Chapter 20: Rapid Positioning

OVERVIEW

A CNC machine tool does not always cut material and make chips. From the moment the cutting tool becomes active in a part program, it goes through a number of motions - some are productive (cutting), others are nonproductive (positioning).

Positioning motions are necessary but nonproductive. Unfortunately, these motions cannot be totally eliminated and have to be managed as efficiently as possible. For this purpose, the CNC system provides a feature called the rapid traverse motion. Its main objective is to shorten the positioning time between non-cutting operations, where the cutting tool is not in contact with the part. Rapid motion operations usually involve four types of motion:

• From the tool change position towards the part

• From the part towards the tool change position

• Motions to bypass obstacles

• Motions between different positions on the part

RAPID TRAVERSE MOTION

Rapid traverse motion, sometimes called a positioning motion, is a method of moving the cutting tool from one position to another position at a rapid rate of the machine. The maximum rapid rate is determined by the CNC machine manufacturer and takes place within the travel limits of the machine.

The common rapid rate for many larger CNC machines is about 450 in/min (11430 mm/min). The modern machines offer a rapid motion up to 1500 in/min (38100 mm/min) or even more, particularly for smaller machines. The machine manufacturer determines the rate of rapid motion for each of the machine axes. The motion rate can be the same for...