, strip, and plate with thickness no more than 50 mm (2 in.). The process is also widely used for cutting the sheet into blanks that
will subsequently be formed or drawn.
Machines for Straight-Knife Shearing
Punch presses and press brakes can be used for shearing a few pieces or are used temporarily when more efficient equipment is not available. Production shearing, however, is usually done in machines that are designed for this operation.
  • 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.
straight edge shearing machine
straight edge shearing machine

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:

hydraulic shearing machine
hydraulic shearing machine
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:

principle of shearing
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
Friction between the punch and the workpiece can, however, increase the punch force significantly. Furthermore, in addition to the punch force, a force is required to strip the punch from the sheet during its return stroke. This second force, which is in the opposite direction of the punch force, is difficult to estimate because of the many factors involved in the operation.

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 major processing parameters in shearing are
  • The shape of the punch and die
  • The speed of punching
  • Lubrication
  • The clearance, c, between the punch and the die.
shearing operation process
shearing operation process

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).

The speed of punching
Edge quality can be improved with increasing punch speed; speeds may be as high as 10 to 12 m/s. As shown in Fig., sheared edges can undergo severe cold working due to the high shear strains involved. Work hardening of the edges then will reduce the ductility of the edges and thus adversely affect the formability of the sheet during subsequent operations, such as bending and stretching.

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Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

One thought on “Shearing Machine – Types, Principle, Working, Operations

  1. Dear sir
    Qusetion :-
    THE UPPER BLADE AND LOWER BLADE THE SAME DIMENSIONS WIDTH AND THICKNESS FOR The guillotine shears, blade length up to 6 m and cutting capacity up to 20-30 mm sheet thickness.

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