Machinery's Handbook, 31st Edition
Broaches and Broaching 1047 space. The pitch of the roughing and finishing teeth should be equal for broaches without separate inserts (notwithstanding the different values of d and F ) so that, after wear makes this necessary, some of the finishing teeth may be ground into roughing teeth. US Units (2a) Metric Units (2b) If the pitch obtained by Formula (2a), or Formula (2b) in metric calculations, is larger than the minimum obtained by Formula (1), this larger value should be used because it is based upon the usable power of the machine. As the notation indicates, 70 percent of the maximum tonnage T is taken as the usable capacity. The 30 percent reduction is to provide a margin for the increase in broaching load resulting from the gradual dulling of the cutting edges. The procedure in calculating both minimum and allowable pitches will be illustrated by an example. Example: Determine pitch of broach for cast iron if L = 220 mm; d = 0.1 mm; and F = 4. Minimum pitch 3 220 0.1 4 28.14 mm × × = = Next, apply Formula (2b). Assume that b = 75 mm and T = 8 metric ton; for cast iron and depth d of 0.1 mm, P = 1586 MPa (Table 2). Then, Allowable pitch 9810 8 = 0.1 × 220 × 75 =1586 33.34 mm = ^ h This pitch is safely above the minimum. If in this case the usable tonnage of an available machine were, say, 7 metric tons instead of 8 metric tons, the pitch as shown by Formula (2b) might be increased to about 38.1 mm, thus reducing the number of teeth cutting simul taneously and, consequently, the load on the machine; or the cut per tooth might be reduced instead of increasing the pitch, especially if only a few teeth are in cutting contact, as might be the case with a short length of cut. If the usable tonnage in the preceding example were, say, 10 metric tons, then a pitch of 26.68 mm would be obtained by Formula (2b); hence, the pitch in this case should not be less than the minimum of approximately 28.14 mm obtained from Formula (1). Depth of Cut per Tooth.— The term “depth of cut” as applied to surface or external broaches means the difference in the heights of successive teeth. This term, as applied to internal broaches for round, hexagonal or other holes, may indicate the total increase in the diameter of successive teeth; however, to avoid confusion, the term as here used means in all cases and regardless of the type of broach, the depth of cut as measured on one side. In broaching free-cutting steel, the Broaching Tool Institute recommends 0.003 to 0.006 inch (0.076–0.15 mm) depth of cut for surface broaching; 0.002 to 0.003 inch (0.05–0.076 mm) for multispline broaching; and 0.0007 to 0.0015 inch (0.018–0.038 mm) for round hole broaching. The accompanying table contains data from a German source and applies specifically to surface broaches. All data relating to depth of cut are intended as a general guide only. While depth of cut is based primarily upon the machinability of the material, some reduction from the depth thus established may be required, particularly when the work-supporting fixture in surface broaching is not sufficiently rigid to resist the thrust from the broaching operation. In some cases, the pitch and cutting length may be increased to reduce the thrust force. Another possible remedy in surface broaching certain classes of work is to use a side-cutting broach instead of the ordinary depth-cutting type. A broach designed for side-cutting takes relatively deep narrow cuts that extend nearly to the full depth required. The side-cutting section is followed by teeth arranged for depth cutting to obtain the required size and surface finish on the work. In general, small tolerances in surface broaching require a reduced cut per tooth to minimize Allowable pitch T dLbP inch = Allowable pitch 9810 T dLbP = mm
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online