I think only you can decide because you need establish the nature of the majority of the parts you are going to need to divide, therefore the chuck best suited to the largest range of them.

However, you could add index holes to the other two back-plates to allow dividing with any of the chucks .

+1 for Nick Clarke 3.

Might it be possible, having fitted it to the 4 jaw, put another backplate into the 4 jaw, centre it and index that as well, for fitting to the 3 jaw? the you can have the best of both worlds, for square, irregular and hexagonal or round material.

Howard

I take Ega's point – 12 holes will cover most indexing, but 24 has that useful extra integer of 8 – octagons and 8-hole pitch-circles are not very common but thinking a little laterally, this also allows setting-off at 45º. It's one of those rare things, an "extra" you might need only very rarely but are glad to have when you do, yet takes up no extra space!

I'd agree too with Old Mart if you elect to use only one indexed back-plate, use it for the 4-jaw.

No reason you can't have stopped holes for screws provided they engage fully. Holes that break out in a corner as you describe do look messy but worse, the breaking-through risks broken drills or taps. I take it the chuck is the pattern with long cap-screws running right though the chuck from its outer face.

'

A tip when indexing: clean the rim with a cloth moistened with meths, then mark the needed holes with a fibre-tipped pen or marking-out fluid. These marks can be cleaned off afterwards in the same way.

{I used similar for circumferentially and axially dividing the 120-off 3mm dia. holes in a strainer made from a bit of standard PVC sink waste-pipe. Having no formal method at the time, the dividing-engine was my EW 2.5" lathe simply standing on a table. The 60-tooth change-wheel on the spindle, with the relevant teeth marked by two differently-coloured felt-tips and aligned by eye with a convenient edge, gave the staggered bands of holes. A second, inked change-wheel acted as lead-screw "dial" for spacing the bands. I used a hand-held battery-drill, with a brass guide-block gripped by the tool-clamp. }

PS:

Overnight, I remembered the idea of using a removable "dividing ring" temporarily fastened to the chuck/backplate via grub screws. One such with 48 divisions was available from Hemingway for use with the Quick Step Mill but could readily be shop-made.

The answer to the original question could then be that dividing could be available on all chucks with the same size backplate and, in principle, with a range of divisions.

There was also a design of spring-loaded detent for use with the division ring.