On 2019-02-21, wrote:

[ ... ]

Hello,

From what I can tell, these parts are not generally interchangeable.
I have a Dalton 7" (number 6125) and the three jaw chuck that isn't
running true (after 100 years, no surprise...) and have been looking
around for jaws or chuck to replace it.

Hmm ... first find out where the runout is coming from. One
possible place in a 3-jaw (or the rare universal 4-jaw) is the scroll
plate. It normally is held on center by a boss into the center of the
plate, and over the years, the ID wears and allows the plate to shift
off center -- thus carrying the jaws with it.

To add to the fun -- if there are multiple key holes, the
direction of the offset depends on the key hole used for the last
tightening. Some chucks have a marker beside one of the (typically
three) key holes, and when new, this was the one which would give
minimum runout.

So -- take the back plate off (assuming that there is a separate
back plate) and whatever else is necessary to access the back of the
scroll plate.

Then clean it all with a reasonable grease solvent, and with the
jaws removed, see whether the scroll plate can be slid around a bit. If
so, slide it to one side, and at the point opposite the contact point,
get your index gages and find which one is the thickest that you can
slide into the gap.

Divide that thickness by two, and get some shim stock of that
thickness, and cut a piece long enough to wrap around the boss in the
chuck, and wide enough to match the thickness of the scroll place, and
put it in the gap between the boss and the scroll plate hub.

Once this is done, try each of the key holes to see which gives
the least runout. Hopefully, this is the one marked by the maker.

Using this one, tighten the jaws onto a ring -- ideally one made
to fit behind the gripping surface of the jaws -- and then mount a
toolpost grinder (or adapt a Dremel or the like to act as one) and
re-grind the inside end of the jaws until all three are clean fresh
grinding. At this point, you will probably have better centering at all
sizes with the reground jaws -- except if the scroll in the plate has
worn at a particular angle of setting, or has had the scroll teeth bent
by overtightening at a particular size. The wearing could be if it was
at one time used in production, almost always tightened on the same
diameter of stock. If that has happened, it really needs a new scroll
plate -- somewhat tricky to make, unless you have a universal mill with
an index head geared to the X-axis leadscrew.

Nothing I've found fits - even
the 1.25" - 12 spindle thread seems to be bespoke. Let me know if you
find something!

1.25" x 12TPI spindle suggests that you need to make your own
backplate for a new chuck, rather than trying to find one which you can
buy.

Good Luck,
DoN.

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