A split nut is a device commonly found on a metal lathe and automatic feed-milling machines. It consists of two halves of a threaded fixture that can be closed around a threaded shaft or opened to slide freely along the shaft when the halves are separated. Commonly linked together with adjusting bolts, the split nut can be tightened to allow the threads to engage a rotating threaded rod, thereby driving the tool holder along the lathe bed by riding on the threaded rod. This enables the split nut to be loosened to disengage the threaded drive rod, which allows the tool holder to be manually slid back and forth along the lathe bed by hand.
By using a split nut, the device retains the simplicity of a hand-operated machine, yet it gains the complexity of an automatic-feed instrument. In extremely complex split-nut designs, the nut is divided into four parts containing both right- and left-hand threads. With this type of nut, the machine can be adjusted to have both forward and reverse automatic-feed capabilities. The most common material to create a split nut from is brass, though nylon is often employed for its ease of use without the need for heavy lubrication. Brass is a soft metal that has some form of self-lubricating properties, making it a good fit for the design as well.
The success of the self-feeding design often directly correlates to the manner in which the nut is held together. While the most basic styles are held together by small threaded bolts, some of the more intricate versions are actually held together by small springs. The halves of the nut are initially engaged by adjusting small screws or bolts, however, the final pressure is provided via a spring or pair of springs located on the adjusting bolts. If the pressure of the springs is too great, the tool bed can bind and stick as the fixture attempts to slide along the drive rod. If the spring is too light, the action can become jerky as the components slip against each other while the threads struggle to remain in contact with each other.
Once the initial set-up and adjustment is complete, most split nut drive devices provide very reliable performance for the operators of the machine. In most cases, two split nut drives are used to achieve forward and reverse operation. By disconnecting a nut on one side of the tray, the connected nut will drive it one direction. The operator simply disengages the current nut and engages the other to send the tool bed back in the other direction.