weird adjust-true 3-jaw

A long time ago I bought a Yuasa 3-jaw chuck with 1½-8 backplate. It was said to
be "adjust-true" and yes, it had radial screws at the back of the sides of the
chuck body. I put it in a drawer and forgot about it. Yesterday I was going
through stuff and found it and took it out and looked at it. Nicely made, but it
only has THREE adjusting screws instead of four! I took the chuck screws loose
and tried tapping it around a little but it didn't want to move very much. So I
took it all apart.
Yup, had a deep central boss on the backplate which the radial screws bear
against, it's an adjust-true all right. I measured it and the recess in the back
of the chuck and WHASSUP?? within .002"! Why the heck would they make it that
way? Barely any clearance to move radially and only 3 adjusting screws, what
WERE they thinking?
Oh, well. Easy to fix. I took a cut off the central boss and a real light facing
cut across the whole backplate, now it's got room to move and should be dead
aligned to my lathe. Put it back on and dialed it in, a little weird with 3
screws instead of 4, but doable. Got it all fixed up, nice little 3-jaw, less
than .001" runout.
In the same drawer was a venerable Jacobs 18N which I recently got an R8 arbor
for. I remembered why I got it so cheap when I put the key in it - didn't fit!
The crown gear piece was too close to the hole! I got it for $10 a long time ago
off a table at some show. I just pressed the cover off a little bit, now it fits
perfect, nice Jacobs 18N chuck for the Bridgeport.
Back to work on the table hoist system I'm making for my 20 ton press ..
Reply to
Grant Erwin
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Grant, I used to run a lathe with a 10" HOWA chuck. It also had the three adjusting screws. Though handy for some things, it was really a pain most of the time because it would not move linearly. It would slide toward or away from you. So the runout would change position. My method for indicating parts was to move the chuck in small increments. If #1 needed to be tightened I would loosen #2 & #3 and then snug them back up. Just so they had a little load on them. Then tighten #1 and check the runout. And keep on this way until the desired TIR is achieved. This method helped to move the chuck in line with the screw being adjusted. If the screw needed to be loosened then the opposite screws would be turned the same amount. I got pretty fast on this chuck because I used it every day practically for 10 years. Still, 4 screws are better than 3. I have a 4" 3 jaw with a 5C shank. It gets a lot of use when the collet closer is in the lathe. I took it apart and made it into an adjusting chuck but there was not enough room for 4 screws so I'm still stuck with a chuck that has only 3 adjusting screws. Cheers, Eric
Reply to
Eric R Snow
Jeez Grant,
With the work you've done as far as facingthe backplate, I'm surprised you haven't drilled and tapped new holes to give it 4 adjusting screws...
FWIW the old geezer I posted about in "dumb things" scratch build a very nice back plate for his 3 jaw with 4 adjusters, works like a charm (and yeah sadly that was back when he was "with it")...
Reply to
Dave August

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