Bison Set Tru Chuck

[Zee]

Hello,
I have recently bought a 6 1/4'' Bison set try chuck. It didn't come with any instructions. I am able to get it centered, although when I start to tighten down on the hex screws on the face of the chuck my dail indicator starts to move. When I'm finished I may be two or three thousands out again. Any ideas on how to fix my problem?
Thanks

[JCHannum]

Welcome to the site.

Two questions, are you using the backplate that came with the chuck? What screws are you loosening and tightening?

The backplate for these chucks has a raised hub that the adjusting screws bear against when adjusting for center. If this feature is not present, the adjustment will not hold.

There are two sets of hex socket screws on the face of the chuck. The inner set holds the chuck together, the outer set holds the chuck to the back-plate. It is the outer set that should be loosened when adjusting. These screws need only to be backed off slightly, just enough to permit adjusting. The adjustments are made with the hex screws on the periphery of the chuck, these screws should be left tight once the adjustment has been made. Then tighten the retaining screws, I tighten them in a criss-cross pattern in a couple of passes. First snug them up then tighten them. They do not require a great deal of torque when tightening as the adjusting screws hold the chuck in position.

[Zee]

Thanks Jim,

Yes I am using the backing plate that came with the chuck. I bought this chuck used, although I think its in very good condition. I have never used a set tru chuck before. First I loosened off the four periphery screws, just a little bit. Then I loosened off the three screws that hold the chuck together on the face of the chuck. I didn't loosen them much, just enough to adjust the chuck. Then with my fail indicator on my work I'm able to remove all runout with the four set screws on the periphery of the chuck.

Then when I tighten the three screws on the face of the chuck is where I'm getting the runout again. I now have played with this chuck a little bit, after all screws are snug my run out is down to about a half of a thousand. With my limited experience I don't know if I could get it any closer. My work doesn't have to be that precise. Although I would like to do the best I can.

Am I going about this the right way Jim? Should I be doing something else to better myself in centering this work in this chuck?
Thanks,
Kyle Z.

[JCHannum]

Check to make sure all surfaces are clean and true. This is especially important with used tooling. Remove the three mounting screws, loosen the adjusting screws and remove the chuck from the backplate. Check that all surfaces are clean. Remove the backplate and check those surfaces as well.

Once that is accomplished, remount the backplate and indicate that to check that it is running true to the lathe spindle. You might need to take a light facing cut across the face of the backplate and a light cut on the raised hub to clean that up and true it to the spindle.

Bison claims 0.0006" repeatability when removing and reinstalling workpieces in the chuck, and that might be what you are seeing.

[Toolguy]

I have a 6" and an 8" six jaw with the set-tru feature. When adjusting runout, I don't loosen the screws on the face as you're only moving a thou or 2. You can easily scoot sideways a little bit with the face screws tight, then it stays where you put it. I commonly have less than a thou. runout at any given diameter. My particular 2 chucks are very accurate and repeatable. YMMV.

[lane]

Same here. Just leave them tight.

[lakeside53]

Yes, and adjustment is an art... especially as the bison screws are not at 90 degrees to each other.

[Zee]

Thanks for all the help! I really appreciate it.
Kyle

[browne92]

Once that is accomplished, remount the backplate and indicate that to check that it is running true to the lathe spindle. You might need to take a light facing cut across the face of the backplate and a light cut on the raised hub to clean that up and true it to the spindle.

I would have started with this before ever trying to true up the chuck.

Best of luck.

[Old Hat]

I use the nudging screws only to shift the body of the chuck.
after each shift, I back off the nudging screw.
As mentioned the face-bolts are kept at a soft tight and left alone.

When I'm dead nuts, I bring the nudging screws all back to contact and just
a tiny snug, Done.
If heavy ruffing, I may ellect to tighten the face-bolts a tad more than usual.

[JCHannum]

My posts were in response to the OP's questions regarding the initial setup of his chuck on a different machine. When tweaking a chuck to a new workpiece, the adjusting screws are usually all that need be used.

With any used piece of tooling and many new ones too, and especially a lathe chuck, the best procedure is to dismantle and inspect and clean all mating surfaces. With a lathe chuck, checking and truing to the spindle is always recommended. Usually a light cut is all that is needed.

[Old Hat]

I can't remember aquiring a machine, or complex tool, that I didn't do an exploratory surgery on.
Ive had a few that would have been toast in an instant IF I had just done the "plug&play" thing.

[oldtiffie]

My "set true" is not of "Bison" character but works well.

I bore out the hole in the back of the chuck mounting plate to be say 0.002">0.005" clear of the front face of spigot on the mating face of the lathe spindle flange.

I mount (or loosen if already mounted) the chuck-to-bore tightening bolts and just use a 2" diam phosphor-bronze "dolly" that I've had since I was an apprentice (55+ years ago) - it works as a very good "dead blow" hammer and does not damage or dent what it is used upon.

I put a test piece in the chuck and using a good dial indicator to true it up for total indicated run-out (aka: TIR) at or close to zero. I tighten the bolts and re-check - and if OK - go ahead otherwise re-adjust until it is as accurate as required.

All my (bought new) chucks are quite good enough for this.

[JoeLee]

Yes, and adjustment is an art... especially as the bison screws are not at 90 degrees to each other.

What do you mean, the set screws aren't 90 degrees apart???? On my 8" Bison set tru they are, there are 4 of them, unless the 6" has 3.

Zee..... I would also check your spindle for excessive play before driving yourself crazy with the chuck. If the spindle is sloppy you'll never get the chuck adjusted to true.
The guys are right on with the hold down screws, just loosen them a bit, the adjusting screws develop more than enough force to nudge the chuck around on the back plate. It is an art to set one of these and takes a bit of practice as to tightening one side or slightly loosening the other. A word of advise...... don't over tighten any one side as it may start to deform the body of the chuck. Many times I find that I have to gradually tighten down the mounting screws as I play with the adjusting screws to obtain the desired setting.
Also your test piece should be a good piece of ground stock, not a piece of cold rolled round.

JL...............

[lakeside53]

I have two high end 6 1/4 Bison set tru chucks, one three jaw and the other 6. Both have the 4 adjuster screws arranged so they are in pairs at about 75 degrees (between the closer two). Would be a lot easer to make tiny tweaks if they were at 90 like my Rolm, but you get used to them.