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] However, the sheared edge of the workpiece will usually experience workhardening and cracking. If the workpiece has too much clearance, then it may experience roll-over or heavy burring.
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 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]As far as equipment is concerned, the machine consists of a shear table, work-holding device, upper and lower blades, and a gauging device. The shear table is the part of the machinery that the workpiece rests on while being sheared. The work-holding device is used to hold the workpiece in place and keep it from moving or buckling while under stress. The upper and lower blades are the piece of machinery that actually do the cutting, while the gauging device is used to ensure that the workpiece is being cut where it is supposed to be.
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. Typical workpiece materials include aluminum, brass, bronze, and mild steel because of their outstanding shearablity ratings, however, stainless steel is not used as much due to its tendencies to work-harden.
There are also other types of Geometrical Possibilities besides straight shearing though: These include the Squaring Shear, the Angle Shear, the Bow-Tie Shear and the Bar Shear. All of these have many different uses and are all used pretty regularly in certain manufacturing fields.
Tool Materials
- Low alloy steel is used in low production of materials that range up to 1/4 in. thick
- High-carbon, high cromium steel is used in high production of materials that also range up to 1/4 in. in thickness
- Shock-resistant steel is used in materials that are equal to 1/4 in. thick or more
Tolerances and Surface Finish
When shearing a sheet, the typical tolerance is +0.1 or -0.1, but it is feasible to get the tolerance to within +0.005 or -0.005. While shearing a bar and angle, the typical tolerance is +0.06 or -0.06, but it is possible to get the tolerance to +0.03 or -0.03. Surface finishes typically occur within the 250 to 1000 microinches range, but can range from 125 to 2000 microinches. A secondary operation is required if one wants better surfaces than this.
Production Cost Elements
- Time it takes to setup
- Time it takes to unload and load
- Time it takes to change the tools
- The cost of tools/machinery
- Paying for use of tools and equipment
- Tool Maintenance
- Direct Labor Rate
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.
Todd, H. Robert; Allen, K. Dell; Alting, Leo (1994), Manufacturing Processes Reference Guide (1st ed.), Industrial Press Inc.,