Myford set up.Time to get it right.

Paul,

The 'made for myford special chucks' do not have a backplate in the conventional sense, what you are assuming is the backplate is in fact an integral but threaded part of the chuck, as shown in the picture below.

You take it apart by undoing the socket screws and splitting the two parts with a hide mallet.

if inconsistent or large TIR is an issue for you the easiest route is to purchase a new chuck (of the non integral variety) and new unmachined backplate, machine said backplate on your lathe to mount your new chuck. dont expect less than 3-6 thou TIR as a repeatable error (you might get better - just don't expect it!).

Your other more time consuming choices are:

determine the cause of the error -

is it badly worn jaws - if so you could attempt a jaw re-grind to minimise the runout, you should be able to reduce it slightly but not remove it entirely (google / utube is your friend here)

worn or damaged scroll - you are stuffed - you can tell by inconsistent runouts at differing diameters if runout is a problem you will need a new chuck

deformed , (ie bent off axis) chuck mount (hard to believe but entirely possible- that was the problem with the one in the picture) - you can tell by putting a dti on the back of the chuck and check for axial wobble - you may be able to effect some sort of correction by shimming the downside on the rear chuck part.

Thanks for that Martin.Im off to buy a new chuck right now!