This project started when I was inspired by a 3d printed plotter bot design by MakerBlock. He and others developed arduino code that allowed a robot to read g-code and plot with a marker using hobby servos. (Something typically done with stepper motors.) Great idea and perfect for making a low cost simple robot with lots of functionality.
My intent was to leverage his code and create my own set of robot hardware to draw on coffee mugs. The original vision was to create an educational toy like the famous Egg-bot, but one with even lower barriers to copy.
Why a Rack & Pinion Actuator?
I choose a rack & pinion type actuator to keep the vitamins low and the movement speed fast. (As opposed to a slow leadscrew or pulley driven actuator with belts & bearings). Well for those reasons and the fact that hobby servos only have 180 degs of usable motion severely limits your options.
I had to do some math up front to size the gear to get the 80mm of motion I needed to draw on an entire 13 oz coffee mug. The effective diameter is the only variable that affects motion, tooth size did not, so I picked a large (easily printable) tooth size that maintained contact ratio around 2. The tooth mesh benefits from some of the assembly compliance to keep it tight enough to prevent backlash, but squishy to prevent sticking points.
Keeping it Tight:
For a drawing robot to be worth anything it needs to be rigid so you can draw straight lines. The challenge of designing a tight moving assembly without bearings is that to reduce slop you end up making it harder to drive, and hobby servos don't have much torque to waste.
Indeed, my research for an existing rack & pinion to start from yielded lots of wiggly/loose gear rack assemblies that are purpose built. Most of the machines I saw were hacked together, designed by folks gifted with software skills much greater than their mechanical design skills. I consider those projects to be fantastic works of art, rather than repeatable mechanically sound works of engineering. There is a limit to how much slop you can tune out of a machine.
So I had to design these models from scratch and during the iteration process I found it necessary to include some special compliance/adjustability features to facilitate reliably smooth motion. For those interested I wrote an article on my personal blog to share some of the design for assembly tips I used, "Design for Assembly Tips- Building a Better 3D Printed Linear Actuator".
To aid in that effort I included pockets/reservoirs along the moving surfaces to hold grease to further facilitate smooth wiggle free motion.
During the design phase I realized that I could greatly enhance the usefulness of the linear actuator by giving it a standard hole mounting pattern to make it modular. The result allowed me to rearrange the robot components to reuse proven designs and to create even more awesome stuff!
At this point this actuator is a key component of two robots. See images for the prototype Mug-O-Matic & Post-It-Plotter!
Additionally, the 4x screw hole pattern I've selected allows for easy reconfiguration of the parts at 90 deg angles. I have also designed some compatible accessories including a gripper (because fun!), a fishing line pulley (designpending), a lazer light pan-tilt frame, and some other fun bits.
For all of these projects I drive the robots with an arduino nano, expansion board, & 9v battery or wall-wart. Yes it is brutal on the 9V, but it so compact & easy to use that I don't care ;). This setup is great for a classroom environment because it doesn't require messy soldering or complicated bread boarding.
The servos are also great because...Read more »