Red Eye Garage Tip #7 - Machining Polyurethane December 28 2016
We received a set of control arm bushings that needed to be shaved down to fit. We hope you're never in this position, but if you do get stuck and have a lathe, here is a trick to have up your sleeve. We've never machined polyurethane before, but this is what we learned.
We needed the bushing height to be shorter. The attachment point on the chassis was too small for the bushing to fit properly. In the photo below, the shaved bushing is on the left in comparison to unmodified bushing on the right. The manufacturer's name has been blurred to protect the guilty.
We first experimented with 100 grit sand paper to see if we could sand it down to the final dimension, but the sand paper was quite ineffectual, and it would take way too long to be practical. A belt or disc sander may or may not have worked, but it seemed riskier than machining.
We sharpened a HSS tool bit to a sharp point, with a high rake angle. Polyurethane doesn't produce chips like metal, so when the tool cuts through it, the waste stays in one piece. In our case, this produced a disk, as you can see in the photo below. It is useful to know this to plan your cut, as you need to allow space for your waste to flop around. You also don't want the waste to be too large and pull away from the part prematurely, leaving behind a broken surface finish. A plunge cut is fairly straightforward, but we are unsure of how the waste would behave if we were turning down the part.
The softer the bushing material, the more difficult it is to machine. Even work-holding can be difficult, as the bushing will deform under clamping pressure. Some people freeze their bushings to increase machinability, but we did not need to do this for our street compound bushings. They have a Shore A durometer of 80.
We experimented with speeds and feeds, and a relatively high surface speed (approx 400 SFM) and a relatively low feed rate (approx. 0.5 inch/min) is what worked best for us.
A depth of cut of 0.040 inches worked best for us. A thicker 0.080 inch cut produced worse dimensional tolerances, where the depth of cut became deeper further into the cut. As you can see in the photo below, there is a small lip near the circumference of the bushing.
We achieved good results, though the finish definitely wasn't as good as the original finish. So if you need to machine polyurethane bushings, don't be afraid to fire up the lathe and give it a try.