Example 2 Machinability Evaluation Drilling of Sintered Steel

Blind hole drilling test was performed on a wide spectrum of P/M qualities. Cylindrical blanks with a diameter of 80 mm and a height of 10 mm are used in the test. A survey test is performed in order to select the type of drill and cutting conditions. The main comparisons were made under dry condition using a HSS drill with a diameter of 4 mm and a point angle of 118°. Total breakdown of the drill is chosen as criterion, based on the fact that drilling is commonly used as a bulk removal operation. All materials are investigated in order to divide them into four groups. The classification criterion used is the time required to drill 100 holes. Influence of cutting fluid, density, alloying contents, feed rate, and drill type are evaluated for the selected group representatives.

All the tests were carried out in a numeric controlled machining center. Early in the testing procedure, the heat generated in the workpiece during the drilling cycle was recognized to have an effect on the result, and therefore, the time between inserts was increased and temperature measurements were carried out on the workpiece. Key results are noted below.

Tool Life. Evaluation for atomized iron with diffusion bonded nickel (4%), copper (1.5%), and molybdenum (0.5%), addition of 0.8% C regarding influence of length, type of drill, and coating of the drill is shown in Fig. 7. The length of the drill has a large influence on the performance. Evaluation of centered drilling shows improvements in distribution and in performance. It is believed that the first inlet is crucial for the performance. This has its origin from the inhomogeneous microstructure of P/M material.

Fig. 7 Influence of feed rate, tool material, coating, and additives on machinability

Tool material and coating have a strong influence. Geometry changes exist among the three types of drills. Regarding feed rate, the performance is comparable with the result from the turning test presented for HM PVD-TiN tool, which ranges from 0.12 to 0.16 mm/rev. For Distaloy AE 0.5% C using HSS drill, the same tool life exists up to feed rate 0.16 mm/rev. This behavior is unique for P/M material.

Effect of Carbon Contents. Addition of carbon above 0.25% C for atomized iron powder or addition of 2% Cu will decrease the machinability. Micro smearing on the tool is believed to be the reason for the large decrease in machinability (Fig. 8).

Fig. 8 Influence of carbon contents on machinability of ASC100.29 2% Cu. Drill, HSS; point angle, 118°; feed rate, 0.06 mm/rev; D = 4 mm; criteria, total failure

Density, Cutting Fluid, and Additives. Influence from density is regarded as small in the range of 6.7 to 7.3 g/cm3. Result from productivity evaluation of atomized iron powder with 2% Cu and 0.5% C at 100 holes reveals 5% decrease for the range. Use of cutting fluid has no significant effect on the productivity. Water as cutting fluid decreases the productivity. For high performance materials, additives have large influence on the machinability.

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