Basic CNC Programming (Part 3) : Programming System

Programming Systems

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 (Fig. 6). Command codes which tell the machine to move the table, spindle, and knee are explained here using a vertical milling machine as an example:

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  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 work-piece.

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 (Fig. 7). 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.

Source : Computer Numerical Control Programming Basics / Steve Krar, Arthur Gill

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Basic CNC Programming (Part 2) : CNC Machines

CNC Machine Types 

1. CNC Lathe

The engine lathe, one of the most productive machine tools, has always been an efficient means of producing round parts (Fig. 4). Most lathes are programmed on two axes.

Fig. 4 The main axes of a lathe or turning center. (Emco Maier Corp)

• The X axis controls the cross motion of the cutting tool. Negative X (X-) moves the tool towards the spindle centerline; positive X moves the tool away from the spindle centerline.

• The Z axis controls the carriage travel toward or away from the headstock.

2. 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:

Fig. 5 The main axes of a vertical machining center. (Denford Inc.)

• 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.