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The results of laboratory investigations and shop experience related to the machining of the alloy systems of columbium, molybdenum, tantalum, and tungsten are presented. Material properties and their relationships to maching characteristics are discussed, as are certain economic aspects of shop practice. Empirical data are presented to illustrate and quantify the more significant characteristics, and conclusions are drawn for appropriate shop maching conditions. In general, these metals are very difficult to machine; the most notable exceptions are the infiltrated forms of tungsten, whose extraordinary maching characteristics approach those of the light metals, and the molybdenum group, which are readily machinable by certain processes. However, grinding any of the refractory metals is slow and difficult, and very smooth finishes are exceedingly difficult to obtain.

Results are reported of drilling and tapping tests on the high cobalt alloy steel, MX-2, hardened to 50-56 Rockwell C. The empirical data are presented and discussed, and conclusions are drawn as to best shop practice. 52 Rc has been found to be a nominal upper limit to the use of steel cutting tools. At and below this limit, this material may be drilled readily and tapped under appropriate cutting conditions; above 52 Rc, special procedures are required to attain adequate life from high-speed steel tools.

Fabricating studies have been conducted on high-strength titanium alloy 13V-11Cr-3A1 in solution treated and aged condition--workpiece hardness was 45 Rc. The aged alloy was found to be less sensitive to strain hardening during machining than the solution treated form. This material is quite abrasive to cutting tools, when turning this alloy, use of a large side cutting edge angle is advantageous. A 5% aqueous solution of barium hydroxide was far superior to other turning fluids tested. The relationships between the cutting forces and the parameters feedrate and depth of cut have been established; a similar equation has been derived for unit horsepower. For drilling and tapping, a highly chlorinated cutting oil was the best fluid tested. A grinding method which minimizes residual stresses and distortion has been developed. Rough and finish grinding conditions are recommended which result in grinding ratios of 10–20 respectively.

The results of emprical investigations of the drilling, tapping, and surface grinding characteristics of D6aC alloy steel hardened to 54 Rc are reported. This material is machined readily by these processes under appropriate cutting conditions; however, this hardness level appears to be an upper limit to the use of steel drills and taps. A new type of tap, characterized by a unique land geometry, has been developed and tested; this has proved substantially superior to conventional geometries for the present application. Recommendations are formulated for machining this material by each of these processes, and for certain design limitations attendant thereto.