I'm looking at ordering some drill rod to make a test bar when I get my lathe set up. I do have a steady rest, but was debating getting a rod that would just fit through the spindle to save some hassle. What is the max size that I could fit through a 9A spindle? I think I remember hearing somewhere that 3/4" was the max size that would fit through.
As an aside, I only currently have a 3 jaw chuck set up. I have a 4 jaw, but I need to make a back plate for it. I'm starting to think that I would be better off by using some centers in the headstock and tailstock to get it close, and then make the back plate. Then make the test bar once I can use the 4 jaw to make sure it's spot on. I don't yet know what kind of runout my 3 jaw has. Thoughts?
You would use a test bar to help get your lathe set up. Trying to make one to test your own accuracy wouldn't work out.
The test bar will help you see how close to dead center your head stock and tail stock are pointing at each other. Also using one in head stock, and another in tail stock, will help you see how close to true that both head stock and tail stock are to the ways. Instead of tilted up or down slightly, or side to side.
You'll also see how much wear on ways, as usually close to chuck is wore down a little more, but as saddle moves away from chuck, it will gradually rise.
Trying to use a dead center in each, may get you at center at a specific spot, but you won't know if either head stock or tail stock is tilted or true to ways. And as head stock and tail stock move away from each other the in-accuracy would grow. With test bars you indicate off the side of shafts to see how true to ways you are.
Also a test bar in spindle will let you see if you have any run out.
You don't use a test bar in a chuck, a four jaw chuck can be adjusted to get your work close to spinning without appearing out of round regardless. For a test bar, you would remove chuck, and end of spindle should be tapered. Test bar would be put into that taper. Roll spindle by hand with dial indicator mounted on saddle to indicate off test bar. Also move saddle back and forth the length of test bar.
Depending on readings, you may shim headstock, or one end of head stock. And also make adjustment to tail stock, be it add or remove shims, or shift it side to side of it's base. But the general idea would be to get everything pointed at center, as well as close to true to ways as possible. The closer everything gets to dead center and true to ways, the more accurate your work will turn out.
Not sure your spindle thread, on a 9a I believe, but buying a partially machined back plate on ebay, or such is ok. the threads are already done. You just cut it down and true face to fit whatever chuck you want, maybe cut a boss for chuck to sit on.
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