Powder Metallurgy Materials Machinery's Handbook, 31st Edition
1550
Poor Good Fig. 22. Countersink in Clearance ( Source: Metal Powder Federation)
Flanges: Flanges or overhands can be produced by step in the die. Too long flanges a causes ejection difficulties. A long flange should incorporate a draft around the flange, a radius at the bottom edge, and radius at the juncture of the flange and component body to reduce stress concentrations and the likelihood of fracture. Hubs: The PM process can readily produce hubs, which are complementary part sec tions to gears, sprockets, or cams. It is important to include a generous radius between the hub and flange section and to maximize space between the hub and the root diameter of the gear or sprocket. Slots and Grooves: Grooves can be pressed into either end of a part from projections on the punch face, with the following general caveats (Fig. 23). Curved or semicircular grooves are limited to maximum depth of 20 percent of the overall part length. Rectangu- lar grooves are limited to a maximum depth of 15 percent of thickness of the workpiece in the pressing direction, surfaces parallel to the pressing direction have up to 12º draft, and all corners are radiused. Deep narrow slots and grooves require fragile tool members and should thus be avoided.
A
r
up to 12°
H
H
r
r
r
r
Detail A-A
Good
Poor
Fig. 23. Semicircular and Rectangular Groove ( Source: Metal Powder Federation) Undercuts: Undercuts on the horizontal plane (perpendicular to the pressing direction) cannot be made since they prevent part ejection from the die. Annular grooves must be machined as a second operation (Fig. 24a). For a part such as that shown in Fig. 24b, where a juncture undercut is needed to allow fit-up to a “dead corner”, an alternative approach is shown in Fig. 24c. Tolerances: For reasons of economics, tolerances no closer than necessary should be specified. Table 6 illustrates tolerances characteristic of PM and competitive near-net- shape forming methods.
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