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Universal joint for Stack-a-bot

This documentation is for version 1.0, which is the first working prototype made of acrylic plastics and nylon. Needed for each gearbox approx:

Design

The idea of Stack-a-bot is thoroughly described in the Final project sector of this homepage. When the idea of robotic arm built of modules came to me, I realized that I could design the motorized joints in several ways. The most obvious was to put in a stepper motor, that would probably be the easiest way out, both when it comes to design and programming. The problem though is the torque of the stepper motors, as I was planning to lift an arm and a camera and move it around an object. Stepper motors were thus thrown of the design table early in the process. I looked at several gearhead DC motors in the hope that some would fit my needs of slow turning, high torque and fit into a 50mm tube, with no luck. All the motors I found in my search would have needed further gearing down, either for more torque or slower speeds. Since I needed anyways to design a gearbox I decided to go with cheap DC motor and my pick is the Reliapro 12V, 26500rpm three buck motor that is in the standard inventory of Fab Lab. The first design of my gearbox was an adapted design from a previous project. I went with moving shafts and gears being able to place only one gear on each shaft. I thought this was the way to go, as I could make the most of the space within the tube my joint is made of. This design was soon proven to be unusable since I had only enough space for four 18 tooth gears in the gearbox, as each shaft had to move freely without interfering with gears. For a while I was on my way of designing stackable gearboxes, so I could just add a new four gear box to the first one to accomplish my goals. I therefore turned my view towards traditional gearboxes like you see in cars where the gears turn freely on two shafts, with only one gear coupled to the output shaft at each time. I designed the gearbox with this in mind, setting up two static shafts where the gears could turn freely. I managed to set up a gearbox with seven gears turning on these shafts, each with the gear ratio of 3/1 (24/8) contributing to the total rat io of the gearbox to 37/1 or 2187/1. The output shaft is not geared at all, but it turns a gear on the output rod with the gear ratio of 2.5/1 (20/8) adding to the gearing of the gearbox to 5467.5/1 taking the turning of the motor from 26500rpm down to 4.85rpm. The torque of the motor is increased from 88.5 g/cm to 483.8Kg/cm with this gearing but gears made of acrylics will not hold at high pressure.

Cutting , milling and 3D printing

All the acrylic plastic components are cut with Epilog Laser cutter. Use the settings that suit your laser for the thickness of the plastics. I keep the 1.5mm shimmer black in the print out and raster it, to make it a little thinner. When it comes to cutting the tubes, I use the glass turner that comes with the Epilog laser to cut the 50mm tubing. I use the small wheel to hold the tube in place and to secure even movement of the tube during the cutting process. The end units of the gearbox are milled with ShopBot from 16mm thick nylon. Two end units are needed for each gearbox. Four different profiles are needed for the milling of the end unit, but I use the same tool for every profile or 1/8 inch, four flute endmill. As the nylon is quite tough but a little flexible material you need a slow milling speed or /s in order to keep your tools intact. If you use the same tool on all profiles, you can actually decide the order of the work as you please as long as you take the cutout-outside profile last. I choose to do the pocket profiles first, and the process here described will reflect that. The first profile is the 8mm pocket for the bearing and connectors of the toolbox. I take the whole middle section down to 8mm as that is a little easier in setting the profiles though it takes a little longer time. Next I do the 12mm pocket on the outside. This pocket has to extend a little over the outside cutline; otherwise you will get a thin collar of plastic at the edge of the unit. Next it is the cut profile in the middle. This profile has to be set at inside milling. It is possible to let the machine start cutting lower since the 8mm pocket has already been made. The last profile is the outside cut that releases this object from the bulk of plastic that it once was.
For 3D printing I made a 3D model of the end unit in Autodesk 123D and printed it with PP3DP printer from about 11 grams of plastic.

Assembly

Glue together two 8 teeth gears, and then six parts of 8 teeth gear and 24 teeth gear. When gluing together gears, it is very important to make sure they are not offset. Use a shaft to set them before you glue them together. Glue a 4mm shaft collar to one end of each shaft. (Shaft collars are those small rings cut out in the cutting process marked blue in the drawings). Glue the shorter shafts by the collars into holes x and y in wall 1 in the gearbox. Place the longest shaft in the hole q on wall 1 and glue the 3mm collar on the shaft on the other side of the wall. Be sure that you dont glue the shaft solid to the wall. Stack the gears onto shafts x and y according to drawing 1. Place the 3mm thick 8 tooth gear on shaft q in line with the last 24 tooth gear and glue it solid. (See drawing 2). Add wall 2 to the assembly and glue the last 8 tooth gear to the end of shaft y. Add the sidewall A with the output gear facing the short end. Place the half-circle-bore gear (orange) on the motor shaft and glue it tight. Place the motor in wall 3 and screw it tight with xxx bolts/screws. Add wall 4 to the end of the motor and place it all on the sidewall A with its gear inside the gearbox. Lock it together with the second sidewall. If you are building the gearbox with bearings on the output shaft, you need to press the bearings into walls 1 and 2 before you start assembling the gearbox. In this version of the gearbox you dont need to cut out all the shaft collars and the output shaft only needs to be 43mm long. Assemble the output rod by stacking the 20 tooth gear with the two square holed circles. The largest diameter circle is set in the middle. Use the 6x10mm square piece to assemble all the parts and glue it together. Insert this assembly into the 20mm tube and use glue to secure it. Parts for this are marked green in the drawing.

Designing the bearing

This gearbox is designed to use a standard 6804 bearings for the output rod. If you dont have one at hand, or just want to build your own bearing, this could be a guide to it. I designed the bearing since the cheap bearings I bought on Ebay, had not arrived in time for my delivery of the assignment. I made it the same diameter as the 6804 bearings and the same thickness by using 4mm and two 1.5mm thick acrylic sheets. The diameter for a 6804 bearing is 32mm and the bore is 20mm. This means that the space for the outer ring, rollers and inner ring, as usually found in a bearing is only 6mm. My experience from laser cutting tells me that working with very small objects can be difficult, especially if the diameter of the object is smaller than the thickness. A conventional bearing with an inner ring was thus hard to build. I therefore decided to use the output rod as an inner ring, with 4mm rollers attached to the outer ring, standing 1mm out of its slots. I therefore drew first a 32mm circle, with a 22mm cutout centralized. I then made two 4mm circles, centered them horizontally and placed them with 20mm distance in between. I grouped the two circles and centered them with the big circle horizontally and vertically. I then made a duplicate and rotated it 45, repeating it until I had eight circles around the 22mm cutout. I then ungrouped the small circles and withdraw them from the big circle. The rollers are cut from 3m m thick acryl, and I made them 4.1mm in diameter to substitute for the loss of material in laser cutting, making the bearing tighter.

Building the bearing

Use the cutout from the 4mm bearing core as a mold to avoid the bearing and the outer shield to be offset. Put an outer shield into the mold, and the 4mm core on top of it. Glue it together and wait for it to dry. Place all the bearing rollers into its places and place the second outer shield onto the bearing. Turn it upside down and place it in the mold. Glue it in one place on the inside of the bearing and wait for it to dry. Pick it out of the mold and glue it in several places around the edge. Be careful to use as little glue as possible to avoid gluing the rollers tight.