Again G Codes

7/31/2019 Again G Codes

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RS274NGC G-CODE PROGRAMMING BASICS

Contents

IntroductionBlocks

Numbers

Words

Line Numbering Words

Axis Words

G "Preparatory" Words

M "Miscelaneous Words"

F, S, T, "Control" Words

Modal Codes

Credits

Introduction

RS-274D is the recommended standard for numerically controlled machines developed by

the Electronic Industry Association in the early 1960's. The RS-274D revision wasapproved in February, 1980. These standards provide a basis for the writing of numeric

control programs.

There are a number of historical sidelights to this standard, many having to do with the

original use of punched paper tape as the only data interchange medium. The 64-characterEIA-244 paper tape standard is now (thankfully) obsolete, and ASCII character bit patterns

are now the standard representation. This old tape standard had specific characters used for

'searching' for specific lines (program blocks) on the tape, 'rewinding' the tape, etc.Ocasionally this obsolete language is still used when referring to some cnc control tasks.

The full NIST Enhanced Machine Controller is nc programmed using a variant of the

RS274D language to control motion and I/O. This variant is called RS276NGC because it

was developed for the Next Generation Controller, a project of the National Center forManufacturing Science. The version of RS274 used by EMC adheres closely to the

publications of the NCMS wherever those publications produce an unambiguous set. In

some cases reference to other implementations of RS274 had to be made by NIST.

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Blocks
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The basic unit of the nc program is the 'block', which is seen in printed form as a 'line' of

text. Each block can contain one or more 'words', which consist of a letter, describing a

setting to be made, or a function to be performed, followed by a numeric field, supplying avalue to that function. A permissible block of input is currently restricted to a maximum of

256 characters.

The following order is required for the construction of a block.

1. an optional block delete character, which is a slash / .2. an optional line number.

3. any number of segments, where a segment is a word or a comment.

4. an end of line character.The interpreter allows words starting with any letter except N (which denotes a line number

and must be first) to occur in any order. Execution of the block will be the same regardless

of the order.

An example of a program block would be

/N0001 G0 X123.05This block is constructed using three words, N0001, G0, and X123.05. The meanings of

each of these words is described in detail below. In essence, the n word numbers the line,

the g0 word tells the machine to get to its destination as quickly as it can, and the finalposition of the x axis is to be 123.05. Since it is constructed with a preceeding slash, this

block could be deleted during a run if optional block delete were activated.

There are some general considerations when writing nc code for the EMC:

The interpreter allows spaces and tabs anywhere within a block of code. The resultof the interpretation of a block will be the same as it would if any white spaceswere not there. This makes some strange-looking input legal. The line "g0x +0. 12

34y 7" is equivalent to "g0 x+0.1234 y7", for example.

Blank lines are allowed in the input by the interpreter. They are ignored.

The interpreter also assumes input is case insensitive. Any letter may be in upper or

lower case without changing the meaning of a line.

Whenever you write nc programs, you would do well to be considerate of others who may

have to read that code, even though the interpreter itself does not care about white spaceand case. Unless your are really up against the 256 digit block size limit, white space

between words and the absense of it within words makes a block much easier tounderstand.

There are a number of limitations about the number or types of words that can be strungtogether into a block. The interpreter uses the following rules:

A line may have zero to four G words.

Two G words from the same modal group may not appear on the same line.


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A line may have zero to four M words.

Two M words from the same modal group may not appear on the same line.

For all other legal letters, a line may have only one word beginning with that letter.

Don't worry to much about modal codes or the order of execution of the words within a

block of nc program just yet. These will become clear as you work your way through thedefinitions of the permissible words listed in the next unit.

For now it is enough to remember that a program block is more than the words that arewritten in it. Various words can be combined to specify multi-axis moves, or perform

special functions. While a block of code has a specific order of execution, it must be

considered to be a single command. All of the words within a block combine to produce asingle set of actions which may be very different from the actions assigned to the same

words were they placed in separate blocks. A simple example using axis words should

illustrate this point.

n1 x6 - moves from the current x location to x6n2 y3 - moves from current y location to y3 at x6

n3 z2 - moves from current z location to z2 at x6 and y3

n10 x6 y3 z2 - moves on a single line from current x, y, z to x6 y3 z2

The final position of the first three blocks (n1-n3) and the (n10) block are the same. The

first set of blocks might be executed in sequence to move the tool around an obstacle whilethe path of the tool for the combined block (n10) might run it into the part or the fixture.

