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Old 05-02-2017, 10:54 PM
PJS PJS is offline
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Default Concentricity issue

I'm cutting out some parts that resemble the adaptor insert on an old 45 RPM record. For some reason, I can't get the center hole to be concentric with the OD. In some cases, the difference is close to 0.125" off on a 4.0" diameter. The material is 0.5" PVC, I'm using a 0.125 bit running about 150 fpm at 10K spindle speed. I machine all the holes first then the cutout last. The holes are cut with a spiral motion. Not sure where to turn to fix the issue. This isn't the first time I've had concentricity issues. I tried to include some pictures but they are too large. Suggestions are welcome.
Paul
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Old 05-02-2017, 11:10 PM
Kyle Stapleton Kyle Stapleton is offline
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You may be losing step, if that is possible on a Cobra. You are running a high chip load .007 (.125 bit with 2 flutes, if you are running a 1 flute bit you are super high .015) try turning up your Rpm's to about 18000 which will give a chip load of .004 with a 2 flute bit.
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Old 05-03-2017, 08:46 AM
ktringle ktringle is offline
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I had the same issue with a project awhile back. I believe my problem was bit deflection. I slowed my feed rate down and the circles were cut properly.
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Old 05-03-2017, 08:54 AM
de5 de5 is offline
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Quote:
Originally Posted by PJS View Post
I'm using a 0.125 bit running about 150 fpm at 10K spindle speed.
Did you mean 150 ipm?
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Old 05-03-2017, 03:09 PM
PJS PJS is offline
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Yes, IPM. These machines are good but there is a limit!

The hole diameter is 1.0". I can jack up my spindle speed, no issue. I did try to slow the feed speed down to less than 100 but thought that was too slow. I'm using a PVC bit but want to make sure the material doesn't fuse back together. The direction of the error is usually the same on all the parts. I can stack the pieces in the same orientation as they were cut and be a little off. If I rotate them 60 degrees in either direction, then the center holes are off. Weird thing is, they are not all off the same amount. All are off at least a little but some are worse than others.
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Old 05-03-2017, 05:03 PM
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Gary Campbell Gary Campbell is offline
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Paul...
Have you eliminated mechanical backlash as the cause?

Items to check would be anything that moves or rotates between any motor and the ballscrew or pinion gear that it drives
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