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Compensating for test bar error in a tailstock

jwearing

Member
When aligning a lathe headstock with a test bar in the spindle, it's possible to compensate for runout and error in the test bar by indicating the end of the test bar and rotating the spindle. The extremes of the indicator travel are noted and then the spindle is rotated until the indicator reads the midpoint. At this point the test bar cab be swept, the path the indicator sweeps will be fairly accurately parallel to the axis of rotation.

My question is:
How do I do the same thing on the tailstock? I want to mount a test bar in the tailstock quill taper, and sweep it with an indicator to align the tailstock. But if there is any error in the test bar, that error will be transfered to the tailstock when I scrape it. I can't use the trick mentioned above because I can't rotate the tailstock quill.

I have tried rotating the test bar 180º and taking the measurement a second time, and averaging the result. But I don't think this will be that accurate. Is there a better way?
 

TGTool

Active member
If I'm following correctly, you're looking at scraping the tailstock base and wanting to make sure it's coming out parallel with the ways.

Richard will have the definitive answer, but my abbreviated thought it to check against the tailstock quill, not a test bar in the taper. The quill should be straight so simply extending it and sliding on the ways will tell you if it's true, ignoring wear in quill fitting.
 

jwearing

Member
Ok, I went back to the Connelly book and you're right, the quill is used as the reference for alignment. So in that case I don't have to worry about transferring any error to the TS base when I scrape it.

But nevertheless the test bar is still important, it's needed to measure the alignment of the taper. And I'm still curious if there is a way to measure its error or compensate for it. Connelly lists the tolerance of the taper alignment as .0005" in 12", so I think the error in even a quality test bar is still significant.
 

TGTool

Active member
Pull the quill, mount the test bar in it and check with V-blocks. Yeah, there can be error accumulation but it's all a matter of doing the best you can within limitations.

And you have to consider that if the taper is misaligned in the quill there's not a lot you can do about it. You'd have to chuck it in the lathe, verify straightness of the quill and the lathe itself, set the taper, take a truing cut and probably finish with a Morse taper reamer. Standard procedure if you were reproducing the quill, but in this instance you've got an even chance of introducing another error somewhere along the line and you have to worry about taking out so much material in getting realigned that that the taper socket is now much deeper than you want. It doesn't take much on the diameter at this angle to shift the profile a long ways longitudinally.
 
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Mcgyver

Active member
Have you done anything about the tailstock quill/bore? That is often a very worn area and imo needs treatment one is putting the effort into scraping the rest of the lathe. Having confidence in the geometry of quill and bore plays into scraping the base as well.

I've been lapping or sunnen honing the bore, then hard chroming and grinding (have the plater do the grinding if you can't) the quill.

The the base gets scraped parallel to the quill. Lastly, check the tailstock quill to tailstock taper and regrind if necessary. You need to be able to hold and rotate the quill for that.
 

jwearing

Member
I haven't done much scraping on the tailstock yet, so far I have just been taking measurements. The original quill actually fits well, although it is pitted from corrosion and the taper is badly misaligned. I have second quill too, from a parts machine, but it is undersized and the fit is sloppy. I have a piece of TGP bar stock that I put in the quill bore and I'm using that for alignment. I don't know if it's bad enough to require honing, but if I have to make a new quill anyways maybe it's worthwhile.

Thanks for the comments everyone.
 

Mcgyver

Active member
To tell if its bad enough to need honing, check the bore with a bore gauge. Measure at various places along it. I'd be a bit surprised if the lathe was worn enough to need scraping but the quill/tailstock bore was fine. Not meaning to press it OT, just saying if that needs doing, and there's a good chance it will, it needs doing before you reference to it for scraping so survey it, measure etc.

I just did one, got the desired 3 tenths clearance with a round straight bore and round straight shaft. With a bit of oil, you can hold the tailstock vertically and the quill doesn't fall out. Ahhhhhh, goosebumps! Its kind of what you want and need if you going to the (very considerable) trouble of scraping a lathe
 

Tyrone Shoelaces

Active member
What makes you think the test bar has an error ? Have you checked it out ? Bought test bars are usually pretty accurate.

Regards Tyrone.
 

Richard King

Active member
TC has said what I would say....plus it seems as if the OP knows his answers before he asked them. There are dozens of this subject in the archives too. Also Mcgiver said...the bore.
 








 
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