basic motion types of cnc

While your particular CNC machine may have more motion types (depending on your application), let's concentrate on becoming familiar with the three most common types of motion. These three motion types are available on almost all forms of CNC equipment. After briefly introducing each type of motion, we'll show an example program that stresses the use of all three.

These motion types share two things in common. First, they are all modal. This means they remain in effect until changed. If for example, several motions of the same kind are to be given consecutively, the corresponding G code need only be specified in the first command. Second, the END POINT of the motion is specified in each motion command. The current position of the machine will be taken as the starting point.

Rapid motion (also called positioning)

This motion type (as the name implies) is used to command motion at the machine's fastest possible rate. It is used to minimize non-productive time during the machining cycle. Common uses for rapid motion include positioning the tool to and from cutting positions, moving to clear clamps and other obstructions, and in general, any non-cutting motion during the program.

You must check in the machine tool builder's manual to determine a machine's rapid rate. Usually this rate is extremely fast (some machines boast rapid rates of well over 1000 IPM!), meaning the operator must be cautious when verifying programs during rapid motion commands. Fortunately, there is a way for the operator to override the rapid rate during program verification.

The command almost all CNC machines use to command rapid motion is G00. Within the G00 Command, the end point for the motion is given. Control manufacturers vary with regard to what actually happens if more than one axis is included in the rapid motion command. With most controls, the machine will move as fast as possible in all axes commanded. In this case, one axis will probably reach its destination point before the other/s. With this kind of rapid command, straight line movement will NOT occur during rapid and the programmer must be very careful if there are obstructions to avoid. With other controls, straight line motion will occur, even during rapid motion commands.

Straight line motion (also called linear interpolation)

This motion type allows the programmer to command perfectly straight line movements as discussed earlier during our discussion of linear interpolation. This motion type also allows the programmer to specify the motion rate (feedrate) to be used during the movement. Straight line motion can be used any time a straight cutting movement is required, including when drilling, turning a straight diameter, face or taper, and when milling straight surfaces. The method by which feedrate is programmed varies from one machine type to the next. Generally speaking, machining centers only allow the feedrate to be specific in per minute format (inches or millimeters per minute). Turning centers also allow feedrate to be specified in per revolution format (inches or millimeters per revolution).

A G01 word is commonly used to specify straight line motion. Within the G01, the programmer will include the desired end point in each axis.

Circular motion (also called circular interpolation)

This motion type causes the machine to make movements in the form of a circular path. As discussed earlier during our presentation of circular interpolation, this motion type is used to generate radii during machining. All feedrate related points made during our discussion of straight line motion still apply.

Two G codes are used with circular motion. G02 is commonly used to specify clockwise motion while G03 is used to specify counter clockwise motion. To evaluate which to use, you simply view the movement from the same perspective the machine will view the motion. For example, if making a circular motion in XY on a machining center, simply view the motion from the spindle's vantage point. If making a circular motion in XZ on a turning center, simply view the motion from above the spindle. In most cases, this is as simple as viewing the print from above.

Additionally, circular motion requires that, by one means or another, the programmer specifies the radius of the arc to be generated. With newer CNC controls this is handled by a simple "R" word. The R word within the circular command simply tells the control the radius of the arc being commanded. With older controls, directional vectors (specified by I, J, and K) tell the control the location of the arc's center point. Since controls vary with regard to how directional vectors are programmed, and since the R word is becoming more and more popular for radius designation, our examples will show the use of the R word. If you wish to learn more about directional vectors, you must reference your control manufacturer's manual.