3.1
Mechanics of Shearing
SHEARING PROCESS
3.1 MECHANICS OF SHEARING
The shearing process involves the cutting of flat material forms, such as sheets and plates. The cutting may be done by different types of blades or cutters in special machines driven by mechanical, hydraulic, or
pneumatic power. Generally the operations consist of holding the stock rigidly, while it is severed by the force of an upper blade as it moves down past the stationary lower blade.
During the shearing process, three phases may be noted: In phase I, because of the action of the cut- ting force
the stress on the material is lower than the yield stress This phase is that of elastic
deformation Fig. 3.1. To prevent the movement of material during the cutting operation, the material is held by the material holder at force
In phase the stress on the material is higher than the yield stress
but lower than the UTS. This phase is that of plastic deformation In phase
the stress on the material is equal to the shearing stress
=
The material begins to part not at the leading edge, but at the appearance of the first crack or breakage in the material. Fracture of the material occurs in this phase.
The amount of penetration of the upper blade into the material is governed by the ductility and thick- ness of the work material. If the material is thicker and more brittle, the first crack will appear earlier, so
there is earlier disjunction of the material. The sheared edge is relatively smooth where the blade pene-
trates, with a considerably rougher texture along the torn portion.
3.2 SHEARING FORCES
Knowledge of the forces and the power involved in shearing operations is important, and they may be cal- culated according to the edge types of the cutters. There are three types of cutters:
23
24
Shearing Process straight parallel cutters,
straight inclined cutters, and rotary cutters.
Work material Upper blade
Phase
I
Phase Phase
111
Fig. 3.1
Schematic illustration of the shearing process.
3.2.1 Shearing with Straight Parallel Cutters
The shearing force F with straight parallel cutters Fig 3.2 can be calculated approximately as:
where:
=
shear stress,
A
=
cutting area. The cutting area is calculated as:
where:
b
=
width of material T
=
thickness of material This calculated shearing force needs to be increased by
20 to 40 depending on the:
obtuseness of the angle of the cutter edge, enlarged clearance between cutters,
varations in the thickness of the material, and other unpredictable factors.
The real force of the shearing machine is:
Shearing Process
25
The crosscut force at the cutters Fig. 3.2 is:
F
For shearing without a material holder, the turn angle of the material is:
=
and the crosscut force is:
=
For shearing with a material holder, the turn angle is: and,
the crosscut force is:
Work material Work material
Fig. 3.2
Schematic illustration of shearing with straight parallel cutters.
3.2.2 Shearing with Straight Inclined Cutters
Shears with straight inclined cutters are used for cutting material of relatively small thickness compared with the width of cutting. Using inclined cutters reduces the shearing force and increases the range of
movement necessary to disjoin the material. The penetration of the upper blade into material is gradual, and as result, there is a lower shearing force.
The shearing force can be calculated approximately as:
0.6 UTS
E,,
-
F
=
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