Turning Heads, and Other Parts: The Engine Lathe

Today we’re going to take a look at my engine lathe. It’s down in my basement shop, and it’s one of the most important tools I own. Let’s get to know its parts and what they do.

The Major Parts

Lathes work by rotating the piece to be worked on against a cutting tool. This particular lathe is a MicroLux 7″ x 14″ lathe, sold by Micro-Mark. (They have discontinued this model in favor of a slightly larger version, but they’re similar.) This is quite a small lathe, as lathes go, though there are certainly smaller ones around. The lathe is actually made in China by Seig. It is imported to the USA by many companies, in a number of configurations, and at different levels of fit and finish. They’re generally referred to as “mini lathes”. This one is one of the better grades of Seig lathes, based on what I’ve seen. I have had it for about 18 years, and it has served me well.

The Headstock

The heart of the lathe is the headstock. The main spindle of the lathe, which turns the work, is here, along with the transmission for selecting drive gear, the transmission for driving the leadscrew (more about that later), and the motor controls.

The motor controls consist of a switch that selects off, forward, and reverse and a knob that controls the motor (and thus spindle) speed. The green pilot light indicates that the system has power, and the yellow indicates a fault. Above the knob is the main power/emergency off switch.

When the motor is in “forward”, the chuck will spin counterclockwise when viewed from the tailstock of the lathe.

There are two very important controls on the back. The headstock shift lever selects between high gear and low. High gear spins the chuck faster at a given throttle level, at the cost of low-speed torque. The leadscrew shift lever selects between forward, reverse, and neutral. It controls the direction of the leadscrew with respect to the direction of the motor and spindle.

The Change Gears

In normal practice the change gears hide behind their cover.

The gearing system hides back here and drives the leadscrew. Since it takes power from the spindle the speed of the leadscrew is proportional to the speed of the spindle, and it slows down or speeds up in concert with the speed at which the work is spinning. The relative speeds of the two are controlled by the change gears, which are the bottom four gears in the photo below.

The Chuck

The chuck holds the work to be shaped. There are other ways to hold the work as well, depending on the part and the level of precision needed. Chucks remain the most common, however, particularly for home shops when you aren’t making large numbers of the same part.

This particular chuck is a 3-jaw scroll chuck, meaning the jaws work a lot like the chuck in your drill. They move in and out together, so the work remains centered in the chuck. I also have a 4-jaw chuck where the jaws move individually and independently. It allows me to hold irregular shapes, and also to hold the work deliberately off-center, to machine features that are off-center to the work.

The Lathe Bed

The lathe bed is the foundation of the machine. It holds the headstock (and thereby the spindle) in alignment with the rest of the lathe. When I installed the lathe in the shop I had to carefully level the bed using a high-precision level to ensure that the lathe was straight and untwisted. If it weren’t, it wouldn’t cut accurately. The lathe bed is cast iron.

The Leadscrew

As we’ve discussed, the leadscrew turns proportionally to the spindle. It runs the length of the lathe bed. I can stop it, alter the direction it turns, and alter the rate at which it turns in proportion to the spindle.

The reason that’s important is about to become clear.

The Carriage

The carriage controls the cutting tool. It can move forward and backwards across the axis of the lathe. That movement controls how much material is cut from the workpiece at once. The compound slide can pivot on the cross slide, so it can move at an angle to the axis of the lathe. You can make tapered shapes using the compound slide.

Finally, the toolpost sits atop the compound slide. It can also be angled to present the tool at an angle to the work. The tool is held in the tool holder, which is attached to the toolpost. My lathe has a “quick change toolpost”, which allows me to quickly switch between cutting tools while making a part.

The feed handles for both the cross slide and the compound have measuring dials graduated to one thousandth of an inch. This allows fine precision. The dials can also be re-zeroed so you don’t have to do a lot of mental math to figure out how deep to make each cut.

The carriage is attached to the apron.

The Apron

The apron makes the carriage move left and right. It can move via the handwheel, which turns a gear that runs on the gear rack. It can also move via the half-nut. The half-nuts (there are two) surround the leadscrew, one on top and one below. The half-nut lever can spread the halves further apart, or bring them close together. When they’re spread apart, the leadscrew is disengaged from the apron, and the apron moves via the handwheel. When they’re close together, they actually mesh with the leadscrew. That, in turn, forces the apron to move at a rate proportional to the turning of the spindle and chuck (and thus the work). Since the carriage is attached to the apron, it goes along for the ride.

The lead screw and half nuts allow us to move the carriage along at a known rate. We use that fact to our advantage when we cut threads, and we also use it to achieve a nice, even finish on our parts.

The Tailstock

The last important part we will talk about is the tailstock.

The tailstock has a spindle that moves in and out (left and right with respect to the operator). In defiance of its name, the spindle does not spin. It just moves in and out. The handwheel, however, turns a screw that controls this in and out motion of the spindle. The spindle lock prevents the spindle from moving, and the locking lever keeps the whole tailstock from moving.

The spindle is hollow, and the inside walls form a taper. This taper will mate with a shaft bearing the same taper. That allows a tool to “stick” in the taper and stay put. You can release the tool by fully retracting the spindle with the handwheel. This causes the screw to protrude enough into the taper to push out the tool.

It’s important to note that the spindle lines up exactly (in theory) with the spindle of the headstock, both up and down and forward and back. That means you can support long workpieces with a tool in the tailstock so they don’t bend.

The most common use of the tailstock is to drill holes in the end of a workpiece, either to accomodate the live center, or just because you need a hole for something.

And that’s a lathe!

That was a lot to absorb, wasn’t it? The lathe is pretty complicated, but really, what it does is make things round. Sometimes it’s an object. Sometimes it’s a hole. But when you work on a part in a lathe, you’re making something round.

Next time we’re going to try to bring this together by showing how parts are cut on the lathe. We’ll see you then!