Mechanical Technology

G-codes are also called preparatory codes, and are any word in a CNC program that begins with the letter 'G'. Generally it is a code telling the machine tool what type of action to perform, such as:

• rapid move
• controlled feed move in a straight line or arc
• series of controlled feed moves that would result in a hole being bored, a workpiece cut (routed) to a specific dimension, or a decorative profile shape added to the edge of a workpiece.
• change a pallet
• set tool information such as offset.

There are other codes; the type codes can be thought of like registers in a computer

X absolute position

Y absolute position

Z absolute position

A position (rotary around X)

B position (rotary around Y)

C position (rotary around Z)

U Relative axis parallel to X

V Relative axis parallel to Y

W Relative axis parallel to Z

M code (another "action" register or Machine code(*)) (otherwise referred to as a "Miscellaneous" function")

F feed rate

S spindle speed

N line number

R Arc radius or optional word passed to a subprogram/canned cycle

P Dwell time or optional word passed to a subprogram/canned cycle

T Tool selection

I Arc data X axis

J Arc data Y axis.

K Arc data Z axis, or optional word passed to a subprogram/canned cycle

D Cutter diameter/radius offset

H Tool length offset

(*) M codes control the overall machine, causing it to stop, start, turn on coolant, etc., whereas other codes pertain to the path traversed by cutting tools. Different machine tools may use the same code to perform different functions; even machines that use the same CNC control.

• Partial list of M-Codes

M0=Program Stop (non-optional)

M1=Optional Stop, machine will only stop if operator selects this option

M2=End of Program

M3=Spindle on (CW rotation)

M4=Spindle on (CCW rotation)

M5=Spindle off

M6=Tool Change

M7=Coolant on (flood)

M8=Coolant on (mist)

M9=Coolant off

M10=Pallet clamp

M11=Pallet un-clamp

M30=End of program/rewind tape (may still be required for older CNC machines)

A standardized version of G-code known as BCL is used, but only on very few machines.

G-code files may be generated by CAM software. Those applications typically use translators called post-processors to output code optimized for a particular machine type or family. Post-processors are often user-editable to enable further customization, if necessary. G-code is also output by specialized CAD systems used to design printed circuit boards. Such software must be customized for each type of machine tool that it will be used to program. Some G-code is written by hand for volume production jobs. In this environment, the inherent inefficiency of CAM-generated G-code is unacceptable.

Some CNC machines use "conversational" programming, which is a wizard-like programming mode that either hides G-code or completely bypasses the use of G-code. Some popular examples are Southwestern Industries' ProtoTRAK, Mazak's Mazatrol, Hurco's Ultimax and Mori Seiki's CAPS conversational software.