Lathe Spindle/Chuck Concentricity

Hi Joe

For the sake of completeness I've taken a few measurements:-

Spindle Nose

Register nose dia = 1.2496"

Dia over thread = 1.1226"

No2 MT Adapter

Register nose dia = 1.2498"

Dia over thread = 1.1258"

Myford supplied blank

Register nose dia = 1.2497"

Dia over thread = 1.1240"

This bears out the consistancy in accuracy of the register claimed by Myford . The small variation in dia. over the thread is probably due the fact that I have never (before to day!) screwed anything on to the blank and only very infrequently onto the MT adapter whereas of course chucks go on and off the spindle frequently. In both cases I guess the thread crests on the adapters need to be eased ( polished?) a little as they are very tight at present.

The lathe is a very low use S7

Regards

Norman

Interesting. So, the blank and adapter are within +/- a thou of nominal thread size whereas your spindle proper is 2.4 thou under nominal. At least our findings agree that the range of variance is around 4 thou.

On balance, I still think it would be sensible to make a gauge that closely matches the target spindle rather than rely on the blank. Above all, you don't want to lovingly machine the register recess/female thread on the backplate at a single setting (relative to the blank) only to find that the thread is far too loose/too tight when fitted on the target.

Joe

Edited By joegib on 18/10/2012 17:44:22

Joe

Surely though if the register is right and the chuck / faceplate / backplate etc. is a really close fit on this register then the small amount of clearance in the thread is of no consequence once it is screwed up tightly? Does the screw thread have any function other than to guide the chuck on to the aligning register and then hold it back against the shoulder ?

I agree about the blank - I bought it thinking it might be useful for something but to date I've not really found just what that useful might be!

Norman

Am I mistaken? I thought that the dimantions given were all less than 1thou out on the register. If I've read them correctly they were all on the minus side and were 2tenths, 3tenths and 4tenths down repectivly.

Now I've been heard to boast that I talk in tenths and work in sixteenths, but I hope that I would notice 2.4THOU down.

Alan.

Well Alan

I can't vouch for the accuracy of my mic.but I guess that ,even if it is not as accurate in absolute terms as it apperars to be, a variation of between - 0.0002" and - 0.0004" from the nominal 1.25" does seem to confirm Myfords claim that "registers are held to very close limits" !

Norman

I must have a look, a number of years ago there was an Artical in ME that discussed spindle noses of different makes of lathe, there were a number of different means of location. Ian S C

I must have a look, a number of years ago there was an Artical in ME that discussed spindle noses of different makes of lathe, there were a number of different means of location. Ian S C

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My old 15/90 was made for high precision work on the UK Thunderbird missile program of the 1960s

They use a square key protuding from a short nose cone. Behind all this is a loose ring finely threaded on the inside which cannot fall off

The chuck fits onto the nosecone and the male thread is on the back of the chuck

The loose ring then screws it all together from the back

It's all quite fiddly and not at all easy to keep clean

**LINK**

Model 15/90 used a captive screwed ring behind the spindle-nose flange that resembled a miniature version of the American L Series long-taper key fitting employed on industrial-size, general-engineering lathes.

Lathe Spindle fittings

**LINK**

http://www.lathes.co.uk/latheparts/page8.html

Edited By Ady1 on 19/10/2012 10:41:29

Alan,

In the first para of my post at 17:42 I referred only to the thread size, not the respective register sizes. If we take the 'nominal' thread size to be 1.1250", Norman's actual spindle thread size at 1.1226" is 2.4 thou under nominal. Further, Norman's MT2 adapter has a thread diameter of 1.1258" and the thread diameter of his lathe spindle is 1.1226". Thus, the difference between the largest and smallest thread diameter in his set of devices is 3.2 thou. This is roughly comparable to the thread size variance of 4 thou found among my own 3 specimens.

We're all agreed that Myford register sizes in the tested specimens match or very closely approximate to nominal.

Norman,

I agree that we want an 'easy' thread fit between the spindle and backplate threads so as not to constrain the recess from seating correctly on the spindle register. But it's a question of degree.

