In more than 20 years I’ve never needed to use a chuck on a rotary table. Much simpler to use a spigot, or similar, in the centre hole for locating the work. On my rotary table that is possibly easier as the centre hole is 1″ diameter parallel rather than tapered.

Andrew

I’m not sure how to say this without it sounding patronising, but please believe me … it’s one of the most important things I learned about using the Rotary Table:

Never lose sight of the fact that centring the R/T under the spindle is one thing, and using the mounted chuck [or any other fixture] to position the workpiece correctly is a completely separate activity.

Unless you are intending to produce offsets, the R/T must remain concentric with the machine spindle.

0.004” is not a good number … so I suspect that you are somehow allowing these adjustments to conflate.

MichaelG.

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Edit: __ expert comments appeared whilst I was still trying to compose these few words.

On 8 June 2024 at 21:21 Dr_GMJN Said:

[…] Im asking how to centre the chuck on the already centred R/T. […]

All I did for the BCA was to make a close-fitting plug for the chuck [*], bored through as a close sliding fit on a pin which fits the central hole in the table.

… Assemble all the components and tighten the screws, then remove plug and pin.

… as and when required for use.

[*] more strictly, for the thick adapter plate upon which a small independent 4-jaw gets mounted.

MichaelG.

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Note: __ the central hole in the BCA table is [I think] 1/2” diameter, and the ‘lead-in’ of the pin is VERY slightly tapered to make insertion easy.

Yes – it will help if you can establish whether the socket in the R/T is actually concentric with it’s centre of rotation.

Also whether the mill spindle nose is concentric about it’s axis.

If you can find the ‘truest’ reference to measure off there are subtly different ways of setting up – say mounting the DTI on the R/T platen and using that to ‘read’ the inside of the mill spindle nose – that may help you get a better result – it is after all the Cs of R that you need to coincide, initially.

Once that part of the job is set up, I usually locate the chuck by holding a (known ‘Good’) pin in a collet in the mill spindle, tighten the jaws of the RT chuck onto it, an nip up the bolts.

If you are going to interpose an ER chuck, just as well check concentricity of that too.

This is the only photograph of the Rotary table chuck adaptor plate that I made when I had my Emco FB2 that I have.

The Plate was centred in the 4 Jawed Chuck and a register was turned to fit closely the register in the back of the Chuck. At the same time a hole was bored and reamed in the centre of the plate.

A Top Hat shaped Spigot was then turned to fit both the Rotary Table location diameter, (the Brim of the Hat), and the bored and reamed hole in the adaptor plate, (the body of the hat). This can be made a press fit if desired but I chose to leave mine to be a nice sliding fit. The Chuck can then be mounted directly to the table if required, by removing the location spigot.

One thing to note is that it helps if the stylus of the clock is on the centreline of the work. This makes setting up the Rotary Table much easier as the stylus of the clock can be used to sight the X & Y positions with reference to the Tee slots. Provided of coarse they are in line with the X & Y axis, which they were on the FB2.

Regards

Gray,

On 9 June 2024 at 10:03 Dr_GMJN Said:

That’s what I’m doing, but tapping the mount plate and moving it in only x or only y is impossible on my setup, so I – literally – go around in circles correcting one axis and simultaneously introducing an error into the other.

One possibly stupid suggestion! If you rotate the table about 45 degrees from where it is in the photos so that one pair of bolts are along the x axis and the other pair along the Y axis, it should make it far easier to “tap” one axis at a time.

On 9 June 2024 at 12:40 Peter Cook 6 Said:

On 9 June 2024 at 10:03 Dr_GMJN Said:

That’s what I’m doing, but tapping the mount plate and moving it in only x or only y is impossible on my setup, so I – literally – go around in circles correcting one axis and simultaneously introducing an error into the other.

One possibly stupid suggestion! If you rotate the table about 45 degrees from where it is in the photos so that one pair of bolts are along the x axis and the other pair along the Y axis, it should make it far easier to “tap” one axis at a time.

^^^ This. Most definitely. The way the chuck plate is set up with slots at 45 degrees in the pic is making a simple job hard.

The other thing is, use a dial indicator with the magnetic base attached to the milling table next to the RT, not dangling from the spindle as in the pic. When you are tapping the chuck to move it, it could be moving the mill table also and give a false reading off that dangling dial gauge. Dial gauge attached to the table will move with the table and still give a true reading of concentricity of the item held in the chuck as the RT is rotated.

And use the “quick method” to set the item gripped in the chuck to run true, same as when using a 4 jaw chuck in the lathe:

Rotate the job through 360 degrees and set the dial gauge bezel to zero at the lowest point of the needle.

Rotate again and note the highest reading on the gauge.

Rotate job until the needle sits exactly halfway between 0 and the highest reading.

Set the dial gauge bezel so zero lines up with the needle.

This is now your zero point to set the job to. Adjust your backplate, (or chuck jaws in the lathe) so each of the four cardinal points, (12 o’clock, 3, 6 and 9) reads zero the job’s good.

It’s amazing how much time and mucking about this methodical approach saves over the traditional random tapping and spinning and needle waving and swearing.

Then once you have the job set to rotate concentrically in on the RT, and your chuck plate bolts tightened up, then you can go back to dangling the dial gauge from the spindle to check the job is centred under the spindle and adjust the mill table position to suit.

I suggest first to check that the table and chuck back surfaces are flat, to the extent that if one (only) clamping bolt is nipped up, it is impossible to slide a 0.05mm feeler under any of the other fixing bolt positions. Check this for all four fixings similarly clamped. Alternative to a feeler, lay thin paper pieces under the non clamped fixings, and check they remain clamped when each single bolt in turn is nipped up.

Assuming this procedure is passed, work as follows. Start by getting down to a few thou concentricity as previously. Now find which axis across a pair of opposite fixings is worse, then nip up lightly one of the other two fixings, leaving all three others loose. Tap the chuck (which will pivot on the nipped bolt) to centre the “worse” axis. Now nip up a bolt in that axis, and loosen the first bolt to do the same in the second axis, and so on. Repeat until you get to full concentricity, which should only take a few more steps, but as good centring is approached start to nip up the other fixing bolts a bit more lightly than the pivot.

This should control things such that only one axis at a time can move. Hopefully final clamping will not result in distortion, but allowance could be made for this too provided it is consistent.

Part looks good … but my brain remains befuddled.

MichaelG.