Tailstock adjustment : go for the drill chuck or live center ?

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compact8

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I have an Emco Compact 8 bench lathe and I recently found that the live center is 0.03 mm ( slightly more than a thou ) above the spindle while the 3-jaw chuck is 0 ~ 0.02 mm below the spindle depending on the size of the drill shank. I have already tried mounting the drill chuck at different angular positions but the center of the chuck is alway beneath that of the spindle. I believe this is due to the weight of the chuck

This is probably no big issues but if I want to fine tune it by shimming or milling the lower half of the tailstock, should I aim at perfect alignment for the live center or the drill chuck ?
 
I wouldn't trust either one of those items if I wanted to accurately measure the TS alignment on my machine. Both are only going to be so-so references. Get yourself a dead center that fits in the TS quill and use that.

The fact that the numbers vary for the drill chuck suggests that either the chuck or the drills aren't perfect. Probably neither is. And a live center has to have enough free play so it can rotate. Another word for free play: "slop". Some live centers are better than others, but.....why not just go with a dead center and bypass all those variables?
 
The tolerances for lathes provide for the tailstock quill to be high (above center) by a certain amount when new, this provides for allowance for future wear. I refer to Schlesinger's universal tolerances for machine tools; remember that nothing in this world is perfect, nor does it need to be prefect.
 
I have tried such as that, found that the weight of the indicator can effect the accuracy of the reading,
 
compact8 said:
I have an Emco Compact 8 bench lathe and I recently found that the live center is 0.03 mm ( slightly more than a thou ) above the spindle while the 3-jaw chuck is 0 ~ 0.02 mm below the spindle depending on the size of the drill shank. I have already tried mounting the drill chuck at different angular positions but the center of the chuck is alway beneath that of the spindle. I believe this is due to the weight of the chuck

This is probably no big issues but if I want to fine tune it by shimming or milling the lower half of the tailstock, should I aim at perfect alignment for the live center or the drill chuck ?
Click to expand...

I think that you are already pretty "fine tuned". Moving the tailstock up or down that small amount will acomplish nothing tangible in a measured part. Tailstocks are made high when they're new, and they wear towards low.

You can do the math if you want on what errors the height differences you have will make, but if you round off the results based on what the lathe is capable of, and what measuring tools are capable of outside of a temperature controlled laboratory, the discrepancies are going to round to zero.
 
There is quite a bit to unpack here.

First: it is not the indicator that is sagging due to gravity - it is the indicator stand. A misapprehension spread by a famous person whom I won't mention.

You eliminate this by chucking your indicator in the chuck (or better yet a collet). Sweep the opening in your tailstock with a half thou (or better) TEST indicator. This will tell you 90% of the story. No harm will come from your tailstock being 1 thou high. (your indicator will always rotate in the centre of the headstock no matter how out your chuck is)

You can adjust the tailstock by removing material - if you have the skills and equipment. DO NOT POINT THE TAILSTOCK DOWNWARDS TO GET YOUR CENTRE TO BE LOWER. The movement of the quill will make the quill drop more as you play it out, and will cause you to break small drills. (and other not-so-good side effects)

The process to lower a tailstock requires a surface plate, scraping equipment, a tenths indicator with a stand that is appropriate, such as a quality height gauge with the adapter, or a surface gauge with a large vertical shaft. You have to blue in the base to the top first, as many have very little contact area (!!). Even on a 50k$ Leblond this is true (mine is in showroom condition, with less than 200 hrs on it). After refining the fitting, it may well be that you are already on center.

Measuring the angle is an intricate procedure, but there are shortcuts. I would have to write a 10 page pamphlet on tailstock aligning, so there is no room in a short reply. Sorry about that.
 
Jake M said:
I think that you are already pretty "fine tuned". Moving the tailstock up or down that small amount will acomplish nothing tangible in a measured part. Tailstocks are made high when they're new, and they wear towards low.

You can do the math if you want on what errors the height differences you have will make, but if you round off the results based on what the lathe is capable of, and what measuring tools are capable of outside of a temperature controlled laboratory, the discrepancies are going to round to zero.
Click to expand...
From what I have read, a misaligned center will create bending stress on the workpiece that changes in direction rapidly as the spindle turns resulting in bad things such as wearing out of the work piece at the clamping points although the cutting force does the same thing anyway. After reading all the replies, this is probably over worrying
 
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