Planet Gearbox for Fine Mirror Adjust, parametric

thingiverse

Version 1.1 Adjusting mirrors or beamsplitters on an optics bench can be difficult. This device sits between the mirror and the mounting post, and enables fine adjustments with a 5.33:1 gear ratio, so 10 degrees on the dial results in 2 degrees of mirror motion. The gearbox is made up of 5 parts: a captured planetary gear system (with herringbone teeth); a dial; a planet-gear carrier; a mounting post holder; and a threaded connector ring. The entire set is printed at once, and then assembled. The connector ring holds the carrier between the gearbox and the post-holder. See the exploded picture. The files are completely parametric, so using OpenSCAD, you can make (almost) any gear ratio you want. I've included two STL files: a 2" diameter 1:5.33 gear box, and a 3" 1:9.8 gearbox, though I haven't yet printed the 2nd version to see how strong it is. Print Settings Printer Brand: Ultimaker Printer: Ultimaker 2 Rafts: No Supports: No Resolution: normal Infill: 20 Notes: Because the threads require many facets, they slow down (or crash) the OpenSCAD rendering on my machine. There is an option in the Customizer to put in "fake threads" or disks to get rapid viewing, but prints ought to be done from "real threads". PLA was used for all the pieces. But I've been getting a lot of snapped posts on the carrier. I upped the wall thickness to 1.2mm in Cura, because otherwise the thin posts were hollow and too weak. Despite selecting a #6-32 bolt-hole for the central shaft, trying to put a #6-32 into it caused the post to shatter. I've been making do with a #4-40. Whatever you try, just don't force it--the printed posts aren't as strong as extruded plastic pieces. I fixed the Customizer interface, but found that it still took too much processing for the cloud server to finish. You're probably better off downloading the .scad file yourself and installing OpenSCAD to run it. The gear ratios get bigger when the sun gear gets smaller, but then the drive shaft also shrinks. At some point the plastic is too thin and weak to drive the output. I've pushed the design almost to the limit of thin drive shafts. The 1:5.33 design uses #6-32 (M4) drive shaft in a 2"(51.7mm) gearbox, and the 2nd design achieves 1:9.8 with #4-40(M3) drive shafts in a 3" (77.4mm) gearbox. I haven't printed the 2nd design yet to see if it can handle the higher torque, but the first design is a bit overbuilt (15mm high) for the light mirrors in our lab. Post-Printing Cleanup & Assembly Threads have to be cleaned well. The side of the gearbox with the Sun gear protruding for the dial is the top. Breaking the gear loose simply required snapping the dial onto the Sun gear (logo facing out) and twisting. Excess flashing appeared in one spot on the herringbone teeth on my first print only.This made one spot on the dial rotation very stiff. A penknife was needed to clean that gear (difficult to get access) but 5 minutes of work and the action was very smooth. The 2nd and 3rd prints with different filament didn't have the flashing and rotated right off the glass, even with 0.1mm tolerances. The Planet-carrier is inserted into the bottom. I had to sandpaper the 5 posts to get them to slide in smoothly--lots of flashing left on my print. Do this gently, I destroyed about 3 carriers (then upped the wall thickness to 1.5mm to get solid, not-hollow, posts). The connector is threaded onto the gearbox to cover the threads. The post-holder is then threaded into the connector to hold the Planet-carrier in place. That's it! OPTIONAL Metal Hardware I have included 3 optional metal bolts and 1 nut (colored pink in OpenSCAD view): 1)The post-holder has a set-screw and nut for tightening it up on the post. #4-40 (M3) with nut work nicely, 2)The output shaft has a threaded hole for bolting on the mirror/beam-splitter optics equipment. 3) The Planet-carrier has a threaded hole for a #4-40 (or tinier!) woodscrew that can provide a low-friction pivot up against the post-holder it it needs to be screwed on more tightly. I've been backing off a quarter turn on the post-holder assembly and that seems to make the friction tolerable without a pivot. How I Designed This Planetary gear design There are 3 gears in a planetary gearbox: Sun, Planets (riding on a carrier), and Ring. Any one of them can be held fixed, and the other two become an input/output pair. This provides no less than 6 gear ratios. If two gears are fixed, then the system behaves as a 1:1 (a feature used in automatic transmissions). Formulas for teeth and gear ratios can be derived by considering the ratios of circumferences of the gears or downloaded from the internet--they are also displayed in the OpenSCAD program. The biggest gear ratio is obtained when the Ring is fixed, the Sun is input, and the Planet-carrier is output. However, the Planets are in-between the Sun and Ring. So to make this accessible, the carrier has an output shaft that comes up through the Sun gear. This shaft is threaded for a bolt that attaches to the optics mirror/beam-splitter. But since most designs drive the Sun gear with an interior shaft, I had to use a sun driver on the outside of the Sun gear teeth, which is where the dial snaps on. To keep the carrier friction low, I made the Planet-carrier posts as skinny as possible, and put 1mm friction rings (bearings) on the bottom of the posts. In operation the carrier is held in place by being sandwiched between the post-holder and the gearbox. At larger gear ratios, the Sun gear shrinks, making the output shaft very skinny. Using the 2" (51.7mm) gearbox diameter, I was able to get a 3.7:1 ratio using a 1/4-20 (M6) bolt, a 5.33:1 ratio using a #6-32 (M4) bolt, and a 6:1 ratio using a #4-40 (M3) bolt. For the smaller bolts, I also had to reduce the number of Planets from 5 to 4 to 3, otherwise the planet teeth overlapped. I've also made a 3" gearbox with #4-40(M3) bolt that achieved 1:9.8 for all you extreme gearheads out there. Larger diameter gear boxes should be able to get even higher gear ratios. If higher gear ratios are needed a 2nd set of gears will be needed, this design uses only one planetary gear set. Update Log 10/22/2016 Added a 1:9.8 gear ratio, 3" (77.4mm) gearbox, which pushes the limit of torque possible with thin PLA shafts. The connecter ring now has 10-deg tick marks for helping align, and the logo scales with size. 10/21/2016 Fixed some bugs in the .scad file, now it displays "fake threads" correctly, and warns not to print. If unintentionally printed, however, the parts may still be assembled.