CNC Motion Control and its Three Common Types

Numerous CNC machines are capable of operating unattended during the machining process, freeing up operators for other tasks. This advantage leads to various benefits, such as reduced operator fatigue, minimized errors due to human oversight, and consistent, predictable machining times for each workpiece. As CNC machines operate under program control, they also demand a reduced skill level from operators compared to traditional machine tools.

At the core of CNC technology lies motion control, a fundamental element

CNC's Fundamental Function: Motion Control

The fundamental function of any CNC machine is to achieve automatic, precise, and consistent motion control. CNC machines possess two or more axes for various directions of motion. These axes are automatically and accurately positioned over their specified travel distances.

Instead of manually turning cranks and handwheels, as is typical with conventional machines, CNC machines utilize servomotors under CNC control to execute motion. Part programs guide this motion, encompassing types of motion such as rapid, linear, and circular, axis movement, the extent of motion, and the rate of motion, also known as the feed rate. These aspects are programmable in nearly all CNC machines.

Commands executed within the CNC control are typically designed to instruct the drive motor to rotate a specific number of times. The rotation of the drive motor subsequently drives the ballscrew, which, in turn, drives the spool. On the opposite end of the roller, a feedback device allows the control to verify that the specified number of rotations has taken place.

Although somewhat simplified, this basic linear motion can be likened to turning the vise handle of an ordinary bench vise. As you rotate the handle, the lead screw turns, driving the movable jaws of the vise. However, linear axes in a CNC machine are exceptionally precise, with the number of revolutions of the axis drive motor governing the exact linear motion along the axis.

Direction of Motion (Axis)

CNC programmers must be familiar with the programmable directions of motion, referred to as axes, available for CNC machines. These axes' names may vary between different machine types, but they are universally represented by letter addresses. Common axis names include X, Y, Z, U, V, and W for linear motion and A, B, and C for rotary motion.

As mentioned previously, CNC programs consist of commands, which, in turn, consist of words. Each word comprises a letter address and a numeric value. The letter address indicates the word's type. CNC control manufacturers may differ in how they define word names (letter addresses) and their meanings. New CNC programmers should consult the manufacturer's programming manual to understand word names and meanings. Here is a brief list of some word types and their typical letter address specifications:

O: Program number (for program identification)

N: Serial number (for line identification)

G: Preparation function

X: X-axis designation

Y: Y-axis designation

Z: Z-axis designation

R: Radius designation

F: Feed rate designation

S: Spindle speed

H: Tool length offset designation

D: Tool radius offset designation

T: Tool designation

M: Miscellaneous Functions

For rotary axis offsets, a letter address (usually A, B, or C) and the endpoint of the motion are required. However, the endpoint for rotary axis motion is specified in degrees, not inches or millimeters.

The Three Most Basic Types of Motion

While CNC machines may feature additional motion types depending on specific applications, we will focus on the three most common types found in nearly all CNC equipment. Following a brief introduction to each type of motion, we will provide a sample program that emphasizes all three.

Rapid Motion (Positioning)

This type of motion commands the machine to move at its maximum speed, minimizing unproductive time during machining cycles. It is employed for positioning the tool at the cutting location, clearing fixtures and obstructions, and any non-cutting motion within the program.

Linear Motion

Linear motion enables the machine to move in a straight line, as discussed earlier in the context of linear interpolation. This motion type also allows the programmer to specify the feed rate to be used during the motion. Linear motion can be used for various applications, including drilling holes, turning diameters, facing, tapering, and milling straight surfaces.

Circular Motion

This type of motion guides the machine in a circular path, primarily used to create radii during machining. The considerations related to feed rate that apply to linear motion also pertain to circular motion.

The continuous evolution of CNC technology will continue to drive progress in manufacturing, offering more opportunities and solutions for various industries.