To make the specification of an allowable line of code precise, NIST defined it in a

production language (Wirth Syntax Notation). These definitions appear as Table *** at theend of this chapter. In order that the definition in the appendix not be unwieldy, manyconstraints imposed by the interpreter are omitted from that appendix. The list of error

messages elsewhere in the Handbook indicates all of the additional constraints.

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Numbers

Since every nc word is composed of a letter and a value. Before we begin a serious

discussion of the meaning of nc programming words we need to consider the meaning ofvalue within the interpreter. A real_value is some collection of characters that can beprocessed to come up with a number. A real_value may be an explicit number (such as 341

or -0.8807), a parameter value, an expression, or a unary operation value. In this chapter all

examples will use explicit numbers. Expressions and unary operations are treated in thecomputation chapter. The use of parameter values or variables are a described in detail in

the Using Variables chapter.
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EMC uses the following rules regarding numbers. In these rules a digit is a single character

between 0 and 9.

A number consists of :

an optional plus or minus sign, followed by zero to many digits, followed, possibly, by

one decimal point, followed by

zero to many digits provided that there is at least one digit somewhere in thenumber.

There are two kinds of numbers: integers and decimals. An integer does not have a decimal

point in it; a decimal does.

Some additional rules about the meaning of numbers are that:

Numbers may have any number of digits, subject to the limitation on line length. A non-zero number with no sign as the first character is assumed to be positive.

Initial and trailing zeros are allowed but not required.

A number with initial or trailing zeros will have the same value as if the extra zeroswere not there.

Numbers used for specific purposes in RS274/NGC are often restricted to some finite set of

values or to some range of values. In many uses, decimal numbers must be close to

integers; this includes the values of indexes (for parameters and changer slot numbers, forexample). In the interpreter, a decimal number which is supposed be close to an integer is

considered close enough if it is within 0.0001 of an integer.

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Words

An nc program word is an acceptable letter followed by a real_value. Table 2 shows the

current list of words that the EMC interpreter recognizes. The meanings of many of these

words are listed in detail below. Some are included in and in the chapter on tool radiuscompensation and the chapter on canned cycles.


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Table 2Words acceptable to the EMC interpreter

D

FG

HI

J

K

L

tool radius compensation number

feedrategeneral function (see below)

tool length offsetX-axis offset for arcs

X offset in G87 canned cycleY-axis offset for arcs

and Y offset in G87 canned cycle

K Z-axis offset for arcsand Z offset in G87 canned cycle

L number of repetitions in canned

cyclesand key used with G10

M

NP

Q

R

S

TX

Y

Z

miscellaneous function (see below)

line numberdwell time with G4 and canned cycles

key used with G10Q feed increment in G83 canned cycle

R arc radius

canned cycle plane

S spindle speedT tool selection

X-axis of machine

Y-axis of machineZ-axis of machine

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Line Number Words

A line number is the letter N followed by an integer (with no sign) between 0 and 99999.

Line numbers are not checked except for to many digits. It is not necessary to number linesbecause they are not used by the interpreter. But they can be convenient when looking over

a program. N word line numbers are reported in error messages when errors are caused by

program problems.

Line numbers can be confusing because they are not the number that is displayed as beingexecuted. Nor are they the number used to restart an nc program at a line other than the

start. That number is the number of the current block in the program file with 0 being the

first block.

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G10 coordinate system origin setting

G17 xy plane selection

G18 xz plane selectionG19 yz plane selection

G20 inch system selection

G21 millimeter system selectionG40 cancel cutter diameter compensationG41 start cutter diameter compensation left

G42 start cutter diameter compensation right

G43 tool length offset (plus)G49 cancel tool length offset

G53 motion in machine coordinate system

G54 use preset work coordinate system 1G55 use preset work coordinate system 2

G56 use preset work coordinate system 3

G57 use preset work coordinate system 4

G81 drilling canned cycle

G82 drilling with dwell canned cycle

G83 chip-breaking drilling canned cycleG84 right hand tapping canned cycle

G85 boring, no dwell, feed out canned cycle

G86 boring, spindle stop, rapid out cannedG87 back boring canned cycleG88 boring, spindle stop, manual out canned

G89 boring, dwell, feed out canned cycle

G90 absolute distance modeG91 incremental distance mode

G92 offset coordinate systems

G92.2 cancel offset coordinate systemsG93 inverse time feed mode

G94 feed per minute mode

G98 initial level return in canned cycles

Tool diameter compensation (g40, g41, g42) and tool length compensation (g43, g49) are

covered in a separate page. Canned milling cycles (g80 - g89, g98) are covered in their ownpage. Coordinate systems and how to use them is also covered in a separate page. (g10,

G53 - G59.3, G92, G92.2)

Basic Motion and Feedrate

G0 Rapid Positioning

Using a G0 in your code is equivilant to saying "go rapidly to point xxx yyyy". This code

causes motion to occur at the maximum traverse rate.

