When machining parts from TEFLON PTFE resins, either manually or automatically, the basic rule to remember is that these resins possess physical properties unlike those of any other commonly machined material. They are soft, yet springy. They are waxT, yet tough. They have the cutting "feel" of brass, yet the tool-wear effect of stainless steel. Nevertheless, any trained machinist can readily shape TEFLON PTFE to tolerances of +0.001 inch and, with special care,to +0.0005 inch.
Choose Correct Working Speeds
One property of TEFLON PTFE resins strongly influencing rnachining techniques is their exceptionally low thermal conductivity. They do nor rapidly absorb and dissipate heat generated at a cutting edge. If too much generated heat is retained in the cutting zone, it will tend to dull the tool and overheat the resin. Coolants, then, are desirable during machining operations, particularly above a surface speed of I50 m/min (500 fpm).
Coupled with low conductivity, the high thermal expansion of TEFLON PTFE resins (nearly IQ times that of metals) could pose supplemental problems. Any generation and localization of excess heat will cause expansion of the fluoropolymer material at that point. Depending on the thickness of the section and the operation being performed, Iocalized expansion may result in overcuts or undercuts, and in drilling a tapered hole .
Machining procedures then, especially working speeds, must take conductivity and expansion effects into account.
Surface speeds from 60-150 m/min (200-500 fpm) are most satisfactory for fine-finish turning operations; at these speecls, flood coolants are not needed. Higher speeds can be used with very low feeds or for rougher cuts, but coolants become a necessiry for removal of excess generated heat. A good coolant consists of water plus water-soluble oil in a ratio of 10:1 to 20:I.
Feeds for the 60-150 m/min (200-500 frm) speed range should run berween 0.05-0.25 mm (0.002-0.010 inch) per revolurion. If a finishing cut is the object of a high-speed operation (e.g., an automatic screw-machine running at240 m/min [800 fp-]), then feed musr be dropped to a correspondingly lower value. Recommended depth of cut varies from 0.005-6.3 mm (0.0002- 0.25 inch).
In drilling operations, the forward travel of the tool should be held to 0.13-0.23 mm (0.005-0.009 inch) per revolution. It may prove advantageous to drill with an in-out motion to allow dissipation of heat into the coolant.