McGraw the Pen Plotter

McGraw is a pen plotter at DoES Liverpool, that was converted from a laser engraver.

Because of this former life as a CNC machine, McGraw expects to be fed G-Code files. So if you want it to draw anything, you’ll need to convert your artwork to G-Code format. This is where the scripts in this repo come in!

Requirements

• Python 3

How to use this

First, run the script to create a virtualenv and install the required Python packages into it. Open a Terminal, cd to this directory, and run:

script/setup

A little while later, it will have set everything up.

Now activate the virtualenv:

. ./venv/bin/activate

Eventually, we want the code in this repo to work for any SVG or G-Code file. But for now, you are expected to either pass a perfectly-formed G-Code file to ./send-serial.py, or to export some G-Code from Inkscape then fix it up with ./fix-gcode.py and send it with ./send-serial.py.

For example, assuming you have a G-Code file that has been exported from Inkscape (in this case, test/a3-landscape-test.gcode):

cat test/a3-landscape-test.gcode | ./fix-gcode.py | ./send-serial.py

Note: send-serial.py will attempt to guess McGraw’s device path on your system—defaulting to common paths like /dev/tty.usb* and /dev/ttyUSB*—but you may need to provide a path explicitly:

… | ./send-serial.py test/a3-landscape-test.gcode --device /dev/cu.usbserial1

McGraw will now start drawing your file.

Tips

You can send one-off G-Code commands to McGraw by piping them into ./send-serial.py, eg:

echo 'M3 S05' | ./send-serial.py   # pen up
echo 'M3 S120' | ./send-serial.py  # pen down

When fixing up your G-Code, you can provide a custom speed parameter (the default is 800):

… | ./fix-gcode.py --speed 1200 | …

If any of the scripts give you a ModuleNotFoundError when you run them, then you’re probably running them outside the virtualenv. Activate the virtualenv, and re-attempt:

. ./venv/bin/activate
cat test/a3-landscape-test.gcode | ./fix-gcode.py | ./send-serial.py

You can stop send-serial.py partway through a file by pressing Ctrl-C. Note: This will prevent further commands being sent to McGraw, but McGraw will continue whatever commands have already been sent – the only way to prevent that is to cut power to McGraw using the red button on its circuit board.

How it works

svg-to-gcode.sh is meant to be a friendly wrapper to a complex vpype command. Vpype is a Python utility creating and modifying vector graphics like SVGs. In case, we’re using the vpype-gwrite plugin to output G-Code. The settings in mcgraw-config.toml tell vpype-gcode to use the specific commands (eg: for pen up, pen down) that McGraw the Pen Plotter expects. But it doesn’t work right now.

svg-to-gcode.py is meant to use the vpype and vpype-gcode Python libraries to read the specified SVG files, optimise them for plotting (combining lines, sorting lines by proximity), and then export them as G-Code files. The settings in mcgraw-config.toml tell vpype-gcode to use the specific commands (eg: for pen up, pen down) that McGraw the Pen Plotter expects. But it doesn’t work right now.

fix-gcode.py automatically performs the replacements that we used to do by hand (see Converting from Inkscape GCodeTools output, below), making G-Code files exported from Inkscape’s built-in GCodeTools “Path to Gcode” extension suitable for drawing on McGraw.

send-serial.py reads G-Code commands from the given file, or from whatever’s piped into it on standard input, establishes a Serial connection with mcGraw, and sends each line of the file to McGraw, printing McGraw’s responses. Since it’s using a “call and response” method (meaning that McGraw has to wait for the next command after finishing the current one, rather than already having the next command in its internal queue) send-serial.py potentially slows down McGraw, especially when drawing lots of small line segments. Perhaps one day we could re-write send-serial.py to use a batched approach.

Jackie’s “flavour file”

Jackie provided a Juicy G-Code flavour file, with the following command snippets:

begin

expression	what it does
G17	select the XY plane (this probably isn’t required, as G17 is the default in grbl)
G21	use millimeter units (this probably isn’t required, as G21 is the default in grbl)
G90	use absolute distances (this probably isn’t required, as G21 is the default in grbl)
G28	rapid move back to home
G1 F400	linear move, at feed rate 400
M3 S05	set spindle speed to 5 – McGraw interpets this as “pen up”
G4 P0.3	dwell for 0.3 seconds – we use this to avoid “ticks” caused by the pen remaining in contact while the head moves

end

expression	what it does
M3 S05	set spindle speed to 5 – McGraw interpets this as “pen up”
G4 P0.3	dwell for 0.3 seconds – we use this to avoid “ticks” caused by the pen remaining in contact while the head moves
G1 X.000 Y.000	linear move to XY position 0,0

toolon

expression	what it does
M3 S120	set spindle speed to 120 – McGraw interpets this as “pen down”
G4 P0.3	dwell for 0.3 seconds – we use this to avoid “ticks” caused by the pen remaining in contact while the head moves

tooloff

expression	what it does
M3 S05	set spindle speed to 5 – McGraw interpets this as “pen up”
G4 P0.3	dwell for 0.3 seconds – we use this to avoid “ticks” caused by the pen remaining in contact while the head moves

Converting from Inkscape GCodeTools output

The DoES wiki explains how to replace some expressions generated by GCodeTools, with equivalents that McGraw understands. These are archived here for convenience:

before	what it does	after	what it does
G00 Z5.000000	rapid travel to XYZ 0,0,5	M03 S05; G04 P0.3	set spindle speed to 5 (pen up) then dwell for 0.3 seconds
Z-0.125000	a coordinate 0.125mm below the surface		nothing (so 0)
G01 F100.0(Penetrate)	linear move at feed rate 100	G04 P0.3; M03 S120	dwell for 0.3 seconds then set spindle speed to 120 (pen down)
F400	speed 400	F800	speed 800