Centering R/T chuck

[Dr_GMJN]

On Dr_GMJN Said:

<p style=”text-align: left;”>Thanks Michael – Was it the first image that befuddled you, or the whole thing?</p>
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So I think the R/T isn’t too bad, and as ever it’s the mill that is introducing the main errors.

It’s O.K. … My head is content with your latest explanation.

What was previously disturbing me was your suggestion that, although you could centre the R/T it was in some way to blame for the failure to centre the chuck … that simply ‘did not compute’ for me.

MichaelG.

[Dr_GMJNParticipant @dr_gmjn]

No I wasn’t suggesting that.

I think what’s been happening (and I only realised this afternoon) is that adjusting head height and locking the z-axis between setting the chuck using a spigot in a collet, and then swinging a dti in the spindle to check concentricity, has changed the y-setting of head by a few thou. That makes it impossible to get consistent readings using that method, so I never stood a chance.

Initially setting the R/T to the taper, then removing the head spindle as a setting check eliminates that issue.

But of course, even now, with the chuck centred on the R/T to a thou or so, any work that involves subsequent z-axis movement is going to be inherently inaccurate due to my inability to properly setup the z-axis gibs.

I really need to find a proper small milling machine (easier said than done) which would make my model engineering life a lot more enjoyable.

 

 

 

 

[Dr_GMJN]

On Dr_GMJN Said:

No I wasn’t suggesting that.

[…]

 

 

 

 

Apologies then … I must have misinterpreted this:

Still had 0.004” error, which I’m beginning to think is inherent to the R/T; I’ve never got better than that since buying it several years ago.

… I’m probably just too weary to read stuff properly at the moment !

Pleased to know you have sorted things out anyway.

MichaelG.

[HopperParticipant @hopper]

You are putting the cart before the horse by trying to centre the RT under the spindle and then trying to adjust the chuck for concentricity to the RT.

First thing you need to do is set the chuck concentric to the RT by the method/s you have described, ie tapered spigot and then check with dial indicator mounted to the table or RT base. It could even be done on the bench or anywhere.

Second, check repeatability of concentricity of chuck to RT by turning it through a good number of revolutions, locking and unlocking the locks each time, and make sure it continues to run true. If it does not, you have some slack in the RT pivot that needs addressing. If it does continue to run true with the chuck plate bolts fully tightened, the RT/Chuck needs no further tinkering with. It is set for all time. No further action required.

Third, you mount the RT on the mill table and hold the job or same sized spigot in the chuck and centre it under the mill spindle. Lock the quill in your working position first. Mount the DTI on the spindle and measure concentricity of the spigot or job in the chuck. Adjust x and y table handles to achieve concentricity and lock table and check again and adjust as needed.

Any further adjustments to get the locked quill and spindle concentric to the job should by made by the x and y handles on the table. NOT by moving the chuck on the RT. Once set concentric to the RT, the chuck should never need resetting again. Ever.

That said, I note you have a 4 jaw self-centring chuck. Two possible problems with these:

One, many of them are made for woodworking lathes and are not made to a level of precision needed for model engineering work. Easy enough to check if there is a change in concentricity to the RT when holding different diameter spigots in the chuck.

Two, if the job being held is out of round, oval or egg shaped etc, three jaws will all contact it, four jaws will not necessarily. One can ride a bit high. But that is mostly of concern when gripping rough stock etc, not turned bar etc.

 

[Michael Gilligan]

On Michael Gilligan Said:

On Dr_GMJN Said:

No I wasn’t suggesting that.

[…]

 

 

 

 

Apologies then … I must have misinterpreted this:

Still had 0.004” error, which I’m beginning to think is inherent to the R/T; I’ve never got better than that since buying it several years ago.

… I’m probably just too weary to read stuff properly at the moment !

Pleased to know you have sorted things out anyway.

MichaelG.

Ah I see. Yes, that is in fact what I was meaning there:

At that point I’d been centring the R/T by rotating  a dti in the mill spindle, with the finger in the central R/T taper (not turning the R/T), Which was fine.

However, when I subsequently tried to centre the chuck by clamping a stub shaft it the jaws, and simultaneously in a mill spindle collet, subsequent checking by rotating the R/T gave a significant error. This, I atttibuted – possibly – to runout on the R/T.

I now think (pretty sure) that the action of unlocking and moving the mill head up – in order to clamp the stub shaft in the collet to centre the chuck – moves the head slightly in y. Therefore the chuck axis and R/T axes are not coincident.

That’s why I always got a runout error with the chuck fitted, because I always had to reposition the mill head to work through the process.

[HopperParticipant @hopper]

And that is why you should centre your chuck on the RT first, then leave it alone after that.

Once chuck is known to be concentric with the RT, you use the x and y table handwheels to move the job back to concentric with the spindle after any time you have to move the head.

[Hopper]

On Hopper Said:

And that is why you should centre your chuck on the RT first, then leave it alone after that.

Once chuck is known to be concentric with the RT, you use the x and y table handwheels to move the job back to concentric with the spindle after any time you have to move the head.

Is moving the R/T after every height change realistic?

What happens when your central datum isn’t accessible?

