General Guidelines

Representative machining methods are turning, milling, drilling, tapping, and grinding. The basic guideline is to start with cutting conditions for wrought and cast parts and tailor them for the application. The recommendation applies in particular to lower density parts. High-density parts (>92% of theoretical density) machine like wrought metals.

Smearing of self-lubricating porous parts can be a problem. Recommended practice involves the use of sharp tools and light cuts in single-point machining, such as turning or boring.

Coolants are preferred in most machining operations. Coolant pickup can be a problem. The rate of pickup is directly related to the amount of porosity. Ideally, all machining except grinding should precede deburring. Retained deburring abrasive can cause excessive tool wear. Ceramic and cubic boron nitride (CBN) inserts are usually run dry; performance is typically better than or at least equal to that obtained with coolants.

Material Selection. Powder metallurgy carbon steels are selected primarily for parts with moderate strength and hardness, combined with machinability. Iron-copper and copper steel materials are produced from admixtures of elemental iron powder and elemental copper powder with or without graphite powder (carbon). When secondary machining is required, combined carbon contents of less than 0.5% should be specified. Copper-infiltrated iron and steel materials offer improved machinability because of reductions in interrupted cuts, and machined parts have a smooth surface finish. Among stainless steels, SS-303 is preferred when parts require extensive secondary machining. Brass, bronze, and nickel silver parts usually have good machinability.

Additives. Free-machining benefits can be obtained by means of small additions to a standard powder composition. Additives for ferrous powders include lead, sulfur, copper, or graphite; for nonferrous powders, lead is used. The advantages of changing composition in this manner can be at least partially offset by side effects. Additions can cause problems, such as dimensional changes of parts during sintering and deterioration in the properties of parts.

Prealloyed manganese sulfide powders appear to avoid those shortcomings in ferrous alloys. Manganese content is intentionally high to ensure that all sulfur is present in the form of manganese sulfide inclusions. When these inclusions are extensively deformed in the shear plane and in the flow zone adjacent to the tool surface, they contribute to higher cutting speeds, longer tool life, good surface finish on parts, and lower tool forces. In addition, chips are more readily handled than those produced by conventional P/M materials. Oil or resin impregnation of porous P/M parts also improves machinability (see the article "Resin Impregnation of Powder Metallurgy Parts" in this Volume).

Design. Certain types of holes, undercuts, and threads are examples of features that cannot be accommodated by the P/M consolidation (pressing) process and therefore require machining. Holes in the direction of pressing, produced with core rods that extend up through the tools, are readily incorporated in parts, but side holes (those not parallel to the direction of pressing) cannot be made in the same way and are generally produced by secondary machining.

Undercuts on the horizontal plane (perpendicular to the die centerline) cannot be produced if they prevent the part from ejecting from the die (Fig. 1). Annular grooves around a part are produced by machining or by making the part in an assembly of two pieces. Likewise, a part with a reverse taper (larger on bottom than on top) cannot be ejected from a die. Because threads in holes and on outside diameters prevent a part from being ejected from a die, they cannot be made with conventional P/M methods; machining is required.

Pre)erred design

Pre)erred design

Fig. 1 P/M part design considerations. (a) Undercuts on horizontal plane cannot be produced in P/M process. Machining is required to obtain such features in parts. (b) Example of undercut in flange that is beyond capability of P/M process. (c) Alternative to part in (b) that can be made without secondary machining

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