Take the case of of a novice presented with your equipment, charged with preparing a backplate and who, crucially, takes your Myford spindle blank as a reliable gauge. Relative to your spindle blank's thread size of 1.1240" lets say he adds a couple of thou for clearance, achieving an effective size of 1.1260". When offered up to your actual spindle nose (thread size 1.1226"  the backplate will rattle home nicely but the overall thread clearance will be 3.4 thou. Arguably, that's too much but might well be acceptable.*

Now take my case. Relative to my spindle blank thread, sized at 1.1210", the novice likewise adds a couple of thou for clearance, achieving an effective thread size of 1.1230". Trouble is, my lathe spindle is sized at 1.1250" so when presented with this backplate, it just ain't gonna go! Poor old novice is going to have to remount the backplate, somehow pick up the thread and pare or chase off another couple of thou, while praying that he achieves a fit at the next removal.

All I'm saying is that these spindle blanks can't be treated as gauges when preparing backplates unless there's close correspondence in the measured thread sizes of target spindle and blank. If not, then I suggest it's preferable to prepare a gauge based on the actual target spindle.

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* I'm a cut and try kinda guy so I'd actually do this job using a reliable gauge and a process of fit and feel rather than measurement. It may be that the clearances I've used in the above examples are unrealistic in real world terms. I'm happy to accept correction on that.

Joe

Edited By joegib on 19/10/2012 10:56:14

The articals in ME: 1998 Vol 180 no., 4067 by Don Ainsley,"Machine Tool Spindles". Catalogues the types of spindle in common use. Lathe spindles (English), Lathe spindles (American). Milling Machine spindles.

Year 2000 Vol 185 no., 4134, Letter to a Grandson, by M.J.H. Ellis. Ian S C

Hi Joe

"All I'm saying is that these spindle blanks can't be treated as gauges when preparing backplates unless there's close correspondence in the measured thread sizes of target spindle and blank. If not, then I suggest it's preferable to prepare a gauge based on the actual target spindle."

Yes I agree with you but  I didn't actually suggest this - Andy did . This was before I measured it and, had the dimensions been the same as my spindle nose, then it could have been used for that purpose. That was not the reason I bought it however. Without a clear purpose in mind I just thought it might be useful to mount my chucks on  some future project. - and it was cheap! I still don't have a project in mind but, who knows? - Maybe one day it will be just the thing I need. At the moment though it's in my study acting as a paper weight - perhaps that's its destiny!

Regards

Norman

Edited By NJH on 20/10/2012 17:25:05

Dirt in the thread will throw off the cocentricity which the register will limit to within its tolerance of accuracy but the thread does play a role if you let it. It's more important the thread is concentric with the register rather being a few thou undersize. It's also important that the rear surface of the back plate is true where it butts up to the vertical surface of the spindle. It really depends what standards of accuracy you're happy with. The register keeps things within a certain tolerance, perfectly good for the vast majority but it is theoretically possible that with a true rear surface and clean well formed thread on chuck and spindle that the register doesn't touch the inside of the backplate at all. If there's a half thou clearance between spindle register and backplate, there could be a quarter thou clearance all around once fitted as the thread does the centring.

Agreed — on a theoretical level — in which case the register could be dispensed with. Indeed, such high precision could be expected if the lathe is a Hardinge (the lathe maker cited, I think, in the thread I referred to) and if, say, Pratt-Burnerd is the supplier of the chuck /backplate combo specified to match that spec.

In reality, many of us find ourselves with Myfords of variable thread size, needing to machine backplate castings for East European or Far Eastern chucks. Even with our modest equipment (outside and inside mikes are fairly cheap) we can achieve easily measurable close fits between register and recess. It's not so easy as between male and female threads. Perhaps skilled people can do it by fit and feel or using fancy measuring equipment. As Michael Williams suggested early on, the practical approach is follow Myford's lead by using the register.

Of course, much of this is academic. A 'good' 3-jaw chuck is said to have no more than 3 thou TIR. So any gains in concentricity at the register/recess or thread-to-thread interfaces will probably be lost at the chuck jaws. 4-jaw chucks don't care about register/thread eccentricity. Nor do collets using the lathe's native taper. Hence, no doubt, the counsel of perfection that if you want utmost concentricity, use a 4-jaw chuck!

Joe