Getting Started with G-Code

Getting Started with G-Code

Programming is a fundamental skill for all types of CNC machining, even as automation and new technology seem to be replacing programming tasks. 
Every machinist still needs to understand how their programs and tools work. 
Whether you're new to CNC programming and its most common language, g-code, or you've been writing code by scratch for years, CNC codes can still feel like a foreign language. 
And to make things worse, every machine speaks a different dialect you have to understand. 
Do you understand what they're saying? Here are the g-code basics you need to know to efficiently understand and write programs that produce high quality products.

What is G-Code?

G-code is a programming language for CNC that instructs machines where and how to move. 
Most machines speak a different “dialect” of g-code, so the codes vary depending on type, make, and model. 
Each machine comes with an instruction manual that shows that particular machine's code for a specific function.

G-code stands for “geometric code,” and follows some variation of the alpha numeric pattern:

N## G## X## Y## Z## F## S## T## M##

N: Line numberG: MotionX: Horizontal positionY: Vertical positionZ: DepthF: Feed rateS: Spindle speedT: Tool selectionM: Miscellaneous functionsI and J: Incremental center of an arcR: Radius of an arc

Alpha numeric codes are used for programming as they are a simple way to:
Define motion and function (G##)
Declare a position (X## Y## Z##)
Set a value (F## and/or S##)
Select an item (T##)
Switch something on and off (M##), such as coolant, spindles, indexing motion, axes locks, etc.

For example,

G01 X1 Y1 F20 T01 M03 S500

would generally indicate a linear feed move (G01) to the given XY position at feed rate of 20. 
It is using Tool 1, and the spindle speed is 500. 
Miscellaneous functions will vary from machine to machine, so in order to know what the m-code means, the machine's instruction manual will need to be referenced.

Machine Motion

Everything a machine can do is based on three basic types of motion:
Rapid move: a linear move to an XYZ position as fast as possible
Feed move: a linear move to an XYZ position at a defined feed rate
Circular move: a circular move at a defined feed rate

Every g-code tells the machine which variation of these basic motions to perform, and how to perform it.

X and Y are Cartesian coordinates for horizontal and vertical position, and Z represents the depth of the machine. 
These alpha numerals will follow the motion/function command (G) to declare the position of the machine.

Next, F determines the feed rate (for feed moves or circular moves), while S determines the spindle speed. 
T is used to select a tool. 
Other alpha numerals used in programming might include I, J, and R, which have to do with arc centers and radii.

Miscellaneous Codes

The line of a program might also include m-codes, which are generally codes that tell a machine how to perform an action. 
While not guaranteed to be the same across machines, some common, standard m-codes are:

M00: Program stop
M01: Optional program stop
M02: End of program
M03: Spindle on clockwise
M04: Spindle on counterclockwise
M05: Spindle stop
M06: Tool change
M08: Flood coolant on
M09: Flood coolant off
M30: End of program/return to start
M41: Spindle low gear range
M42: Spindle high gear range

Modality
Just like a light will stay on until it's turned off, g-code functions (on controllers that support modality) will remain active until they are deactivated by another code. 
In other words, only one function can be active at any given time. 
To deactivate a function, just select a new function.


For example, say a code begins with a linear rapid move at X1 Y1 (G00 X1 Y1). 
If the next function is another linear rapid move, it is not necessary to write G00 again. 
All that is needed on the next line of code is the new position (say, X2 Y2) because the modal condition is the same. 
Then, to change the function to a linear feed (G01), programming G01 on the following line would deactivate the linear rapid move and activate the linear feed.

Once a condition is set, it stays active until it is turned off or another condition overrides it.

Canned Cycles
Canned cycles are a kind of modal condition that incorporate all the motions to complete a common task into one code.


For example, oftentimes G81 is code for a basic drilling function. 
In the case of basic drilling, the tool would have to be 1) moved to the starting point of the hole's location, 2) rapid to the clearance plane, 3) fed to the depth, and 4) rapid out. 
That would be four lines of code in the program that would have to be repeated for every new drill position! With the canned cycle G81, only the hole locations need to be specified after activation. 
Canned cycles like G81 significantly reduce the amount of code by incorporating multiple motions into one code.


