Hi Eric,

The graduations may relate to an amount taken off the Diameter of the hole.

I know mine does, though mines in thousandths of an inch, very accurate too, but can't remember from where I purchased it.

So really you will need to take a cut at one division, and measure the difference from the original hole size.

Bernard.

From Sunny Scunny…

Why take it apart? Stick it in the mill, bore a shallow hole, wind it out 10 divisions and measure again.

Easy.

Armchair job. Presumably Arc sold imperial versions a long time ago (nearly all metric items these days). Neither are they difficult to disassemble. It was only a suggestion. I would likely have experimented myself.

If new, ring Ian on Monday?

Whether graduations are in tens per rev or, say, eight might even give a clue as to metric or imperial if the tool was really old….?

As a quick check, I just picked up my 2” boring head, removed the Morse taper and proceeded to measure exactly ten turns of adjustment with my digi-calipers – quite easy to do. Also, the adjusting screw thread is sufficiently visible to measure the pitch (had my thread gauges happened to be in the house🙂 ).

I can assure you that simply taking a measurement with a thread gauge, or two with a caliper is even more simple than fitting the holder to the machine, setting up a dti (a dial gauge would be better) and taking measurements.

The whole problem is not exactly rocket science (no disrespect intended to the OP). I try to offer ideas of how to do it, not necessarily just the answer – which basically teaches nobody anything. Since when has anyone learned how to do long multiplication by using a calculator? Being given the answer does nothing towards learning a skill.