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CNC Motion Control and Three Common Types

Many CNC machines can be left unattended throughout the machining cycle, freeing the operator for other tasks. This benefit allows CNC users to provide several side effects, including reduced operator fatigue, errors due to operator error, and consistent and predictable machining times for each workpiece. Because the machine will run under program control, the skill level required of a CNC operator is also reduced compared to that of a machinist who produces a machinist with a conventional machine tool.

The heart of CNC: Motion Control

The most basic function of any CNC machine is automatic, precise, and consistent motion control. All forms of CNC machines have two or more directions of motion, called axes. These axes can be precisely and automatically positioned along the length of their travel.

Instead of manually turning cranks and handwheels in the manner required to cause motion on conventional machines, CNC machines allow motion to be actuated by servomotors under CNC control and guided by part programs. In general, motion types such as, rapid, linear and circular, axis movement, amount of motion and rate of motion or feed rate are programmable with almost all CNC machines.

CNC commands executed within the control are usually programmed to tell the drive motor to rotate a precise number of times. The rotation of the drive motor in turn rotates the ballscrew. The ball roller drives the spool. A feedback device on the opposite end of the roller allows the control to confirm that the commanded number of rotations has occurred.

Although having a rather crude analogy, the same basic linear motion can be found in an ordinary watch vise eye. As you rotate the vise crank, you rotate the lead screw, which in turn drives the movable jaws on the VISE. In contrast, the linear axes on a CNC machine are very precise. The number of revolutions of the axis drive motor precisely controls the amount of linear motion along the axis.


Direction of Motion (Axis)

The CNC programmer must be aware of the programmable directions of motion (axes) available to the CNC machine. The names of the axes will be converted from one machine type to the next. They are always submitted via letter address. Common axis names are X, Y, Z, U, V and W for linear axes and A, B and C for rotary axes.

As mentioned above, programs consist of commands and the commands consist of words. Each word has a letter address and a numeric value. The letter address tells the word type.CNC control manufacturers do differ in how they determine word names (letter addresses) and their meanings. Beginning CNC programmers must refer to the maker's programming manual for the control to determine word names and meanings. Here is a short list of some word types and their general 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 effect designation

S-Spindle speed

H-Tool length offset designation

D-Tool radius offset designation

T-Tool designation

M-Miscellaneous Functions

Rotary axis offsets still require a letter address (usually A, B, or C) as well as the endpoint of the motion. However, the endpoint of the rotary axis motion is specified in degrees (not inches or millimeters).

The three most basic types of motion

While your particular CNC machine may have more types of motion depending on your application, let's focus on the three most common types found on almost all forms of CNC equipment. After a brief introduction to each type of motion, we'll show a sample program that emphasizes all three.

Rapid Motion (also known as Positioning)

This motion type is used to command motion at the fastest rate of the machine. It is used to minimize unproductive time during a machining cycle. Common uses for rapid motion include positioning the tool to the cutting position, moving to clear fixtures and other obstacles, and generally, any non-cutting motion during the program.

Linear Motion

This motion type allows the programmer to move in a completely straight line as discussed in the case we discussed during our discussion of linear interpolation. This motion type also allows the programmer to specify the rate of motion (feed rate) to be used during the motion. Linear motion can be used at any time with linear motion, including when drilling holes, turning diameters, faces, or tapers, and when milling straight surfaces.

Circular motion

This type of motion causes the machine to move in a circular path. As mentioned earlier during our demonstration of circular interpolation, this type of motion is used to generate radii during machining. All of the feed rate related points made during our discussion of linear motion still apply.

The continued development and innovation of CNC technology will continue to drive manufacturing forward, providing more opportunities and solutions for a variety of industries. We hope this article has helped you better understand the fundamentals of CNC motion control and its types.

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