jstr1963,
There is no question in my mind that the problem concerns the internal threads in your 3-jaw chuck.
I could see a little in your video and they looked pretty rough. I have no idea how that could happen, but it's there. At this point, I think you need to find some way to clean up the threads or get a new chuck. The threads are designed to have a little slack and yours don't. If you are 100% sure the threads are clean, you need to find someone with a tap to fit it or get someone to chuck it in a larger lathe and carefully scrape the threads with an internal threading tool. Disassemble the chuck and work with the back portion only. You can use the front portion of the backplate to serve as a registration surface. If there isn't any runout with it, you are pretty safe there won't be any runout when you get it back together. You may be able to get an idea where it is sticking, too.
If you still have runout after the chuck goes all the way to the face on the spindle, I believe you can shave a small amount from the face of the backplate to fix it since the body of the chuck registers against that face. If I were you, I wouldn't fool with it until you get better at using the lathe and metalworking in general. If the chuck isn't useable as it is, you can't make it worse, but if you aren't any good at it, you can't make it better, either.
You could also have a problem with the jaws. There is no way to tell any of this until you can get the chuck to seat against the shoulder on the spindle.
In the video, there also appears to be a couple of dings on the edge of the registration face on your spindle.
Robert D. was referring to a separate faceplate, not a part of your chuck. I think you are talking about an adapter plate for a plainback chuck. What he was saying is that you should mount a faceplate and see if it screws against the registration shoulder, which you show it does. six thousandths out isn't bad for a faceplate that size. The first thing I do when I get one is to skim the surface to eliminate or at least minimize the runout.
You don't have to use an endmill to check runout. You can use any straight, smooth rod. Drill rod works as well as any other steel or aluminum rod that is relatively smooth. The rod doesn't even have to be straight. It does make it easier and you don't need any math.
To verify that you don't have any issues with the jaws on your 4-jaw chuck, you still need to chuck up a piece of rod, center it as best as you can and check it again at least an inch away from the initial location. Even if the jaws are not parallel to the centerline of the spindle, you will still be able to adjust the work to zero runout at any given point. If the jaws are parallel to the spindle, the runout will be identical all along the work. If they aren't parallel, the runout will be different in different locations along the work. Think of it as wobble. It is hard to explain, for me anyway, but it is an important concept.
Hope all this makes sense,
Steve Fox
