Machinery's Handbook, 31st Edition
1408
Shearing
Driven cutter
c
Work material
T
Shear line
Fig. 4. Schematic Illustration of Shearing with Rotary Cutters Shearing force with rotary cutters can be calculated approximately as
tan F n k UTS T 2 2 λ φ =
(17)
where φ = acos 1 µ − + : D ; μ = lap of cutters (in.); T = thickness of material (in.); D = diameter of cutter (in.); n = 0.75 to 0.85 (for most materials); k = 0.7 to 0.8 (ratio UTS ⁄ τ ); and, l = relative amount of penetration of the cutters (Table 3). The real force of a shearing machine with rotary cutter is (18) Rotary shearing machines are equipped with special holding fixtures that rotate the work material to generate the desired circle. D T Clearance.— Clearance is defined as the space between the upper and lower blades. With out proper clearance, the cutting action no longer progresses. With too little clearance, a defect known as “secondary shear” is produced. If too much clearance is used, extreme plastic deformation will occur. Proper clearance may be defined as that clearance which causes no secondary shear and a minimum of plastic deformation. The clearance between straight blades (parallel and inclined) is: c = (0.02 to 0.05), mm. The clearance between rotary cutters with parallel inclined axes is (19) where T = material thickness (in.). Cutoff and Parting Cutoff.— Cutoff is a shearing operation in which the shearing action must be along a line. The pieces of sheet metal cutoff are the blanks. Fig. 5 shows several types of cutoff opera tions. As seen in the illustration, a cutoff is made by one or more single line cuts. The line of cutting may be straight, curved, or angular. The blanks need to be nested on the strip in such a way that scrap is avoided. Some scrap may be produced at the start of a new strip or coil of sheet metal in certain cases. This small amount is usually negligible. The use of cutoff operations is limited by the shape of a blank. Only blanks that nest per fectly may be produced by this operation. Cutoff is performed in a die and therefore may be classified as a stamping operation. With each cut, a new part is produced. More blanks may be produced per stroke of the press ram by adding more single-line cutting edges. . F F 13 M = . T = 01 to 02 ^ h . c
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