Originally written by Carla Satra in an owwm.org forum discussion. This was in the context of leveling a Monarch lathe but the advice is readily adapted to any precision machine.Get or borrow a precision level, such as a Starrett #199 or equivalent, a '10 second' level, which is graduated to 1/2 thou (.0005) per foot accuracy.
Also, get or borrow a set of 1-2-3 blocks, or any two blocks 1" or so in thickness which have been ground together, i.e., are exactly the same dimension. Use these on the flats of your lathe bed, after carefully touching the bed with a very fine stone to remove any slight raised burrs, as might be caused when a tool-bit or a part was dropped on the ways. Those can be impossible to see, but you can feel them with a fine stone, and remove them without taking metal off the way surfaces.
You'll need the matched set of blocks to allow the level to 'bridge' over the 'V' areas of the ways.
As you'll notice, there are four levelling screws on the headstock base, and another four on the tail base.
Put some pieces of flat steel between the levelling screws and the concrete floor, to prevent the screws from just digging into the concrete, and snug up the screws, so that each one is taking its share of the weight of the machine.
The lathe should be level longitudinally, of course, but the important consideration is that its levelled across the bed, taking one reading as close to the headstock as you can get, and another at the far right end of the bed.....look closely for burrs, and remove any you find there, as it is common for operators to lay tools and stock across the bed at the right end, and subtly scar the way surfaces.
You would not think that a casting as heavy and strong as a Monarch lathe bed could twist by having its ends supported out of level, on a non-level concrete floor, but it can, and does, and that almost certainly has happened to yours. When I say 'twist', of course, I mean by only a few thou, but thats enough to cause the lathe to turn taper.
After carefully levelling the lathe, check it monthly until it 'settles in', which may take a year or more. You may have to adjust the levelling several times before the machine is 'settled-in'.
Almost every old lathe you'll find will have been used, or stored, out of level, and it could take some time for the induced strains to work themselves back out of the castings. The strains which have subtly warped the bed for years will take at least months to 're-settle' themselves, once the bed is correctly levelled.
Those Monarch lathe beds were ground to 'tenths in a foot' accuracy when new, and yours should at least work within .001 per foot unless badly worn or abused by careless operators in times past. (the number of 'tenths in the foot' seems to vary with whose story of working at the Monarch one chooses to believe, but it is a small number........with the EE model, they claimed a spec of 'millionths in the foot'.
Depending on the amount of use your machine has seen, it may have some wear on the tail bottom. If you don't have/can't borrow a proper test-bar, then sweep the Morse taper hole in the tail spindle with an indicator held in the chuck. You may need to put a thou or two of shim between the tail casting and the tail bottom, to bring the tail centre height into alignment with the spindle centre-line.
Your 12" is probably a 700 rpm machine, that is, 700 being the fastest spindle speed. Per the old Monarch specs, it can be re-speeded up to 1200, which was offered as a factory option, but thats not really worth doing, unless you are planning on production work in small diameters. You may need to take up a little on the Timkens, but, if you do, be very careful, and feel them for warming when running at higher speeds. A little warming is alright, but no more than that.
If your machine wasn't badly worn or abused, once you've gotten it levelled up and the gibs set to your liking, and have 'made friends with' the lathe, you can easily work 'within the thou', and get good finishes.