I am working on a project to fully automate my telescope, and one problem I had was adjusting the focus. After some discussion, the best course of action is probably to use a stepper motor with an encoder (which I am using for the alt/azimuth as well). But I wanted to try an idea I had to modify a high torque servo to see how well it would work in this role.
The problem I had was my servo could only rotate 180° while the focus required about 570° for the full extent. My initial idea was to get a larger pinion gear for the rack and pinion mechanism, which would make the focus only require 180°, but I couldn’t easily (or cheaply) find a replacement gear that would fit. So instead I decided to modify the servo for continuous rotation by removing the metal stopper on the gear. This would allow for 570° rotation (or more) but I would lose position control. So the solution here was to replace the servo’s potentiometer with a linear pot and attach it to the slide for position feedback. In the video below you can see it in action. It’s not pretty, I just hacked it together with zip ties to hold everything in place for now for a POC.
One tool thats becoming a must have for the home hacker is the CNC machine. I have been looking into these machines for the past few months and have been trying to come up with a design that will suite my needs. People use these machines for many different applications, but my main reason for wanting one is for milling PCB’s, while also wanting to make some small parts from plastic/wood. This being the goal, I finally came to the conclusion that accuracy/precision is more important to me than size. So my final design has only a 4″x4″x4″ x/y/z range with 1 mil/step theoretical accuracy (or 1/8 mil/step if using micro-stepping. This should be large enough for most of the prototype PCBs I make while still large enough to make some parts and things from plastic and wood.
Initially I was trying to build the machine from scratch using wood. Ultimately I was not satisfied with the accuracy I was (not) able to achieve. Also, I was trying to go the route of using of the shelf parts from hardware stores, such as threaded rod and nuts for the lead screw and stainless steel rod for the linear rails. I found that it was very difficult to avoid binding and the friction from the threaded rod and nut seemed too much. Later I found I could use some high quality drawer sliders for the rails and this worked quit well, yet I still had the lead screw problem.
Finally I found some linear actuators at HSC Electronics that suited my needs perfectly. As much as it hurts me to give in and buy pre-made parts, I could not resist the $40 price 🙂 Also its better than anything I could build and I have been quit happy with them. Ultimately, the goal for this project is not the project itself, but to have a working CNC machine that I can use to facilitate my other projects, so I don’t mind using some pre-made parts.
The drivers I am using are the EasyDriver’s from sparkfun. These are okay, but really kind of under powered for these steppers I am using. The Linear axis have a homing swith and an encoder that I am not currently utilizing, but I plan to in the future.