I'm happy to report that Enco's Interstate 6" 3-jaw chuck PN 270-4020 and Shars fully machined 6" backplate with 1 1/2-8 threads PN 202-1609 are a match. The hole circles, bolt holes and number of holes are the same. I had to cut about 3/8" of the shoulder off , bore a bit of the threads out in order to match my spindle then flip it face it off and cut the shoulder to match the chuck. I also had to make a sleeve c. 1/4" wide in order to properly machine the threads spindle interface.
When it's all done I've a 0.004 TIR, I might have done better if I'd known then what I know now as I didn't initially give quite enough relief for the unthreaded portion of the spindle hence wasn't snugging up to the spindle shoulder tightly. I'd already machined the chuck side of the back plate before I realized the problem and had to remachine both sides. I've still got about 1/16" to play with so may try to get it closer but it's so much better that the chuck that came with the lathe that I'm tickled.
Mike
When it's all done I've a 0.004 TIR, I might have done better if I'd known then what I know now as I didn't initially give quite enough relief for the unthreaded portion of the spindle hence wasn't snugging up to the spindle shoulder tightly. I'd already machined the chuck side of the back plate before I realized the problem and had to remachine both sides. I've still got about 1/16" to play with so may try to get it closer but it's so much better that the chuck that came with the lathe that I'm tickled.
Mike
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