Example:

N100 G0 X10.00 Y5.00

This line of code causes the spindle to rapid travel from wherever it is currently to

coordinates X= 10", Y=5"

When more than one axis is programmed on the same line, they move simultaneously untileach axis arrives at the programmed location. Note that the axes will arrive at the sametime, since the ones that would arrive before the last axis gets to the end are slowed down.

The overall time for the move is exactly the same as if they all went at their max speeds

and the last axis to arrive stops the clock.

To set values for rapid travel in EMC, one would look for this line in the appropriateemc.ini file:


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[AXIS_#] MAX_VELOCITY = (units/second)

The previous value for the rapid rate, [TRAJ] MAX_VELOCITY, is still used as the upper

bound for the tool center point velocity. You can make this much larger than each of theindividual axis values to ensure that the axes will move as fast as they can.

One thing to remember when doing rapid positioning, is to make sure that there are no

obstacles in the way of the tool or spindle while making a move. G0 code can makespectacular crashes, if Z is not clear of clamps, vises, uncut parts, etc.....Try to raise thetool out of the way to a "safe" level before making a rapid.

I like to put a G0 Z2.0 (Z value depending on clamp height) towards the beginning of my

code, before making any X or Y moves.

Example:

N100 G0 Z1.5 ----move spindle above obstacles

N110 G0 X2.0 Y1.5 ----rapid travel to first location

G1 Linear Interpolation

G1 causes the machine to travel in a straight line with the benefit of a programmed feed

rate (using "F" and the desired feedrate). This is used for actual machining and contouring.

Example:N120 Z0.1 F6.0 ----move the tool down to Z=0.1 at a rate of 6 inches/minute

N130 Z-.125 F3.0 ----move tool into the workpiece at 3 inches/minute

N140 X2.5 F8.0 ----move the table, so that the spindle travels to X=2.5 at a rate of 8

inches/minute

G2 Circular/Helical Interpolation (Clockwise)

G2 causes clockwise circular motion to be generated at a specified feed rate (F). The

generated motion can be 2-dimensional, or 3-dimensional (helical). On a common 3-axismill, one would normally encounter lots of arcs generated for the X,Y plane, with Z axis

motion happening independently (2 axis moves in G17 plane). But, the machine is capable

of making helical motion, just by mixing Z axis moves in with the circular interpolation.

When coding circular moves, you must specify where the machine must go and where the

center of the arc is in either of two ways: By specifying the center of the arc with I and Jwords, or giving the radius as an Rword.

I is the incremental distance from the X starting point to the X coordinate of the center of

the arc. J is the incremental distance from the Y starting point to the Y coordinate of thecenter of the arc.


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Examples:

G1 X0.0 Y1.0 F20.0 ----go to X1.0, Y0.0 at a feed rate of 20 inches/minute

G2 X1.0 Y0.0 I0.0 J-1.0 ----go in an arc from X0.0, Y1.0 to X1.0 Y0.0, with the center ofthe arc at X0.0, Y0.0

G1 X0.0 Y1.0 F20.0 ----go to X1.0, Y0.0 at a feed rate of 20 inches/minute

G2 X1.0 Y0.0 R1.0 ----go in an arc from X0.0, Y1.0 to X1.0 Y0.0, with a radius of R=1.0

G3 Circular/Helical Interpolation (Counterclockwise)G3 is the counterclockwise sibling to G2.

