Blanking and Punching Machinery's Handbook, 31st Edition
1410
Characteristics of the blanking process include: 1) Ability to produce workpieces in both strip and sheet material during medium and mass production. 2) Removal of the workpiece from the primary material stock as a punch enters a die. 3) Control of the quality by the punch and die clearance. 4) Ability to produce holes of varying shapes quickly. Characteristics of the punching process include: 1) Ability to produce holes in both strip and sheet material during medium and mass production. 2) Ability to produce holes of varying shapes quickly. There are three phases in the process of shearing during blanking and punching as illus trated in Fig. 8. Punch Work material
Phase I
Phase II
Phase III
Die
Fig. 8. Phases in the Process of Shearing In Phase I, work material is compressed across and slightly deformed between the punch and die, but the stress and deformation in the material does not exceed the plastic limit. This phase is known as the elastic phase. In Phase II, the work material is pushed farther into the die opening by the punch; at this point in the operation the material has been obviously deformed at the rim, between the cutting edges of the punch and the die. The concentration of outside forces causes plastic deformation at the rim of the material. At the end of this phase, the stress in the work mate rial close to the cutting edges reaches a value corresponding to the material shear strength, but the material resists fracture. This phase is called the plastic phase. During Phase III, the strain in the work material reaches the fracture limit, and micro- cracks appear, which turn into macro-cracks, followed by separation of the parts of the workpiece. The cracks in the material start at the cutting edge of the punch on the upper side of the work material, and at the die edge on the lower side of the material; the crack propagates along the slip planes until complete separation of the part from the sheet occurs. A slight burr is generally left at the bottom of the hole and at the top of the slug. The slug is then pushed farther into the die opening. The slug burnish zone expands and is held in the die opening. The whole burnish zone contracts and clings to the punch. Blanking and Punching Clearance.— Clearance, c, is the space (per side) between the punch and the die opening shown in Fig. 9, such that: c D d 2 d p = − Ideally, proper clearance between the cutting edges enables the fractures to start at the cutting edge of the punch and the die. The fractures will proceed toward each other until they meet. The fractured portion of the sheared edge then has a clean appearance. For optimum finish of a cut edge, correct clearance is necessary. This clearance is a function of the type, thickness, and temper of the material. When clearance is not sufficient, additional layers of the material must be cut before complete separation is accomplished. With correct clearance, the angle of the fracture will permit a clean break below the burnish zone because the upper and lower fractures will extend toward one another. Excessive clearance will result in a tapered cut edge, because for any cutting operation, the opposite side of the material that the punch enters after cutting will be the same size as the die opening.
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