Machining PU Raw Material
With SuperPro polyurethane raw material you can make special bushes yourself. The material behaves differently from metal or hard plastics. These notes help with machining.
Hardness grades
The raw material is available in 70, 80, 90 and 95 Shore A. For control arm bushes, 80 Shore (road) and 90 Shore (track) are usually used. Hardness mainly affects durability under high load; handling depends less on it because SuperPro polyurethane is progressive: restoring force rises with load.
Shore values are not comparable with those of other manufacturers or original bushes, because the undersized bore and the tension created by the pressed-in tube also play a part.
What does not work
- Melting and re-casting: the material is mixed with a hardener at the factory and must cure at the correct temperature. After re-melting it no longer cures evenly and loses its properties.
- Milling: at machining temperatures the material begins to smear. Milling is not possible.
So the only way to machine it is on a lathe.
Turning: tooling and settings
- Soft grades (70 and 80 Shore): put the raw material in the freezer overnight and machine it straight after removal. Cold makes it firmer and cuts cleanly before it warms up.
- Harder grades (90 and 95 Shore): hard enough to machine without cooling or lubrication.
- Tooling: the parting tool should be razor-sharp like a knife blade; the turning tool ground with about 30° rake and equally sharp. A dull tool squashes instead of cutting.
- Cutting speed: as a guide, about 150 m/min (about 2000 rpm on 1 inch / 25 mm bar).
- Geometry: compact parts with a length-to-diameter ratio of about 2:1 are easiest.
- Even feed, no pauses during the cut, clear chips regularly. Wear safety glasses.
Designing a bush – notes
- Make the bore slightly smaller than the tube: the tube is pressed in, the polyurethane is preloaded and sits firmly.
- Grooves in the bore (e.g. spiral or honeycomb) reduce friction and store grease.
- For pivoting mounts (e.g. control arms) provide clearance into which the material can expand. Torque-strut mounts do not need to pivot.
- Lubricate all contact surfaces between polyurethane and metal with SuperPro silicone grease (see fitting guide).
Note: solid polyurethane is generally unsuitable for engine mounts because it transmits more vibration than rubber. Have safety-relevant parts checked by qualified personnel.