Thread: Truing a chuck back plate

A forum member sold me a nice 4.5" Taylor conical lathe chuck, on a Hercus style back plate.

Its jaws feel tight, with very little slop against the scroll, so it should be much better than my worn out Pratt 4.25", or my "has runout from new" TOA 125mm.


However, when mounted up on my Hercus 9C, it consistently has 7 thou run out. I marked the side that is high. I can remove it from the spindle, clean the threads, spin it back on, and that marked side is still 7 or 8 thou high.

So, time to investigate. The back of the plate has a nick where some of the iron has been chipped away:
IMG_2328.jpg

so I thought that might just need truing up. Take it off the chuck:
IMG_2329.jpg

It was a good tight fit. Strangely, there was lots of oil in there!
Note that the bolt holes are not symmetrical. Note also the inside of the chuck:
IMG_2331.jpg
It has a raised lip near the shoulder edge, that the flat of the back plate locates onto.
Stamped into the chuck is PLATE MUST CLEAR THIS FACE.



I screw the back plate onto the spindle, and do some tests:
IMG_2330.jpg
50 microns axial run out on the edge of the spigot. So, that might be 2 of my 7 thou?


I also measure the flatness of the outer face, where it bolts onto the back of the chuck. 180 microns, or 7 thou wobble. OK, that is the issue. I could just machine it flat now, but I figure the chip on the back should come out first, so screw the back plate on backwards...
Except that it won't screw on all the way. Too tight.
Only goes on about 3/8" before I decide to stop forcing it. I check the wobble:
IMG_2332.jpg
55 microns. Different to when screwed on using the register.



For a while, contemplate machining that little surface with the plate only half screwed on, but decide on a plan B - the surface grinder...


IMG_2333.jpg 30 microns removed

IMG_2334.jpg 50 microns

90 microns IMG_2335.jpg and it is flat, relative to the other side.


Try back on the spindle:
IMG_2336.jpg IMG_2337.jpg
60 microns axial wobble (slightly more than the 2 thou before), and 80 microns lateral wobble (down from 180).


The lateral wobble is easy to correct - just machine off in the lathe. The axial wobble is a little harder. I'm trying to avoid machining too much off the face, so I try to correct the wobble by grinding a slight slope on the register-locating face of the back plate.

66234400554__15326891-F9F8-46A0-88E1-12B28BBA429C.jpg
Note the sheet of paper on one side. Sadly, this actually made the axial wobble worse -
I think I might have been removing metal from the wrong side - so I had to re-do it.
Sadly, it made no difference. Still 2 thou axial wobble on the locating spigot. Sigh. Go home for the day.



Going to have to do it the sensible way. Re-machine the bolt-attaching face:
IMG_2339.jpg

mount the chuck up to test. Just how tight should I torque a vintage chuck?
IMG_2340.jpg
About 6 thou TRO. Not much better.

Going to have to re-cut the spigot:
IMG_2345.jpg
I made the bare minimum cut - about 1mm off the outside, and 0.5mm from the spigot face.


Re-mount the chuck. Two of the bolts aren't going in smoothly - the two that were on the high side:
IMG_2346.jpg
I wonder if that was a problem all along, twisting the chuck a tiny bit?
File the holes a little, find 3/8 NC tap and die to clean the bolts and chuck threads. Attach chuck.

There is about 21 thou gap between the chuck and the part of the back plate where the bolts attach, which I might later correct (by machining 17 or 18 thou off the flat of the locating spigot), but this does allow me to spend time carefully tightening the bolts to minimise run out.
I started by correcting the chuck body, then refined with a ground rod in the chuck.
End result:
IMG_2351.jpg
23 microns. Less than a thou!!!




So, this is a pretty good old chuck, but I am perplexed that the back plate fitted so badly.



Then I had the thought to check the register.
Using an old (uncalibrated?) 2" mic., my Hercus spindle is 1.471":
IMG_2348.jpg
but this back plate's register is a little larger:
IMG_2349.jpg IMG_2350.jpg
at 1.488",


so I wonder if this was from a Southbend, Myford, or Boxford?

The so-called register on Hercus 9/10 inch lathes is not used to centre the chuck.
I came to this conclusion after measuring, as you did, the diameters of the register area of the spindle, and the corresponding area on the chuck, on both my Hercus lathes. There is clearance, not a close sliding fit as would be expected if it was doing anything to keep the chuck and spindle on the same axis.
However, something does do this, and it must be the spindle threads - actually the angled flanks of the thread faces that contact each other.
I read about this on a US forum about South Bend lathes.

The registration of the Myford lathe chuck was discussed years ago in the British model engineering press, and they came to the same conclusion as described by nadroj.

I delved into this subject when my Sheraton 9A had a 4-jaw chuck that would not register correctly.
A slight bore adjustment cured it.

nigelpearson,
Great detective and correction work.
Congratulations on using the 21st Century dimension "micron" so profusely.

The best to all for the New Year,
John.

Originally Posted by electrosteam

Congratulations on using the 21st Century dimension "micron" so profusely

Well, all my lever DIs, and the borrowed surface grinder, are metric.
If I had an imperial lever DI, the language might have been different



P.S. my toolmaker friend says that if I can't measure Microns,
I shouldn't talk about them?
...but I ain't gonna start throwing "hundredths" around




Happy Gnu Deer (er, New Year!)

Id probably just make a new backplate where all the registers fit properly and you can trust it? But if you want to save that one i would spot face where all the bolts sit round the back so they are square too.

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