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G-code is a common name for the programming language drive NC and CNC machine tools. It was developed by EIA in the early 1960s, a final revision was approved in February 1980 as RS274D.

Due to the lack of further development, the sheer variety of machine tool configurations, and little demand for interoperability, few machine tool controllers (CNCs) adhere to this standard. Extensions and variations have been added to it independently by manufacturers, meaning that operators have to know the dialects and quirks of the particular machines they use, and CAM systems have had to limit themselves to the lowest common denominator of all the tools that they support.

Many manufacturers tried to overcome this difficulty of remaining compatible by following the lead of a machine tool controller built by Fanuc. Unfortunately, Fanuc does not remain consistent with RS-274 or its own previous standard, and has been slow at adding new features and exploiting the increase in computing power. For example, they changed g70/g71 to g20/21; they used parentheses for comments which caused difficulty when they introduced mathematical calculations; they started to use nanometers just recently (requires 64 bit); they introduced the nurbs to overcome slow fetching of blocks from memory (instead of caching).

Examples

G-code is also the name of any word in a CNC program that begins with the letter G, and generally is a code telling the machine tool what type of action to perform, such as:

• rapid move
• controlled feed move in 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 position

Y position

Z position

M code (another "action" register or Machine code(*))

F feed rate

S spindle speed

N line number

R Radius

T Tool selection

I Arc data X axis

J Arc data Y axis.

K Arc data Z axis.

(*) 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, however, may use the same code to perform different functions: even machines that use the same CNC control. Some have stated that "Fanuc" G-code is standard, but this is in fact a myth.

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

G-code is understood by Gerber photoplotters, machine tool controls, and some machinists.

G-code files are output by CAM software such as Smartcam, Gibbscam, Featurecam, Edgecam, Surfcam, Mastercam, OneCNC, etc. 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 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.

Example

This is a generic program that demonstrates the use of G-Code to turn a 1" diameter X 1" long part. Assume that a bar of material is in the machine and that the bar is slightly oversized in length and diameter and that the bar protrudes by more than 1" from the face of the chuck. (Caution: This is generic, it might not work on any real machine! Pay particular attention to point 5 below.)

Several points to note:

1. There is room for some programming style, even in this short program. The grouping of codes in line N05 could have been put on multiple lines. Doing so may have made it easier to follow program execution.
2. Many codes are "Modal" meaning that they stay in effect until they are cancelled or replaced by a contradictory code. For example, once variable speed cutting had been selected (G97), it stayed in effect until the end of the program. In operation, the spindle speed would increase as the tool neared the center of the work in order to maintain a constant cutting speed. Similarly, once rapid feed was selected (G00) all tool movements would be rapid until a feed rate code (G01, G02, G03) was selected.
3. It is common practice to use a load monitor with CNC machinery. The load monitor will stop the machine if the spindle or feed loads exceed a preset value that is set during the set-up operation. The job of the load monitor is to prevent machine damage in the event of tool breakage or a programing mistake. Furthermore, to some extent it can warn of a tool that that is becoming dull and needs to be replaced or sharpened.
4. It is common practice to bring the tool in rapidly to a "safe" point that is close to the part - in this case 0.1" away - and then start feeding the tool. How close that "safe" distance is, depends on the skill and comfort level of the programmer.
5. If the program is wrong, there is a high probability that the machine will crash! That can be very costly. It is possible to intersperse the program with optional stops (M01 code) which allow the program to be run piecemeal. The optional stops remain in the program but they are skipped during the normal running of the machine. Thankfully, most CAD/CAM software ships with CNC simulators that will display the movement of the tool as the program executes. Many modern CNC machines also allow programmers to execute the program in a simulation mode and observe the operating parameters of the machine at a particular execution point. This enables programmers to discover semantic errors (as opposed to syntax errors) before losing material or tools to an incorrect program.

Basic ISO CNC Code

|

M03, M04, M05 Spindle CW, Spindle CCW, Spindle Stop

|

M08, M09 Coolant/lubricant On, Coolant/lubricant Off

M02 Program Stop

M30 Program end, rewind

M99 Subprogram end

M00, M01 Program stop, optional stop

|

G96, G97 Constant surface speed, Constant Spindle speed

G50 Maximum spindle speed

G18, G19 Feed mm pr revolvation, feed mm/min

G00, G01 rapid movement, Linear Interpolation (cutting in a straight line)

|

F Feed

S Spindle Speed

|

direction Coordinats X Y Z A B C

Example of a simple CNC lathe program

O1234

G50 S2000

G96 S300 M03

G00 T0606 (ROUGHT TURN TOOL)

G18 X37. Z0.

G01 X-1. F0.2

Z1.

G00 X30.

G01 Z-20.

X33.

X35. Z-21.

Z-25.

X37.

G00 X150. Z300.

M01

T0101 (18MM DRILL)

G97 S1000

G19 M03

X0. Z5.

G01 Z-25. F100

G00 Z5.

X150. Z300.

M05

M30

Example of a simple CNC milling program

A simple example might be a 4" x 2" rectangle. The basic code might read something like:

N1X0Y0T01
N2X0Y2000
N3X4000Y2000
N4X4000Y0
N5X0Y0
N6M00

Line 1 (N1) tells the machine to traverse to grid point X0Y0 and to pick tool #1

Line 2 tells the machine to traverse to grid point X0Y2.000

Line 3 tells the machine to travel to grid point X4.000Y2.000

Line 4 tells the machine to travel to grid point X4.000Y0

Line 5 returns the machine to origin

Line 6 stops the machine

Note that the program does nothing to define the tool cutting path. If the machine is a router and uses a 1/8" radius cutter, the actual part will end up 1/4" smaller than designed (1/8" per side). To compensate, a G-code command (in this case) may be used to adjust the tool path.

N1G44M0125
N2X0Y0T01
N3X0Y2000
N4X4000Y2000
N5X4000Y0
N6X0Y0
N7M00

In this case, the controller sees the first line and adjusts the location of the cutter to .125 (or 1/8") to the outside of the cutting profile. Now the machine will make a part that matches the one designed. Depending on the cutting tool, the compensation can be set as needed. For example, a laser with a very fine beam might have a compensation of .005", while a waterjet with a .060 inside tip diameter may need a compensation of .030.

See also

• Direct Numerical Control
• Gerber file

External links

• The NIST RS274NGC Interpreter

Template:Metalworking - CNC, CAD, and CAM

• CNC, CAD, and CAM
• CAD file formats
• Domain-specific programming languages
• Encodings