The lathe is known as the King of Machine Tools for a reason. There are very few things that you can’t make with one. In fact, people love to utter the old saw that the lathe is the only machine tool that can make itself. While catchy, I think that’s a bit disingenuous. It’s more accurate to say that there are parts in all machine tools that (arguably) only a lathe can make. In that sense, the lathe is the most “fundamental” machine tool. Before you harbor dreams of self-replication, however, know that most of an early lathe would be made by hand scraping the required flat surfaces. So no, a lathe can’t make itself really, but a lathe and a skilled craftsperson with a hand-scraper sure can. In fact, if you’ve read the The Metal Lathe by David J. Gingery, you know that a lathe is instrumental in building itself while you’re still working on it.
We’re taking trip through the machining world with this series of articles. In the previous article we went over the history of machine tools. Let’s cut to the modern chase now and help some interested folks get into the world of hobby machining, shall we? As we saw last time, the first machine tools were lathes, and that’s also where you should start.
With that bit of pedantry out of the way, let’s talk about why lathes are fundamental. Remember how I said that machine tools cleverly create parts that have greater precision than they themselves do? The lathe is the primary example of that.
First and foremost, the machine spins the work while the cutting tool remains stationary. This may seem arbitrary, but it most certainly is not. The earliest lathes held the workpiece between two sharp points. This is a clever way to achieve extremely high precision on one axis. So much so that turning “between centers” is still the go-to technique today when maximum precision is needed. Two points form a straight line, and by supporting the work this way, we eliminate all sources of “run out” or imprecision in the drive mechanism. This in turn reduces the precision problem to figuring out how to move the tool back and forth while maintaining a well-controlled distance to the work. This is a much easier problem to solve than any other mechanical geometry a machine tool might use, and thus why lathes are king.
The lathe is the place to start for getting into machining as a hobby because it teaches you the fundamentals that apply to all machine tools, while also being the most generally useful. The deeper you get into lathe work, the more you realize how little there is that it can’t make. Most other machine tools exist not to do things the lathe can’t, but rather to make those operations simpler to set up or faster to perform.
Sizing Up Your First Lathe
Okay, so you’ve made the wise decision to buy a lathe. Where should you start? The first decision to make is size. The rule of thumb among Crusty Old Machinists™ is that you must buy the largest machine you have space for. The saying goes that you can make small things on a large lathe, but you can’t make large things on a small lathe. Like most old sayings, it’s only kinda true. Using a machine suited to the size of work you’re doing makes sense. If you want to make clocks, RC cars, or models, get a small bench-top machine. If you want to blueprint an engine for a racecar or repair the town’s historic steam locomotive, get a large floor-standing machine.
Lathes are measured in “swing” and bed length. A designation like “7×20″ means something 7″ in diameter can be swung around without hitting the bed, and the bed itself is 20″ long. Like most numbers applied to consumer products, these are both misleading. While a 7″ object may technically fit, the machine probably doesn’t have the horsepower or toolpost reach to actually work on something at that limit. Similarly, that 20” bed length quickly gets eaten up by the tailstock, drills, and other tooling that need to be inside your work envelope. In that sense, the old saying is true — get a machine that is as big as you think you can fit within the category of work you want to do.
I’ll leave you now to think about how big your machine should be. Next time we’ll get into what to look for in a machine, how to buy one, where to put it, and so forth. Stay tuned!