Machinery's Handbook, 31st Edition
602 MACHINING PLASTICS machining of plastics requires dust control, adequate ventilation, safety guards, and eye protection. Like some metals, plastics may need to be annealed before machining to avoid warpage. Some commercially available bar and rod stock are sold already annealed. If annealing is necessary, instructions can be obtained from plastics suppliers. Plastics moduli are small fractions—2 to 10 percent—of those of metals, and this lower stiffness permits much greater deflection of the work material during cutting. Thermoplastics materials must be held and supported firmly to prevent distortion, and sharp tools are essential to minimize normal forces and avoid excess heat generation. Plastics recover elastically during and after machining so that drilled or tapped holes often end up tapered or of smaller diameter than the tool. Turned diameters also can end up larger or smaller than the dimensions measured immediately after the finishing cut. The low thermal conductivity of plastics causes much of the heat generated in cutting to be absorbed by the tool. Heat in the plastics tends to stay near the surface. The heat must be removed by an air blast or a liquid coolant for optimum surface finishes and close tolerances. A non-aromatic, water-soluble misting or flood coolant is suggested. Plastics have thermal expansion coefficients some 10 times higher than those of metals so that even though actual heat generation during machining may be less than with metals there can easily be more expansion. Adequate tool clearances must be provided to mini mize heating. Compared with most structural metals, temperatures at which plastics soften, deform and flow are quite low. Allowing frictional heat to build up causes gum ming, discoloration, poor tolerance control, and rough finishes. These effects are more pronounced with plastics such as polystyrene and polyvinyl chloride that have low melting points than with plastics that have higher melting points, such as nylons, fluoroplastics, and polyphenylene sulfide. Sufficient clearances must be provided on cutting tools to pre vent rubbing contact between the tool and the work. Tool surfaces that will come into contact with plastics during machining should be polished to reduce frictional drag and resulting temperature increases. Proper rake angles depend on depth of cut, cutting speed, and the type of plastic being cut. Large rake angles should be used to produce continuous- type cuttings, but they should not be so large as to cause brittle fracture of the work and resulting discontinuous chips. Turning and Cutting Off: High-speed steel and carbide tools are commonly used with cutting speeds of 300–600 and 600–1200 ft/min (91–183 and 183–366 m/min), respectively. Water-soluble coolants can be used to keep down temperatures at the shear zone and improve the finish, except when they react with the work material. Chatter may result from the low modulus of elasticity and can be reduced by close chucking and follow rests. Box tools are good for long, thin parts. Tools for cutting off plastics require greater front and side clearances than are needed for metal. Cutting speeds should be about half those used for turning operations. Drilling: This is the most common machining operation because small-diameter holes are more easily drilled than molded. However, plastics are rather difficult to drill without some damage. Many difficulties not encountered in drilling metals, such as gumming, burning in the drilled hole, cracks around the edges or growth of cracks after drilling, can occur. Two reasons for these difficulties are that swarf flow (chip removal) in drilling is poor and cutting speeds vary from the center to the periphery of the drill, so that drilling imposes severe loading on the workpiece. Some drill types used with plastics are shown in Fig. 26. Drills of high-speed steel or premium high-speed steel (T15, M33, or M41-M47) are recommended, with low helix angles, point angles of 70–120 degrees, and wide, highly polished flutes to ease chip exit. Normal feed rates are in the range of 0.001–0.012 in/rev (0.025–0.3 mm/rev) for holes of 1⁄16 to 2 inch (1.6–51 mm) diameter, and speeds of 100– 250 ft/min (30.5–76.2 m/min), with lower speeds used for deep and blind holes. Point angles of 60 to 90 degrees (included) are used for many plastics, but an angle of 120 degrees should be used for rigid polyvinyl chloride and acrylic (polymethyl methacrylate).
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online