Aqtvlxl机械制造及自动化毕业设计外语文献翻译(5)

Aqtvlxl机械制造及自动化毕业设计外语文献翻译

difficult to machine than brass.

Magnesium is very easy to machine, with good surface finish and prolonged tool life. However care should be exercised because of its high rate of oxidation and the danger of fire (the element is pyrophoric).

Molybdenum is ductile and work-hardening, so it can produce poor surface

finish. Sharp tools are necessary.

Nickel-based alloys are work-hardening, abrasive, and strong at high

temperatures. Their machinability is similar to that of stainless steels.

Tantalum is very work-hardening, ductile, and soft. It produces a poor surface finish; tool wear is high.

Titanium and its alloys have poor thermal conductivity (indeed, the lowest of all metals), causing significant temperature rise and built-up edge; they can be difficult to machine.

Tungsten is brittle, strong, and very abrasive, so its machinability is low,

although it greatly improves at elevated temperatures.

Zirconium has good machinability. It requires a coolant-type cutting fluid,

however, because of the explosion and fire.

20.9.3 Machinability of Various Materials

Graphite is abrasive; it requires hard, abrasion-resistant, sharp tools.

Thermoplastics generally have low thermal conductivity, low elastic modulus, and low softening temperature. Consequently, machining them requires tools with positive rake angles (to reduce cutting forces), large relief angles, small depths of cut and feed, relatively high speeds, and

proper support of the workpiece. Tools should be sharp.

External cooling of the cutting zone may be necessary to keep the chips from

becoming “gummy” and sticking to the tools. Cooling can usually be achieved with a jet of air, vapor mist, or water-soluble oils. Residual stresses may develop during machining. To relieve these stresses, machined parts can be annealed for a period of time at temperatures ranging from 80 C to 160 C (175 Fto315 F), and then cooled slowly and uniformly to room temperature.

Thermosetting plastics are brittle and sensitive to thermal gradients during