Chuck backplate error

[Chris VParticipant @chrisv]

Good morning all.

Recently I attempted to make a second hand chuck backplate a better fit to my lathe. I have made several from scratch successfully but this did not go as planned. I turned too much out of the Rear of the backplate Recess where it fits over the register on the spindle nose. So it threads on with a wobble but sits tight up to the shoulder on the spindle. With the chuck fitted, its a 4 jaw independent, the front face runs pretty true.

In use is this going to be an issue or should I start again?

Cheers

Chris.

[Chris VParticipant @chrisv]

Whats the important bit?

[Tony Pratt 1Participant @tonypratt1]

Impossible to tell if a heavy cut would shift it, try it out and see how it goes.

Tony

[not done it yetParticipant @notdoneityet]

Do you really need to start again? Bore out, sleeve and re-machine?

[Chris VParticipant @chrisv]

Thank you both. Yes I suppose I could try boring it out.

What would be the best material for a sleeve, I have brass in stock but guess that would wear more quickly….?

Mild steel or should it be cast iron like the backplate…or bronze if I have to buy anything?

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Chris, well you only need to bore out the register part and just put a ring of steel in and re-bore the register to size, a piece of tube would do if you have some large enough.

Regards Nick.

[Chris VParticipant @chrisv]

Thanks Nick, Ok seems sleeving it is the way to go so I will give that a try when I get some time.

Many thanks all,

Chris.

[old martParticipant @oldmart]

It is the centralising of the thread against the vertical face of the spindle that makes a backplate repeat its location every time it is refitted. We have at least 8 choices of backplates for the Smart & Brown model A with its 1 3/4" X 8 spindle thread. The register clearance varies from 0.0005" to 0.020" and they all repeat perfectly. The only exception to this is if your lathe spindle has square threads, they rely on the register being a good fit.

[Chris VParticipant @chrisv]

Thank you old mart.

A member very kindly offered me a slice of CI to sleeve it so I went and checked what size I would need. But on further investigation I found the thread was more undersize than the register I had cut, so the thread was making it all seem way too far off. The chuck & backplate are second hand, sold as to fit a Drummond and as there's different size threads for these its no great surprise it didn't fit. There was no register turned in the backplate when I got it, so I put it down to another learning experience.

But I have a new Myford backplate on the shelf which fits well, so I will use that.

Thanks again

Chris.

[Martin ConnellyParticipant @martinconnelly55370]

The lathes.co.uk web site states that the register is not too important for the repeatability of the backplate position. I can't remember exactly how it's worded but it does support old mart's view.

Martin C

[Tony Pratt 1Participant @tonypratt1]

Posted by Martin Connelly on 06/01/2021 17:01:09:

The lathes.co.uk web site states that the register is not too important for the repeatability of the backplate position. I can't remember exactly how it's worded but it does support old mart's view.

Martin C

There has been many an argument about this & I am in the camp of the back plate controls concentricity & the screw just holds things together, others will differ

Tony

[david bennett 8Participant @davidbennett8]

Chris, if your backplate goes on with a wobble it suggests your backplate thread is "drunken" This is very bad for repeatability.

Dave.

Edited By david bennett 8 on 06/01/2021 18:39:50

[old martParticipant @oldmart]

The wobble is a concern, but all backplates should be screwed up several times and then the front face skimmed till true. The only time I didn't have to true up was when I bought a 9" faceplate for the museum's Smart & Brown model A from a seller on ebay who had a low starting price, but had no idea what he was selling. One of his photo's showed the casting number and I took a chance and got lucky. I fitted it to the lathe and was just going to face it off, when something made me decide to check it first, it had a tir of 0.0005".

So few people have a first hand experiance of actually running registers, but there are many experts about.

Here is a list of the chucks and faceplates used on the museum's lathe.

6" Toolmex lightweight 4jaw independent. 160mm Chinese 4 jaw independent.

er25 collet.

100mm Pratt Burnerd 3 jaw scroll.

125mm Chinese Huhhot 3 jaw scroll.

2 off 5" Pratt Burnerd 3 jaw scroll.

160mm Chinese 3 jaw scroll.

6 3/4" Pratt serrated 3 jaw scroll.

7" faceplate with bolted on boss.

9" faceplate with bolted on boss.

9" faceplate.

As already mentioned, the register clearance varies from 0.0005" to 0.020", and the tightest one is hardest to fit.

The toolmex and the 5" PB's also fit on the rotary table without their backplates fitted.

[david bennett 8Participant @davidbennett8]

The point re. a drunken thread is that is not possible to turn the vertical register true to the thread at the same setting. This is essential. The horizontal register is not important.

Dave.

[old martParticipant @oldmart]

I can't figure how the op managed to produce a drunken thread, or maybe it is just a bit loose.

[david bennett 8Participant @davidbennett8]

A loose fit is unlikely to produce a wobble. Dave.

