Almost all woodworking lathes are supplied with a single-phase induction motor. The motor typically mounts inside the stand or at the back of the headstock and is connected to a pulley on the spindle by a belt. For small lathes, l hp is adequate, whereas bigger machines require 1 hp or even 1/2 hp. Mini lathes can get by with as little as l/4 hp. Machines imported from Asia frequently have power specifications on the motor nameplates that are optimistic. Often such machines are quite serviceable otherwise, so the best course is to replace the anemic motor with a U.S. model if it lacks the desired power or burns out.
Increasingly, manufacturers are putting variable-speed direct-current (DC) and alternating-current (AC) motors with solid-state controllers on lathes. Today’s solid-state circuitry allows the use of controllers that efficiently make DC current from ordinary single-phase household current. This makes it possible simply to dial a speed, which is much more convenient than having to move a belt by hand to change speed. Such a controller cannot be used with a standard induction motor; it requires a DC motor. DC variable speed requires both a controller and a DC motor.
For variable-speed AC, a solid-state single-phase controller is attached to a standard three-phase motor. The controller takes your 60-cycle single phase electrical house (or shop) current and delivers three-phase current at an any cycle rate between about 2 and 65 cycles. The speed of AC induction motors is controlled by the cycle rate of the current, so slowing the cycle rate slows the motor. With this system, the motor is inexpensive but the controller is about the same price as a DC motor and controller combined.
Either AC or DC solid-state controllers work beautifully and are my preferred form of power. The drawback to both AC and DC variable-speed power is cost. They are expensive-$500 and up. Still they offer the easiest (and often the cheapest) way to add variable speed to a lathe that does not have this feature.
The original drive system for connecting lathes to a power source was a flat leather belt. A three- or four-step set of matched pulleys gave a good range of speeds for the turner. Although flat leather belts gave very constant speed with no surging, they tended to slip, thus wasting power.
Most lathes made in recent years use V-belts for power transmission. V-belts drive positively because greater tension on the belt causes it to wedge tighter in the pulley groove. An additional advantage of V-belts is that manufacturers can provide variable speed by installing a variable width pulley set. A mechanical control adjusts the width of the drive pulley, which effectively changes the pulley diameter and thus the speed. Moving the two halves of the drive pulley apart decreases the diameter (and decreases the speed) ; squeezing the halves back together does the reverse.
The mating pulley on the headstock is similarly split but is spring loaded so it automatically adjusts to the state of the drive pulley. This setup gives a wide range of infinitely variable speeds. The only drawback to such a speed-control system is that it wears out belts faster, necessitating annual belt replacement for a lathe that gets moderate to heavy use.
A recent innovation is the poly V-belt, which is a flat rubber belt with a series of small V-ribs machined on the inside surface. This design gives the belt the positive drive characteristics of a V-belt with the constant velocity of a flat leather belt. Many newer lathes run on poly V-belts.