Burnishing. When the clearance between a shaft and a P/M bearing is ±0.012 mm (±0.0005 in.) or less, burnishing the bearing bores after they have been installed in the housing is preferred for correcting the bore size. No more than 0.002 mm/mm (0.002 in./in.) of diameter should be displaced, and the smallest amount of displacement that will produce the true diameter is desirable. The type of burnishing tool recommended for this operation is illustrated in Fig. 10.

Fig. 10 Ball broach for burnishing bores in P/M parts. Dimensions given in inches

Given a finished bore diameter of 38 mm, +0.005 mm/-0.0000 mm (1.500 in., +0.0002 in./-0.0000 in.) (B in Fig. 10), the diameter of the starting end of the burnishing tool then becomes 38.10 mm, -0.050 mm (1.5000 in., -0.0020 in.), or 38.05 mm (1.4980 in.), and bearings would be bored to 38.075 mm, +0.0125 mm/-0.0000 mm (1.4990 in., +0.0005/-0.0000 in.). Thus, there would be a minimum clearance of 0.025 mm (0.001 in.) at the entering end of the tool, and the first land would be a line-to-line fit. The tool then becomes progressively larger, and the bearing is expanded. If there were no springback, the operation would be stopped at the fourth or fifth tool land. However, the bearing would ordinarily be burnished to 0.010 mm (0.0004 in.) oversize to allow for springback.

Roller burnishing is a cold-working operation that compresses metal rather than removes it. The technique is suitable for sintered (not heat-treated) powder metal materials for which maintenance of open surface porosity is not critical. A significant improvement in surface finish can be obtained using a roller burnishing tool. In addition, the tool is adjustable to match individual product specifications as well as to compensate for wear on the rolls and mandrel. Both through holes and blind holes can be roller burnished. Hole size tolerance depends on the input tolerance of the hole; that is, a prepared tolerance of 0.050 mm (0.002 in.) can be reduced to 0.025 mm (0.001 in.), or a ±0.0025 mm (±0.0001 in.) tolerance can be held if the input tolerance is 0.010 mm (0.0004 in.). Surface finishes of 0.25 /'m (10 /'in.) are common after roller burnishing, A lightweight, low-viscosity lubricating oil is recommended for most P/M materials.

Honing and Lapping. Holes requiring extreme accuracy can be honed or lapped by normal techniques if retention of porosity is not required. However, size control of holes in P/M parts can usually be obtained more economically by reaming or burnishing.

High-density ferrous metal parts, especially when hardened, have been successfully honed and lapped using conventional procedures.

Diamond- and CBN-plated bore finishing tools are recommended for precise hole size control. These tools can be used on standard drilling or honing machines, as well as on multiple-spindle or numerically controlled machines. The use of an adjustable sleeve attached to a mating tapered mandrel increases tool life. The selection of diamond grit size determines the metal removal rate and the surface achieved. The amount of material to be removed from the hole diameter can be determined by: surface finish (start)--surface finish (after honing)/l 00,000 = required stock removal. If the existing finish is 1.25 /'m (50 /'in.) and the desired finish is 0.25 /''m (10 /'in.), then 0.010 mm (0.0004 in.) should be removed from the hole diameter.

Honing of infiltrated parts is seldom practical, because the stones become loaded. Neither lapping nor honing is recommended for porous parts, because either of these processes will cause the pores to become filled with abrasive particles. For special applications that require the use of lapping or honing, ultrasonic or solvent cleaning should be performed following grinding.

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