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Machining acetal washers

G

gdgambler

Hot Rolled
Joined
Oct 1, 2006
Location
K. Falls, Oregon
I am machining thousands of acetal washers .120 thick,1/2" dia. with a .320 hole in them. I have tried different speeds and feeds but the washer is .120 on one side and .119 on the other side. I have even made thin ones with the same results. Is this common with cnc machining acetal parts? My tools are razor sharp also.
 
Is it .119 at the outer dia. And .120 by the .320 dia thru hole? Is this on CNC? If so, program to compensate for that taper.

I guess it’s possible that the plastic is more dense on 1 side of the bar than the other... if that’s the case, part most of the way then face and then finish parting may help?

.001” on a plastic washer shouldn’t matter but I’ve seen engineers request worse...
 
I am working with plus or minus .005 but just wondering why I have a thinner side. I measure the thickness on one side then measure the thickness 180 degrees on the other side in the same area.
 
I would think the stock is not running concentric as your parting these off,it's creating a "whipping effect"
mark a jaw and several parts see if the high and low are in the same areas.
 
2000 rpm f.0015 turning and f.0035 cutoff I have tried different feeds but this works the best so far. I can even run them at 4000 rpm's but the part seems to get worse. I will mark the collet and part to see if I am getting a whipping effect. I have slowed the rpm's down even lower thinking that the whipping effect would be less and the part seems to stay the same.
 
Material stick-out? Have you tried running a few out of aluminum or some other material to see if it's the same? Is it possible your spindle is bent?
 
Just before parting off the washer put a sharpie line to indicate at chuck angle the thin and thick parts are. If they all line up the same way it would be interesting. I wouldn't know what to do with that information, but it could be interesting anyway.
 
Can you reduce the collet clamp pressure, have seen that cause similar issues, other thing to easily try is anneal some plastic and try it again. Maybe also try a different collet if you have one, even slightly out of round clamping can force it to bend by differing amounts as you part slices off the end.

Plastic and single thou level precision everything needs to be right. things that would never ever cause issues on metal matter on plastics.
 
Hate working with plastic of any ilk. First is the stock .5 consistently round or lobed, second try a different collet. Either could be causing pressure differences and when the material relaxes you get this showing up in the thickness.
 
I would run some parts and be happy I was only using 10% of my tolerance. I know that's crazy, but that's what I'd do.
 
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