Motion is defined as a continuous change in the position of a body (the cutting tool) relative to a reference point (the part), as measured by an observer (the operator) in a particular frame of reference. A frame of reference is the perspective from which space is observed. Specifically, in physics, it refers to a provided set of axes from which an observer can measure the position and motion of all points in a system, as well as the orientation of objects in it. This is important for the CNC machine because every movement it makes has to be measured and calculated.
The Cartesian coordinate system
Also known as the rectangular coordinate system it is used to define each point in space through three numbers, usually called the x, the y and the z coordinate of the point.
Linear and rotational motion
There are two kinds of motion, linear and rotational. Linear movements are produced by the different axes moving along their rails. Diagonal lines, and arcs and circles, are the result of two or more axes moving at the same time in a synchronized manner. Rotational movements are the result of the part or the head rotating around an axis.
3 & 5 axis
3-axis machines move in a Cartesian manner along x, y and z. A fourth axis often takes the form of a rotating device for the part being cut, similar to a lathe or an indexing head along the spindle that permits the controlled rotation of an aggregate tool. 5-axis machines have a much greater range of motion and can move in a manner similar to the human hand. These machines often have a deep z stroke to be able to work in a large three dimensional area. There are 6 possible axes of motion, 3 linear and 3 rotational, one of which is considered to be the cutter spinning in the spindle. The CNC machine must calculate the direction, speed and acceleration on each of the axes in order to cause the proper movement to be made. This is done with the help of complex mathematical formulas called algorithms.
Rapid vs. cutting motion
There are other terms that machine manufacturers refer to when talking about motion. Rapid motion refers to a motion that the machine makes when travelling only and not cutting. This is used when going from one point to another away from the material being cut. There is no need to limit the speed because it won’t affect the quality of the cut. Vendors often erroneously refer to this as being the top speed of the machine. The theoretical top speed of cut will be set by the cutter used and the material being cut, as well as by the length of the cut, more so than by the speed of the machine unless quality and precision of cut are not important. When making a short cut, the cutter almost never has enough time to accelerate to its top speed before having to decelerate ahead of coming to a full stop.
Absolute vs. incremental motion
Absolute motion and incremental motion are the terms used to describe the point of reference used by the machine to follow a specific path. Absolute motion is measured from the base reference point specified earlier in the program. This may or may not be the zero of the machine. Incremental motion, on the other hand, is measured using the last known point as the reference. In other words, measurements are made from the previous instructions. Care must be taken when working in incremental mode not to skip over any point because every subsequent measurement will be affected.