20th Aug 2019, 11:50 AM #1
Checking centre alignment on a new or secondhand lathe
One of the things to check when buying a new or secondhand lathe is whether the head and tailstock centres are aligned. Centre misalignment can be a significant problem for most wood lathe operations.
I wouldn't expect any of the following issues on a new quality brand lathe that is installed on an even floor or benchtop, but not everyone can afford one (or two) of those.
I recently provided some of the following details in response to another thread which I misunderstood as a lathe centre alignment issue. I thought why leave that buried away inside a thread where it is unlikely to be found. So, I'm reproducing it here in its own thread with a few supplements.
Typically centre alignment is checked by bringing the tailstock with a centre in it up to the headstock with another centre in that. If the centre points don't kiss precisely there is misalignment.
If there is misalignment, the next thing to check is whether the points are offset horizontally or vertically?
If the misalignment is vertical, then you have either a manufacturing fault or a severe wear issue with the tailstock and/or bed. If the tailstock is low then this can be fixed by shimming it up, but a challenge with still allowing the tailstock to slide. We had this problem with a new but cheap lathe at the local men's shed. The solution there was to put a full length shim under and attached to the tailstock casting. Depending on how the headstock has been manufactured, it may be possible to raise that if that is low, but a more challenging task I expect in most instances.
If the misalignment is horizontal and you have a fixed headstock and there is no slop in the tailstock then you have a manufacturing fault that is probably not readily fixed and that lathe would best be avoid. But, if there is slop in the tailstock the misalignment might be resolved with lateral fettling and shimming, but you would have to love doing that sort of thing to go down that road.
If you have a swivel headstock and there is horizontal misalignment when the headstock is locked into the pre-set inboard position, loosen off the locking pins and levers and see if a double ended Morse taper will readily fit and bring the head and tailstock into alignment. If so, lock in place without the pre-set pin. If not, there could be a more significant problem and another lathe to avoid.
If there is both horizontal and vertical misaligned you may have a twist issue. This may not be obvious on the showroom floor or the sellers premises, but manifest itself when the lathe is relocated to your workshop. Cast iron lathes can twist if on an uneven floor. Place winding sticks either end of the bed to check for bed alignment. If there is twist, raise the leg that is down to bring the bed back into alignment. Then check again for centre point alignment. I had to do this for my Woodfast C1000X on my less than fully flat workshop floor.
The above so far has dealt with parallel or offset misalignment, but there is a shortcoming in the above method of checking for true alignment. You can do all of the above and get the centre points kissing but there can still be a misalignment problem. The quickest way to check for this misalignment is to put a long piece of wood firmly into chuck jaws and bring the tailstock with a dead centre in it (or a pencil in a Jacobs chuck) up to the end of the wood while turning slowly. If the dead centre or pencil scribes a small circle in the end of the wood you have this form of misalignment in the headstock. This is caused by the following misalignment, which I have exaggerated to illustrate the cause.
So, in both the case of a fixed headstock or swivel headstock lathe, the points can kiss when brought together but there isn't true axial or angular alignment. This becomes more of a problem the further the tailstock is positioned away from the headstock. Definitely an issue if the shaft through the headstock isn't parallel vertically and horizontally with the lathe bed on a fixed headstock lathe or with kissing centres only being achieved with angular misalignment on a swivel head lathe. The manufacturers of cheaper lathes might ensure that the points kiss when brought together (by whatever means), but their engineering shortcomings can become an issue when the head and tailstock are well apart.
If the circle scribing test outlined above indicates angular misalignment, use a bored through Morse taper and sight through it with the head/tailstock well apart to diagnose how it is misaligned. Without ready access to a bored through Morse taper, devise something by way of an aperture front and back of the taper bore to sight through.
The twist covered above can also cause this problem to a lesser extent, so return to check that before accusing a manufacturer of poor manufacturing; it could just be your uneven floor or benchtop. But, if there is no twist in the bed and the headstock bore is in fact misaligned with the bed it may be a problem that is not readily rectified.
There are many other things to check when buying a lathe, like a headstock thread that causes runout, which are separate issues that I've not covered here, but are more likely to be covered elsewhere on this and other forums.
So, those are the issues relating to centre alignment that I'm familiar with brought together here.
Others with experience on this may have something to add.Stay sharp!
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22nd Aug 2019, 12:11 PM #2
Simple and effective for that lathe, which only gets a few hours work each week.Stay sharp!
22nd Aug 2019, 06:38 PM #3
On any lathe with a machined bed (eg. not round tubular ways) one of the first things I do, before even checking the kiss, is run a short straight edge across the ways.
Not along the bed - although that's also a good idea - but across it, at several points, checking that the machined sections of both ways are in the same plane. While I'm at it, I take note of whether the spacing between the two ways is also generally consistent.
I've encountered lathes where one of the ways is twisted slightly in relation to t'other, which can introduce all sorts of problems if you're doing long-hole boring, etc. I'm no metal machinist but I'm given to understand that under certain circumstances the metal can move after machining (much like wood) to relieve stresses.
What this'll do is make the tailstock lean back 'n forth as it's moved along, again moving the tail spur off centre. Using a laser pointer to check, it may kiss at both extremes yet be off a few mm in the middle!
It only takes 30 seconds to check, then I'll know whether it's even worthwhile wondering about whether the spurs align...
Just as an aside, in spindle turning this sort of misalignment rarely matters... it's usually in bowl turning where one tool or another is fixed to the tailstock that it becomes a PITA.
- Andy Mc (AKA "Ghost who posts." )
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