Array extruder

Here is a way of using linear rails and bearings to get the paint extruder working.

As you can see in the diagram, there are two direct drive possibilities for the motor.

The shaft to the right is pushes a piston into a cylinder to dispense the paint out of the bottom.

It needs 12″ of 6mm linear rail ($12), four linear bearings ($20), 24″ of 1/4-20 shaft ($2), one stepper ($12), one stepper driver ($25), one coupler ($20). So that’s about $90 bucks without the acrylic parts and fasteners and stuff. So to run five of these I probably need to make 10 to have backup and “hot reloading”. So it’s about a thousand dollars to make the paint head for the painting robot. I’ll probably needs some sort of arduino microcontroller and other electrical to control it. So the thousand dollars is pretty conservative. There will of course be all the parts for the gantry system (linear rails bearings, other material, cnc control and so forth). I’m supposing that may be a couple thousand dollars. So the first prototype will probably be about 3000.00 at this rate.

This configuration seems promising since 12″ of rail and four bearings gives a stroke of 3.5″. 1/4-20 threaded rod gives us 4000 steps per inch (assuming 200 steps per revolution with a typical step angle of 1.8 degrees).

So if we go with a large bore of say 1″ we get a total displacement of 11 cubic inches (180cc or  0.180L). We get a “step displacement” of .000785 cubic inches (0.0128cc).  I suspect that’s about 1/20th of what we need so a typical delivery will need  20 or more steps. That’s pretty good resolution and it will get better if we go with a smaller bore.

So with a bore of about 1″ we have a displacement of about 9 cubic inches. With 1/4-20 threaded rod and 200 step/rev motors, we’re down to 4000 steps per inch.

As you can see in the left diagram these can stack. That stack is about 10 inches wide and 10 inches tall.

So the tubing coming off the bottom to deliver the paint will be very short for the center extruder and only a few inches for the far left and far right extruder.

This is all assuming that I can make a syringe type extruder that will work with acrylic paint. The first thing I will try is to make cyclinders from acrylic tubing. I can weld the front end with a cap and hose connector. The piston can be a sandwich of three acrylic disks. The center one slightly smaller so a rubber o-ring can act as a ring. Perhaps even two rings but I don’t want too much friction.

I bought a bunch of acrylic paint to try some experiments with already.

I still need to work out the details of:

  1. How to change the paint repositories or how to fill them without getting air into the chambers.
  2. How to deliver the paint that comes out the bottom to the canvas in a reasonable controlled manner. 🙂
  3. Set up a microcontroller to receive commands and run the 5 stepper motors.
  4. How much burp is needed between each paint delivery to prevent running.
  5. How to control the overall machine. My plan is to create a three axis system using a gantry (three more steppers, a bunch of linear rail and bearings). I’m supposing this can be seen as an 8 axis system since there are three for the gantry and five for the paint dispenser. Or distill the paint sensor into a virtual that accepts just one channel of control and some microcontroller orchestrates the paint colors being delivered. Perhaps send it an RGBKKVVVVV number and it figures out what to do. My acronym is Red Green Blue Key Key Volume Volume Volume Volume Volume. So there are the three primaries red, green and blue, two keys (black and white) and five volume parameters for each of the five paints.
  6. How to process the source images into tool paths or g-code.