How to build an 2-axis Arduino CNC Gcode Interpreter

Computer model of where the 3-axis CNC router will move to cut the shape


3D printers, laser cutters, water jet cutters, robot arms, delta robots, stewart platforms, the Makelangelo: all of these are examples of computer numerical control (CNC) machines. CNC machines need to move accurately and on command. Stepper motors are a great way to move accurately – they move a predictable amount and then stay where you put them. To command the stepper motors we need a way to easily turn our human desires into machine instructions into stepper motor steps. In effect we need our robot brain to be an interpreter. I’m going to show you a simple interpreter written for Arduino that lets you move stepper motors for your robots.


This tutorial is meant for people who have an Arduino and understand the basics of programming in C. That means variables, methods, while(), if(), switch(), and manipulating strings. There’s even a few ?: style if statements.


In the very beginnings of computers the programmers talked to the machines in machine languages – the ones and zeros of binary. They memorized the binary machine language and wrote programs in that machine language. Each program probably did one small job because they were so hard to write.

Then somebody got smart and wrote the first interpreter – a binary program that could turn a more human language (assembly) into binary. Every language since then has been a different flavor of the same ice cream: trying to find easier ways of letting humans tell machines what to do. Most programs today are written using interpreters that were built using interpreters that were built using the original interpreters. Ours will be, too. So Meta!


I can’t build something without a clearly defined goal. I need to see the target in order to hit it.

I could write a 10-line program that does one pattern of motor movements but then – just like the binary programmers of yore – I would have to write a new program for every pattern. Compiling, uploading, and testing a program is a time-consuming process. My goal is to write an interpreter that can listen and respond in real time to any pattern.

Before you can run an Arduino program you have to compile and upload it. When our program is done you won’t have to compile anything. You will send the gcode to the Arduino through the serial connection and the Arduino will listen, understand, and obey.


My NEMA 17 stepper motors are controlled by an L293 motor shield riding on top of an Arduino UNO. On the motor shield I attached a female power plug so I could easily plug in a 12v power supply. I put a piece of tape on the motor shaft of each stepper so I can easily see it moving. Later I can replace that with pulleys and belts.

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For the sake of simplicity I’m going to assume that our motors are moving orthogonal to each other. That means on a grid that one motor moves along the X axis and one moves on the Y axis, also known as a cartesian coordinate system. (Delta robots, stewart platforms, and robot arms all use much more complicated systems to arrive at the same effect.)

Because we’re moving in a grid I can use Bresenham’s Line Algorithm to move each motor at same time with different relative speeds. That means in our cartesian system I can draw lines from anywhere to anywhere and they’ll be nice and straight. I can draw curves by chopping the curve into lots of tiny lines that approximate the curve shape.


The Arduino will need to:

  • Setup:
    • start listening to the serial connection
    • set up the stepper motors
    • Tell whoever is listening that we’re ready for more instructions.
  • Loop forever:
    • Wait for a message
    • Read the message
    • Interpret the meaning
    • Act on that meaning
    • Tell whoever is listening that we’re ready for more instructions.


The messages I send to the Arduino could be in any language. I could even make up a language! To keep life simple I’m going to use a language other people know and understand, called gcode.

The rules of gcode – the punctuation, syntax, grammar, and vocabulary – are very easy to explain to a machine. Gcode commands all consist of an uppercase letter followed by a number. Here are the codes I’m going to build into the interpreter.

Command Meaning
G00 [X(number)] [Y(number)] [F(number)]
G01 [X(number)] [Y(number)] [F(number)]
Absolute mode: Move in a line to (X,Y) at speed F
Relative Mode: Move (X,Y) amount at speed F
G04 P(number) Do nothing for P seconds
G90 absolute mode
G91 relative mode
G92 [X(number)] [Y(number)] change logical position
M18 turn off power to motors
M100 print out instructions for the human
M114 report position and feedrate
  • Every (number) is assumed to be a float – a number that might have a decimal place and an exponent. 2.015e-5 is a float.
  • Anything in [brackets] is optional.
  • G and M commands cannot be combined in a single instruction.
  • Arduino software has a nice way to send messages to the PCB through the serial interface window. It’s the magnifying glass on the right hand side of the Arduino window. Unfortunately the serial interface window doesn’t send the return key (\n) to the PCB. Instead of return I’m going to use semicolon (;) to tell the machine “this is the end of an instruction”.


