Shearing is a critical element in cutting and manufacturing sheet metal. Understanding the forces and cutting options available will help every person involved in the quoting and manufacturing process.
Table of contents
Shearing Process Background
Shearing is the process of cutting flat material into sheets, plates or strips. A special machine performs this process and they typically utilize a cutting tool moving along a straight line. The process involves an upper and a lower cutting tool with the upper blade of power shears often being inclined.
Forces at Play
The sheet metal cutting process puts material through 3 different phases:
- Plastic Deformation: As the upper blade moves down onto the workpiece, the force is not great enough to fracture the material. Henceforth, the stress on the material is lower than the yield stress.
- Penetration: As the blade makes contact with the material, the blade begins to push through it. Consequently, the stress on the material is higher than the yield stress, but lower than the ultimate tensile strength.
- Fracture: As the blade punctures through the material it fractures it and creates two separate pieces of material. The stress on the material is equal to the shearing stress resulting in a clean break.
Sheet Metal Cutting Options
There are three different types of tools available for sheet metal cutting:
- Straight parallel blades: This occurs when both blades are parallel and moving in opposite directions. The shearing force with straight blades can be calculated as:
Where F= force, t= shear strength of material and A= cutting area. The area can be calculated as:
Where b=length of the cutting material and T= thickness of the material
- Straight inclined blades: This occurs when one blade is at a slight incline relative to the working surface. This strategy is typically used to cut thin material. The shearing force can be calculated as:
F= n * k* UTS * Lambda*(T^2/tan theta)
Where n=.75-.85 , k= 0.7-0.8, Lambda = amount of upper blade penetration into material, sigma= angle of inclination of the upper blade. Please refer to the table below for lambda values.
- Rotary cutters: This shearing process is very similar to inclined blades but the top blade rotates. The upper cutter pinches the material and causes it to rotate between the two cutters. A variety of formulas are available to calculate the shearing force, but it largely depends on your angle and cutters.
Sheet Metal Clearance:
In conlusion, clearance needs to be accounted for whenever sheet metal is being cut. If too much clearance is used, plastic deformation will occur and if there isn’t enough then a defect called “secondary shear” is produced. Additional information about cuts and parting can be found in our article about dies and punching.
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