A #1050 3 Jaw Chuck Jaw Boring Test

I thought it would be interesting to look at #1050 3 jaw chuck boring.

A relatively new #1050 chuck, with unbored jaws. I tested runout using dowel pins, which may not be perfect compared to gage pins, but will do for this. With the unbored jaws I found the runout was:

1. 3/8" dia. pin, .001" runout
2. 1/2" dia. pin, .004" runout
3. 3/4" dia. pin, .008" runout

Boring the jaws using the factory supplied washer. I made sure to set the carriage depth stop so that the boring bar wouldn't hit the washer.

After boring.

I thought it was odd that the bored jaws didn't have a uniform width. I'm not sure why. Could be that there's enough play in the jaws that the jaw lifts, throwing off the boring. Or it could be that the jaws were slightly deformed, as I'd been using them for a while unbored.
In any case the results were:

1. 3/8" dia. pin, .003" runout (+.002" over unbored)
2. 1/2" dia. pin, .000" runout (-.004" over unbored)
3. 3/4" dia. pin, .006" runout (-.002" over unbored)
   

So a slight improvement was gained except in the case of the 3/8" pin.

Next I inserted a 1/4" pin far at the back of the jaws.

And bored them out with a smaller boring bar.
The results were:

1. 3/8" dia. pin, .000" runout (-.001" over unbored)
2. 1/2" dia. pin, .002" runout (-.002" over unbored)
3. 3/4" dia. pin, .000" runout (-.008" over unbored)
   

Which is pretty interesting. Note that when I say .000" I mean the needle didn't move even a quarter a division on a .001" indicator but may have wiggled a few tenths.

Next I bored out the jaws without any preload at all, just opened them up about an inch and bored them.

The results were:

1. 3/8" dia. pin, .002" runout (+.001" over unbored)
2. 1/2" dia. pin, .000" runout (-.004" over unbored)
3. 3/4" dia. pin, .002" runout (-.006" over unbored)

Then I rebored the jaws with the factory washer and found that things had changed again...

The results were:

1. 3/8" dia. pin, .002" runout (+.001" over unbored)
2. 1/2" dia. pin, .000" runout (-.004" over unbored)
3. 3/4" dia. pin, .001" runout (-.007" over unbored)

So what did I learn? I have no idea except that it's probably worth experimenting yourself to find the optimal diameter of the preload washer for a given diameter or range of diameters of workpieces. It does definitely suggest that leaving the jaws unbored is not a good idea.

It may be that it's a good idea to undercut the back of new jaws before placing the washer and boring them as any uneveness in the jaws will cause poor preload?