Chuck Run Out - What Caused It?

prasad

prasad

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A few months ago I bought a new 6 inch 4-jaw independent jaw chuck with its mounting plate for my G4000 9x19 lathe. I mounted it properly after turning the mounting plate on the lathe. TIR was under 0.001" on the mounting plate as well as the chuck. I have been using the chuck whenever needed. I have mounted and removed the 4-jaw several times. I had not checked the TIR after the initial check. Today I checked and found it to be around 2.5 thousandths. I dont see anything else wrong with the lathe. Spindle does not show any play when I try to shake it. All gibs are properly adjusted. Any suggestions?

I dont see any runout when I mount my original 3-jaw chuck. That may confirm that my lathe is OK.

I do understand that this is a 4-jaw chuck and I can always center any object that I mount and it will overcome this runout. I am only curious to understand what changed.

Thanks
Prasad
 
I believe it is a thread on type mount. Might be seating a little different with repeated use, also make sure the mating surfaces are clean when mounting the chuck. Also agree that the mounting bolts to the back plate might have changed ever so slightly or seated. If you are measuring the chuck body, 0.0025 run-out is meaningless. Even the best chucks have some run-out on the body, and an independent chuck it doesn't even matter.
 
Agree with the above comments. My first thought was mating surfaces being clean. Slight turning of the chuck on the mounting plate when in use, or even when tightening up the jaws as you align the part. But I agree it is a moot point with a 4 jaw chuck anyway since you dial in the part. Like you said since your 3 jaw repeats when installed the lathe is not the problem.
 
I'm confused by what you mean by run-out????? - since you can dial in a component in pretty well any plane to be 0.000 (close to the chuck, 3" from the chuck, 6" from the chuck). If you set the bar perfectly at one plane (say 1" from the chuck), and then check the run-out further along (say 6" from the chuck) - then there probably will be some run-out. You are supposed to dial in the component in your 4 jaw in the plane that works best for that project (close to the chuck? at the far end to get an existing center to run ture? Whatever the job requires.).

The issue is the alignment of the center-line axis of the chuck jaws and how parallel that is with the axis of the travel of your carriage. No chuck is going to be perfect (see responses above by others) and usually you are going to do some machining on the component you just put in the chuck (so if it had any run out, it soon won't). That issue is no different whether it is a 3 jaw, 4 jaw, 6 jaw, collet chuck - you get the idea. If you really need something to be straight (as in an existing component and you are doing some finishing work on it?) make sure you mark the orientation in the chuck before you remove it. Or don't use a chuck, hold your work between centers, or on a mandrel, . . . . many ways to hold your work piece.

If axial alignment of the chuck is still an issue: after you have gone through the lathe alignment exercise, after you have gone through tuning the chuck back plate to the spindle, after you have skimmed the mounting plate for the chuck (as you did when you first mounted the chuck). You can skim the mounting surface in the back of the chuck. I did this on my 4 jaw to improve the angular alignment. I have done the basics described above, and I still was getting about 0.006" runout a foot from the chuck, when dialed 0.000" right close to the chuck. So, I gripped a bar in my largest chuck, took a skim cut, then mounted up my regular 4 jaw backwards, and skimmed the mounting face. Now my run out a foot from the chuck is about 0.003" - it is better.

In fairness I must confess that I'd owned the lathe / chucks etc. for about 30 years and used them a lot, with no issues. However, once I knew the axial alignment issue was there - my OCD would not allow me to continue using the chuck without at least trying to improve it.

DSCF1978.JPG DSCF1981.JPG DSCF1985.JPG
 
Wow, many replies (and ideas). Many thanks. Let me answer one by one. No, all bolts are tight. Yes, it is a screw on mount to the spindle. I understand your point and I now realize that it is very much likely that there could be something in the threads that caused this run out. I will have to go back and check it. Right now I have a job in the chuck and I am half way done. It has to be completed before I can unmount the chuck.

I know that this run out does not really matter because it is a 4-jaw independent chuck. I was trying to understand what made the change in the system. It must be something in the threads. I mounted the DI to measure on the surface edge of the mounting plate (6" diameter) and also diameter on the chuck. No, this run out is not causing any trouble to me. My question was purely to understand the behavior, a part of self education.

Thank you every one and wish you all a very happy and prosperous New Year 2016.

Regards
Prasad
 
Your adaptor to chuck . Does your chuck have a shoulder to mate to? How does your bolts fit the holes in the backing plate?
 
David VanNorman said:
Your adaptor to chuck . Does your chuck have a shoulder to mate to? How does your bolts fit the holes in the backing plate?
Click to expand...

Yes, it does have a shoulder and the chuck sits tight on it. The run out was the same on both - adapter plate edge or the chuck. I now think the run out is coming from some debris caught in the threads of spindle. I will know it only after I can unmount the chuck. It might take another couple of days until I finish the task that is now in the chuck.

Thank you
Prasad
 
Hi

I bought a secondhand Pratt 4-jaw in December 2015. When I tried to mount the chuck, I had the same problem. I dailed the face and diameter in to 0.000. I had a runout of 0.004 at my mounting surface(like in the 3rd picture of your set) . I finished mating my mounting plate to my chuck, set it in my lathe and removed the jaws.
I proceed to square my chuck face, replaced the jaws and squared them to by means of a ceramic insert, round stock and an outer ring. Now my secondhand Pratt looks and perform like a brand new chuck for less than a fifth of the price

Sent from my SM-T111 using Tapatalk
 
you have to mark your jaws!!!!! sure they go in as long as they are in order but if they are moved side to side your center may move
 
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