CNC Graphics Tablet V2

This instructable demonstrates a major software upgrade to the graphics tablet described in https://www.instructables.com/CNC-Graphics-Tablet...

The software upgrade supports the following:

freehand drawing,
straight lines,
clockwise arcs,
counter-clockwise arcs,
automatic closure of loops.
image scaling.
choice of g-code file format [1]
GRBL plotters such as that described in

Operation is simple ... decide whether the next outline is straight / curved and touch the appropriate menu button ... or draw freehand.

The estimated cost of construction is less than $25.

Images

The cover image shows the image to be traced
Photo 2 shows the traced image outline in the Processing3 graphics window
Photo 3 shows the Universal Gcode Sender Visualizer view
Photo 4 shows the original and scaled images side by side
The video shows the entire process from tracing the image to plotting

Notes

[1]

The tablet is intended for use with CNC plotters ... not CNC mills.

Parts and Construction

Parts

Parts are the same as for https://www.instructables.com/CNC-Graphics-Tablet-1/ apart from a paper “menu” taped to the underside of the screen.

Construction

Construction details for the graphics tablet are described in https://www.instructables.com/CNC-Graphics-Tablet-1/
Download and print the attached file “menu.jpg” (photo 4)
Tape the menu to the underside of the graphics tablet screen as shown in photos 2 and 3 above.

The estimated cost of construction is less than $25.

Images

Photo 1 shows the paper menu
Photo 2 shows the menu taped to the underside of the screen
Photo 3 shows the underside off the screen

Software

Downloads

Download and install the following software on your computer:

Arduino IDE (Integrated Development Environment) from https://www.arduino.cc/en/software
Processing 3 from https://processing.org/download/ [1]
Universal Gcode Sender (Classic Version) from https://winder.github.io/ugs_website/download/

Download each of the attached files

touchpad_sender.ino
touchpad_receiver_v2.pde

Arduino file installation

Download the attached file touchpad_sender.ino
Copy the file contents to a new Arduino sketch.
Save the sketch as “touchpad_sender” (without the quotes)
Compile and upload the sketch to your touchpad Arduino

Testing the Arduino software

You can test this sketch as follows:

Open your Arduino “Serial Monitor”
Set the “baud” speed to 115200

You should see a stream of “S's" flowing down your monitor screen. This is the Arduino trying to “Sync” to the Processing sketch which has yet to be loaded.

Send an upper-case letter “S” from the Serial Monitor ... the stream of “S”s will stop ... the arduino now thinks it has the attention of Processing 3
Now send another “S” while touching the touchpad with a toothpick (or similar) ... the Arduino should respond with the XY touch coordinates.

The Arduino and Processing software use two-way handshaking when data is being sent ... to see another coordinate you will have to send another “S”

Close the Arduino Serial Monitor once you have finished testing.

Processing file installation

A Processing sketch is very similar to an Arduino sketch ... the only visual difference is that the Arduino loop() is replaced with a draw() loop.

Download the attached file touchpad_receiver_v2.pde [2]
Copy the file contents to a new Processing sketch.
Save the sketch as “touchpad_receiver_v2” (without the quotes)
Your Processing installation is now complete
Testing the Processing software
Plug your Arduino touchpad into your PCs USB port
Launch Processing3
Open Processing sketch “touchpad_receiver_V2.pde”
Left-click the Processing “Run” button at the top-left corner of the screen
The display should look like look like photo 1 [3]
Follow the on-screen instructions

Notes

[1]

The software should work with Processing 4 Beta 1.0 which is now available.

[2]

The included software uses variations of:

Bresenham’s Line Drawing Algorithm
the Three Point Circle Algorithm and
an original Arc Drawing Algorithm

The formula for drawing a line through three points is derived in this Instructable.

Formulas for drawing straight lines and arcs have previously been derived in https://www.instructables.com/CNC-Drum-Plotter/:

Step 5: Bresenham’s Line Drawing Algorithm
Step11: Arc length from gcode I,J biarc values

[3]

The numbers highlighted in red in photo 2 must match ... you will get an error message if your COM port has a different number.

Replace the [0] in your setup() code to match the your COM port number if you get an error message.

Downloads

touchpad_sender.ino

touchpad_receiver_v2.pde

Three Points on a Circle

This step derives the touchpad formulas and may be ignored.

The formula for an offset circle is :

r^2 = (x-h)^2 + (y-k)^2 .......................................................................................... (1)

Where:

r = radius
x = absolute x position along the X-axis
y = absolute y position along the Y-axis.
h = the horizontal offset the circle center is from the (0,0) XY coordinate.
k = the vertical offset the circle center is from the (0,0) XYcoordinate.