Some other common canned cycles exist for peck drilling, counter boring, and tapping.



Modal Code Groups
Modal code groups allow there to be multiple codes on a single line, but there can only be one code from each group on a line. 
This is because codes within a group will override each other.

The modal groups for g-codes are:
Group 1 (motion): G00, G01, G02, G03, G80, G81, G82, G84, G85, G86, G87, G88, G89
Group 2 (plane selection – XY, YZ, ZX): G17, G18, G19
Group 3 (absolute/incremental mode): G90, G91
Group 5 (feed rate mode): G93, G94
Group 6 (units – inches/millimeters): G20, G21
Group 7 (cutter radius compensation – CRC): G40, G41, G42
Group 8 (tool length offset – TLO): G43, G49
Group 10 (return mode in canned cycles): G98, G99
Group 12 (work coordinate system selection – WCSS): G54, G55, G56, G57, G58, G59)


Postprocessors
A postprocessor is a translator that translates the calculated image of a toolpath on your computer screen into the language for a machine control. 
You can create a post processor by searching for a sample program that aligns closely with your machine, and then amending the program to precisely fit your machine. 
Before you can do this, you'll need to know:

Get Started with G-Code
Ready to start using g-code to program your machines? Begin by reviewing your machine's unique coding chart. 
Remember, every machine is a little different. 
A Haas' code for a function might not be the same as an Anilam's code for that function. 
You have to know which codes your specific machine uses for the tasks you want it to perform.

The program will often start with an initialization code (%), followed by a program number. 
Then, there will be a line of safety codes. 
Next will be a line for tool change. 
This puts the appropriate tool in the machine and set the speed to be used. 
The bulk of the program will then be the machine movements and positioning.

If using line numbering, it's a good idea to name each line in increments of at least five. 
This way, if you need to add lines of code later, the lines will still be labelled in order.

When you've finished programming your g-code, you'll typically end the program with a series of functions that stop and reset the machine so it's ready for the next time.

G-Code Tips to Note

Some machines and controllers ignore spaces. 
G01 X1 Y1 Z1 might mean the same thing as G01 X1Y1Z1.
The Z-axis is positive in the up direction. 
Z1 will bring the tool up, while Z-1 will bring the tool down.
Your machine's g-code dialect will specify if a leading zero is necessary (as in G01, as opposed to G1).
The dialect will also determine if decimal points are always necessary (ex. G01 X1. Y1. Z0.5)
It's a good idea to run the sample programs that come in your machine manual before you try to run a big program. 
Oftentimes, the sample programs do not work and you will need to note the issues and set your own benchmarks.

When used correctly, g-codes are an invaluable tool for CNC machinists, allowing you to take full advantage of your machine's capabilities. 
And integrated CAM solutions continue to streamline the CNC coding process, so you don't have to program parts by hand using g-codes. 
Producing high quality parts has never been faster.

G-code shouldn't be “all Greek” to you. 
Even as automatic coding and other manufacturing advancements take shape, understanding CNC language will bridge the manufacturing borders that have been holding you back, helping you make the best products possible.

CNC G codes, M codes
Common G codes and M codes for CNC
Not all codes are available on all controls, and some controls have other codes.
See your machine manual for detailed explanations.

