Machinery's Handbook, 31st Edition
Dowel Pins 1885 Assembly with Dowel Pins.— Hardened and ground dowel pins are made to a high degree of accuracy. Dowel pins are used to position and align machine components, provide a positive stop, add structural rigidity to an assembly, or to create a part nest. When components are assembled with dowel pins, the dowel is a very close fit with its mating part. Machines or tools made with dowel pin construction can be disassembled for maintenance, engineering changes or repair, and reassembled, maintaining the original positional accuracy. Holes for dowel pins are carefully produced to obtain either a close sliding fit or a press fit for the dowel. When using dowels with fasteners, proper machine shop practice dictates that the sizes should be the same whenever possible. For example, if 3 ⁄ 8 -inch socket head cap screws are used, 3 ⁄ 8 -inch diameter dowels should also be used.
B
A
A
A
SECTION A-A
Fig. 1. Fig. 1 is a typical example of an assembly using socket head cap screws and dowels. The fasteners and dowels are positioned as far apart as possible for rigidity and locational sta bility. Engineering drawings often show dowel locations as a circle with a partly filled-in cross. The dowel holes on this assembly are relieved in the top plate and have a smaller knockout hole in the bottom plate. The hole for the socket head cap screw is counterbored deep enough so that the screw is recessed 0.03 inch below the surface of the plate. Making dimension “A” different from dimension “B” prevents the plates from being assembled backwards. When dowels are used for positioning: • Position dowel pins as far apart as possible. • Length of engagement for the dowel should be twice the dowel diameter in each component. • Avoid pressing a dowel into a blind hole. If options are limited, pull dowels have a threaded hole that can be used to remove the dowel from a blind hole. • Avoid pressing a dowel into a hardened part. • Provide a knock out hole. • Relieve deep holes. • Dowels should be a press fit in one component and a slip fit in the other. Fig. 2 shows a cross section of two plates with four different assembly possibilities for dowels.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online