Machinery's Handbook, 31st Edition
1458 Steel Rule Dies Heat treatment of the high-carbon-steel rules is designed to produce a hardness suited to the application. Rules in dies for cutting cartons and similar purposes, with mostly straight cuts, are hardened to 51 to 58 RC. For dies requiring many intricate bends, lower- carbon material is used, and is hardened to 38 to 45 RC. And for dies to cut very intricate shapes, a steel in dead-soft condition with hardness of about 95 RB is recommended. After the intricate bends are made, this steel must be carburized before it is hardened and tempered. For this material, heat treatment uses an automatic cycle furnace, and consists of carburizing in a liquid compound heated to 1500 ° F (816 ° C) and quenching in oil, followed by “tough” tempering at 550 ° F (288 ° C) and cooling in the furnace. After the hardened rule has been reinstalled in the die block, the tool is loaded into the press and the sharp die is used with care to shear the sides of the pad to match the die contours exactly. A close fit, with clearances of about half those used in conventional blanking dies, is thus ensured between the steel rule and the punch. Adjustments to the clearances can be made at this point by grinding the die steel or the punch. After the adjustment work is done, the sharp edges of the rule steel are ground flat to produce a land of about 1 ∕ 64 in. (0.40 mm) wide ( B in Fig. 65), for the working edges of the die. Clearances for piercing punches should be similar to those used on conventional piercing dies. Pipe and Tube Bending The difference between a pipe and a tube is how they are measured, and ultimately what they are used for. A pipe is a vessel; a tube is structural. A pipe is measured by the inner diameter; a tube is measured by the outer diameter. Generally, a tube will have a consistent outer diameter and its inner diameter may have varying wall thicknesses to increase its strength. However, a pipe will have a consistent inner diameter and its outer diameter may have varying wall thicknesses. The terms used in tube bending are defined in Fig. 66a, and those used in pipe bending are defined in Fig. 66b.
A
B
SECTION A-A
B
A
SECTION B-B
a) Tube
b) Pipe
d Fig. 66a. Dimension and Terms: Tube
d Fig. 66b. Dimension and Terms: Pipe
The radius of the bend R is defined with respect to the centerline of the tube or pipe. When the tube or pipe is bent, fibers at the outside wall are in tension and fibers at the wall on the inside bend are in compression. This condition of tensile stress causes thinning and elongation of the wall at the outside, and the compression stress causes thickening and shortening of the inner wall. As a result, the cross section of the bent section of the tube is flattened. The oval distortions grow stronger if thinner tube/pipe walls and smaller bending radii of the workpiece have been selected. Ovality can be calculated by the following formula: (52) % u D D D 100 max min = −
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