CNC G codes
Code	Category	Function	Notes
G00	Motion	Move in a straight line at rapids speed.	XYZ of endpoint
G00 - Positioning at rapid speed; Mill and Lathe
G01	Motion	Move in a straight line at last speed commanded by a (F)eedrate	XYZ of endpoint	 
G01 - Linear interpolation (machining a straight line); Mill and Lathe
G02	Motion	Clockwise circular arc at (F)eedrate	XYZ of endpoint IJK relative to center R for radius	 
G02 - Circular interpolation clockwise (machining arcs); Mill and Lathe
G03	Motion	Counter-clockwise circular arc at (F)eedrate	XYZ of endpoint IJK relative to center R for radius	 
G03 - Circular interpolation, counter clockwise; Mill and Lathe
G04	Motion	Dwell: Stop for a specified time.	P for milliseconds X for seconds	 
G04 - Mill and Lathe, Dwell
G05	Motion	FADAL Non-Modal Rapids	 	 
G09	Motion	Exact stop check	 	 
G09 - Mill and Lathe, Exact stop
G10	Compensation	Programmable parameter input	 	 
G10 - Setting offsets in the program; Mill and Lathe
G12 - Circular pocket milling, clockwise; Mill
G13 - Circular pocket milling, counterclockwise; Mill
G15	Coordinate	Turn Polar Coordinates OFF, return to Cartesian Coordinates	 	 
G16	Coordinate	Turn Polar Coordinates ON
G17	Coordinate	Select X-Y plane	 	 
G17 - X-Y plane for arc machining; Mill and Lathe with live tooling
G18	Coordinate	Select X-Z plane
G18 - Z-X plane for arc machining; Mill and Lathe with live tooling
G19	Coordinate	Select Y-Z plane	 	 
G19 - Z-Y plane for arc machining; Mill and Lathe with live tooling
G20	Coordinate	Program coordinates are inches	 	 
G20 - Inch units; Mill and Lathe
G21	Coordinate	Program coordinates are mm	 	 
G21 - Metric units; Mill and Lathe
G27	Motion	Reference point return check	 	 
G27 - Reference return check; Mill and Lathe
G28	Motion	Return to home position	 	 
G28 - Automatic return through reference point; Mill and Lathe
G29	Motion	Return from the reference position	 	 
G29 - Move to location through reference point; Mill and Lathe (slightly different for each machine)
G30	Motion	Return to the 2nd, 3rd, and 4th reference point	 	 
G31 - Skip function; Mill and Lathe
G32	Canned	Constant lead threading (like G01 synchronized with spindle)	 	 
G32 - Thread cutting; Lathe
G33 - Thread cutting; Mill
G40	Compensation	Tool cutter compensation off (radius comp.)	 	 
G40 - Cancel diameter offset; Mill. Cancel tool nose offset; Lathe
G41	Compensation	Tool cutter compensation left (radius comp.)	 	 
G41 - Cutter compensation left; Mill. Tool nose radius compensation left; Lathe
G42	Compensation	Tool cutter compensation right (radius comp.)	 	 
G42 - Cutter compensation right; Mill. Tool nose radius compensation right; Lathe
G43	Compensation	Apply tool length compensation (plus)	 	 
G43 - Tool length compensation; Mill
G44	Compensation	Apply tool length compensation (minus)	 	 
G44 - Tool length compensation cancel; Mill (sometimes G49)
G49	Compensation	Tool length compensation cancel	 	 
G50	Compensation	Reset all scale factors to 1.0	 	 
G50 - Set coordinate system and maximum RPM; Lathe
G51	Compensation	Turn on scale factors	 	 
G52	Coordinate	Local workshift for all coordinate systems: add XYZ offsets	 	 
G52 - Local coordinate system setting; Mill and Lathe
G53	Coordinate	Machine coordinate system (cancel work offsets)	 	 
G53 - Machine coordinate system setting; Mill and Lathe
G54	Coordinate	Work coordinate system (1st Workpiece)	 	 
G54~G59 - Workpiece coordinate system settings #1 t0 #6; Mill and Lathe
G55	Coordinate	Work coordinate system (2nd Workpiece)	 	 
G56	Coordinate	Work coordinate system (3rd Workpiece)	 	 
G57	Coordinate	Work coordinate system (4th Workpiece)	 	 
