Thread: Screw on backing plates etc.

Yes

Was that yes to all of the above,or to something specfic.

Originally Posted by pipeclay

For this example I would expect the clearance to be only .0005"to max .001".

I have much more clearance at the register (the small unscrewd portion) than that, but the chuck appears consistently concentric when screwed up. I don't know just how concentric, and it's possibly not meaningful to try to find out. A chuck isn't a collet.

Even if the thread is also loose, I believe it will pull itself into line, when it butts up to the shoulder - not that I'm advocating a sloppy fit.

I was glad to find out that however badly out of round a chuck might be, it can still make accurate parts, if all the important machining can be done without having to remove and refit to the chuck.

Jordan

Thanks Pipeclay for your request on the subject.
I have been pursueing this subject for a long time & can give you some first hand experience with my Hercus model 260.purchased new from Hercus 20 yrs ago.
The outside diameter of the shoulder of the threaded headstock spindle is 1.750"
I have 2 backing plates made by Hercus that came with the lathe they are fitted to a 6 inch dia Rohm 3 jaw self centreing chuck & a 4 jaw chuck.
The recess in the backing plate measures 1 756"
The 3 jaw chuck has a repeatabilty of .002" on each set up.
I also have a Burnerd Multisize collet chuck & it is fitted to a Hercus made backing plate with a bore register of 1.756" it has a repeatablity of runout of .0005" or less (thats half a thousandth of an inch or better)
Also I checked the bore register of a drive dog plate & a faceplate (Hercus manufactured) & they both meausure 1.756" dia , so I think this must be the standard bore that hercus make their backing plates to.
I have a 4 inch dia "Fuerda" brand 3 jaw self centreing chuck that I machined the cast backing plate for myself. I made the bore register on it is 1.752" dia & I screwcut the internal in the lathe.
The repeatability of this chuck is .002"
So in summary I can say that all of these backing plates were mounted on the spindle thread, up hard against the shoulder & originally machined to be a close wringing fit into the back of their respective chucks.
It would seem not too critical to have an extremely close fit or tolerance on the thread bore register ( the backing plate to headstock spindle) to maintain accurate repeatable concentricity readings.
For extreme accuracy & second operation work I use a collet set up in the 4 morse taper bore of the lathe with adaptors to take either 4C or 3C collets & a draw tube. These wiill repeat a concentricity of .0005" half a thou or better.
I hope this will be of some assistance to you.
regards
Bruce
ABRATOOL

Thankyou to all,I was under the impression that the fit between the unthreaded section of the spindle and the recess in the adaptor plates/backing plates had little or no effect on the repeatability of the work holding method being used,these answers have at least confirmed what i think.
This comes about from my attempt with the screw on collet chucks.