Machining UHMW

I doubt it. You have a very large thermal expansion rate which will be against you, as well as general softness of the material which will give distortion when clamped as well as spring-back at the point of cut (a VERY sharp cutter will help this). Also, this material is formed under great pressure so there are likely to be internal stresses that are unbalanced when the hole is made. Everything seems to be against you in the finished part. You may be able to come close though if EVERYTHING is on your side that day. On short bores (2") we can hold within +/-.002 without really trying. An 8" long hole is another thing altogether.

As to the black material: Is it virgin material with carbon black added to improve UV resistance or is it re-grind material with black added to hide the misc crap that tends to get into the re-grind stuff? Always go with virgin material. I have seen re-grind with floor sweepings, metal filings, sawdust, etc mixed in.

Koz

James Fehn wrote:

That sounds do-able, even if it's three thou TIR. Your biggest source of variation may well be the temperature of the part. I would suggest you run one, allow it to cool to operating temperature, and then check the bore dimension.

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Possible, perhaps. But by no means easy. Lots of internal stresses in UHMW, and it tends to spring back after cutting if the tool is not very sharp.

Here's some recommended cutter geometry.

- an 8° clearance on cutting edge

- side clearance 8° to 10°

- rake 12°

- all forming tools - 8° to 10° rake

Ned Simmons

I don't think moisture is a big issue woth polyethelyne, I worked in vacuum forming for a while and this was one material we did not pre-dry prior to forming. Heat on the other hand might have caused the problem, think tupperware in the dishwasher with a hot dry cycle.