To move the carriage forward and backward, a spiral gear placed on a shaft the length of the lathe bed is employed. A typical machine's speed and feed will be coarse because the spindle is driven by an induction motor and speed is controlled by varied ratio gear meshing. The motor and gears are housed in a frame that can be moved back and forth on the lathe bed.
A spur gear (not shown) is mounted on the end of the output shaft of the motor. This drives another gear called a drive gear which in turn drives the spiral gear. As the motor turns, the drive gear turns which in turn turns the spiral gear which in turn moves the carriage forward or backward.
Spiral gears were first used on lathes about 1910 and have been popular ever since. They offer many advantages over other types of gearing including better wear characteristics, less noise, and easier maintenance. Disadvantages include higher cost and lower efficiency than other forms of transmission such as helical gears and worm gears.
The figure below shows how a spiral gear works on a lathe. In this example, the motor drives both the spiral gear and the carriage gear through two separate gears called carrier gears. The carrier gears mesh with their corresponding gears (one carrier with one spiral gear and one carrier with one carriage gear) but they do not touch each other. This allows the motors to run at different speeds without affecting the movement of the carriage.
Gearboxes enable correct cutting speeds or torque to be obtained at the spindles as necessary in the case of cutting drives or desired feed rates in the case of feed drives by providing a broad range of cutting speeds and torque from a constant speed power input. They are used instead of direct drive on many types of lathes.
On turret-type lathes, the gearbox usually fits inside the headstock and is mounted so it can turn freely but remain close to its corresponding spindle. The gear teeth of the box mesh with those of the spindle, thus allowing the output shaft of the gearbox to drive the spindle at a ratio determined by the number of gear teeth on each component.
On fixed-head lathes, the gearbox is usually mounted on the end of the main body opposite the head. It contains the same number of gears as there are spindles and when it is put into gear by turning the handle or wheel that controls it, it will drive all the spindles simultaneously. These machines are most often used for heavy-duty work such as grinding wheels and drill bits. Fixed-head machines are more stable than turret-type machines because they do not depend on a separate support structure for their stability. Also, the user has better control over where the cutter is pointing during use.
A secondary speed change gearbox is installed beneath the headstock to alter the speed, allowing for variable feed rates for threading and automated carriage lateral movement. The feed rod is utilized for the majority of turning operations, whereas the lead screw is used for thread cutting. Both devices are controlled by a foot pedal or remote switch.
The speed change gearbox consists of two parts: a housing which contains all of the components, and a cover which fits into the opening at the top of this housing. Inside the cover is a cylindrical drum with notches cut in its outer surface. This cylinder is called a "sector". The number of notches on the sector determines the number of speeds available from the gearbox. As the sector turns, each notch comes into alignment with a ring gear attached to the end of the main shaft. When you press down on the foot pedal, it lifts up the sector, causing that many teeth to come into mesh with the ring gear. Thus, you can use different combinations of speeds by pressing down on the pedal as needed during a job.
Secondary drive mechanisms are available for most lathes, including belt-driven models and electric-powered ones. These devices allow for variable-speed operation without changing gears, which saves time when you are threading small holes or performing other tasks that require constant speed.
As seen in the image below, all Geared Lathes have three or more shafts placed therein. The gears are designed to achieve a variety of speeds, up to and including 18 spindle speeds. The center shaft has three gears, D, E, and F, which operate as a single unit and hence revolve at the same speed. The outer two shafts may have as many as 11 gears on each.
Geared lathes were developed around 1870 by John Deere in Illinois. They were an improvement upon earlier hand-powered lathes such as Thomas Jefferson's design that used a belt and pinion system for its speed control. Geared lathes were very popular into the 1920s when they were being replaced by electric power tools.
Gears are shaped like wheels with teeth cut into them. The number of teeth on each gear determines how fast it will spin. The greater the difference between the numbers of teeth on one gear and another, the faster those gears will rotate. On most modern geared lathes, the center shaft has no more than seven or eight small gears attached to it. These small gears mesh with larger gears mounted on the inner and outer shafts. As these large gears turn, they also turn the smaller ones, thus creating a continuous circle. This is different from early models where all the gears were the same size and only one or two would be needed to drive the other(s).