Machinery's Handbook, 31st Edition
Powder Metallurgy Materials 1549 Design Details.— Successful design for PM rests on an understanding of how the unique aspects of the technology affect the countless details that make up the design of any struc tural part. Holes: Holes in the pressing direction can be round. Tooling members, which create holes, are called core rods. Lightening holes are frequently added to large parts to reduce projected pressing area, thus making parts lighter and easier to press. Blind holes, blind steps in holes, and tapered holes are readily produced. Side holes have to be produced after a sintering operation, usually by machining. Wall Thickness: Die fill is all-important. As a general rule, walls should not be made any thinner than 0.057 in. (1.45 mm). One should avoid designing long thin walls; they re- quire tooling that is complicated, and the parts themselves have a tendency toward density variation. Flatness: Total measured flatness depends on part thickness and surface area. Thin parts tend to distort more than thick parts during sintering or heat treatment. Repressing im- proves flatness. Projection basses are easier to flatten than entire face areas. Taper and Draft: Drafts are generally not required or desired on sides of parts. While drafts on outer sections for ejection are sometimes helpful, producing one demands care- ful timing of the tools and slower production rates. Fillet and Radii: Generous fillet radii are most desirable. Tooling with such fillets is more economical and longer lasting. Parts made with fillets have greater structural integ - rity (Fig. 20). Parts with generous radii are more easily and more quickly made.
Fillet Radius
Fillet Radius
Poor
Good
Poor
Good
Fillet Radius
Fillet Radius
Poor
Good
Poor
Good
Fig. 20. Examples of PM Parts Showing Poor and Good Designs ( Source: Metal Powder Federation) Chamfers and Bevels: Chamfers are necessary on front edges to prevent burring. For example, on bushings (Fig. 21) a 30° to 45° chamber and 0.005 to 0.015 in. (0.13 to 0.38 mm) flat to eliminate feature edges are the preferred.
30°
b
b = 0.13 to 0.38 mm (0.005 to 0.015 in.) Fig. 21. Chamfers are Preferred on Parts Edges ( Source: Metal Powder Federation)
Countersinks: A countersink is a chamfer around a hole for a screw or bolt head (Fig. 22). When the countersink is formed by a punch, a 0.010 in. (0.25 mm) nominal flat is essential in order to avoid sharp, flanged edges on the punch.
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