Dual-Task CoreXY DrawBot

When I saw a COREXY (or H-Bot) plotter, I thought I would make it someday. Since most of its components were made from 3D printers so I have postponed it. Fortunately, once again the PVC pipes gave me an idea to build it with a special version. My Dual-Task CoreXY DrawBot can perform double tasks simultaneously, eg: plotting 2 same drawings at the same time, or using pen plotting and laser engraving simultaneously.

Please check my introduction video before getting started.

a. Main materials:

⦾ 1pcs x DFRduino UNO R3 - Compatible with Arduino Uno

⦾ 1pcs x Arduino CNC Shield V3 GRBL.

⦾ 1pcs x 3 Axis Control Board GRBL With Laser Engraver Supported. (Optional if we use Laser engraving mode)

⦾ 1pcs x 2500mW Laser Module (Optional if we use Laser engraving mode).

⦾ 4pcs x Stepper Motor Driver A4988

⦾ 2pcs x Stepper motor NEMA 17.

⦾ 2pcs x CD/DVD Rom Player Drive.

⦾ 2pcs x 50 mm L Stepper Motor Support.

⦾ 3meter x GT2 6mm Timing Belt.

⦾ 4pcs x GT2 5mm Bore Aluminum Toothless Idler Pulley for 6mm Width Timing Belt.

⦾ 1pcs x GT2 Idler Timing Pulley 5mm Bore 20 Teeth.

⦾ 2pcs x GT2 Timing Pulley 20 Teeth.

⦾ 4pcs x Round Shaft Diameter 8mm, Length 500mm.

⦾ 8pcs x Ball Flanged Shielded Bearings 8 x 22 x 7mm.

⦾ 8pcs x Horizontal Ball Bearing Bracket or Vertical Ball Bearing Bracket.

⦾ 2pcs x Aluminum Flexible Shaft Coupling, Inner Hole Size: 10mm x 10mm.

⦾ 1pcs x Power Supply 12/24VDC.

⦾ 2pcs x Clear/White Acrylic, size A3, thickness at least 5mm.

⦾ 4pcs x Copper Brass Pillars L-10mm.

⦾ 2 meter x 8P/16P Rainbow Ribbon Cable.

⦾ 1pcs x 5mm DC Female Power Plug.

⦾ Some small cable ties, cable spiral wrap, bolts and nuts and small zinc wires.

b. PVC pipes and fittings:

⦾ 14pcs x PVC Pipe Tee Ø21mm.

⦾ 16pcs x PVC Three Way Tee Ø21mm.

⦾ 4 meter x PVC Pipe Ø21mm.

c. Tools:

⦾ Drilling machine.

⦾ Hand saw.

⦾ Soldering machine.

d. Firmware & Softwares:

⦾ GRBL.

⦾ Inkscape.

⦾ LaserGRBL.

⦾ Engraver Master.

⦾ Universal Gcode Platform (UGS).

My DrawBot is based on GRBL firmware and used built-in CoreXY kinematics in GRBL. Its mainframe and supports are made of PVC pipes and acrylic sheets. It has working range about Y350mm x X320mm and detail configuration is shown below:

It has two Z axes ("A" is cloned from Z axis) so it is possible to draw two identical images or texts at the same time.

My DrawBot movement is described as follows:

⦾ X and Y rotate clockwise: Y-

⦾ X and Y rotate counterclockwise: Y+

⦾ X rotate clockwise and Y rotate counterclockwise: X-

⦾ X rotate counterclockwise and Y rotate clockwise: X+

Go to the next steps to see how I built it.

Firstly, I joined 4 pcs x PVC three-way tees Ø21mm together. I made two sets like that (called Y supports) for 2 ends of the Y axis. Later, two stepper motors X and Y would be mounted on them.

To build Y slider, I connected 2 pcs x PVC tees Ø21mm together and inserted ball bearings into 4 ends of this assembly.

Then I jointed 10pcs x PVC tee Ø21mm together. It was not only used to connect two Y supports, but also used as a footbase for my CNC DrawBot.

There're 4 open holes for each Y support. I inserted locked bearings into 2 open holes of each assembly. I threaded 2pcs x T8 shafts, length 500mm through Y slider and they were fixed by two Y supports at open holes with locked bearings. To keep the frame strong enough, the remaining two open holes of the Y supports are connected to 10-tees support assembly above.

The bottom view of Y structure.

I cut 2 acrylic sheets, dimension L180 x W120 x T5mm and drilled 8 holes. Four holes were used to mount stepper motors by L supports and the other for connecting to Y supports.

I drilled 4 holes in each Y supports corresponding to 4 holes on the acrylic motor supports above.

The first acrylic support was mounted to Y axis frame at one side.

And the second acrylic support was done in a same way at opposite side. The bottom level of PVC frame and acrylic sheets should be the same, making sure that both rested on a flat floor.

These acrylic sheets keep the DrawBot from falling down when the X axis moves far away from the DrawBot's centerline. They also fix the Y pipe supports, pulleys and shafts do not move when the DrawBot is in operating.

Same as Y axis, I made two sets of X supports which were built from 4 pcs x PVC three-way tees Ø21mm. Detail’s as follow:

⦾ The first X support: inserted lock bearings at 2 open holes, and installed one idler timing pulley 20 teeth as below picture:

⦾ The second X support: inserted lock bearings at 2 open holes, and installed two bolts M4 – L20mm. The distance between them was equal to the diameter of idler timing pulley 20 teeth - about 12mm. They were used to fix the belt system.

X slider was contructed from 2 pcs x PVC tees and ball bearings were inserted into 4 ends of this slider.

Then I cut 2pcs x PVC pipes Ø21mm, length about 500mm.

