Installing a new lathe chuck

I'm buying a new lathe chuck for my S7B, and the one I want is not available in Myford fitting so I'll have to machine a backplate for it. I've done this once before years ago but am considering the job again new. It looks like a suitable 5" backplate is available pre-machined to fit the spindle (Chronos list one), and while last time I machined everything from a raw casting this might be a time-saver if the quality is good.

The chuck is a TOS 5" 6-jaw, selected mainly because I do a fair bit of work on alloy tubes and a 6-jaw is better suited to gripping thin tubing. I would love a Griptru Burnerd 6-jaw chuck but the price new is silly (over £800 new), and I already have a 5" TOS 3-jaw for the S7B and I've never had an issue with the build quality of it.

I was thinking for alignment, after machining the backplate register to fit the recess, I could put a 2MT test bar in the headstock and grip that with the chuck jaws while the holes are spotted through (it's a front mounting chuck), remove the backplate for drilling and tapping, then re-mount and use the test bar again to hold the chuck whilst the bolts are tightened.

I know I should be relying totally on the rear machined register of the chuck to provide the alignment datum point but then thought it might be an idea to use an actual workpiece clamped in the jaws as being a more representative/useful datum? In fact, if the backplate were machined a couple of thou undersize there would be room for subequent adustment should the chuck start to run out after a period of use. I figure the chances of it shifting in use are minmal with 3x8mm bolts fully tightened.

If anyone has their own ideas as to how to go about this job for best results I would be interested to hear them.

Chris

Edited By Chris Heapy on 01/06/2013 10:23:45

Chris,

Unless I have misunderstood why would you need to use the 2MT Test bar when you will have machined a register for the chuck to locate on.

Martin P

Given the ingenuity you have shown on other threads how about designing your own version of the Griptru adjustment mechanism,
Since you will have to allow some slack in the jaws holding the bar in order to slide it you will have lost someof the advantage already. I think in the end the usual dial indicator and gentle tapping is just as good.

I would go with Micheal and machine a tight fit with the register, also put an alignment mark on both chuck and backplate so that you put the two parts together exactly as you have set out the tapped holes.

I specifically want the 6-jaw for holding alloy tube which tends to distort in a 3-jaw, but it will come in handy for other work too - particularly delicate rings and discs. Yes, there are alternatives and I've been using them for years - which is why I recognise their limitations and want the chuck.

It isn't too heavy for the S7, especially with fairly lightweight workpieces. I would be reluctant to run it at top speed perhaps but all in all the 5" 3-jaw I posess has performed faultlessly and is the same weight. I would rather not have the added overhang caused by fitting a backplate but it's unavoidable.

The register on my existing chuck I machined the backplate a close fit in its recess so it cannot readily be adjusted by the 'loosen the bolts and tap with soft mallet' method. That is why I was thinking of not having it such a close fit on this one and indeed having that technique available to me. I think we have to assume a decent chuck is machined axially true - there isn't much we can do about it (well, not easily anyway).

Martin, the register is only nominally concentric with the gripping circle it may not be optimal in a 'less than premium' chuck. The quoted accuracy is 0.025mm TIR and there is no Griptru-type of adjustment. If it were a Burnerd I would be more inclined to trust it (but note that even with such a premium chuck there is apparently a benefit in having it adjustable - Griptru/Set-tru etc.) There may be something to be gained by tweaking the mounting - but quite possibly not if the run-out is not stable at all diameters as can be expected on a cheaper chuck.

Bazyle - you've stirred the memory now! I seem to recall a method for a DIY Griptru ajustment published years ago. I'll have to search the archives.

I suppose I should add that I tend to use the 4-jaw a lot for this type of work, and whilst a definite improvement over a 3-jaw SC chuck (an extra jaw for support, better accuracy) it is slow to set up and there is less 'feel' about whether you have over-tightened the chuck, additonally much more work if you need to remove the workpiece and re-mount it a few times. Cutting threads on tubes/rings for optics is the main job here - lots of different diameters, some fairly thin tubes, precise threads required that don't take well to being squished in chuck jaws. I'm not looking for perfection - just an improvement and ease of use.

I've just fitted (literally!) a Zither 4" 3-jaw SC chuck from Arc Euro Trade to my mini lathe. I had to turn the front register on the backplate. It was tough steel that turned well, and also tapped nicely for the M6 threads. I used quality 25mm stainless screws with the heads turned off for the backplate to spindle.

It came with a test chart that doesn't give chuck specific dta but confirms that the chuck is within the spec given. This is 0.03mm (>1 thou) on the outer body diameter and 0.080mm runout (3 thou) at 50mm from the chuck if tightend with the outer pinion.

I had hoped the body would turn dead tru, but it has an eccentricity of about 0.0005" - still better than spec.

I first tried the 50mm test with a 3./8" stainless bar, but it was a bit battered and I got a pretty dismal result about 8 thou

I repeated the test witha clean 1" silver steel bar pushed right home into the chuck. I was astounded to see no perceptible movement of the DTI needle at all (my DTI is a quality German one and reads to 0.0005" graduations). This is better that the (still very good in my opinion) 80 mm Chinese chuck supllied with the lathe.

I'm very chuffed (gobsmacked!) and all I can say is if you have a reasonable chuck, make the backplate as close a fit for the chuck as you can.

Neil

The problem with relying on the chuck manufacturers precision of concentricity is that you are limited by it. So in my opinion Chris, you'll be OK undercutting the register by a few thou' and doing the final centering with a clock to obtain whatever degree of accuracy you want. I do this myself and have never had a chuck move on its backplate, despite quite heavy cuts.

Martin.

Chris

Have you considered THIS or something similar?

Norman

Looks like Norman wins the prize I think that would do just fine - provided it doesn't add to the overhang too much.

Chris

Bazyle wrote "Given the ingenuity you have shown on other threads how about designing your own version of the Griptru adjustment mechanism"

The concept rings a vague bell but I have tried unsuccessfully to search the archives and find the post.

Hoping someone will fill in the blank(s).

Thanks

Dunc, I too have seen it somewhere, but can't remember where.

There is a similar but less complicated method here: **LINK**

Andy

There was a construction article in ME , Dec 1991, about making an adjustable backplate with micro adjustment screws. The article was titled "Super Con". Could be one way to go.

Michael

Are single copies of ME that old still available? Or a reprint of the article from the publisher? Don't want a digital subscription for just one article.

Found this reference

http://warhammer.mcc.virginia.edu/ty/7x10/vault/Lathes/7x10-7x12-Projects/SetTruBackplate/

but the one I vaguely recall was much swimpler to implement

If your primary objective is to securely hold tube without damage, and you have some ER collets, then have you considered these **LINK**?

Just get the hex version and hold it in your 3-jaw - its much cheaper than a 6-jaw.

If you've bought the chuck already, why not fit it to a close fitting spigot and then check it's accuracy? Why go to the bother of more complex solutions if it turns out to be OK as it is? Also this would let you know the basic accuracy of the chuck and you can later reduce the register to allow the chuck to be moved if you want, but you can't make it bigger!

Neil

Ooo - new kit arrived

and this is the type of thing it will be asked to hold for machining. Just waiting for a backplate to arrive now.

I fitted the backplate and tested the chuck this afternoon. Maybe I was just lucky but there is negligable run-out near the chuck jaws and about 0.06mm TIR about 4 inches out. I don't know that I could make any improvements on that so I'll just use it for a while and check again.

Another attempt at a video

Edited By Chris Heapy on 07/06/2013 17:24:15