The business end of any lathe-the part that drives the work-is the headstock assembly. The headstock is fixed permanently at the left end of the bed and consists of either a casting, a welded steel body, or an extrusion that holds a spindle set in bearings. A pulley on the spindle is connected by a belt to a motor, which is normally mounted below or behind the headstock.
Spindle – The spindle, a threaded shaft mounted horizontally, is the heart of the headstock. It accepts the drive centers, fa ceplates, and other accessories that hold and power the work. Spindles are either hollow or solid and range in size from 1/2 in. to 11/2 in. dia.
The spindle size you need depends on the type of turning you intend to do. For turning spindles between centers, you can get by with a small diameter spindle. A common spindle size that is adequate for medium-duty work is 1 in. For heavy-duty faceplate turning and architectural turning, you will need at least a 1 1/4 in. spindle that will not flex under load. Watch out for spindles with odd thread sizes or odd taper sizes-what may seem like a bargain will be no bargain at all if you can’t easily obtain accessories to fit the spindle. The chart above lists common spindle sizes for which you have a good chance of readily finding accessories.
You should give much consideration to the spindle when choosing a lathe. The most important thing to look for is a hollow spindle that is machined to accept Morse-taper accessories. Morse tapers lock in place when inserted into the matching tapered socket in the spindle.
More than any other feature, Morse tapers separate good lathes fr om bad. Even if your turning needs are very casual, the advantages of Morse tapers are enormous. A Morse-taper socket in the spindle makes for quick and easy mounting of drive centers and a host of other chucking accessories. Morse tapers lock when driven home and release with an equal opposite force. They’re universal, so you’re not dependent on the manufacturer for replacement accessories. By contrast, on a lathe that has a solid spindle, all accessories have to be screwed on, which is not only time consuming but also limits the range of accessories available and ties you to the manufacturer.
Morse tapers are available in sizes #0 through #7. They have been widely copied in the forms of other locking tapers, such as the American Standard taper, the British Standard taper, the Brown and Sharps taper, and the Jarno taper.
A further consideration is the height of the spindle above the bed, which dictates the swing of the lathe and the diameter of the work that can be turned. At its simplest, swing is double the height of the spindle center over the bed. For example, a lathe with a center height of 6 in. will swing 12 in. You have to be careful of manufacturers’ claims because they sometimes quote the swing over a “gap,” which is a short dip in the bed just ahead of the headstock. This gap, which is typically about 2 in. deep, allows you to turn larger-diameter fa ceplate work in this area. The problem is that when work extends into the gap you can work only on the face (the exposed side) of it.
Unfortunately, twice the center height above the bed is not a true measure of capacity. A better yardstick is to measure the distance from the top of the tool base to the center of the spindle. Doubling this will give you the true swing, which is the diameter of work the lathe will swing between centers. A lot of faceplate turning also requires placing the tool base under the work.
Bearings – Bearings hold the spindle rigidly in place and allow it to turn with a minimum of fr iction. They’re an important consideration when buying a lathe. The problem is that bearings are much harder to judge on cursory examination than features such as the construction method or spindle type.
Historically, lathes have run on plane cast-iron bearings, sleeve bearings, and babbitt bearings (see the illustration on p. 16), but most lathes made today have rolling-element bearings. These include ball, roller, and tapered-roller bearings (see the photo below right ). Of these, ball bearings are by far the most common type used in lathe construction. Each bearing consists of an inner race and an outer race that are separated by a series of steel balls. The opposing ends of the spindle ride on these rings of balls, which provide a precise hold and allow the spindle to turn with a minimum of effort. The majority of the power from the motor can now be used for productive work.
Outboard turning feature – Because some work will b e too large in diameter to swing over the bed, lathe manufacturers often design the headstock so that work can be mounted on the outboard side. Outboard turning is accomplished in two basic ways. One is to put a left-hand thread on the outboard end of the headstock spindle and mount the work so that it faces away from the lathe. The other option is to design the headstock so that it pivots at right angles to the bed.
When work is mounted on the outboard end of the headstock, the swing of the lathe becomes double the distance from the center of the headstock spindle to the floor. Although this may sound like an ideal solution for turning large-diameter work, outboard turning presents a number of problems, the greatest of which is speed. Many lathes do not have a low enough speed range to turn anything greater than 1 2 in. in diameter. For large-diameter turning, you need a bottom speed of 1 5 0 rpm to 300 rpm.
Another problem with outboard turning is that there is no place to support the tool rest. You have to use a floor-stand rest (typically a tripod stand that holds the rest) , but this device is not as solid as a standard rest. In addition, this method of outboard turning sometimes requires a second set of left-hand-threaded faceplates, and it demands that you do everything in reverse of your normal way of turning. (Delta makes many of its faceplates threaded in both directions, which means they can be screwed on either end of the spindle.)
Some lathes that use the outboard end of the spindle for outboard turning have a special tool base attached rigidly to the lathe, which is a much better setup. In effect, this creates a small bowl lathe that is a mirror image of the actual lathe.
The second option for outboard turning, the swing-head design, is well adapted to light- and medium-duty lathes. In a swing-head lathe, the headstock pivots at right angles, and the work is turned in front of the lathe (see the photo at left on p. 1 8 ) . A special tool base attaches to the base of the headstock into which the tool rest fits. Although such a setup allows only a limited outboard swing of about 1 6 in. to 20 in., it’s a better arrangement for a number of reasons. First, the diameter of the work is limited to within the low-speed range of the lathe. Second, the tool base is attached rigidly to the lathe bed, making turning sure and safe. Third, turning is in the same direction and orientation as takes place over the bed. Finally, a second set of left-hand-threaded faceplates is not required.
It has been my experience both in selling lathes and advising buyers of lathes that turners place unnecessary emphasis on turning outboard. In reality, outboard turning is something that most people will do only once or twice in their lifetimes, if at all. Don’t have tunnel vision about the outboard feature, ignoring all the other useful features that a lathe should have and that you will use every turning session. You can always rig something up or borrow a lathe for the once-in-a-lifetime big job.
Index heads Some headstocks are fitted with an index head, which is a mechanism that allows the spindle to be locked at equal intervals so that layout or auxiliary operations can be performed. Examples of such applications include laying out a clock face or a fluting pattern on a bowl or milling reeds or flutes in a column with a router (see pp. 1 02-1 04 ) . The most common setup for indexing is a series of holes drilled in the headstock drive pulley (see the photo above right) , which is mounted on the spindle. A pin in the headstock casting slides into the appropriate hole and locks the spindle in place. Common hole patterns are 1 2 , 24, and 60. I like the 24-stop configuration because it allows me to divide the circumference of a workpiece into many different but equal sets of parts: 24 (for fluted bowls and the like ) , 1 2 (for clock faces ) , 8 (common for period furniture shapes) , 6 (also for furniture) , and 3 (I have never used this).
Indexing i s a feature that may or may not b e of value t o you. I have spoken to many turners who have never used the index head. For certain types of period-furniture turning, such as fluted legs, it is essential. If you are considering a dandy lathe at a bargain price but it lacks indexing, buy it anyway. An index head can be added later if you need it. There are several after-market chucks that incorporate the feature, or you can rig something up yourself. Note that on most lathes the index head should not be used as a spindle lock for removing faceplates because doing so may bend the indexing stop. Most lathes have other provisions for locking the spindle for faceplate removal.