[mike robinson 2Participant @mikerobinson2]

In case this helps: Generally, it will take as long as it does with a 4 jaw chuck set up. With low cost R/T’s (i.e. Vertex or similar) the centre of rotation isn’t always exactly the same as the taper or plain sleeve centre in the rotary plate (error 1). When you mount the chuck on its base plate that won’t be exactly in the centre either (error 2) and finally the chuck jaws will, most likely, be somewhat eccentric (error 3). Finally there will be some ovality of the material, even silver steel (error4). Mount a piece of 3/4″ or 20mm silver steel in the chuck. Mount the chuck and back plate to the RT but don’t clamp it yet) get it roughly centred by clocking the outer chuck dia and lightly nip (finger tight) the mounting plate fasteners. Put the DTI in the mill spindle making sure you can rotate the spindle by hand without collisions. Keep the mill spindle stationary and rotate the RT while tapping the chuck to get it centred, in the same way as you would for a 4 jaw chuck ( + and – and X and Y). When you think its centred then having not disturbed the mill spindle or the indicator rotate the mill spindle to check your previous settings. No need to strain a muscle, jut nip up the fasteners, the friction will prevent movement. I only have these photos to hand……on the left its a slightly paralax view.

[Dr_GMJNParticipant @dr_gmjn]

Thanks all. I think I’ve got a good enough method of setting the chuck now.

[Dr_GMJN]

On Dr_GMJN Said:

On Hopper Said:

And that is why you should centre your chuck on the RT first, then leave it alone after that.

Once chuck is known to be concentric with the RT, you use the x and y table handwheels to move the job back to concentric with the spindle after any time you have to move the head.

Is moving the R/T after every height change realistic?

What happens when your central datum isn’t accessible?

It’s the only workaround you have while your mill does not hold position when moving the head or quill up or down. The only other alternatives would be get out the scraper, blue and dial gauges and overhaul the mill (not a beginner project) or buy a better mill.

If central datum is not available or the round job is not accessible to the dial gauge for some reason, you might have to work off the OD of the chuck body. Which is not necessarily true to the job held in the jaws on a wonky chuck, but if you test it and make a note of it when setting the chuck concentric on the RT, it could be done.

PITA but that is life with cheap machine tools, or old worn out ones. You become a fully fledged member of the Bodges R US club.

[JasonBModerator @jasonb]

As someone who regularly uses chucks on both of my rotary tables this is how I mount them.

The Soba has a MT socket, so I turned down a blank MT Arbor to leave a pin. Onto this I can slip various locating rings, the largest of which fits snugly into a hole in the rotary table’s mounting plate. So just put the arbor and ring into the R/T drop on the chuck with it’s mounting plate and screw it down to the table.

The Sieg R/T has a shallow parallel recess above the MT socket and the backing plate supplied has a matching close fitting socket which locate the two.

Then I just grip a bit of rod or reversed milling cutter in the ER collet, close the chuck jaws around that then bolt the R/T to the mill table and zero dials or DRO and I’m good to go. Works on all three of the Sieg mills.

 

EDIT if you really want to be chasing tenths then leave the spigot of the arbor that is holding the work oversize and mill to final dia on the R/T then it will be spot on to the R/T axis

[Howard LewisParticipant @howardlewis46836]

Being a total simpleton, I use Jason’s method to centre the backplate on the R T.

NOT suggesting that Jason is a simpleton, far from it  I just take the easiest way out.

Using a Morse Taper arbor to hold the backplate, in the lathe, I would turn a register to locate the chuck to the backplate.

Using these methods, The chuck will be centred on the backplate as accurately as possible (Ideally make the register size /size with the chuck register, so that it is a TIGHT fit) and the Morse Taper arbor in the R T will then centre the backplate on the RT.

Using a self centering 4 jaw, IMO, will produce no better concentricity than using a self centering 3 jaw; for the same reasons.  That’s why I don’t use one.

There has to be clearance between each jaw and the scroll, so accuracy will depend up the accuracy/ comsistency of the scroll pitch and the jaw to scroll fit.

With the scroll having to have clearance within the chuck body as well the scroll / jaw clearances, great accuracy, or repeatability is unlikely to be good enough for your requirements.

Clearnces are like tolerances, they add up to bigger nunbers than you imagine!

The more clearances that are involved the less the chances of accurate or consistent location.

It is acknowledged that the concentricity provided by a 3 jaw will vary according to the diameter being used; i. e. varying according to the consistency of the pitch of the scroll.

If you want to centre work under the spindle, forgetting any discrepancies in the mill and it’s spindle, the technique, surely is to use the morse taper / register method to centre a 4 jaw independant chuck on the backplate, (Which hopefully is centred on the R T which, in turn, is centred under the mill spindle) and then to clock the work and bring it to centre as one normally would with a 4 jaw independant

Any innaccuracies will then be those within the R T and the mill.

On the basis of “Greater fleas have lesser fleas”, ultimately the accuracy and repeatability will depend on those of the machines used to manufacture the Mill and R T.

So where do stop? Presumably where cost /budget and acceptable accuracy coincide for each one of us.

Howard

[Graham MeekParticipant @grahammeek88282]

I did mention early on a similar solution to Jason’s.

It is so easy to make such an adaptor plate, and once made it takes only the time to bolt the chuck onto the table to be concentric. This method stood me well for over 40 years.

Regards

Gray,

 

 

[Dr_GMJNParticipant @dr_gmjn]

Thanks all.

I put the spare MT I had directly into the ML7’s headstock spindle, and drilled it out to IIRC 5mm. It wasn’t easy to drill, but I got about 25mm deep.

Then turned the stepped stub shaft, the smaller diameter of which is a tight push-fit into the MT hole.

I assemble this taper/stub into the RT, grip the chuck on it the larger diameter, and tighten down.

Apart from ideally boring a larger hole into the MT (for rigidity) and using a series of stepped shafts (spanning a few different potential workpiece diameters), it’s a pretty good method the way I’ve ended up doing it?

At least it compensates to a degree the chuck inaccuracy, and saves having to centre the chuck on its backplate (not sure how I’d do this anyway since the mounting bolts are under the plate).

[Author]

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