Machinery's Handbook, 31st Edition
1414
BLANKING PRESSURE
4"
1"
0.8"
Ø0.8" 2 PLACES
2.45 "
0.85"
0.8"
0.5"
3.5"
Fig. 11. Dimensioned Drawing of Part to be Blanked Perimeter of blank = 4 + 2.45 + 3.5 + 0.8 + 0.5 + 0.85 + 1 + 0.8 = 13.9 in. Circumference of holes = 2( π × 0.8) = 5.026 in. Total perimeter = 13.9 + 5.026 = 18.926 in. Pressure required = P = A × T × S = 18.926 × 0.125 × 17 = 40.22 tons When metric dimensions are used, the same formula applies: where P = pressure required (kg) A = perimeter of blank (mm) T = thickness of material (mm) S = shear strength of material (kg per mm 2 ) Note: The general rule for punches that have a shear, where the cutting faces are ground at an angle, is to halve the tonnage for material thicknesses of up to 0.25 inch or 6 mm. For material thicknesses over 0.25 inch or 6 mm, two-thirds of the tonnage should be used. Fine Blanking.— This process uses special presses and tooling to produce flat compo - nents from sheet metal or plate, with high dimensional accuracy. According to Hydrel A. G., Romanshorn, Switzerland, fine-blanking presses can be powered hydraulically or mechanically, or by a combination of these methods, but they must have three separate and distinct movements. These movements clamp the work material, perform the blanking operation, and eject the finished part from the tool. Forces of 1.5–2.5 times those used in conventional stamping are needed for fine blanking, so machines and tools must be designed accordingly. In mechanical fine-blanking presses, clamping and ejection forces are exerted hydraulically. Such presses generally are of toggle-type design and are lim- ited to total forces of up to about 280 tons. Higher forces usually require all-hydraulic designs. These presses are also suited to embossing, coining, and impact extrusion work. Cutting elements of tooling for fine blanking generally are made from 12 percent chro mium steel, although high-speed steel and tungsten carbide also are used for long runs or improved quality. Cutting clearances between the punch and die as a percentage of the thickness of material are given in Table 8. Table 8. Values for Clearances Used in Fine-Blanking Tools as a Percentage of the Thickness of the Material Material Thickness Clearance, % (in.) (mm) Inside Contour Outside Contour <0.040 <1.016 2.0 1.0 0.040–0.063 1.016–1.600 1.5 0.063–0.098 1.600–2.489 1.25 0.098–0.125 2.489–3.175 1.0 0.125–0.197 3.175–5.004 0.8 0.5 0.197–0.315 5.004–8.001 0.7 0.315–0.630 8.001–16.002 0.5
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