CNC lathe - off center machining


We have to make some 2" diameter pins 5" long.

There is a hole for a grease fitting that is drilled in from the end a

1/2" off of the centerline of the diameter.

We would like to do the drilling and tapping for this in our cnc lathe.

The pins would already be faced to length and chamfered before they got tot this machine.

Has anyone ever machined off of the centerline ( offset the part ) in a hydraulic 3 jaw chuck ?

Is it possible to offset 1 chuck jaw a 1/2" from center and adjust the other 2 jaws accordingly ?

Can it be done ?

How else could it be done ?

I know it would be easier to do it in a cnc / manual mill but that is not an option.

I'm open to any ideas.

Thanks in advance.

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I would make a bushing with the I.D. off center that would fit into your chuck or collet. you can put a flange on it to keep it from pushing backwards. Then, you want to split it down the length on one side with a band saw. Slip your pin into the bushing and then into the chuck. If the wall is extra heavy, you may need to cut into the opposite side of the bushing as well. Just don't cut it in half. I find that bronze will work pretty well for this sort of thing. The only concern I would have is vibration from the offset weight. Don't run it too fast. You may put a lot of wear on your spindle bearings.

Good Luck


Reply to
Jim Z

If you use the same material the offset wont matter if you pu it all the way in the pot chuck. (no vibration) We did this all the time.

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Depending on how accurate you need the offset, it could be done.

This little proggie would probably help:

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"Need to turn an eccentric and don't have a four-jaw chuck (or don't want to set it up for a quick job)? Packing the work in the three-jaw will handle many jobs. This tool calculates the amount of packing needed. A more elegant method employing a slotted tube is also described along with a program for making the required calculations."

Thanks to Dan for the link.

Reply to
Jan Nielsen


If there is a significant out of balance situation for any reason (odd shaped parts, eccentric turning, etc.), you might try to balance the load by using longer chuck jaw hold down bolts and clamping some weight under the bolts. They'd still hold the jaws fine. Got to watch for clearance of the added weight though.

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That is 4 jaw chuck operation, unless you can "rig" something. Michael

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you can adjust the jaws to get it close. I recently ran some square parts in my cnc lathe by placing the jaws in different positions on the serrations and got lucky enough to be in print!!


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:) "D"

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