#define BAUD (57600) // How fast is the Arduino talking?
#define MAX_BUF (64) // What is the longest message Arduino can store?

char buffer[MAX_BUF]; // where we store the message until we get a ';'
int sofar; // how much is in the buffer

 * First thing this machine does on startup. Runs only once.
void setup() {
  Serial.begin(BAUD); // open coms
  help(); // say hello
  set_feedrate(200); // set default speed

 * display helpful information
void help() {
  Serial.print(F("CNC Robot "));
  Serial.println(F("G00 [X(steps)] [Y(steps)] [F(feedrate)]; - linear move"));
  Serial.println(F("G01 [X(steps)] [Y(steps)] [F(feedrate)]; - linear move"));
  Serial.println(F("G04 P[seconds]; - delay"));
  Serial.println(F("G90; - absolute mode"));
  Serial.println(F("G91; - relative mode"));
  Serial.println(F("G92 [X(steps)] [Y(steps)]; - change logical position"));
  Serial.println(F("M18; - disable motors"));
  Serial.println(F("M100; - this help message"));
  Serial.println(F("M114; - report position and feedrate"));

 * prepares the input buffer to receive a new message and 
 * tells the serial connected device it is ready for more.
void ready() {
  sofar=0; // clear input buffer
  Serial.print(F("> ")); // signal ready to receive input

The only mystery here should be F(), a special Arduino-only macro. It tells the compiler to put the string in program memory instead of RAM, which can sometimes be the difference between a program that fits on an Arduino and a program that doesn’t.


 * After setup() this machine will repeat loop() forever.
void loop() {
  // listen for commands
  if( Serial.available() ) { // if something is available
    char c =; // get it
    Serial.print(c); // optional: repeat back what I got for debugging

    // store the byte as long as there's room in the buffer.
    // if the buffer is full some data might get lost
    if(sofar < MAX_BUF) buffer[sofar++]=c;
    // if we got a return character (\n) the message is done.
    if(c=='\n') {
      Serial.print(F("\r\n")); // optional: send back a return for debugging

      // strings must end with a \0.
      processCommand(); // do something with the command

I tend to write my comments first, then I write the code between the comments. That way I get my ideas organized and the code is better documented when I’m done.

Interpreting Commands and Responding

 * Read the input buffer and find any recognized commands. One G or M command per line.
void processCommand() {
  // look for commands that start with 'G'
  int cmd=parsenumber('G',-1);
  switch(cmd) {
  case 0: // move in a line
  case 1: // move in a line
    line( parsenumber('X',(mode_abs?px:0)) + (mode_abs?0:px),
    parsenumber('Y',(mode_abs?py:0)) + (mode_abs?0:py) );
  // case 2: // clockwise arc
  // case 3: // counter-clockwise arc
  case 4: pause(parsenumber('P',0)*1000); break; // wait a while
  case 90: mode_abs=1; break; // absolute mode
  case 91: mode_abs=0; break; // relative mode
  case 92: // set logical position
    position( parsenumber('X',0),
    parsenumber('Y',0) );
  default: break;

  // look for commands that start with 'M'
  switch(cmd) {
  case 18: // turns off power to steppers (releases the grip)
  case 100: help(); break;
  case 114: where(); break; // prints px, py, fr, and mode.
  default: break;

  // if the string has no G or M commands it will get here and the Arduino will silently ignore it

parsenumber(key,default) searches for the letter ‘key’ in buffer. If it finds key it return the number that follows immediately after. If it doesn’t find key it returns ‘default’.

Drawing lines

I first learned about Bresenham’s line algorithm from André LaMothe in one of his books back in the early 90’s. I think it was “The Black Art of 3D Game Programming”? It’s supposed to be used for drawing graphics on a computer screen. It works just as well for our purposes, and it can be extended to any number of motors all moving at once.

The slope of a line can be expressed as dx/dy. Let’s pretend for a moment that dx/dy is 1/3. Three steps on Y equals one step on X. Bresenham’s algorithm starts stepping along Y and adds dx to a counter. when the counter >= Y, step once on X. Here’s the best part: by using the slope of the entire line we can do all the math with whole numbers, which means we shouldn’t get any rounding errors and our Arduino can do the math extra fast. Whole number math nearly always faster than floating point math.

