Precision Tailstock Alignment

There are at least 2 problems with this approach:

1. What about the case where the tailstock bore is higher than the headstock?
2. How do you compensate for the weight of the tailstock and bar? Tailstocks are quite heavy and the bar will be sure to bend a little.

Center-drill both ends of some rod stock, turn both to the same diameter with a finish cut at the tailstock end, to make the center hole and OD concentric, then saw or part them off in a chuck and turn

+file the cut faces smooth.

If you hold these buttons together with the head and tailstock centers any misalignment will clearly show at the edges. It will change as you extend the tailstock if the bore isn't in line with the spindle. You can also see any shift from tightening the clamps.

Vertical misalignment isn't too serious. My nearly 50 year old lathe was abused at a trade school and doesn't have the originally-fitted tailstock spindle, which had been misused as an anvil horn. It drills a centered hole only with a short Collis center drill holder, not with a chuck which droops. But usually the larger drill bit in the chuck plainly jumps up to center height and doesn't wobble. If it does I bore to a continuous chip and resume drilling with a larger bit.

jsw

I forgot to mention that my tailstock is about 0.5mm low. I thought about the effect of tailstock weight. If the morse turned bar is as short as possible the deflection should be small, but could be considerably reduced or entirely eliminated as follows: weight the headstock prior to fitting it to the morse taper. After fitting to the morse taper, arrange a spring balance to pull upwards on the tailstock by the amount just weighed. I'm not certain where the spring balance should be attached on the tailstock for perfect counteraction of gravity effects, but I suspect it's at the point that would suspend it horizontally with no other influences acting on it.

Scrim

Thanks - interesting idea. I think the idea that vertical tailstock misalignment isn't a big problem only applies for taper turning by offsetting the tailstock. For drilling purposes, vertical offset is as troublesome as horizontal. My tailstock is about 0.5mm low and so small drills are severely stressed and large drills cut oversize holes and with only one cutting edge - i.e. only one flute fill with swarf!

Scrim

Center-drill both ends of some rod stock, turn both to the same diameter with a finish cut at the tailstock end, to make the center hole and OD concentric, then saw or part them off in a chuck and turn

+file the cut faces smooth.

If you hold these buttons together with the head and tailstock centers any misalignment will clearly show at the edges. It will change as you extend the tailstock if the bore isn't in line with the spindle. You can also see any shift from tightening the clamps.

Vertical misalignment isn't too serious. My nearly 50 year old lathe was abused at a trade school and doesn't have the originally-fitted tailstock spindle, which had been misused as an anvil horn. It drills a centered hole only with a short Collis center drill holder, not with a chuck which droops. But usually the larger drill bit in the chuck plainly jumps up to center height and doesn't wobble. If it does I bore to a continuous chip and resume drilling with a larger bit.

jsw

Scrim,

Having been involved with machine tools most of my working life I have seen the reworking of Industrial lathes by machine tool rebuilders and if the bedways are reground the headstock and tail stock castings are remachined to realign with each other, having looked at Arceuro's website for your lathe it shows that it is likely that the headstock can be removed from the bedways. You first need to determine what actual errors you have and to do this I would turn a piece of bar in the three jaw chuck into a 60 deg point and mount a centre in the tailstock then with a DTI determine the errors by using a ground bar with centres drilled in each end mounted between the centres. once you have the figures you can machine the base of the headstock using the readings to lower the headstock to align with the tailstock.

Martin P

I've heard that the manufacturing tolerance on tailstock height intentionally biases it slightly low to avoid high ones, since you can more easily shim up in the offsetting joint than cut down. Likewise faceplates are slightly dished if they can't be perfectly flat, so the work will seat at the edges where it's clamped.

jsw

I haven't fully characterised my lathes error since it's been ok for me except for the tailstock - so I thought I'd just try to fix that. In retrospect I guess I'd be better off ascertaining the alignment of the headstock to the bed, and how flat the bed is first. Interesting idea to lower the headstock to match the tailstock - the result would be much neater than shimming the tailstock so I might just do it that way, thanks!

Scrim

Lowering the headstock to match a re-aligned tailstock is a recognised method much favoured by some of the most respected machine (re)builders. A word of caution however, make sure you have checked and corrected any droop in the tailstock first or you may be aiming at a moving target so to speak.

The subject you are discussing is complex and extensive, but that's not intended to put you off, but you need to consider quite carefully what you do first and the ramifications in terms of subsequent work. One thing I can guarantee is that if you try glueing the tailstock in place hanging off a MT bar in the headstock it will be wrong, but more difficult to fix than it is now.

If you want a fairly serious read on the subject, there are pushing

500 pages here: Although the thrust is, as the title suggests, applications of hand scraping, a significant part of the book is how to measure, check, correct and align machine tools; including mills lathes and grinders.

Also, although specifically a test sheet for a particular lathe, these give you an insight into the measurements you need to make and the sort of figures you might like to aim for.

Bear in mind the machine in question is a pretty high class piece of old English iron so you are most unlikely to achieve these results with a minilathe which is an altogether different beast.

Happy reading Richard

Thanks very much for that,

Scrim