I bought a two 5" back plates for the Select recently, both were Myford, and for the very planning you had, a single pass bores the old threads out and one small finishing pass brings them to the 1 1/4" pre-thread bore.
?I also did something I'd never done before, I made a test spud with both the register diameter on one end and the spindles threads, using a thread mic to a very close fit, on the other end. This was used when creeping up on both in the back plates, they are without a doubt the best fitting back plates I've ever made.
?Both the 4 jaw and the 3 jaw were extremely cheap.
?To turn the 3J plate into a "Adjust-Tru" type I expanded on an idea I saw on youtube, a guy made the register of a purchased "made to fit" back plate another 3/8" longer and threw away the dust cover inside so it would fit.
?That gave him lots of room to have the four 3/8 adjusting setscrews something to push against, as the register on the original was only 1/8" or less.
?I knew what he didn't seem to be worried about, that without the dust cover the pinion gears and the crown gear at the back of the scroll would get plugged with swarf quickly! So I re machined the dust cover to leave it in place with a much smaller diameter, which left lots of depth outside of that in the chucks back recces.
?Then I trepanned a recces and made a lug ring to replace the register, 1/16" deep but leaving 3/8" sticking out for the setscrews to push against.
?This ring was a light press fit into the face of the backplate and is held in place with 4 flathead socket screws from the back.
?Not difficult? to do but very effective!
?The OD of the of the ring I then turned down .015" smaller than the original register so the chuck could be adjusted. Also, the front of the ring almost touches the back of that dust cover, so no swarf can get in to the gears.
?But, (Back to the Dalton back plates) you have said in the past that Dalton threads varied in pitch diameter, maybe as taps wore? Anyway, that could make problems, right?
?Can you take a thread mic to a bunch of Dalton spindles and see how much they vary, that and the spindle's register? I'd like to have a drawing if possible, the spindles themselves would be better.
