Machinery's Handbook, 31st Edition
1048 Broaches and Broaching work deflection resulting from the pressure of the cut. See Cutting Speed for Broaching starting on page 1142 for broaching speeds. Pitch Face Angle Land Clearance Angle
Depth
Radius Terms Commonly Used in Broach Design
Face Angle or Rake.— The face angle (see diagram) of broach teeth affects the chip flow and varies considerably for different materials. While there are some variations in prac- tice, even for the same material, the angles given in the accompanying table are believed to represent commonly used values. Some broach designers increase the rake angle for finishing teeth in order to improve the finish on the work. Clearance Angle.— The clearance angle (see illustration) for roughing steel varies from 1.5 to 3 degrees and for finishing steel from 0.5 to 1 degree. Some recommend the same clearance angles for cast iron, and others larger clearance angles varying from 2 to 4 or 5 degrees. Additional data will be found in Table 1. Land Width.— The width of the land usually is about 0.25 × pitch. It varies, however, from about one-fourth to one-third of the pitch. The land width is selected so as to obtain the proper balance between tooth strength and chip space. Depth of Broach Teeth.— The tooth depth as established experimentally and on the basis of experience usually varies from about 0.37 to 0.40 of the pitch. This depth is measured radially from the cutting edge to the bottom of the tooth fillet. Radius of Tooth Fillet.— The “gullet” or bottom of the chip space between the teeth should have a rounded fillet to strengthen the broach, facilitate curling of the chips, and safeguard against cracking in connection with the hardening operation. One rule is to make the radius equal to one-fourth the pitch. Another is to make it equal 0.4 to 0.6 the tooth depth. A third method preferred by some broach designers is to make the radius equal one-third of the sum obtained by adding together the land width, one-half the tooth depth, and one-fourth of the pitch. Total Length of Broach.— After the depth of cut per tooth has been determined, the total amount of material to be removed by a broach is divided by this decimal to ascertain the number of cutting teeth required. This number of teeth multiplied by the pitch gives the length of the active portion of the broach. By adding to this dimension the distance over three or four straight teeth, the length of a pilot to be provided at the finishing end of the broach, and the length of a shank which must project through the work and the faceplate of the machine to the draw-head, the overall length of the broach is found. This calculated length is often greater than the stroke of the machine, or greater than is practical for a broach of the diameter required. In such cases, a set of broaches must be used. Chipbreakers.— The teeth of broaches frequently have rounded chipbreaking grooves located at intervals along the cutting edges. These grooves break up wide curling chips and prevent them from clogging the chip spaces, thus reducing the cutting pressure and strain on the broach. These chipbreaking grooves are on the roughing teeth only. They are staggered and applied to both round and flat or surface broaches. The grooves are formed by a round-edged grinding wheel and usually vary in width from about 1 ⁄ 32 to 3 ⁄ 32 inch (0.79 to 2.38 mm) depending upon the size of broach. The more ductile the ma- terial, the wider the chipbreaker grooves should be and the smaller the distance between
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