G-code

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G-Code, or preparatory code or function, are functions in the Numerical control programming language. The G-codes are the codes that position the tool and do the actual work, as opposed to M-codes, that manages the machine; T for tool-related codes. S and F are tool-Speed and tool-Feed, and finally D-codes for tool compensation.

The programming language of Numerical Control (NC) is sometimes informally called G-code. But in actuality, G-codes are only a part of the NC-programming language that controls NC and CNC machine tools. The term Numerical Control was coined at the MIT Servomechanisms Laboratory, and several versions of NC were and are still developed independently by CNC-machine manufacturers. The main standardized version used in the United States was settled by the Electronic Industries Alliance in the early 1960s. A final revision was approved in February 1980 as RS274D. In Europe, the ISO standard DIN 66025 is often used instead.

Due to the lack of further development, the immense 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 independently by manufacturers, and operators of a specific controller must be aware of differences of each manufacturers' product. When initially introduced, CAM systems were limited in the configurations of tools supported.

Today, the main manufacturers of CNC control systems are GE Fanuc Automation (joint venture of General Electric and Fanuc), Siemens and Maho (Philips), but there still exist many smaller and/or older controller systems.

Some CNC machine manufacturers attempted to overcome compatibility difficulties by standardizing on a machine tool controller built by Fanuc. Unfortunately, Fanuc does not remain consistent with RS-274 or its own previous versions, and has been slow at adding new features, as well as exploiting increases in computing power. For example, they changed G70/G71 to G20/G21; they used parentheses for comments which caused difficulty when they introduced mathematical calculations so the use square parentheses for macro calculations; they now have nano technology recently in 32-bit mode but in the Fanuc 15MB control they introduced HPCC (high-precision contour control) which uses a 64-bit RISC (reduced instruction set computer) processor and this now has a 500 block buffer for look-ahead for correct shape contouring and surfacing of small block programs and 5-axis continuous machining.

This is also used for NURBS to be able to work closely with industrial designers and the systems that are used to design flowing surfaces. The NURBS has its origins from the ship building industry and is described by using a knot and a weight as for bending steamed wooden planks and beams.

[edit] Examples

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.

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.

[edit] 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.)

Tool Path for program

Several points to note:

1. There is room for some programming style, even in this short program. The grouping of codes in line N06 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 (G96), 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 programming mistake. On small or hobby machines, it can warn of a tool 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 of the programmer and maximum material condition for the raw stock.
5. If the program is wrong, there is a high probability that the machine will crash, or ram the tool into the part under high power. This can be costly, especially in newer machining centers. It is possible to intersperse the program with optional stops (M01 code) which allow the program to be run piecemeal for testing purposes. 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.
6. For pedagogical purposes, line numbers have been included in the program above. They are usually not necessary for operation of a machine, so they are seldom used in industry. However, if branching or looping statements are used in the code, then line numbers may well be included as the target of those statements (e.g. GOTO N99).

[edit] Basic ISO CNC Code

|

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

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

M02 Program Stop(most commonly used)

M30 Program end, rewind

M98 Subprogram call

M99 Return to call program

M00, M01 Program stop, optional stop

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G96, G97 Constant surface speed, Constant Spindle speed

G50 Maximum spindle speed

G95, G94 Feed mm per revolution, feed mm/min

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

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F Feed

S Spindle Speed

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direction Coordinates X Y Z A B C U V W

[edit] See also

• Direct Numerical Control
• Gerber file

[edit] External links

• Tutorial for G-code
• Free Machinist Infomation, Calculators, and Forums
• [http://www.isd.mel.nist.gov/personnel/kramer/pubs/RS274NGC_3.web/RS274NGC_3TOC.html The NIST RS274NGC

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