Screwdriver
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Not clear from your description how the back plate fits to the dividing head. The most important part is the registration of the chuck to the precise centre of rotation of the dividing head. If there is a stepped register in the dividing head fitment, then your PCD should ideally be machined in the same process. So if you are machining the register on a lathe, typically a machinist would mark out the PCD in the same setup to keep everything concentric.
If the three mounting holes for the back plate define how the chuck is registered then clearly they need to be fitted prior to working out how to register the new chuck/backplate as a component fitting to the dividing head. I don't understand from your post where the four holes "mounting to the T slots" are. Unless I am misunderstanding, if they're bolting into a T slot, they can be as sloppy as you like
If the three mounting holes for the back plate define how the chuck is registered then clearly they need to be fitted prior to working out how to register the new chuck/backplate as a component fitting to the dividing head. I don't understand from your post where the four holes "mounting to the T slots" are. Unless I am misunderstanding, if they're bolting into a T slot, they can be as sloppy as you like
