Can’t properly adjust bison 6 jaw set tru

Swerdk

Swerdk

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I know that to adjust a bison set tru 6 jaw chuck, you lightly loosen screws in front of chuck( loosening back plate). I am having problems making the 4 set screws work. Are the set screws moving the jaw? Sounds stupid but it has to right? . I was under the impression that the four set screws work against each other. But I can’t get them to move jaws. I know with a 4 jaw, after an indicator is attached, you loosen the low end and then tighten the high end. This doesn’t seem to work with my 6 jaw bison set tru. In another post from 2011, a member stated that an adjust tru works opposite a 4 jaw but didn’t build on statement. Can someone explain how a 6 jaw properly gets adjusted.


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When adjusting a adjust-tru chuck you first mount your stock in the chuck and tighten jaws. Then loosen face screw a touch to allow the ENTIRE chuck body to be adjusted by tightening and loosening the opposing screw on the diameter of chuck body. Once you have centered you tighten bolts on chuck face and recheck to make sure nothing moved. Chuck jaws do not adjust chuck body moves. Hope that helps.
 
The do not move the jaws. The backplate registration step is cut undersized by about 0.005" in radius betwen it an the chuck body. The whole body of the chuck moves, not the jaws. It is the bolts that mount the chuck to the backplate that are just sightly loosened, you then adjust the opposing pair of jack screws to adjust the chuck so that accurate diameter rod has the least amount of TIR. I often use a precision ground 1" bar. If you reverse the jaws you need to recheck the TIR.
 
So the independently adjusting 4 jaw only have the jaws move correct ? Is that the difference?


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Yes, correct.

I had nearly no movement on my 3 jaw set-tru. I had to reduce the diameter of the step on the backplate to give myself more room to adjust.
 
What I am finding is when i counterclockwise one screw the opposite does not move clockwise . I end up with one screw tight and one loose with still very little movement on indicator. What is maximum movement allowed by chuck body?


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That's exactly what mine did. You'll have to measure the step ring on the backplate and the inside back of the chuck to determine how much movement is there. Mine was nil to none until I machined the backplate to give more room.
 
My Pratt-Burnerd has exactly 1 mm movement out of center. That's all it needs, else it's 4-jaw time.

Way it works you have to back off the opposite screw before tightening the screw you're on. Like this: remove 1/4 turn off of a screw, rotate chuck to 180 degrees, add 1/4 turn to the first screw's opposite. In other words, create a gap opposite, rotate, close gap. It is like you are adjusting a fully tightened 4-jaw, can't move metal into a space without opening up some clearance on the opposite side.
 
Sorry my last post made more sense in my head then it does on Paper. What I mean Using a 4 jaw as an example; When I’m loosening one set screw I expect the opposite side to enable me to tighten that set screw there by moving the indicator - does that idea not carry over to a bison 6 jaw set tru ? Because when I loosen one set screw - ( on 6 jaw) The opposite side is not allowing me to tighten it equal to the amount I loosened the first side. I appreciate everybody’s advice I have to wait till this weekend to re-try fiddling with the chuck.


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Sorry Pontiac 428
I just saw your post after I posted mine which sounds redundant now


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