This is a bit of a saga.


My sad old ex-school Hercus has a sad little Pratt chuck. Its jaws aren't parallel – splaying out toward the tailstock (from youth grabbing little things very tightly?), and its about 4thou out of true.


Last week, I decided to try improving it.


First, chuck off the backplate, and clock its front. It was wobbling by nearly 100 microns!
This chuck and plate must have been swapped from another lathe spindle?

Easy, I think. Re-machine the "spigot" to fit the chuck, and we should run true.

A pleasureable 10 minutes to remove 2mm, remount the chuck, remeasure.

Hmmm. Body & face about 60 microns out.



Second, machine the jaws. Find a ring to expand the jaws out into, put pressure on, and bore a little off them with the sharpest tool I can find - an internal threading insert:
IMG_1572.jpg

5 or 6 passes, at a fraction of thou each time. About 1.5thou total D.O.C. until the front inside of the jaws got a little round machined in their middle.

I even decided to re-cut the little deep grooves, for the next time I'm boring out washers.
My 16IR was a little too pointy for that. Had to borrow a different internal thread tool:
IMG_1573.jpg

Now, vacuum out the swarf, and mount a 40mm precision round for testing, and...


still nearly 100 microns, or 4 thou out

I try a different round – a 16mm ejector pin – it comes in at nearly 2 thou.

I try tapping the high side, and notice light under the front of the jaws.
Not parallel? What the?


Remove the round, clock the freshly machined groove along the middle of one jaw as best I can
(it is hard to get good access in there).
Inconsistent results.


I fiddle around with the jaws a bit. They have always been loose in the scroll, but today I notice there is also slight wobble in the T slots. Maybe machining them while clamping outside won't work, because the jaws are rotating in the T slots?


Scratch my head a bit. Decide to re-machine the front while grabbing something deep in the chuck. Find 20mm round slug, put it deep inside, past the front of the chuck body, and re-do the boring.

Another 2 thou D.O.C., and I have jaws with a big step in them. Think about milling, Dremeling, hand filing, but decide to surface grind:
IMG_1574.jpg

Tricky to hold and shim for the level/taper, but end result looks something like:
IMG_1575.jpg


So, mount them back, expecting perfection, and... still nearly 2 thou out of round.
Plus, there is still a slight step in the back of the jaws when grabbing small round stock.


Another day, work out a way to machine that out in the chuck. I find a little steel sleeve, loosely grab it at the front of the jaws, bore extra clearance so it doesn't foul on my "boring" tool, and gently take the last of the step out:
IMG_1576.jpg

which seems to not line up with the previously machined front of the jaws, so I also grab my 20mm round inside the chuck, and finish the front of the jaws again.


Clamp small precision round, and check:
IMG_1577.jpg


Yay! At last some improvement.



Test with larger round stock:
IMG_1578.jpg



Aurrrrrrgh!!!!!!!!!!!! Worse than before.


After two days of careful machining and grinding, I have a chuck which is better at some radii than others, and has a total of over 6 thou removed (so this chuck will never grab very small rod again unless I mill/grind the bevels on the jaws).


The scroll is too badly damaged – time to cut my losses.


Toolmaker takes pity on me, and gives me an unused – still in box – 125mm TOS 3 jaw. Onto part 2...
Attached Images




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