Wobbly chuck, how to diagnose?

[G-ManBart]

Why backwards? I face my backplate by putting it on as it would normally be used.

First face the face, use light cuts. Use power crossfeed to do most of the facing, at the end go manual .
You should be less than 1/2 a thou ideally 0 run out on the face. Then do the registration boss for the chuck.

At least two things come to mind...I'll go from general to specific.

(I know you know most/all of this, but others will likely read it at some point).

I'm not claiming to be an expert, but I've fit quite a few brand new adapter plates to lathes with threaded spindles. In every case I can recall except for one, the unthreaded section intended to go on the spindle register diameter was undersized, so it wasn't possible to mount the plate as it would normally be mounted. On the other hand, they will normally thread on just fine backwards. You get good OD and depth dimensions on the spindle register diameter, mount the plate backwards and then open up the ID and either face off to the depth you want if there's a lot of excess, or make a couple of cleanup facing cuts to ensure it's square to the spindle. At that point you mount the plate normally and make the typical facing passes to square the face with the spindle, turn down any register/shoulder needed for the chuck and you're done.

There is one potential pitfall with that scenario that people need to be aware of and that is how the adapter fits when threaded on backwards. In most cases they are fine, but I have seen one that was off by quite a bit when mounted that way. The solution was to put a thick washer between the spindle register and the adapter...then the adapter was square.

In this case the chuck fits on the adapter plate just fine, but the adapter doesn't register properly on the spindle. That suggests the primary issue is with the register on the adapter plate, not the face of the adapter plate. Since this is a used machine there's now way of even knowing that the chuck was ever even properly fit to this lathe in the first place. It's a lot easier to just face the register then follow up with a light facing pass of the face...the total amount of metal removed should be less, and it should take less time.

I would put the adapter on backwards, face the register, then mount it normally and make a light facing pass, mount the chuck and be done.

 

[WobblyHand]

The solution was to put a thick washer between the spindle register and the adapter...then the adapter was square.

Not any old washer but one that is turned flat! Otherwise you are baking in the error of the washer face parallelism. Not meant for G-ManBart, but for others reading this.

 

[G-ManBart]

Not any old washer but one that is turned flat! Otherwise you are baking in the error of the washer face parallelism. Not meant for G-ManBart, but for others reading this.

I actually was thinking about that, then got distracted by fixing a spelling or grammar error!

 

[AlexPeel159]

Took the faceplate back off. Tested it and it was 0.00075 out of flat 1/2" inch from the outside edge. That seems pretty good to me, unless anyone else thinks its not? Kinda surprised I got that across a 6 inch disc on my first facing operation. Much better than the original 0.013 out of flat.

Took about 0.010 more off the inner register face to give me a bit more room to shift it after it was back together, but after reassembling it I was still unable to get better than 9 thou runout 1" from the jaws. Not sure if I am unable to shift it more because the retaining bolts themselves dont have the clearance to move or if something else is wrong.

 

[AlexPeel159]

To end this saga. Buck adjust tru chuck came in the mail today. Much better condition than the other chuck. Put it on and out of the gate it was only showing 0.0015" runout. Was able to get it dialed into to 0.0001"~ish runout without to much trouble. I only have a 0.0005 dial test indicator but it seemed like it was not going more than a fifth of the way from zero to the next tic. Extra nice is that the buck came with both sets of jaws. I dont think I will be using my old chuck anymore.

 

[SeattleJack]

Recently purchased a Logan 1922, with a threaded 2 1/4 - 8 spindle. Came with a 3 jaw chuck, Logan 430 6". The chuck has a serious visible wobble to it. Not a "I am not sure I think it might have a wobble" wobble, but a "oh yeah that is wobbling" wobble. I have tested the registration face of the spindle and the spindle interior. It was about 0.00075 - 0.001 runout for both. When I tested the chuck with a 0.5 inch gauge pin I got up to 0.018 runout at 1" away from the jaws. I have cleaned and deburred the spindle (nothing large just some small nicks). I also cleaned out the chuck threads, checked for any damage in the thread area, which there doesn't seem to be any, and ran a precision stone across the back face. Again nothing obviously bad, and while nicked up quite a bit nothing seemed raised. The chuck has a lot of visible external wear so it seems like its been well used but notthing major outside of something looks like it hit the top face of the jaws. Not experienced enough to be able to identify the damage but its not smooth like a facing operation. Probably crashed something into it. I have tried seating it several times and it contacts the registration face of the spindle smoothly, nothing binding on the threads, BUT I noticed that it only fully contacts on one side. On the other I was able to fit a 0.003 feeler gauge into the gap.

I have purchased a 6" 4 jaw, also an old Logan, and it threads on fine and does not have the same apparent wobble and seems to seat fully on the spindle all the way around. Seems pretty smooth on rotation like one would expect.

Question is how do I proceed from here? I was planning on taking it apart and cleaning it, which I am sure it needs, but I am thinking that wont help the fact it does not even seat flat on the spindle. Are there more tests I should be doing? Should I attempt to resurface the back registration where it meets the spindle? If so how would I go about doing this? The only other thing I have is the 4 jaw.

It's not really the thread that is supposed to be locating the chuck position. It's the counterbore in the backplate fitting on the spindle boss diameter for the radial location and the back face of the backplate registering on the spindle boss front face. If you really have a .003 gap between the back of the backplate and the face of the spindle perhaps the front of the spindle boss is bottoming out in the backplate counterbore. Check in the bottom of the counterbore in the backplate to be sure there's nothing trapped in there. Check the counterbore dimensions and the spindle boss to make sure someone hasn't 'improved' them in the past.

If all that checks out OK then I would propose checking the chuck backplate bore and aft face for runout by mounting your 3-jaw onto a test bar mounted and turned in your 4-jaw. Perhaps that's not clear. Install your 4-jaw on the lathe. Put a substantial piece of bar stock (1 1/2 to 2 inches) in the 4-jaw that sticks out of the 4-jaw by a couple of inches. Take a couple of skim cuts on the test bar to create a diameter with zero runout. Install the 3-jaw onto the test bar and check the counterbore and aft face for runout.