turning boring uhmw

[hackmeister]

I recently did a job where I had to turn, drill , bore, and part off some rollers that were made out of uhmw. The uhmw was white (if it makes any difference)and it was a solid 3in. dia. bar. Anyway, I had a problem with the parts growing in size after they were out of the machine. I would check the bore dia. while it was in the machine and it would be right (1.848 dia.), but as soon as it was parted off it expanded about .003 Now here is where it gets weird. The parts would continue to expand about another .007 over the course of several hours.
I made sure all the tools were sharp, and I used the coolant on each operation. The chuck pressure was at 150 lbs.
Any guesses about why the parts got bigger?
Thanks so much.

[tony978]

I work at a place that did a long run with parts made out of that stuff and I wouldn't try making any thing but a paper weight out of that stuff .
i want to say if you don't encase it , it will move to ware ever it wants even years later . I hope this helps , I would rework it out of omething that is knowen not to move if your looking to hold any tolerence .

[BOATDUDEGUY]

IM CURRENTLY IN A PLASTICS SHOP AND DO PART OFF WORK
PARTS FURTHER AWAY FROM JAWS ARE FINE, AS YOU MOVE IN TOWARDS THE PRESSURE OF THE JAWS AFTER YOU CUT OFF THEY EXPAND DUE TO THE RELEIF OF NOT BEING GRIPPED, AND WE USE MANUAL CHUCKS, WE DO ALOT OF GLASS, MOLY,191, CARBON,GRAPHIT,PEK, DELRIN,UHMW,POLYPRO
ALL OF THE STUFF CLOSE TO THE CHUCK STRESS RELEIVES AFTER THE CUT, DELRIN WILL SHRINK, AND THERES STILL ALOT I HAVE YET TO DISCOVER
HOPE I HELPED

[BOATDUDEGUY]

Originally Posted by hackmeister I recently did a job where I had to turn, drill , bore, and part off some rollers that were made out of uhmw. The uhmw was white (if it makes any difference)and it was a solid 3in. dia. bar. Anyway, I had a problem with the parts growing in size after they were out of the machine. I would check the bore dia. while it was in the machine and it would be right (1.848 dia.), but as soon as it was parted off it expanded about .003 Now here is where it gets weird. The parts would continue to expand about another .007 over the course of several hours. I made sure all the tools were sharp, and I used the coolant on each operation. The chuck pressure was at 150 lbs. Any guesses about why the parts got bigger? Thanks so much.

CHUCK PRESSURES TOO HIGH, IF AT ALL POSSIBLE EXTEND IT AS FAR AWAY FROM THE FACE OF THE JAWS AS YOU CAN AFTER CUTTING OFF IT STRESS RELIEVES
WE USE MANUAL CHUCKS, TIGHTEN BY HAND AND THE STUFF STILL BLOWS UP WHEN ITS CLOSE TO THE CHUCK!AND WE DONT COME NEAR TO 150LBS OF PRESSURE

[hackmeister]

Thanks BDG. I was wondering if the problem was related to the chuck squishing the stock. I havent had to run that job again yet ,but if I do I'll remember your advice.

[BOATDUDEGUY]

its funny cuz if we have a part that needs to get passed and is out of tolerance we will sit it outside in the sun if we need it bigger or sit it in the frige to get it smaller however we need to get it passed, cuz when it shipps in ups the temp gets way hotter than the ac shop
plastic is a different animal
i never had to use diamond coated inserts before i came to this shop
191 carbon with some graphite when its cutting it dosnt look or sound like a big deal but regular carbide inserts will just get sanded down before we fininsh one peice of material!