In the above diagram:

r1^2 = r2^2 = r3^2 ..................................................................................................(2)

From (1) and (2) we get:

r1^2 = r2^2

Substituting all knowns:

(x1-h)^2 + (y1-k)^2 = (x2-h)^2 + (y2-k)^2 ........................................................... (3)

Expanding (3) and cancelling terms we get:

x1^2 - 2*x1*h + y1^2 – 2*y1*k = x2^2 - 2*x2*h + y2^2 – 2*y2*k ........................ (4)

Rearranging (4) so that the unknowns are on the left:

(x2 – x1)*h + (y2-y1)*k = (x2^2 - x1^2 + y2^2 - y1^2) / 2 .................................. ( 5)

Similarly for r2^2 = r3^3 we get:

(x3 – x2)*h + (y3-y2)*k = (x3^2 - x2^2 + y3^2 - y2^2) / 2 .................................. ( 6)

Let

x1 = 1st touchpad X coordinate
y1 = 1st touchpad Y coordinate
x2 = 2nd touchpad X coordinate
y2 = 2nd touchpad Y coordinate
x3 = 3rd touchpad X coordinate
y3 = 3rd touchpad Y coordinate
A = x2 – x1
B = x3 – x2
C =y2 – y1
D = y3 -y2
E = (x2^2 - x1^2 + y2^2 - y1^2) / 2
F = (x3^2 - x2^2 + y3^2 - y2^2) / 2

Equations (5) and (6) become:

A*h + C*k = E ......................................................................................................... (7)

B*h + D*k = F ..........................................................................................................(8)

Multiply (7) by D and (8) by C:

A*D*h + C*D*k = D*E ............................................................................................ (9)

B*C*h + C*D*k = C*F ............................................................................................(10)

Subtract (10) from (9) and solve for h:

h = (D*E – C*F) / (A*D – B*C) .......................................................................... ... (11)

Substitute (11) into (8)

k = (F – B*h) / D .................................................................................................... (12)

Substituting h and k into (1)

r^2 = (x1 - h)^2 + (y1 – k)^2 ................................................................................ (13)

From which

r = sqrt( (x1 - h)^2 + (y1 – k)^2) .......................................................................... (14)

We have now solved for all of the unknowns h, k, and r

But we are not quite done yet

Arc commands have the following format:

G02 X12 Y34 I56 J78 ............................................................................................ (15)

Where

G02 = plot a clockwise arc
X12 = next X cooridinate
Y34 = next Y coordinate
I56 = offset of current location from the circle center
J78 = offset of current location form the circle center

I and J are calculated using following formulas:

I = h - x1 ................................................................................................................ (16)

J = k - y1 ................................................................................................................(17)

Operation

Before starting

Decide what g-code format you require

Setting the boolean GRBL flag in your header to “false” produces the code shown in photo 1

Setting the boolean GRBL flag in your header to “true” produces the code shown in photo 2. This code is suitable for use with the GRBL plotter described in:

Creating your “drawing.gcode” file

Touch the “*” menu button to start and follow the on-screen instructions
Touch any of the “mode” buttons to change modes while tracing an outline.

For example to draw a rectangle with rounded corners:

touch the “*” button
touch the “Line” button
touch the line start
touch the line end ... a continuous line will be drawn between the two points.
touch the “Arc1” button and follow the on-screen instructions
touch the “Line” button to draw the next line”
repeat this sequence for each corner
touch the “*” button to start a new outline

Touch the “Save” button once you have finished all outlines. This creates a file called “drawing.gcode” in your Processing “touchpad_receiver_v2” folder [1]

Plotting your “drawing.gcode” file

plug your plotter into a spare USB port
position the pen over the lower-left corner of the paper
launch “UGS” (Universal G-code Sender)
set the baud speed to 115200 and “connect” to your plotter
click “browse”.
find your “drawing.gcode” file, then click “open”
click “Visualize” to see the plot paths
click “Send”

Note

[1]

Be aware that the “drawing.gcode” file is overwritten whenever you touch “Save”. Rename this file if you wish to keep your gcode.

Summary

This Instructable describes how to make a CNC graphics Tablet from an Arduino and an 8 inch 4-wire resistive touchpad.

The gcode output from the tablet is intended for use with plotters ... not CNC mills.

The software supports GRBL plotters.

The touchpad has a resolution of 600 x800 pixels which translates to a 600mm x 800mm drawing if your target resolution is 1mm/pixel.

The project comprises an Arduino sender and a Processing receiver.

The Arduino is responsible for sending each touchpad coordinate to a PC which is running Processing3.

Processing3 displays what you have traced and generates a valid g-code file called "drawing.gcode" that is compatible with most CNC plotters.

The g-code file “drawing.gcode”must be renamed if you wish to preserve your work as it is overwritten each time you click "Save".

The estimated cost of construction is less than $25.

Click here to view my other instructables.