Powder metallurgy is the process where by metal parts in large quantities can be made by compressing and sintering various powdered metals such as brass, bronze, stainless steel, and iron. Self-lubricating sintered metal bearings are produced by powder metallurgical methods. Powder metals are fed into a die and axially compacted. The resulting "green parts" are sintered in a protective gas atmosphere at temperatures below the melting point. Through diffusion, a bonding results at the points of contact of the powder particles. This bonding defines the strength of components. Through subsequent calibration, the specified tolerances -- primarily in the bore -- are met without blocking up the pores. Vacuum impregnation with a suitable lubricant is the final stage of the process. To improve the feed in automatic bowl feed systems, there is normally an additional operation to produce a dry-to-the-touch bearing and supplementary heat treatments may be employed.