[old martParticipant @oldmart]

Re reading the first post still leaves me guessing what exactly went wrong, it was not his first backplate.

I have made several, including 1 from billet and 2 from rough castings. I prefer to make them from Boxford backplates as long as the boss has enough meat in it to produce 1 3/4" threads and register. The less machining cast iron the better, messy stuff. The threads, register and rear face are all produced without any disturbance or removing the backplate, so everything is in line. I have removed metal from the rear face on 3 backplates to match them up for the reverse locking mod on 4 backplates, and the front faces only needed about 0.001" tir cleanup.

[Chris VParticipant @chrisv]

Hi Old Mart, The chuck back plate is second hand.

I have not touched the thread, which I have now discovered is a little over size for my spindle nose thread. (I don't know why this is, maybe it could be worn, or maybe its previous owner had a slightly larger spindle, either way its a bit bigger than on my other chucks). So its looser when screwing it on the spindle than my other chucks.

The back plate did not have a rear register recess turned into it which I knew was incorrect so I turned a recess in it aiming for it to mate well with the spindle register…however I took a tad too much off. Now the back plate screws on and sits tight and square to the spindle register, however I know that the recess I turned is slightly too large a diameter. Added to the over size threaded section I was concerned it would not be sufficiently accurate enough once I get to do some fine tolerance work.

Sleeving the recess I made would solve part of the issue but the thread would still be a little loose and of a little concern to me. So as mentioned I will put it on one side for now and use a new factory supplied back plate that fits my spindle well. This way I can have confidence in my chuck, ….if not my turning ability!

Hope that clarify's things for you.

Cheers

Chris.

[old martParticipant @oldmart]

I would remove the chuck and check for runout on the fromt face and for radial runout in any register shoulder. Remove the backplate and refit several times and check whether there is any measurable change. The mating parts will have to be clean and well lubricated to be sure any foreign bodies do not influence things. If the backplate repeats well, then fit the chuck and use it, then take it off and recheck the backplate. You may be lucky and save a lot of rework or replacement.

[Ian HewsonParticipant @ianhewson99641]

I may be wrong, but I always understood the the registers on the spindle nose were for alignment, the thread was for holding the Chuck on only.

[old martParticipant @oldmart]

The rear shoulder is vital for alignment, but as long as the thread is vee shaped, it will bring the backplate into the same position radially each time. The wear of the threads will eventually cause variations, but that would be after thousands of fittings considering the very coarse type of thread used. I do check the alignment every time a chuck is dismantled for cleaning and have never had to re skim the front of the backplate. I have made several backplates and like to allow 0.001" clearance as tighter fits are hard to assemble and remove. The looser ones up to 0.020" have been bought of inherited, but repeat just as well.

Edited By old mart on 08/01/2021 18:05:50

[Anders BeParticipant @andersbe]

I like to add a smal aditional question I'm thinking about: How does the clearance affect the bending stiffnes of the chuck (i.e stiness from the moment arised on the chuck from the cuting force).

Best regards

Anders B

[HopperParticipant @hopper]

Posted by Anders Be on 08/01/2021 19:00:53:

I like to add a smal aditional question I'm thinking about: How does the clearance affect the bending stiffnes of the chuck (i.e stiness from the moment arised on the chuck from the cuting force).

Best regards

Anders B

It doesnt have any effect.. The thread rides up on the V form of the thread and against the register face on the spindle and stays there under normal cutting forces. It is best to have some healthy clearance on the thread to make getting the chuck on and off in a swarfy environment easier. (Is swarfy a word?)

[LathejackParticipant @lathejack]

It is the two registers on a threaded spindle nose that are really intended for location, thats why they are there. The radial register for centralizing and the face or shoulder for squaring things up, the thread is just for securing the chuck, backplate or whatever on, but a close fitting thread will help centralize if the radial register has too much clearance.

There are plenty of lathes with spindles that have an integral mounting flange with just the face and radial register for location, but using separate bolts instead of a threaded spindle for securing.

Chris V said that the front face of his chuck runs true, which it will once the backplate is butted up to the spindle shoulder, but because of the sloppy fitting radial register and thread it is unlikely to centralise repeatedly.

 I have had my Smart and Brown Modal A for over 20 years now, and all new backplates machined to fit it have a little clearance on the thread but a close fit on the register to repeatedly locate them accurately. All machining of the backplate shoulder face, internal register and thread are done at the same undisturbed setting.

Edited By Lathejack on 09/01/2021 04:18:30

[Danny M2ZParticipant @dannym2z]

Maybe the interesting article in MEW # 292 ( April 2020) by Jacques Maurel re: Adjustable 3 Jaw Chucks could be of use for people seeking precision?

Given all that, I found that soft jaws are an easy alternative solution and easy to make.

* Danny *

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