I threaded 2pcs x T8 shafts, length 500mm into X slider and they were fixed by two X supports at open holes with lock bearings. To reinforce the X frame, the remaining two open holes of X supports were connected to 2 pipes Ø21mm above.

Now I had 2 main seperated parts of a DrawBot.

I cut an acrylic sheet, dimension L120mm x W120 x T5mm, drilled and installed 4 toothless idler pulleys in the center. The distances between them were still 12mm.

I disassembled X & Y sliders from their PVC frames, then did following works:

⦾ X slider: drilled 4 holes at bottom and connected 4 copper brass pillar L-10mm on these holes.

⦾ Y slider: drilled 4 holes on top.

Connect X slider to acrylic belt support through 4 copper brass pillars.

Cut 2 more acrylic sheets which had same size with acrylic support above (dimension L120 x W120 x T5mm)

⦾ Black one: reinforce the slider system and it was install at Y slider bottom.

⦾ Clear one: mount Arduino Uno and GRBL shield.

Acrylic belt support was mounted to Y frame.

Thread 2pcs x T8 shafts, length 500mm into X slider.

I cut 2 acrylic sheets, dimension L120 x W100 x T5mm, drilled holes to mount 2pcs x CD/DVD frame and installed 2 aluminium flexible coupling 10 x 10mm for clamping the pens.

I disassembled X supports from X frame and drilled 4 holes on the each support.

Mount CD/DVD acrylic frame on X support.

Install it to DrawBot PVC frame.

Mount the second CD/DVD frame on the other side.

I measured and cut 2.5m GT2 6mm timing belt.

Thread the timing belt through stepper motor pulleys, idler pulleys.

I tightened the timing belt as much as I could and locked two ends by cable ties.

I couldn't wait to finish it in perfect way so when I got to this step I did wirings temporarily, clamped 2 pens on Z axis, and tested how it worked.

My project used 4 stepper motors and A4988 drivers. Because Z axis operated with 2 CD/DVD stepper motors, I had to add 2 jumpers as below picture to configure the 4th axis. In this case, Z-Axis was cloned to the 4th stepper driver A4988 which is marked as "A" on the CNC shield.

We should pay attention to measure & set the current limit of stepper motors by adjusting the trimmer potentiometers on the A4988 drivers, especially for CD/DVD stepper motors – compact size.

a. Enable CoreXY kinematics in GRBL firmware:

⦾ Download GRBL firmware files.

⦾ Copy GRBL to C:\Users\Administrator\Documents\Arduino\libraries\

⦾ Go to C:\Users\Administrator\Documents\Arduino\libraries\config.h. Uncomment the command line "#define COREXY" to enable CoreXY kinematics.

⦾ Homing Configuration: I have referred to https://github.com/gnea/grbl/wiki/Two-Axis-System-Considerations.

Because we can't set-up homing both X and Y axis at the same time in CoreXY systems as usual. So I corrected homing configuration for CoreXY as follow:

// NOTE: The following are two examples to setup homing for 2-axis machines.
// #define HOMING_CYCLE_0 ((1<<X_AXIS)|(1<<Y_AXIS))  // NOT COMPATIBLE WITH COREXY: Homes both X-Y in one cycle. 

#define HOMING_CYCLE_0 (1<<X_AXIS)  // COREXY COMPATIBLE: First home X
#define HOMING_CYCLE_1 (1<<Y_AXIS)  // COREXY COMPATIBLE: Then home Y

b. Upload GRBL firmware to Arduino Uno:

Take note that we should modify GRBL's config.h file properly as mentioned above, before uploading the GRBL firmware to our Arduino.

⦾ Open Arduino IDE, from File menu click Examples ‣ GRBL ‣ grblUpload.

⦾ Select the correct Port and Board (Arduino Uno) ‣ Compile and Upload the code to Arduino Uno.

c. Step/mm setting:

⦾ X & Y axis travel resolution ($100 & $101):

The X & Y stepper motors steps per revolution: 200.

Micro-stepping: 8.

No of pulleys teeth: 20.

Timing belt pitch: 2mm.

➽ $100 & $101 = (200 steps * 8 microsteps) / (2mm pitch * 20 teeth pulley) = 40 step/mm

⦾ Z axis travel resolution ($102):

The Z stepper motors steps per revolution: 20.

Micro-stepping: 1.

Leadscrew pitch: 3mm.

➽ $102 = (20 steps x 1 microsteps)/ (3 leadscrew pitch) = 6.667 step/mm.

I used following softwares to create the G-code files from texts or images:

• Inkscape and Gcodetools extension.
• LaserGRBL.
• Engraver Master.

To send the G-code file that generated by above softwares to controller and monitor the DrawBot's operation, I used Universal Gcode Platform (UGS).

Although my DrawBot has not been completed yet, just temporarily tested, it has given me quite a surprise result.

After temporary testing, I decided to make it neater.

I mounted a clear acrylic sheet which has been drilled on the previous step on the top of belt support.

Because DrawBot PVC frame was all hollow, I could easily drill few holes and threaded the cables inside PVC pipes. But I didn't like this way, so I left the cables exposed and Arduino Uno was placed on the moving part (belt support).

To keep the cables well organized and tangle-free, I used some small sized zinc wires that came along the stepper motor cables and they were covered by cable spiral wraps.

The zinc wire was strong enough to hold the cable in a standing position and also flexible enough to be bent or stretched when my DrawBot was working.

Finally, it looked pretty attractive by this way.

I used two red and blue ballpoint pens to draw some pictures and texts. As you can see, the drawing quality is quite good.

With this DrawBot version, we can also mount one working end with a pen and the other end with a laser module. And we have a CNC machine which perform pen plotting and laser engraving modes in the same time. Of course, we need to tweak the G-Code a little bit.

Thank you for reading my work and hope you enjoy my post this time!!!