The cables have to be put under tension to make the mechanism work- the drive is entirely dependent on the friction between the drive pulley and the cable that will be wrapped around it and the friction is partially a function of the tension. In this low precision application it's acceptable, but in a 3D printer it would be bad.
Also, as the cable walks up and down the pulley, belt segments A and G go slightly out of parallel with the Y axis guide. As the mechanism moves with the cable overlapped, it springs loose at random times and makes plinking sounds that don't sound like a mechanism that's going to last very long. The problem with it is that as the mechanism moves, the cable walks up and down the drive pulley and sometimes overlaps itself on the pulley. The first used the motor mount and drive pulley shown below, with the cables wrapped 3 1/2 times around each of the drive pulleys in order to get enough friction between the pulley and cable that the mechanism wouldn't slip.
To keep things very simple, I decided to use a single level design (unlike UMMD) in which the cables are mostly on one level and cross over each other at the M segments. I modeled all the hardware and t-slot in Fusion360 and went to work on the pulley and motor mounts. Doing so keeps all the cables parallel to the axes. Cable tension is adjusted by moving the corner pulley blocks and the motor mounts along the rails in the Y direction. I designed the system to take maximum advantage of the slots in the t-slot aluminum frame. Regardless of whether the mechanism is belt driven or cable driven, you have to provide some means of adjusting the cable/belt tension. I ended up with three different motor mount designs, the first one awful, the next two much better.
That means you need friction, and the way you get friction is to wrap the cable around the drive pulley multiple times. The key to successful cable drive in this machine is getting sufficient friction between the cable and the drive pulley so that the mechanism moves the way you want without slipping. Only one motor turns when the carriage is moving at exactly 45 or 135 degrees, and both motors turn for all other moves. The entire X axis moves when the carriage is commanded to move in Y. Essentially, there are two fixed motors that drive the carriage in X and Y. Here's a quick review for those who may not be familiar with coreXY mechanisms. Inspired by this mechanism, I decided to use cables to drive this machine. I used a coreXY mechanism similar to the one used in my 3D printer, Ultra MegaMax Dominator (UMMD). I set the overall height at 28" - tall enough that people won't think they're supposed to walk on it (but they may think they should sit on it!). Since this was going to be for a MakerFaire, I decided to keep the height relatively low to the floor to make it easy for kids to see the patterns, and discourage people from putting their hands under the table and possibly getting injured by the mechanism that moves the ball. If something breaks or requires modification or adjustment, I can simply lift the top cover and sandbox off the base and take care of it, without having to vacuum up the sand. The dimensions of the cover and the base are the same, and the sandbox is slightly larger so it fits over the base while the cover fits into the sandbox. A base that contains the robotic mechanism, the sandbox, and the top cover. The framed plastic is 1.880 m x 1.005 m, so I figured that's a good size to use and manageable in terms of the weight, etc.Īfter thinking about what I'm going to do if something goes wrong and how I'm going to fix it, I came up with a scheme that divides the table into three main parts. On a recent scrap yard run I picked up a piece of clear polycarbonate framed in 45 mm square t-slot aluminum, as well as a bunch more of the same t-slot to use for this and other projects.
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