Let’s say you’ve recently bought a lathe and set it up in your shop. Maybe you’ve even gone and leveled it like a boss. You’re ready to make chips, right? Well, not so fast. As real machinists will tell you, you can use all the levels and lasers and whatever that you want, but the proof is in the cut. Precision leveling gets your machine in the ballpark (machinists have very small ballparks) but the final step to getting a machine to truly perform well is to cut a test bar. This is a surefire way to eliminate any last traces of twist in the bed.
There are two types of test bars. One is for checking headstock-to-ways alignment, which is what we’re doing here. There’s another type used for checking tailstock alignment, but that’s a subject for another day.
We start by chucking some stock. You want something of a substantial diameter, because we’re going to have a lot of unsupported stick-out, something you would normally never do. The stock needs to be as rigid as possible on its own. The more stick-out you have, the more precise your measurement of bed twist will be, but the test becomes impossible if stick-out is too much for the stock to remain rigid while cutting. It’s a tricky balance. For this demo on my small bench-top machine, I’m using 1-¼” diameter stock, 5″ long. For a large floor-standing machine, 2″ diameter stock around 10″ long is a good place to start.
Dial it in as close as you can in the four-jaw chuck. The more run-out we eliminate now, the quicker and easier this test will be. If you have stock with a machined surface, that’s ideal, but cold-rolled stock from the factory is generally fine. I’m using mild steel here, but something like 12L14 free-machining steel would make it easier to get a good finish (which helps with the measurements).
The general idea is that we’re making a barbell shape. We’ll make high-precision cuts on the ends, while leaving a narrower area in the middle that we can easily skip over.
With the stock dialed in, turn down a relief area in the center of the bar, leaving about an inch on each end untouched. We’ll only be measuring the ends, so the middle section will just be in the way. Making a relief also minimizes tool wear between cuts (which would affect our test results). A relief of 30-50 thou is sufficient. We want just enough room to clear a few test cuts on each end. Don’t relieve too much, because we need that rigidity in the stock.
Note that we’re not using the tail stock for support here. This is important because the tail stock introduces its own set of variables that affect alignment. We’re only testing headstock-to-ways alignment, so we can’t use the tailstock. This means we have to make very light cuts because our rigidity is very low.
With the relief made, we can now take very light cuts in the two measurement areas. We want just enough to clean up the surface all the way around (so we know we’re inside any runout in the chuck). I’m making two-thousandth cuts on each pass here. Make your pass on both measurement areas, without touching the cross-slide in between. Stop the machine at the end and measure, then wind the carriage back and do another cut as needed.
Once you have a clean cut on both measurement areas, compare the diameters with a high quality micrometer. If they are different, the machine is cutting a taper, which means your bed has some twist. Adjust or shim the lathe’s tailstock-end feet a little bit and make another cut.
A larger tail-end on the bar means the front-right corner of your ways is too low (the toolbit is getting closer to the work as it travels). If the chuck-end of the bar is larger, the front-right corner of your ways is too high (the tool bit is getting further away from the work as it travels).
How close you want to get these measurements is up to you, but a tenth of a thousandth over 5-6″ is likely good enough for anything a hobbyist is going to need. Once you’re done, you can oil and store the test bar for use later on. With a relief cut of 30 thou or so, the same test bar can be reused several times.
That’s all there is to it! Cutting a test bar is an easy hour-long project that will teach you valuable lathe skills and build your confidence in the machine. Once you know you can trust the machine, you’ll know that any future problems exist only between the hand-wheels and the drawing*.