G4 Dwell

Plane selection for coordinated motion

G17 xy plane selectionG18 xz plane selection

G19 yz plane selection

Short term change in programming units

G20 inch system selection

G21 millimeter system selection

Fixture Offsets (G54-G59.3)

Fixture offset are used to make a part home that is different from the absolute, machine

coordinate system. This

allows the part programmer to set up home positions for multiple parts. A typical operationthat uses fixture offsets

would be to mill multiple copies of parts on "islands" in a piece, similar to the figure

below:


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To use fixture offsets, the values of the desired home positions must be stored in thecontrol, prior to running a program that uses them. Once there are values assigned, a call to

G54, for instance, would add 2 to all X values in a program. A call to G58 would add 2 to

X values and -2 to Y values in this example.

G53 is used to cancel out fixture offsets. So, calling G53 and then G0 X0 Y0 would send

the machine back to the actual coordinates of X=0, Y=0.

G53 motion in machine coordinate

system

G54 use preset work coordinate

system 1


system 2

G56 use preset work coordinatesystem 3


system 4

G58 use preset work coordinatesystem 5


system 6

G59.1 use preset work coordinate


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system 7

G59.2 use preset work coordinate

system 8

G59.3 use preset work coordinatesystem 9

Canned Cycles/Drill Subroutines (G80-G89)

Lookhere for a complete reference.

Distance Modes

G90 absolute distance modeG91 incremental distance mode

Feedrate and feed modes

G93 inverse time feed mode

G94 feed per minute mode

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Miscellaneous words

M words are used to control many of the I/O functions of a machine. M words can start the

spindle and turn on mist or flood coolant. M words also signal the end of a program or astop withing a program. The complete list of M words available to the RS274NGC

programmer is included in table 5.

Table 5

M Word List

M0 program stopM1 optional program stop

M8 flood coolant onM9 mist and flood coolant off
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M2 program end

M3 turn spindle clockwise

M4 turn spindle counterclockwiseM5 stop spindle turning

M6 tool change

M7 mist coolant on

M26 enable automatic b-axis clamping

M27 disable automatic b-axis clamping

M30 program end, pallet shuttle, and resetM48 enable speed and feed overrides

M49 disable speed and feed overrides

M60 pallet shuttle and program stop

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Modal Codes

Many G codes and M codes cause the machine to change from one mode to another, and

the mode stays active until some other command changes it implicitly or explicitly . Suchcommands are called "modal".

Modal codes are like a light switch. Flip it on and the lamp stays lit until someone turns itoff. For example, the coolant commands are modal. If coolant is turned on, it stays on until

it is explicitly turned off. The G codes for motion are also modal. If a G1 (straight move)

command is given on one line, it will be executed again on the next line unless a commandis given specifying a different motion (or some other command which implicitly cancels

G1 is given).

"Non-modal" codes effect only the lines on which they occur. For example, G4 (dwell) is

non-modal.

Modal commands are arranged in sets called "modal groups". Only one member of a modalgroup may be in force at any given time. In general, a modal group contains commands for

which it is logically impossible for two members to be in effect at the same time.

Measurement in inches vs. measure in millimeters are modal. A machine tool may be inmany modes at the same time, with one mode from each group being in effect. The modal

groups used in the interpreter are shown in Table 1.

Table 6G and M Code Modal Groups

group 1 = {G0, G1, G2, G3, G80, G81, G82, G83, G84, G85, G86, G87, G88, G89} - motion

group 2 = {G17, G18, G19} - plane selectiongroup 3 = {G90, G91} - distance mode

group 5 = {G93, G94} - spindle speed mode

group 6 = {G20, G21} - units


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group 7 = {G40, G41, G42} - cutter diameter compensation

group 8 = {G43, G49} - tool length offset

group 10 = {G98, G99} - return mode in canned cyclesgroup12 = {G54, G55, G56, G57, G58, G59, G59.1, G59.2, G59.3} coordinate system

selection

group 2 = {M26, M27} - axis clamping

group 4 = {M0, M1, M2, M30, M60} - stopping

group 6 = {M6} - tool changegroup 7 = {M3, M4, M5} - spindle turning

group 8 = {M7, M8, M9} - coolant

group 9 = {M48, M49} - feed and speed override bypass

There is some question about the reasons why some codes are included in the modal groupthat surrounds them. But most of the modal groupings make sence in that only one state

can be active at a time.

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Credits

This page is a rather direct rip off of the relevant portion of the RS274NGC doccument

from NIST. It is a work in progress and always will be a work in progress! This page ismaintained by Dan Falck. Your comments and criticisms are welcome. Examples of real

code with drawings or screen capture would be really nice here.

This page is part of the EMC Handbook and is covered by its GPLD copyright.
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