Shearing (manufacturing)
 |
Shearing is a metalworking process which cuts stock without the formation of chips or the use of burning or melting. Strictly speaking, if the cutting blades are straight the process is called shearing; if the cutting blades are curved then they are shearing-type operations.[1] The most commonly sheared materials are in the form of sheet metal or plates, however rods can also be sheared. Shearing-type operations include: blanking, piercing, roll slitting, and trimming.
Principle
A punch (or moving blade) is used to push the workpiece against the die (or fixed blade), which is fixed. Usually the clearance between the two is 5 to 10% of the thickness of the material, but dependent on the material. Clearance is defined as the separation between the blades, measured at the point where the cutting action takes place and perpendicular to the direction of blade movement. It affects the finish of the cut (burr) and the machine's power consumption. This causes the material to experience highly localized shear stresses between the punch and die. The material will then fail when the punch has moved 15 to 60% the thickness of the material, because the shear stresses are greater than the shear strength of the material and the remainder of the material is torn. Two distinct sections can be seen on a sheared workpiece, the first part being plastic deformation and the second being fractured. Because of normal inhomogeneities in materials and inconsistencies in clearance between the punch and die, the shearing action does not occur in a uniform manner. The fracture will begin at the weakest point and progress to the next weakest point until the entire workpiece has been sheared; this is what causes the rough edge. The rough edge can be reduced if the workpiece is clamped from the top with a die cushion. Above a certain pressure the fracture zone can be completely eliminated.[2]
Straight shearing
Straight shearing is done on sheet metal, coils, and plates. The machine used is called a squaring shear, power shear, or guillotine. The machine may be foot powered (or less commonly hand powered), or mechanically powered. It works by first clamping the material with a ram. A moving blade then comes down across a fixed blade to shear the material. 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 decreases the amount of force required, but increases the stroke. The amount of energy used is still the 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.[3]
The design of press tools is an engineering compromise. A sharp edge, strength and durability are ideal, however a sharp edge is not very strong or durable so blades for metal work tend to be square-edged rather than knife-edged.
Shearing
 |
Shearing is a process that allows for straight line cutting. It is most often accomplished with the use of two blades. One located on top and one on bottom of the surface. One blade usually remains stationary with the other forcibly being passed by the other resulting in a cut.
Is used for straight-line cuts on flat sheet stock. Takes place between the upper and lower shear blades. Produces burred and slightly deformed edges. Blades may be mounted at different angles in order to reduce the force required to make the cut.
The setup and equipment consists of a shear table, shear blades, work piece holding devices. The gage is used in order to insure consistent lengths. The hold-down devices hold the work piece in position. It also prevents the work piece edge form buckling during cutting.
The upper blade is mounted at an angle with respect to the horizontal lower blade. Shearing is limited to only basic straight line cutting. Many different shapes may be produced by inserting the sheet metal at different angles.
Shapes are formed primarily from sheet, plate, bar, and angle stock. There are many different possibilities of shapes that can be produced from sheet metal. Bar and angle materials however cannot have many different shapes. For these shapes this process is usually used only to cut these materials to length.
Tolerances are typically held between +- 0.01in. for sheet materials and +- 0.06in. for bar stock.
There are three types of systems for tool style. Those for sheet and plate material, those for angles, and those for bar stock. Sheet and plate materials use squaring and bow-tie shear. Angle and bar shear are for angle and bar stock material.
There are three main effects on work materials processed with shearing. These include work hardening on the edge of the sheared part, residual cracks, and too much clearance may cause roll-over or heavy burring.
Tool materials include low alloy steel, high-carbon steel, high chromium steel, and shock resisting steel. These are the most common tool steels used in shearing. Low alloy tools are best used when the material is under ¼ in. thick. High-carbon or high-chromium steel is best for high production of materials up to ¼ in. thick. Shock resistant tools are recommended for materials greater or equal to ¼ in. thick.
The shear angle of the upper blade is the most important of factor for the shear and is defined as the amount of rise in inches per feet. This angle should be kept as low as possible to reduce the amount of distortion in narrow work pieces.
References
Notes
Bibliography
- Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4.