That's correct sanddan. This is my 2 cents worth....When you chuck up a bar to machine, and use it for checking spindle to bed alignment... i.e... Headstock alignment, any runout in the chuck or work piece is irrelevant. One you turn down the piece it becomes true to the spindle as well as taking on the spindle to bed accuracy. Any runout in the chuck is canceled out as it rotates. (Because it's making a circle) If its out neg 10 thousands one way, 180 degrees from there it will be out pos 10 thousands. Thus offsetting any effect on the final accuracy. This is assuming though you don't take the piece out of the chuck. Once you do that you have to start over. Because your cutting tool follows the bed of the lathe, it will true the part to the accuracy of the spindle/bed combination no matter how much runout you have in the shaft or chuck. (assuming you are not deflecting the work by taking too heavy of a cut) you just have to make sure the diameter is large enough to makeup for the loss of material you machine off due to runout. Once the shaft is completely cleaned up and concentric, it will give you an exact reading of your headstock to bed accuracy. It seems some people don't understand the logic behind this test.
I shouldn't say lots I guess, but I'm always hearing how you need a "precision" test bar for this. It's nice, but unnecessary. I think people tend to assume that if there's runout at the end of the shaft it will remove more metal then towards the chuck thus affecting the final readings? At least I can see how someone would think that.
Guess I'm weird too. I prefer pictures.I scored very high in spatial acuity and am comfortable thinking in pictures. Yea, I'm weird...
.0003 over 9 isn't too shabby. You could probably just every so slightly crank on the bolt that's more towards the tail stock. Wouldn't take much at all. When you get down to that little, it sometimes feels like you're chasing your tail ya know?? Mine tends to fluctuate between .0002 and .0003 over 6-7 inches depending on the material I'm cutting and the feed rate. I'm curious RIO, is your measurement consistent all the way across the finished surface? The reason I ask is the other day I was machining a 304 stainless shaft and it would be consistent within 2 tenths all the way across on both ends except, for one area 2-3 inches from the chuck, that would randomly increase .001 for a couple inches, then go back to my original setting. It was weird. Think I finally decided it must be something in the material over that area. Normally that would indicate a worn bed. But obviously that can't be the issue here,
I think we end up chasing our tail on this one. Chuck it up a little different you might get something else. Use another chuck, it may be better, but usually worse, the material, cutting tool and depth of cut will also factor in. I just do not have faith that the chuck can hold the material true out to 9" and expect to hit something like 0.0002" repeatably. Let alone change chucks, and trying to get the same accuracy. I have read several posts where gunsmiths have aligned their 4J independent and then never took it off the lathe thereafter.
