Machinery's Handbook, 31st Edition
1416 FINE BLANKING in relation to the die outline, and spacing between adjacent blanked parts. Holes and slots can be produced with ratios of width to material thickness down to 0.7, compared with the 1:1 ratio normally specified for conventional tooling. Operations such as countersinking, coining, and bending up to 60 degrees can be incorporated in fine-blanking tooling. The cutting force in pounds, lb (Newton, N) exerted in fine blanking is 0.9 times the length of the cut in inches (mm) times the material thickness in inches (mm), times the tensile strength in lb f /in 2 (MPa). Pressure in lb (N) exerted by the clamping element(s) carrying the V-projections is calculated by multiplying the length of the V-projection, which depends on its shape, in inches (mm) by its height ( h ), times the material tensile strength in lb f /in 2 (MPa), times an empirical factor f . Factor f has been determined to be 2.4–4.4 for a tensile strength of 28,000–113,000 lb f /in 2 (193–779 MPa). The clamping pressure is about 30 percent of the cutting force, as calculated. Dimensions and positioning of the V-projection(s) are related to the material thickness, quality, and tensile strength. A small V-projection close to the line of cut has about the same effect as a large V-projection spaced away from the cut. But if the V-projection is too close to the cut, it may move out of the material at the start of the cutting process, reducing its effectiveness. Positioning the V-projection at a distance from the line of cut increases both material and blanking force requirements; its location relative to the line of cut also affects tool life. Shaving.— The edges of punched and blanked parts are generally rough and uneven. Shav- ing is the process of removing a thin layer of material, from the inside or outside contour of a workpiece or from both sides with a sharp punch and die, to achieve precise, clean parts. Shaving a Punched Workpiece: It is necessary to provide a small amount of stock on the punched or blanked workpiece for subsequent shaving. This amount, d , is the difference between diameters of the hole after shaving and before shaving. (25) where d = diameter of hole after shaving (inch or mm)and, d o = diameter of hole before shaving (inch or mm). The value of δ is 0.006 to 0.0098 inch (0.15 to 0.25 mm) for a previously-punched hole, and 0.004 to 0.006 inch (0.10 to 0.15 mm) for a previously-drilled hole. The diameter of punch d p can be calculated from the formula: (26) where d = diameter of hole after shaving (inch or mm) e = production tolerance of the hole (inch or mm) i = amount of compensation for tightening of the hole after shaving (inch or mm) ( i = 0.0002 to 0.00067 in. or 0.005-0.017 mm) The diameter of the die D d is (27) Shaving a Blanked Workpiece: Thin layers of material can be removed from a blanked surface by a process similar to punching. If the workpiece after shaving needs to have a diameter D , the punch diameter for the blanking operation is (28) The die diameter for the blanking operation is (29) where D = diameter of final piece (inch or mm) d D p δ = + d d c D c 2 2 d p δ = + = + + d d o δ = − d d p i ε = + + ( . d 120 to130 . ) D d p =
c = clearance between die and punch (inch or mm) δ = amount of material for shaving (inch or mm)
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