A P/M part can be machined to provide shapes that could not be created by P/M compacting or re-pressing, such as some undercuts, side holes, threading or tapping, and so forth. Machining, cutting, or grinding is also done to improve dimensional tolerances. As mentioned previously, a gear can have the hole bored or ground, which removes material to reestablish the centerline to improve concentricity tolerances as well as improving the size tolerance. Parts that require tight thickness tolerances and/or flatness can be surface or double-disk ground in a conventional manner. Likewise, parts that are of a suitable shape can be centerless ground to improve diameter or roundness. Although machining or grinding of P/M parts is similar to that of wrought or solid parts, there are some differences that should be noted:

• Due to porosity, P/M parts may absorb coolant or cutting or grinding fluids. These fluids should contain rust inhibitors.

• Brass or nickel-silver parts traditionally contained small percentages of lead, which improved machinability greatly. However, due to the restraints on the use of lead, other methods are being developed to improve machinability.

• Iron-base parts may have materials like manganese sulfide added to improve machinability.

• Resin impregnation of P/M parts can have a dramatic effect on increasing machinability.

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