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Getting my head round the rotary table

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Nick Welburn25/03/2021 21:40:11
81 forum posts

I’ve treated my self to a rotary table and indexing plates for my new mini mill.

I want to use these to help drill out my Stuart 10v piston ends and stand.
I understand how to get the divisions out, I’m lacking practical experience in getting it all lined up. Today I used a drill in the Chuck of the mill as a datum.

How to I set a repeatable PCD when everything I need to drill with the five bolt pattern has no centre?

Peter Cook 625/03/2021 21:53:07
164 forum posts
50 photos

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.

Emgee25/03/2021 22:00:57
2156 forum posts
265 photos


Depends on what position you select for the holes, if 1 of the holes is on the X axis, 9 o'clock or 3 o'clock, after setting the table to be centered in X&Y axis fix the work to the table at the marked or measured centre and move the table in the X axis by half of the PCD, lock both X&Y axis, you are now at centre of the first hole.
Drill the 5 holes required.


Nigel Graham 225/03/2021 22:21:28
1706 forum posts
20 photos

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.


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.


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.)


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.


Howard Lewis26/03/2021 16:10:51
5298 forum posts
13 photos

Going on from what has just been said, If you mount a blank MT arbor (Remember that you need to remove it afterwards so don't thump it into place! There have been at least two letters in MEW on how to remove a MT from a Rotary Table! ) and rotate the clock around the OD of that.

Or having Zeroed the clock at one point, rotating the table will tell you in which direction to move the table to produce a consistent Zero reading around the OD of the Arbor.

Having centred the Table under the Quill, and clamped (with a final overcheck ) mount the work on the Table and clock the Od, shifting it until the reading is Zero all round again.

At the risk of insult, move the table or work Half of the clock reading, to move towards the centre point. A total reading of 0.5 mm means that the item is 0.25 off centre.

You can offset then off set the Mill Table by the dimension that you need. Again, say 10 mm to drill holes on a 20 mm PCD.

It has been known for the chart supplied with some tables to contain errors, so do make the occasional calculation, or measurement to be sure. It is frustrating to find that what you thought would give 12 equally spaced holes gives 11 equally, but too closely spaced and one a greater distance apart.

It happened to me when trying to produce 13 divisions, and for a long while was convinced that I could not count, before eventually stopping making scrap, checking, and finding an error.

Further checks showed that it was not the only error in the chart.

Hopefully, by now, the charts will have been corrected, and be accurate..

My table has three Division Plates, but does not provide every division between 1 and 100.


John Baron26/03/2021 16:45:13
487 forum posts
189 photos

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.

Dave Halford26/03/2021 17:11:26
1726 forum posts
19 photos

I assume you have turned the edges of the end covers to match the sides of the cylinder, the walls of which are reasonably concentric to each other and you have centred the table as above.

If you mount the cylinder on a turned spigot on the rotary table you can pick out clearly the mid point between the outside and inside walls. Using one axis move the table to the mid point of the walls and lock X & Y

Just be certain that the hole pattern will look symmetrical with the cylinder and does not look like you started in a random location. Do a practise run and ensure that the 6th hole exactly matches the one you started on.

Loctite or superglue etc the end cover to the cylinder and tapping size drill you pattern through both locking the table each time. Remove the cover, flip the cylinder over and repeat for the other cover.

Open out the covers to clearance size away from the cylinder, they might snatch.

That should work, castings being what they are the bolt circle diameter may differ from the drawing. There is no point slavishly following drawings if your holes end up breaking out or the tapping process make the walls bulge.


Edited By Dave Halford on 26/03/2021 17:12:16

Edited By Dave Halford on 26/03/2021 17:14:29

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