 * Uses Bresenham's line algorithm to move both motors
 * @input newx the destination x position
 * @input newy the destination y position
void line(float newx,float newy) {
  long dx=newx-px; // distance to move (delta)
  long dy=newy-py;
  int dirx=dx > 0?1:-1; // direction to move
  int diry=dy > 0?1:-1;
  dx=abs(dx); // absolute delta

  long i;
  long over=0;

  if(dx > dy) {
    for(i=0;i < dx;++i) {
      if(over>=dx) {
      pause(step_delay); // step_delay is a global connected to feed rate.
      // test limits and/or e-stop here
  } else {
    for(i=0;i < dy;++i) { m2.onestep(diry); over+=dx; if(over>=dy) { over-=dy; m1.onestep(dirx); } pause(step_delay); // step_delay is a global connected to feed rate. // test limits and/or e-stop here } } // update the logical position. We don't just = newx because // px + dx * dirx == newx could be false by a tiny margin and we don't want rounding errors. px+= dx*dirx; py+= dy*diry; } /** * delay for the appropriate number of microseconds * @input ms how many milliseconds to wait */ void pause(long ms) { delay(ms/1000); delayMicroseconds(ms%1000); // delayMicroseconds doesn't work for values > ~16k. } /** * Set the feedrate (speed motors will move) * @input nfr the new speed in steps/second */ void set_feedrate(float nfr) { if(fr==nfr) return; // same as last time? quit now. if(nfr > MAX_FEEDRATE || nfr < MIN_FEEDRATE) { // don't allow crazy feed rates
    Serial.print(F("New feedrate must be greater than "));
    Serial.print(F("steps/s and less than "));
  step_delay = 1000000.0/nfr;


Want the entire source in one file, ready to compile? Here you go. I use a version of this code in nearly all Marginally Clever robots.

Edit 2014-09-29: I’ve since added examples for Adafruit Motor Shield v1, Adafruit Motor Shield v2, RUMBA, and RAMPs.


This is an updated version that drives 4 steppers at once. It could be done with even more.

Now you try

  • Add a parsenumber('N') for line numbers. Then your machine can detect if it receives lines in the wrong order, or request specific lines from the computer. You may also need to understand M110 N[number] so the computer can set the line number.
  • Add a ‘;’ on the end of every command, followed by a checksum. Then the arduino can tell if the message is correctly received.
  • Add G02 and G03 for curved lines.

Final thoughts

So there you have it. In 293 lines of code we’ve built a really simple CNC machine Gcode interpreter that handles six G commands and three M commands.

If you’d like me to go into more detail about how to make arcs, or something else then please make a comment below and I’ll post about it.

Please share videos & pictures of your creations to the forums.

See Also

How to choose your controller electronics to build your first CNC.

Only registered users can comment.

  1. I do not understand where parsenumber is to be declared.

  2. Hello, thanks for supporting the cheap AMS v1 for Gcode. Your code is the only one I could find working.
    However it seems like when gcode has Ixxx and Jxxx values for curved lines it seems not responding.
    I dont know if this is my problem or line drawing is the only mod software can parse.
    Thanks in advance.

  3. thanks.
    I was looking fot super-simple g-code parsing code for a robot arm (to avoid the evening of coding my own)

    here I just need to remove these Besenham code since my SPI stepper controllers accept target-positions and not just step+dir

  4. Hi,
    Nice tutorial. I am writing my own 3D printer program I am just not sure about number rounding. I can’t find it in your code, I also can’t find place where you calculate from mm to actual steps. Thanks in advance for help.

  5. thanks for this sir its very helpful for students like me thanks again

  6. Very useful, but may I ask, is there any literature on how to add acceleration to this code? I am making open source-hobby stepper library. I actualy got implemented trapezoidal accelearation on linear movement, but I cannot find out how to implement it for arcs. Thanks for any ideas

    1. this conversation could be long, and should be done in the forums, please.

    2. Deli DD, may I ask how you managed to implement trapezoidal acceleration?

      Also, I am having trouble trying to understand feed rates on the bresenham algorithm. Can anyone recommend any kind of literature on that?


      1. It’s a great question that’s probably better handled in the forums, because it could be a long conversation not relevant to this tutorial.

    3. Atmel has an example for avr on their website which works fine. Single motor only.

  7. I would like to thank you for this post it is very clear to understand! Great job!!
    Just one question: what can I use to send the GCode? Is it compatible with Universal GCode Sender?

    Thank in advance!

    1. It might be compatible. I haven not tested it recently. Please post your results to the forums and I’ll update the post with your results.

  8. I am able to get this program to work on my arduino Mega, but i would like to run it on an Arduino DUE, so i can combine it with my existing program calculating non cartesian coordinates.

    When i load it onto my arduino DUE the serial monitor only responds to the firs command, then stops… What can i do?

    1. Discuss it in the support forums? We don’t officially support DUE because we have none in stock. If you can get it working and submit a github pull request, we’ll include your changes in the official firmware along with your name.

  9. Thanks this is very helpful

    but i dont know if I am the only one who has a problem in code or not ?
    because i took it copy paste on my arduino program and there are errors for declaration like
    so if u have any solution for this problem plz help
    thanks again 🙂

    1. The code in the article may be out of date. Please post a complete error message to the forums and we’ll figure it out together. Then I can update the article.