Machining Tips

Most plastics can be machined using the techniques employed for soft brass. Any machine tool can be used (provided it is capable of high speed without vibration) and any of the tool materials used for metal may be employed - especially high speed steel and carbon tipped tools.

Since machining is relatively expensive (due to the cost of skilled labor), it tends to be used only when the quantity of parts to be produced is small or where other fabricating techniques can not produce the required shapes, tolerances or surfaces - or where high speed automated machining equipment is available. Occasionally, machining may be used as a secondary operation for finishing pre-molded components if the volume of scrap is too high when using semi-finished shapes.

The main differences in machining metals and plastics are the lower thermal conductivity, lower melting point and lower modulus of elasticity of plastics. These give rise to several important rules:

1. Use very sharp tools, preferably with polished surfaces to reduce heat generation and provide good chip clearance. It is advisable to use new tools (tool wear is not so important since wear rates are small). Coolants are generally not necessary to reach high surface quality but they do permit higher cutting speeds.

2. Clamp the component with uniform pressure all around so it is well supported during machining to prevent deflections. Plastics have a much lower modulus of elasticity than metals.

3. Avoid vibration and chatter. The formation of chips serves as an excellent guide to the success of the machining process. Rapid chip clearance is necessary in order to avoid wrapping up the component being machined and to ensure the quick removal of heat, which is largely carried away by the chips. Therefore for rough machining, feed and cutting depths should not be too small.

A smooth surface is achieved by chips in the form of continuous swarf, obtained by top rake angles of 20-30°, feed rates of .05 - 0.1 mm/rev (.002- .004) and cutting depths of I -2 mm = .040" - .080". Shavings and chips should be prevented from wrapping around the component. Rapid chip clearance comes with short, breaking chips obtained by top rake angles of 30-40°, feed rates of .020 " -.020 "/rev. and cutting depths of .156 .312". (This method should be used by experienced workers only since it is possible to damage the component if performed too roughly.)

Cutting speeds should be within the range of 650 - 1,650 ft/minute to avoid excessive heat generation. Regardless of the type or amount of machining, it is best done by rough pre-machining to the approximate size, then a fine finishing to obtain the exact dimensions and a smooth surface. When drilling large diameters, start with a pilot hole of 1/2 " diameter maximum, then use a single point tool to bore the I.D. to size. Remove the drill frequently to assist cooling and to remove chips from the hole and from the drill flutes. After pre-machining, the component should be allowed to cool to ensure accuracy in the second machining step. Where large volumes of material have been removed, components should be stored a few days before post-machining to exact dimensions. This allows the relaxation of residual stresses.

If the final requires a specific moisture content, the component should be conditioned before post-machining.

It should be noted that dimensional tolerances can not be set as close for plastics as for metal components because:

I. The thermal coefficient of expansion of plastics is approximately 7 to 10 times higher than that of steel. (for maximum accuracy always measure dimensions at room temperature).

2. The elastic recovery after relief of clamping or tool pressures can cause inaccuracies.

3. Dimensions can change due to moisture absorption.

4. Relaxation of residual stresses can cause distortion. In general, the expected tolerances should be no closer
    than 1: .002 ". Since this is of the same order as the changes which may arise from environmental conditions
    such as variations in ambient moisture content and temperature (the above tolerance, for example, is
    equivalent to the approximate dimensional change of a (2 ") bore over a temperature variation of +/- 20°F, the
    pursuit of higher accuracies is of no value.