Assuming the table has a central hole (mine has an MT2 taper), put a centre in the hole. Put another centre in the chuck and line up the points – use a magnifier to get it accurate on x & y.

Mount the work centrally on the table using the centre in the chuck to centre the work – without moving the table.

Now the work and table are all centred. move the x (or y) axis by the pitch circle radius. The centre of the chuck is now on the PCD. Rotating the table will now allow the chuck to trace out the PCD.

There are two parts to that: centring the rotary-table on the mill, and centring the work.

1) What you are doing to centre the table is more or less what many of us do, but I would not use a drill in a drill chuck as the centering tool. For better accuracy, use a piece of silver-steel or similar ground material in a milling-cutter collet of the correct diameter for the steel.

The most accurate way, though slower, is to use a DTI on arm held in the collet. Set the RT as central as you can, then swing the DTI gently, by hand not power, round the table's perimeter, being careful not to let the indicator's plunger trip on the T-slots, to determine the cross- and long- travel corrections.

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2) I don't know what the component looks like, but there a few approaches depending on its features.

– If it has a machined hole concentric with the bolt-hole pitch circle you may be able to use an approach similar to the above (having centred the RT first), even if you need turn a plug to fit the bore, with a spigot to fit the collet.

– Again from a concentric machined surface; use the DTI.

– If no bored hole or turned spigot, you may have no choice but to mark out the part as carefully as you can, and use a "sticky pin" to pick up on that.

This is something like a sewing-needle stuck to a rod held in the drill-chuck, and its point gently eased with a small piece of wood into centre by eye (no longer seen describing a circle). The work is then adjusted until the point of the needle is spot-on the marking-out. Use a magnifying-glass to help you.

– If the concentric feature is machined, you might be better both milling that and drilling the ring of holes, from its marked-out centre, all on the RT.

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Once you have centred the table with one milling-machine axis locked and both set to 0, you need move only the other axis by the pcd radius to the first hole. lock that axis, then set the RT to the appropriate angles. Though not essential it is preferable to have Hole Number One at the table's 0º point, or at least on a multiple of the required angular increment.

Write the angles for all the holes from the starting angle, before you start, verify the list, and work to that. If you need run round again, to countersink or tap the holes, it's also a help to mark each point on the scale with a soft pencil or felt-tip pen (both will clean off afterwards with a spot of meths or similar.)

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NB: Once you have centred the table, be careful not to fall into the trap of moving the lead-screws! Move the work on the RT. (How do I know? Errrr….. I didn't know I knew they words!)

On my mill the cross-travel and vertical handles slide off easily, and I use that as a precaution…. with care not to drop the keys into Eternity.

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Hi Nick, Guys,

My mill has an MT3 spindle, so all I do to centre the RT is put a centre in the mill spindle and use the hole in the middle of the rotary table to locate the centre. Then clamp the table down. As far as centring the work I'll leave that to you.