View Full Version : Straightening a Bent Bar of UHMW Polyethylene

01-19-2013, 02:38 PM
I have a 7/8" dia. piece of UHMW polyethylene, 1 foot long. I need to cut it in the lathe but it is bent. When placed on my granite surface plate and both ends of the rod are touching the plate, there is a 0.090" gap between the rod and the surface plate at the center point of the rod. I need the OD of finished part to be 0.853" so there is not enough material to machine out the bow in the rod. I tried putting one end in a vice and gently bending it but that didn't take out the bow. Any suggestions on how to straighten the bow in the rod? Once bent, will it stay straight or will it "pop" back to its bent state? Appreciate any help.

01-19-2013, 03:02 PM
I know nothing of straightening out UHMW, but for other plastics, such as acrylic, the bend is caused by internal stresses that have built up. The only way to permanently remove these stresses is termed 'annealing', where you gently and slowly heat the whole piece up to a certain temperature, just below melting point, then leave it at that temperature for a few hours, then slowly cool it down again to room temperature.

For acrylic this takes a whole day, as the process is a very slow one, and at the end your piece will be a little smaller in all dimensions.

This works for acrylic, but I do not know if it will work for UHMW, and more importantly, I don't know the all-important annealing temperature for UHMW - too high and you get a puddle of plastic, too low and nothing happens.

Considering the price of UHMW, and the time and expense of doing the annealing (if it works at all) I would buy a thicker piece of UHMW

I expect someone else with direct experience of doing this to UHMW will chime in :-)

Richard in Los Angeles

01-19-2013, 05:56 PM
...Considering the price of UHMW, and the time and expense of doing the annealing (if it works at all) I would buy a thicker piece of UHMW.

If I am correct in assuming that you need a finished part that is .853" in dia. and 12" long I think you may have picked yourself an almost impossible task. Starting with a piece of material that is maybe 1-1/4" dia. will help but the bottom line is that a piece of UHMW only .853" in dia. is going to have a lot of flex in it. It will deflect significantly when you machine it. I've machined lots of plastics but never anything with a length to dia. ratio like that. You might get away with using a follow rest but I've never tried one in plastic...

01-19-2013, 06:10 PM
The heating and cooling should work. I'd set it up on a piece of straight angle iron propped up in the oven so it can sag into being straight if it's going to. Many plastics will grow in thickness and shorten in length when heated to the plastic state, so this might be a benefit. All you can do is try it- there's nothing to lose.

You will want to do some test turning to see how the diameter changes with depth of cut. It likely won't be a direct relationship, and the cutter sharpness, relief angles, and setup angles will change the results.

You will need a steady rest. Even a few inches of bar 1 inch in diameter will flex significantly- it might push away from the cutter, or try to climb forward and onto it. You can use a center in the tailstock, but it's still going to bow. Can be very frustrating.

Bob Fisher
01-19-2013, 07:08 PM
I was just faced with a similar problem, trying to put 8 flutes in a 3/4 in cherry dowel. I finally made a router base with a 3/4 in tunnel and passed it over the dowel after finding a way to accurately index the dowel for the next cut. You might be able to do something similar, but there is no guarantee that it will stay straight after. Worked well for my problem, you might be able to do something similar with a way to tun the workpiece while traveling the router. Bob.

01-19-2013, 07:57 PM
Do you need the final piece to be straight? If no:

Advance your piece out of the chuck or 5C holder 1" at a time, and the bend in the rod won't matter.

If yes, you can first try a cold form, where you support the rod at both ends and push down on the middle, overshooting by about 100%. IF it goes back into straight, check a few hours later if it is still straight.

Else you will have to build a jig where you hold it slightly overbent and do some heating and cooling cycles.

You could also just cut first anyway and straighten later. Use a tailstock but still advance out of the chuck 1" at a time.

01-19-2013, 09:31 PM
UHMW isn't going to stress relieve very well. In fact the problem my be that the piece was sitting somewhere where 1 side was warm and some of the plasticizers gassed out causing the part to shrink on that side.

UHM has the interresting property that when machined - say a ring - and let it sit over night. The bore will be bigger, and the OD will be smaller than as machined. the process is called outgassing. Once it happens, it can't be reversed.

01-20-2013, 05:07 PM
Thanks for the ideas. The parts I'm making has flutes running the full length of the outside. The final parts are about 1" long with the flutes on the outside. I was hoping to cut the flutes on the outside of the 1' long piece and then cut the inch long pieces with the fluted outside off of the larger piece. I could shorten the rod to 6" but there are 50 flutes spaced at 7.2 degrees on the outside so cutting them will be very time consuming if it is going to take 100 passes on two 6" long pieces vice 50 passes on a piece 1' long.