Shearing (metalworking)

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.Degarmo, p. 424.] 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 as 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.Degarmo, p. 425.]

traight 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.Degarmo, pp. 426-427.]

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".

References

Notes

Bibliography

*Citation | last = Degarmo | first = E. Paul | last2 = Black | first2 = J T. | last3 = Kohser | first3 = Ronald A. | title = Materials and Processes in Manufacturing | publisher = Wiley | year = 2003 | edition = 9th | isbn = 0-471-65653-4.

External links

* [http://www.e-ci.com/pdf/shears/Shear-Capacities_PT30491.pdf Shearing Capacity Guide]
* [http://www.e-ci.com American Shear Manufacturer - Cincinnati Incorporated]