G58	Coordinate	Work coordinate system (5th Workpiece)	 	 
G59	Coordinate	Work coordinate system (6th Workpiece)	 	 
G61	Other	Exact stop check mode	 	 
G61 - Exact stop check; Mill and Lathe
G62	Other	Automatic corner override	 	 
G63	Other	Tapping mode	 	 
G64	Other	Best speed path	 	 
G65	Other	Custom macro simple call	 	 
G65 - Custom macro call; Mill and Lathe
G68	Coordinate	Coordinate System Rotation	 	 
G69	Coordinate	Cancel Coordinate System Rotation	 	 
G70 - Finish cycle; Lathe
G71 - Rough turning cycle; Lathe
G72 - Rough facing cycle; Lathe
G73	Canned	High speed drilling cycle (small retract)	 	 
G73 - Chip break drilling cycle; Mill
G73 - Irregular rough turning cycle; Lathe
G74	Canned	Left hand tapping cycle	 	 
G74 - Face grooving or chip break drilling; Lathe
G74 - Left hand tapping; Mill
G75 - OD groove pecking; Lathe
G76	Canned	Fine boring cyle	 	 
G76 - Fine boring cycle; Mill
G76 - Threading cycle; Lathe
G80	Canned	Cancel canned cycle	 	 
G80 - Cancel cycles; Mill and Lathe
G81	Canned	Simple drilling cycle	 	 
G81 - Drill cycle; Mill and Lathe
G82	Canned	Drilling cycle with dwell (counterboring)	 	 
G82 - Drill cycle with dwell; Mill
G83	Canned	Peck drilling cycle (full retract),Peck drilling cycle; Mill
G84	Canned	Tapping cycle; Mill and Lathe
G85	Canned	Boring canned cycle, no dwell, feed out, Bore in, bore out; Mill and Lathe
G86	Canned	Boring canned cycle, spindle stop, rapid out, Bore in, rapid out; Mill and Lathe
G87	Canned	Back boring canned cycle, Back boring cycle; Mill
G88	Canned	Boring canned cycle, spindle stop, manual out	 	 
G89	Canned	Boring canned cycle, dwell, feed out	 	
G90	Coordinate	Absolute programming of XYZ (type B and C systems), Absolute programming
G90.1	Coordinate	Absolute programming IJK (type B and C systems)
G91	Coordinate	Incremental programming of XYZ (type B and C systems), Incremental programming
G91.1	Coordinate	Incremental programming IJK (type B and C systems)
G92	Coordinate	Offset coordinate system and save parameters
G92 (alternate)	Motion	Clamp of maximum spindle speed	S	G92.1	Coordinate	Cancel offset and zero parameters
G92 - Reposition origin point; Mill,Thread cutting cycle; Lathe
G92.2	Coordinate	Cancel offset and retain parameters
G92.3	Coordinate	Offset coordinate system with saved parameters
G94	Motion	Units per minute feed mode. Units in inches or mm.,Per minute feed; Mill
G95	Motion	Units per revolution feed mode. Units in inches or mm.,Per revolution feed; Mill
G96	Motion	Constant surface speed control; Lathe
G97	Motion	Cancel constant surface speed, Constant surface speed cancel
G98	Canned	Return to initial Z plane after canned cycle, Per minute feed; Lathe
G99	Canned	Return to initial R plane after canned cycle, Per revolution feed; Lathe


CNC M Codes
Code		Category	Function	Notes
M00	Program Stop (non-optional); Mill and Lathe
M01	Optional Stop: Operator Selected to Enable, Optional program stop; Lathe and Mill
M02	End of Program, Program end; Lathe and Mill
M03	Spindle ON (CW Rotation); Lathe and Mill
M04	Spindle ON (CCW Rotation); Lathe and Mill
M05	Spindle Stop, Spindle off; Lathe and Mill
M06	Toolchange; Mill
M07	Mist Coolant ON	
M08	Flood Coolant ON, Coolant on; Lathe and Mill
M09	Coolant off; Lathe and Mill
M10	Chuck or rotary table clamp; Lathe and Mill
M11	Chuck or rotary table clamp off; Lathe and Mill
M17	FADAL subroutine return	
M19	Orient spindle; Lathe and Mill
M29	Rigid Tapping Mode on Fanuc Controls	
M30	End of Program, Rewind and Reset Modes, Program end, return to start; Lathe and Mill
M97	Haas-Style Subprogram Call, Local sub-routine call; Lathe and Mill
M98	Sub-program call; Lathe and Mill
M99	Return from Subprogram, End of sub program; Lathe and Mill