The Machinists’ Mantra: Level Thy Lathe

Nov 22, 2023

Let's say you’ve gone and bought yourself a sweet sweet metal lathe. Maybe it's one of the new price-conscious Asian models, or maybe it's a lovely old cast iron beast that you found behind a foreclosed machine shop. You followed all the advice for setting it up, and now you’re ready to make chips, right? Well, not so fast. Unlike other big power tools, such as band saws or whatever people use to modify dead trees, machine tools need to be properly level. Not, "Hurr hurr my carpenter's level says the bubble is in the middle", but like really level.

This is especially true for lathes, but leveling is actually a proxy for something else. What you’re really doing is getting the entire machine in one plane. Leveling is a primitive way of removing twist from the structure. It may not seem like a huge piece of cast iron could possibly twist, but at very small scales it does! Everything is a spring, and imperceptible twist in the machine will show up as your lathe turning a couple thousandths of taper (cone) when it should be making perfect cylinders. All this is to say, before making chips, level your lathe. Let me show you the way.

If you have a bench-top machine, start by leveling the bench to pathetic carpenter's standards. That is, level it with any old bubble level such that a pencil won't roll off. Personally, all my benches use grade 8 bolts as feet, which makes leveling a breeze.

The next step is to acquire a quality machinist's level. These differ from hardware store bubble levels in several key ways. First, they are incredibly sensitive, typically reading less than five thousandths of an inch displacement per foot (or less than 0.5mm per meter). Second, they can be self-calibrated. Third, they have precise ground surfaces on at least three sides, and a V-shaped bottom to minimize error from mating with the machine's surface.

Every time you use a machinist's level, you need to calibrate it. The cool thing about a level is that they are "self-proving". Here's how that works.

First, place the level on a clean and dry granite surface plate. Rotate the level until you find an axis that is level. There will always be one, because the granite plate is a plane (as good a one as humans can make anyway) and any plane will have one axis that reads level.

Next, place a heavy straight edge against the level, such as a 1-2-3 block or an angle plate. This holds your reference. Now you can flip the level 180°, place against the reference edge, and check it again. Using the calibration screw on the level, split the difference so the bubble reads halfway between those two readings. You’ll need to go back and forth a few times. Try to touch the level as little as possible. The heat from your hands will warm it up and cause it to change shape sufficiently to lose your calibration.

The goal is to get the level to show "zero" in both directions. As you close in on this, you’ll need to find new axes that are more level to use as reference. Once you get zero both ways, it has self-proven and calibration is complete.

Next, place the level on the ways (the machined parts where the tailstock slides) of the machine, down near the tailstock. Make sure the ways are clean, dry, and free of raised burrs. A 1-2-3 block may be needed for the level to sit flat. If you use blocks for this, measure them first to make sure they are the same. Inexpensive 1-2-3 blocks may not be! Precision-ground gage blocks are better, if you have them.

For a floor standing machine, adjust the feet until you get a level reading. If your machine has a lot of feet, try the "three point" method. Get your machine sitting on only three of its feet (or two feet and a screw jack). A triangle is much easier to get level. Once level, carefully lower the remaining feet until they take weight but don't upset the level.

For a bench-top machine, you’ll typically shim under the feet of the machine as needed. A variety pack of steel shim stock in various thicknesses is helpful here, or sacrifice an old feeler gauge.

Do the same procedure for the ways near the headstock, and also with the level placed lengthwise on the ways. You’ll likely need to go back and forth a few times until every position reads level.

Another school of thought says that, instead of placing the level on the ways, you should place it on the cross-slide, since that's where the cutting tool is. Personally, I think this just introduces new sources of error, but I like to double check on the cross-slide after the ways are level.

Leveling is not a one-time deal, so get comfortable with it. The machine needs to be re-leveled any time it is moved, and periodically as the floor shifts or settles. Even concrete moves, so check your machines from time to time and re-level as needed. You should also check a new machine after using it for a few weeks, as the machine's castings will "settle in" under the vibration of use. Check it again any time you suspect the machine is turning a taper, such as if you’re struggling to hit a dimension on a long part. Older machines are more flexible and will need re-leveling more often.

Once your machine is level, the next step is to really dial it in by cutting a test bar. This is a topic for a future article, so stay tuned for that. In the mean time, stay away from those carpenter types. Apparently that "wood" stuff is ruined by fire. Why would anyone bother with it?