Will your boring head screw off it's shank, if so just put that in the 3 or 4-jaw. Simple straight shank backplate to fit the boring head so chuck can be mounted or even just a bit of bar with a 15mm hole to take one end of bar locked with a grub screw so other end can be turned eccentric to it.

Alternative is to hold work in toolpost or on vertical slide and put boring head into lathe spindle with an external tool and then just use cross slide handwheel to set the offsets

Edited By JasonB on 13/06/2020 08:39:38

You may be overthinking this.

For small offsets its quite satisfactory to simply put the first made end in a four jaw chuck and push it sideways using one pair of jaws. Measure the shift using a dial gauge. Alternatively the the cross slide dial will do the job if you fit an upright to the back side of your cross slide out of the way of the cutting tool and bring that up to touch the work.

For larger offsets put a suitable hole to take the first end in a rectangular block and move the whole thing sideways. You will need a clamp screw or two to hold things. As teh end is to go inside a tube I guess it would be OK to put a flat or two on to help the clamp screw hold. Split and closing screw ould be best but more work.

If you don't have too many to do and have a milling machine it might actually be quicker to make dedicated holders for each offset.

Folk like us tend to be a little too economical with material and fight shy of disposable tooling. if its a one time job its not worth spending ages doing proper tooling. Spacer blocks may be an easier alternative to pushing with the chuck jaw. I have a gash set of gauge blocks kept for this sort of thing. Although they no longer wring tigether any sense they are fine for such work and still way overkill for accuracy.

Clive

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I didn’t realise that you already had a boring head

MichaelG.

You could mount a disc on your 4-jaw chuck key and mark suitable graduations on it to measure exactly the jaw movement and therefore offset. EG if your chuck jaws' thread is 10tpi, scribe 100 graduations on the disc and each one is one thou movement.

Then you set the first job up to run dead true all round. Then using the numbered jaws, loosen jaws 1 and 2 to take the job out, then move jaw 3 by the measured amount on the disc to achieve desired offset. Then tighten jaws 1 and 2 on the new job to be turned offset. Repeat process for each new offset.

Otherwise, ISTR one of GH Thomas's books somewhere has a list of the amount of offset gained in the 3 jaw chuck by inserting various size shims on one jaw. I can't find it though. His indexes are not as comprehensive as his articles.

Edited By Hopper on 14/06/2020 02:42:11