Turning Polyurethane Suspension Bushes
Polyurethane can be tricky to turn in a lathe because it resists cutting and deforms away from the tooling. Turning it at a 2:1 diameter to length ratio (Ø50mm rod protruding 100mm out of the chuck) helps it retain its shape.
PU will melt at approximately 228°C (550°F), so well maintained sharp points of 90° to 110° will help prevent heat build up. For best results turn the rod at approximately 15,250 cm/min [500 ft/min].
When drilling the bores of the bushes, force the drill bit through the rod and quickly retract it again to clear the debris. Reinsert the drill bit but allow it to dwell briefly which will actually melt the PU to the correct diameter!
All in all, it's a lot simpler to turn PU straight from the freezer. At a temperature of -17°C (0°F) it machines quite well and a fine finish can be achieved.
Aim for a sliding fit which will result in a perfect interference fit when the polymer returns to room temperature.