If needed I can implement microstepping at a later date, which will drive my precision down to 0.001563º per step. The total ratio of both gear pairs: 36-1, or 14,400 steps per revolution, or 0.025º per step. the 24-tooth bevel is coupled to a 10-tooth spur that drives a 120-tooth spur. These give an initial ratio of 3-to-1, and also allow me to change the direction of the rotation for easier motor mounting. The first pair is an 8-tooth to 24-tooth bevel gear combination. Instead, I used two pairs of gears to get my precision. Not small enough! The stepper drivers I bought can drive the steppers in 16-step microstepping mode (which would drive our precision up to 6400 steps per revolution), but it comes at a cost to accuracy and power draw so I tried not to rely on it. The stepper motors I bought off of sparkfun have 400 steps per revolution, which works out to 0.9º per step. Now for the gritty details: The motor driveĪs mentioned earlier, I need a large gear reduction. The general advice is: plan out your wires before printing your first piece! This has bit me before, but thankfully wasn't as important with this design. The other easy mistake is to forget about your wire routing. It's so easy to put a bolt somewhere that a tool can't reach. ![]() That way when some small aspect doesn't work out, you don't have to waste as much time and filament fixing the mistake.Īnd there were plenty of newbie mistakes. After a lot of wasted filament, I found that for complex mechanical assemblies like this it's often better to break up the pieces into logical chunks. When you have a 3d-printer and an excess of filament, it's tempting to try to make everything in the fewest number of pieces as possible. The other issue that ended up taking more time than expected: Assembly It takes a lot of calculations, so naturally I found this wonderful OpenSCAD library that does it all for you. What makes a gear "good"? As it turns out, there is a particular tooth shape that works best, and the linear dimensions of the shape depend primarily on the number of teeth and the diameter of the gear. It can be calibrated out, but for smoother movement it's best avoided.īacklash is controlled by how closely your gears fit, and how "good" your gears are. That distance must be covered every time the input shaft changes direction, and introduced a nonlinearity into the controls. With gears comes an aspect that could make my life difficult: backlash.īacklash, or more simply "gear slop", is how far the input shaft to a gearbox must turn before the output shaft begins to turn. ![]() ![]() In particular, I knew I needed an extremely large gear reduction in both axis to get the precision that would be required. I went in thinking "it's all applied geometry" and came out thinking "even the stuff that is applied geometry can be hard". This project took me a little out of my comfort zone. As evidenced by the fact that I generated that model in OpenSCAD. OpenSCAD is a script-based modelling software that has made actual mechanical engineers look at me with a mix of pity and horror.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |