View Full Version : Question RE: Bison 6 Jaw Chuck

05-08-2009, 01:45 PM
I have a new Bison 6 Jaw Chuck that is self-centering with fine adjustment. I was of the impression that once the chuck had been "zeroed", then each time that *in-round stock* is secured in the chuck, the metal will be automatically centered to "zero". Am I wrong in my thinking?

To zero my new chuck, I followed assembly and zeroing instructions. I used *ground and polished* round stock. I easily zeroed the piece and secured the chuck to the backing plate. After all was made tight, I re-indicated and I was still reading "zero". I then opened the jaws and tightened them again and indicated the piece. The stock was now out by 10-15 thou. I revisited the zeroing process and easily zeroed the ground steel. Opening and closing the jaws caused the piece to be out by 10-15 thou. What's going on? I thought it should at least be within .0005" for center.


05-08-2009, 03:21 PM
Are you using all three (or however many there are) sockets for the chuck wrench, using them in the same order, and tightening gradually and uniformly (not being ham-fisted about it)?

05-08-2009, 04:24 PM
Are you using all three (or however many there are) sockets for the chuck wrench, using them in the same order, and tightening gradually and uniformly (not being ham-fisted about it)?

To be honest, I was using whatever socket was close. No I was not "hoging down" but I did apply firm pressure.

Since posting, I was able to finally get in contact with Bison and they pointed out several possible causes after I explained as I explained above. First, I did not indicate for run-out on the back plate. I simply assembled and began indicating so that may be one source of the problem. I may need to take a fine cut to "straighten" the back plate.

The other possibility involves using the "correct socket". I should be closing the jaws by using the socket marked "0". They stated that when accuracy is desired that I should thigten the jaws using only the "0" socket. So when I get home I will indicate the back plate and make any cuts necessary and then assemble the chuck to the back plate and try to use the "0" for accuracy.

Any thoughts on the above?


Thomas Staubo
05-08-2009, 04:48 PM
To be honest, I was using whatever socket was close.

It's a good idea to use one "favourite" socket for best consistent results.
At least that's what I've been told, to try out the three different socket positions and see which one clamps with the least run-out. This advice was for regular three jaw chucks, but should apply to adjust-true chucks too.


Mark Hockett
05-08-2009, 05:20 PM
I have a Bison 6" 6 jaw chuck mounted on a rotary Fourth axis on my CNC mill. I had a 25 part run using the chuck that had a very tight tolerance. I had to check each part with a dial test indicator and each time on my chuck they were all within .0003" throughout the entire run.

Lynn Standish
05-08-2009, 05:21 PM
It may be the way the backplate mounts on the spindle. On mine (D14), the backplate wouldn't "bottom", which took a feeler gage to find. enlarging the female taper on the backplate slightly with emery cloth until it just bottomed, followed by a (very) light facing cut on the backplate when mounted solved the problem. I can zero the chuck to where my Federal .00005" test indicator needle doesn't move when using a .750" dowell pin for checking. It will then zero most pieces to less than .001" without adjustment.

Even a set-true chuck like this only approaches a true zero runout when chucking an object of the same diameter as the one it was adjusted to.

05-08-2009, 09:12 PM
I have a Yuasa 3 jaw "set tru" and a Rohm 4 jack scroll+independant. Neither will perfectly register variable sized stock or even "exact" with same stock when just using the scroll. Yes.. it's better than a regular scroll chuck, but adjusment is required for perfection.

Glenn Wegman
05-08-2009, 10:14 PM
I use two Bison 6 jaw Set-Tru chucks and after a year and a half they are both still within .0003" runout. Wouldn't be without one!

05-08-2009, 11:37 PM
Hi Guys,

I think I have the answer I was needing, i.e. some adjustment will likely be necessary. Sure! Wouldn't it be my good fortune to be one of the lucky few having bought a chuck that maintains "near zero" adjustment over a long time through a variety of diameters. But this may not be the case regarding my chuck. I simply needed to get a "realistic handle" on what I should expect through experiences of others, with similar chucks, and you guys have afforded me the information I needed. If I'm expecting too much then I need to know and if I am resigning to less when I need to expect more then I need to know that too. You've helped me to define "parameters of reasonable expectation".

Since posting this morning, the workday has ended in Alaska and I have returned to my humble home shop. I removed the chuck body from the D1-4 backing plate and indicated the face of the back plate and boss face. Using a .0005" indicator, movement was barely discernible. In fact, there was so little movement that I made a conscious effort to ensure the indicator stem was engaging the face.

I reinstalled the chuck on the back plate and using the ground test rod and the "0" socket for securing the work I re-zeroed the chuck as prescribed. As a test, I opened the jaws and then again close the jaws using the "0" socket and "lo & behold" it was dead nuts on. This was repeated several more time with the same result. Mr. Lakeside 53, your words have not fallen on deaf ears regarding variable diameter stock. It is good to know what I might need to expect and consider.

Thanks to all for sharing your knowledge and experience.:)


05-09-2009, 07:13 PM
The "O" socket is the master pinion.It was that pinion which was used to chuck the backlash ring before the jaws were ground true at the factory.It is the only pinion that will chuck up true and repeat.

05-10-2009, 08:12 AM
It's also a good idea to always mount a d1 chuck in the same holes each time. My d1-3 6-jaw has the hole# stamped on the backplate.

Glenn Wegman
05-10-2009, 10:33 AM
Good point kvom.

My D1-5 spindle came with an index mark on it. When I mounted my chucks, I put a corresponding mark on the adapter plates, and tightened the cam locks in a particular sequence. By repeating this sequence I have perfect repeatability when removing and replacing chucks.

I did the same with my collet closer adapter.

Make sure the adapter plate and spindle nose are spotless when installing the chuck!