I have used something like that extension-plate set-up, not for milling large discs but for drilling the rivet holes round a smokebox over 8 inches outside diameter, in a horizontal mill rather small for such antics.

I bolted the rotary-table to a thick steel bar that had come with the machine, with mounting-slots milled along it: part of a between-centres jig the mill’s previous owner had made for cutting keyways, I would think . The bar itself was bolted across the centre of the table, so the RT was cantilevered by about half its diameter over the front. I did not add extra support, as I recall, but the machining load was very light, simply drilling 3/16″ dia. holes.

So examining your proposal, I would suggest that the diagonal braces are rigid bars with holes on the inboard ends for T-nut screws. Avoiding the complication of tensioning.

I would use angle-section or square-section tube rather than flat bar, for the braces, for extra rigidity.

Where you have a wheel you could use a slide on a plate clamped to the knee, similarly to the table’s cross-feed support on a shaping-machine.

You could mount the RT on the far end of the table too, to get extra distance from the centre to the cutter up to a certain extent. But you would have to do a bit of trigonometry to calculate exact depth of cut or spacing of concentric cuts etc.

If machining full circles only, would it invoke trigonometry?

Pythagoras perhaps – the only dimension involved is the radius and that would be a function of long and cross distances from the spindle axis. Trig. needs angles knowing, but I think here they are not needed.

I am thinking of machines with round column as well as square ones, and applicable whether working from dials or DRO.

If you start at the rotary-table and spindle on a mutual (0,0) position then the radius is the hypotenuse of a triangle formed by the two linear displacements. Work back from the radius and a sensible cross-travel to obtain the longitudinal travel.

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A few years ago I had a slightly similar problem when I needed drill sets of bolt-holes on pitch-circles on several plain discs. These had all been turned and the drilling was the second operation.

The radial accuracy was not too critical, within reason, as long I made the mating parts on the same set-up, and a rotary-table clamped down on a bench-drill was fine for the task. The holes accommodate only ordinary bolts.

I also realised the starting-point angularity did not matter, nor did centring the RT under the spindle, because the radius is always constant from the RT axis.

I marked out the discs, so could swing the head round the column until a centre-drill met the marked circles with the RT reading 0º, clamped all up, measured to verify the setting; then needed use only the degrees to locate the holes.

(I forget the details now but might have used the tool point in the lathe, as a scriber to mark the pitch-circle from the centre.)