Computer Numerical Control | CNC | What Is CNC | How CNC Works | Programming Basics

The term numerical control is a widely accepted and commonly used term in the machine tool industry. Numerical control (NC) enables an operator to communicate with machine tools through a series of numbers and symbols.

NC which quickly became Computer Numerical Control (CNC) has brought tremendous changes to the metalworking industry. New machine tools in CNC have enabled industry to consistently produce parts to accuracies undreamed of only a few years ago. The same part can be reproduced to the same degree of accuracy any number of times if the CNC program has been properly prepared and the computer properly programmed. The operating commands which control the machine tool are executed automatically with amazing speed, accuracy, efficiency, and repeatability.

The ever-increasing use of CNC in industry has created a need for personnel who are knowledgeable about and capable of preparing the programs which guide the machine tools to produce parts to the required shape and accuracy. With this in mind, the authors have prepared this textbook to take the mystery out of CNC – to put it into a logical sequence and express it in simple language that everyone can understand.

 

Milling Machine

The milling machine has always been one of the most versatile machine tools used in industry (Fig. 5). Operations such as milling, contouring, gear cutting, drilling, boring, and reaming are only a few of the many operations which can be performed on a milling machine. The milling machine can be programmed on three axes:
• The X axis controls the table movement left or right.
• The Y axis controls the table movement toward or away from the column.
• The Z axis controls the vertical (up or down) movement of the knee or spindle.

The main axes of a vertical machining center.

Programming Systems | CNC programming Basics | G-Code | M-Code | Incremental and Absolute Programming System

Two types of programming modes, the incremental system and the absolute system, are used for CNC. Both systems have applications in CNC programming, and no system is either right or wrong all the time. Most controls on machine tools today are capable of handling either incremental or absolute programming.

Incremental program locations are always given as the distance and direction from the immediately preceding point

• A “X plus” (X+) command will cause the cutting tool to be located to the right of the last point.
• A “X minus” (X-) command will cause the cutting tool to be located to the left of the last point.
• A “Y plus” (Y+) command will cause the cutting tool to be located toward the column.
• A “Y minus” (Y-) will cause the cutting tool to be located away from the column.
• A “Z plus” (Z+) command will cause the cutting tool or spindle to move up or away from the workpiece.
• A “Z minus” (Z-) moves the cutting tool down or into the workpiece.

In incremental programming, the G91 command indicates to the computer and MCU (Machine Control Unit) that programming is in the incremental mode.

Absolute program locations are always given from a single fixed zero or origin point. The zero or origin point may be a position on the machine table, such as the corner of the worktable or at any specific point on the workpiece. In absolute dimensioning and programming, each point or location on the workpiece is given as a certain distance from the zero or reference point.

• A “X plus” (X+) command will cause the cutting tool to be located to the right of the zero or origin point.
• A “X minus” (X-) command will cause the cutting tool to be located to the left of the zero or origin point.
• A “Y plus” (Y+) command will cause the cutting tool to be located toward the column.
• A “Y minus” (Y-) command will cause the cutting tool to be located away from the column.

In absolute programming, the G90 command indicates to the computer and MCU that the programming is in the absolute mode.

Programming Systems | CNC programming Basics | G-Code | M-Code | Incremental and Absolute Programming System | Interpolation | Linear Interpolation | Circular Interpolation

Interpolation

The method by which contouring machine tools move from one programmed point to the next is called interpolation. This ability to merge individual axis points into a predefined tool path is built into most of today’s MCUs.

 

There are five methods of interpolation:

• linear
• circular
• helical
• parabolic
• cubic

All contouring controls provide linear interpolation, and most controls are capable of both
linear and circular interpolation. Helical, parabolic, and cubic interpolation are used by industries that manufacture parts which have complex shapes, such as aerospace parts and dies for car bodies.

Linear Interpolation

Linear Interpolation consists of any programmed points linked together by straight lines, whether the points are close together or far apart

Curves can be produced with linear interpolation by breaking them into short, straight-line segments. This method has limitations, because a very large number of points would have to be programmed to describe the curve in order to produce a contour shape. A contour programmed in linear interpolation requires the coordinate positions (XY positions in two-axis work) for the start and finish of each line segment. Therefore, the end point of one line or segment becomes the start point for the next segment, and so on, throughout the entire program.

Circular Interpolation

The development of MCUs capable of circular interpolation has greatly simplified the process of programming arcs and circles. To program an arc, the MCU requires only the coordinate positions (the XY axes) of the circle center, the radius of the circle, the start point and end point of the arc being cut, and the direction in which the arc is to be cut (clockwise or counterclockwise)

Codes:

The most common codes used when programming CNC machines tools are

• G-codes (preparatory functions), and
• M codes (miscellaneous functions).

Other codes such as F, S, D, and T are used for machine functions such as feed, speed, cutter diameter offset, tool number, etc.

G-Code
G-codes are sometimes called cycle codes because they refer to some action occurring on the X, Y, and/or Z axis of a machine tool.

Group	Code	Function
01	G00	Rapid Positioning
01	G01	Linear Interpolation
01	G02	Circular Interpolation  clockwise (CW)
01	G03	Circular Interpolation Counter clockwise (CCW)
06	G20*	Inch input (in.)
06	G21*	Metric Input (mm)
	G24	Radius Programming (**)
00	G28	Return to Reference Point
00	G29	Return from Reference Point
	G32	Thread Cutting (**)
07	G40	Cutter Compensation Cancel
07	G41	Cutter Compensation Left
07	G42	Cutter Compensation Right
08	G43	Tool length compensation positive
08	G44	Tool length compensation minus
08	G49	Tool Length Compensation Cancel
	G84	Canned Turning Cycle (**)
03	G90	Absolute Programming
03	G91	Incremental Programming

(*) – on some machines and controls, these may be G70 (inch) and G71 (metric)
(**) – refers only to CNC lathes and turning centers.

M-CODE:
M or miscellaneous codes are used to either turn ON or OFF different functions which control certain machine tool operations.

Code     Function     

M00     Program stop     
M02     End of program     
M03     Spindle start (forward CW)     
M04     Spindle start (reverse CCW)     
M05     Spindle stop     
M06     Tool change     
M08     Coolant on     
M09     Coolant off     
M10     Chuck – clamping (**)     
M11     Chuck – unclamping (**)     
M12     Tailstock spindle out (**)     
M13     Tailstock spindle in (**)     
M17     Tool post rotation normal (**)     
M18     Tool post rotation reverse (**)     
M30     End of tape and rewind     
M98     Transfer to subprogram     
M99     End of subprogram     

(**) – refers only to CNC lathes and turning centers.