Drive systems are an essential part of the CNC technology as they are instrumental in driving the machine along its axis. They are the mechanical components that convert rotary motion to linear motion. By altering components of the drive system, cutting speed, accuracy and precision can be controlled. It is important to understand the various mechanisms that contribute to performance of your machine. The following is a comparison of two components of the drive system:
The Ball Screw
The ball screw is the most common drive for CNC machines. It provides a high level of efficiency at 90-95%. The ball screw runs on ball bearings, which circulate around the screw and nut through helical grooves. Once the balls reach the end of the nut they are returned to the leading end of the circuit and are re-circulated. This allows there to be no physical contact between the screw and nut, which decreases friction and contributes to the high level of efficiency. The force is distributed over a large number of ball bearings, leading to a low load per bearing, and allowing for heavier loads to be processed with smaller motors or less power.
- Higher mechanical efficiency due to the lower friction (90-95%) compared to the 10-80% efficiency range of a lead screw
- Lower power requirements
The lead screw is run on what is called a thread helix which describes the grooves that are around the outside of the shaft. The spiral grooves on the outside of the screw match with ones that are on the inside of the nut. There are various technical considerations such as the direction of the threads, the size of the screw, pitch, number of starts, lead, and TPI. The pitch of the screw is the distance between the threads and the number of starts distinguishes how many threads are on a single screw. The lead, which is the distance the nut travels per revolution, is denoted as the pitch multiplied by the number of starts. Another important measure is the screw’s threads per inch (TPI) which refers to the number of grooves per inch, and can have implications for machine speed.
- Lower cost
- Low maintenance, small number of parts, compact, and simple