Machinery's Handbook, 31st Edition
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MACHINING PLASTICS Table 12. Suggested Approximate Speeds and Feeds for Turning Various Thermoplastics
Roughing 0.150 in. Cut Depth
Finishing 0.030 in. Cut Depth
Material
Speed (feet/min)
Feed (in/rev) 0.018 0.012 0.012
Speed (feet/min)
Feed (in/rev) 0.007 0.006 0.005
ABS
1200 1200 1000 1200 1000 1200 1000 1500 800 1500 900 1200 1200 800
800 800 700 800 700 800 700 1200 800 1200 800 700 800 600
Acetal (POM) Acrylic (PMMA) Polycarbonate (PC) Polyamide, Nylon 6 / 6 (PA6) Polyester (PET) Polyether ether ketone (PEEK) Polyethylene (PE) Polyphenylene sulfide (PPS) Polytetrafluoroethylene (PTFE) Polyvinyl chloride (PVC) Ultem polyetherimide (PEI) Polypropylene (PP) Polystyrene (PS)
0.012 0.012 0.010 0.010 0.018 0.015 0.018 0.010 0.012 0.010 0.012
0.006 0.005 0.005 0.005 0.008 0.007 0.008 0.005 0.005 0.005
0.005 Other Machining Techniques: Lasers can be used for machining plastics, especially sheet laminates, although their use may generate internal stresses. Ultrasonic machining has no thermal, chemical, or electrical reaction with the workpiece and can produce holes down to 0.003 inch (0.0762 mm) diameter; tight tolerances, 0.0005 inch (0.0127 mm); and very smooth finishes, 0.15 m inch (0.381 m m) with No. 600 boron carbide abrasive powder. Water-jet cutting using pressures up to 60,000 lb/in 2 (414 N/mm 2 ) is widely used for plastics and does not introduce stresses into the material. Tolerances of ± 0.004 inch ( ± 0.102 mm) can be held, depending on the equipment available. Process variables, pres sures, feed rates, and the nozzle diameter depend on the material being cut. This method does not work reliably with hollow parts unless they can be filled with a solid core. Machining Hard Rubber: Hard rubber, or ebonite, is the oldest rigid manmade plastic, and has been used since the 1840s. The base material is a highly vulcanized natural or, today, synthetic rubber, empirical formula C5H8S, containing 32 percent sulfur. A range of compositions containing various amounts of fillers is available. Unfilled ebonite has a specific gravity = 1.18, tensile strength = 9,000 psi (62 N/mm 2 ), elongation = 3 percent, and Izod impact strength = 0.5 ft-lb f /in (26.7 J/m) of notch. Its continued use is due to its good insulating characteristics, good chemical resistance, and ease of machining. Tools suitable for steel, high-speed steel, or tungsten carbide may be used with hard rubber with no top or side rake angles and 10 to 20 degrees clearance angles. Without coolant, surface speeds of 200 ft/min (60 m/min) are recommended for turning, boring and facing; they may be increased to 300 surface ft/min (90 m/min) with coolant. Drilling of hard rubber requires high-speed steel drills of 35 to 40 degrees helix angle to obtain maximum cutting speeds and drill life. Feed rates for drilling range up to 0.015 in/ rev (0.38 mm/rev). Deep-fluted taps are best for threading hard rubber and should be 0.002 to 0.005 inch (0.05-0.13 mm) oversize if close tolerances are to be held. Machine oil is used as the lubricant. Hard rubber may be sawed with band saws having 5 to 10 teeth per inch, running at about 3000 ft/min (915 m/min), or it may be cut with abrasive wheels. Use of coolant in grinding ebonite gives a smoother finish. Piercing and blanking of hard-rubber sheet is best performed with the rubber and dies warmed. Straightening of the often-distorted blanks may be accomplished by dropping them into a pan of hot water. Plastics Gears.— Plastics gears may be cut from blanks, as are metal gears, or molded to shape in an injection-molding machine for lower production costs, though tooling may cost more. Cut plastics gears may be of similar design to their metal counterparts, but molded gears are usually of modified form to suit material characteristics, including sen - sitivity to temperature and humidity. Plastics materials also may be preferred for gears
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