Machinery's Handbook, 31st Edition
1406 Designing Sheet Metal Parts for Production Design rules of parts produced by drawing: Avoid very complicated parts. Make diameter of flange D less than three times the diameter of shell ( D < 3 d ), if height h of shell is greater than twice the diameter of shell ( h > 2 d ) as shown in Table 2. Avoid design of rectangular and square shells with bottom radii less than the corner radius in the junction area. The shortest distance between corner radii should be no less than the depth of shell. Shearing Shearing involves the cutting of flat material such as metal sheets, plates, or strips. To be classified as shearing, the cutting action must be along a straight line. The piece of sheet metal sheared off may or may not be called a blank. Shearing is performed in a special machine with different types of blades or cutters. The machines may be foot-, hand-, or power-operated. The shear is equipped with long or rotary blades for cutting. The upper blade of power shears is often inclined to reduce the required cutting force. During shearing operations, three phases (Fig. 3) may be noted: Phase I - Plastic Deformation : As the upper blade begins to push into the work material, plastic deformation occurs in the surfaces of the sheet, and the stress on the material is lower than the yield stress. Phase II - Penetration: As the blade moves downward, penetration occurs, in which the blade compresses the work material and cuts into the metal. In this phase, the stress on the material is higher than the yield stress but lower than the ultimate tensile strength ( UTS ). Phase III - Fracture : As the blade continues to travel into the work material, fracture begins in the material at the two cutting edges. The stress on the work material is equal to the shearing stress. If the clearance between the blades is correct, the two fracture lines meet, resulting in a separation of the work material into two parts.
F
F
Upper blade
F
F d
F d
F d
Holder
Lower blade
Work material
Phase I
Phase II
Phase III
Fig. 3. Schematic Illustration of the Phases of Shearing Shearing is the preferred way to cut blanks whenever the blank shape permits its use. In most cases, however, the limitation of straight lines in the shape of the blank eliminates the use of shears. Shearing is economical because no expensive dies have to be made for cutting out the blanks. Shearing is used for the following purposes: 1) To cut strip or coiled stock into blanks 2) To cut strip or coiled stock into smaller strips to feed into a blanking or drawing die 3) To trim large sheets, squaring the edges of the sheet. Shearing Forces.— Calculating the force and power involved in shearing operations var ies according to the types of blades. There are three types of blades: a) straight parallel blades; b) straight inclined blades; and c) rotary cutters. Shearing with Straight Parallel Blades: The shearing force with straight parallel cutters can be calculated approximately as (13) F A τ =
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