Powder Metallurgy Materials Machinery's Handbook, 31st Edition
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(a) (c) Fig. 24. Undercuts: a) Annular Groves Cannot Be Molded; b) Undercuts Cannot Be Molded; (b) c) Undercut Can Be Molded ( Source: Metal Powder Federation) Table 6. Tolerances that Can Be Produced in PM Process
Typical Dimensional Tolerances ( ± ) (in/in) (mm/100 mm)
Critical Dimensional Tolerances ( ± ) (in/in) (mm/100 mm)
Forming Method
PM
0.002 (0.203) 0.001 (0.102) 0.010 (1.016) 0.010 (1.016) 0.0005 (0.013) 0.002 (0.203) 0.0025 (0.254) 0.02 (2.032) 0.0032 (0.325) 0. 010 (1.016)
0.001 (0.102) 0.0005 (0.051) 0.002 (0.203) 0.003 (0.305) 0.0002 (0.020) 0.0010 (0.107) 0.0010 (0.107) 0.015 (1.524) 0.0025 (0.254) b
PM (secondary forming)
Cold forming Hot extrusion
Fine blanking (single level) Die casting a Magnesium and Aluminum Hot closed-die forging Conventional Zinc PM perpendicular to press axis PM parallel to press axis
0.005 (0.508) a Die parting line tolerance will be two to three times greater than the data shown. b Fixture-quenched forgings, i.e., forgings clamped for quenching during heat treatment im- mediately after hot forging, can hold tighter tolerance. Design of Typical Parts.— Although PM is used for a whole host of parts in many different applications, most parts (typical categories include gears, cams, assemblies, and bear ings) have their own unique design considerations. Gears: Gears are well-suited to PM production. Carbide dies for production gears pro vide long life and good accordance. PM gears can be made with blind corners, thus elimi nating the need for undercut relief. PM gears can be comminuted with other configuration such as cams, and various other components. PM helical gears are possible; copper infil tration is sometimes used to improve teeth density. Since tooth and shape is not a problem, true involve gear forms are more easily produced through PM than with other fabrication methods. One should keep in mind when designing PM gears that hubs or pinions should be located as far as possible from gear root diameters. Cams: Cams are well-suited to PM production: The process provides excellent surface finishing and part-to-part consistency; the natural finish of a self-lubricating PM cam will often outwear a ground cam surface; for radial cams, the shape is formed in the die; for face cams, the shape is formed in the punch faces. Assemblies: Two or more PM parts can often be joined from a unit that is difficult, if not impossible, to make as a single structure. Capitalizing on PM’s flexibility, it is feasible to make assemblies of very difficult materials such as a bronze bearing in a ferrous struc - tural part. PM parts can be joined by conventional methods and also by sintering together
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