Drilling

In drilling, speeds and feeds are 80 to 85% of those for wrought metals of the same composition. For long tool life, nitrided steel, high-speed steels containing cobalt, and carbide-tipped drills are recommended.

Low helix angle drills are not recommended for softer P/M materials because of their poor chip ejection characteristics. Drills with 40° helix angles had twice the tool life of those with 30° helix angles in work performed with soft P/M materials.

Large amounts of coolant are required in drilling medium- or low-density materials; coolant should operate effectively at the drilling point to reduce abrasive wear due to powder particles at the bottom of the hole. A single-nozzle coolant system does not work properly because small or powdered chips do not easily exit through the drill flutes. A ring design system, however, is effective in eliminating the chip-clogging problem. Oil hole drills are the most effective means for removing chips from the cutting zone.

Cutting speeds of up to 25 m/min (80 sfm) and feed rates up to 0.25 mm/rev (0.010 in./rev) are recommended for high-speed steel drills. Cutting speed and feed rate could be as high as 120 m/min (390 sfm) and 0.5 mm/rev (0.02 in./rev), respectively, when solid-carbide or carbide-tipped drills are used. Carbide indexable drills are efficient because margins are eliminated. Abrasive margin wear and the welding of powder chips are also eliminated.

Holes in planes beyond the capability of the P/M process are best made by drilling when parts are in the presintered (partially sintered) state. In this state, drilling properties are similar to those of cast iron. If it is necessary to qualify the position and size of holes after final sintering, a carbide reamer or carbide-tipped reamer should be used. Roller burnishing can be used to meet accuracy and surface finish requirements. Any size change in the operation is related to the preburnished surface finish and the size of the hole. A change of 0.019 to 0.025 mm (0.00075 to 0.001 in.) in diameter is representative for a hole about 25 mm (1 in.) in diameter.

A number of studies have been performed to determine ways of enhancing drill life during P/M machining (Ref 4, 5, 6, 7, 8, 9, and 10). The following example is another. As shown in the following example, drill length is considered as a primary factor for improvements. Additives have strong influence on machinability for high performance materials. Tool life is nearly independent on the feed rate, which ranges from 0.05 to 0.16 mm/rev. Cutting fluid has no significant effect on the productivity. Distribution decreases for coated drills and for short drills.

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