I’ve put off starting a 3D printer for a long time, mostly because I couldn’t think of anything particularly useful to do with it. But over the past year or so I’ve run off a few parts, and the list of things I’d like to have has gotten longer. So for my birthday a couple weeks ago, in a fit of delinquent irresponsibility I pulled the trigger on a Rostok Mini delta printer kit from 3d Printer Czar. Took just over a week from order to the delivery arriving, and it’s been taking up most of my free time since Tuesday afternoon.
Part 1: mechanical build
I managed to complete the basic mechanical build in 3–4 hours in one evening. The directions online are a little rough in parts, but mostly pretty thorough. The kit includes most tools needed, which was nice. But a hex key for the M8 bolts on the top belt bearings was missing, and my hands were very glad to have decent-sized pliers.
First minor issue I had was with bolting things to the printed parts. It’s important not to over-tighten these — I managed to crack one part slightly along the print grain tightening it a little too much. I’m not too concerned about it because it has other bolts nearby to take up the strain, but I was much more cautious after that one.
Second was with the two printed parts attached to the hotend. One seemed to warp slightly inserting the hotend, leaving the end tilted slightly to one side. Not really a showstopper, and worst case I’d be able to print a replacement part, possibly with a slightly wider insert for the hotend..
I only had one major problem after the build was finished. The build instructions don’t place anywhere near enough emphasis on how important it is to make sure the carbon fibre delta arms are the same length. Following the directions to the letter I eyeballed the rods to make sure they were about the same length, cut strips from a leftover screw baggie, wrapped them around the lead screws on my u-joints, and jammed them in to the ends of my rods. That left me with a central effector platform that was visibly a few degrees off level. And shortly afterwards I discovered that the rods were also pretty fragile — the u-joints liable to pull out of the rods with too much force.
I suppose this could be improved by packing the lead screws with thicker plastic, and then I’d be able to match the lengths of the rods more carefully by screwing the u-joints in and out. But decided to scrap that idea, and go with a more permanent solution. I’d build a jig to hold the rods in place and glue them.
Part 2: fixing the arms
Yesterday I unscrewed the arms and measured the rods more carefully. Found a good 1.5mm difference between the shortest and longest arms, which in hindsight is pretty ridiculous. With the effector plate in my hand though I was able to realise that the bolts holding it together were still barely finger-tight. Tightening them up held the whole structure together properly and straightened up the hotend nicely.
Building a jig to hold my arms while they were glued was simple enough. I took the length of the longest arm, added 34mm for the two Traxxas 5349 u-joints (they’re 22mm long and 10mm wide, so it seemed logical that the centre of the hole would be 17mm from the end), and an extra mm for luck. From there one could probably bang a couple of nails in to a block of wood and use that to make a series of arms the same length, but I started to overengineer a little. Starting with LibreCAD I quickly drew up a block with six pairs of screw holes the right distance (uh, 187mm for my arms).
This evening I wandered down to Robots and Dinosaurs, a pretty great maker space in Sydney, and used the laser cutter to cut a chunk of 8mm acrylic to shape. Then added longish bolts and fixed them in place to make my jig. Filed the insides of the carbon fibre rods a little, then used extra strong, slow setting Araldite epoxy to glue the u-joints to the rods. Placed the joints on to the rods, fixed them down with another set of nuts, and then looped rubber bands over the ends to make sure everything stayed together.
I gave the glue a few hours to set, then took the arms off and tested them all on the same pair of bolts to make sure I hadn’t done anything too stupid. Not sure why, because I don’t know what I’d do if the arms were different lengths at this point. But they all slid over the bolts with the same amount of play, so I’m pretty confident I’ve got them accurate enough, and right now I’m pretty pleased with how well it worked.
So, next is to leave the arms overnight to fully cure. Tomorrow I’ll reassemble everything then get back to work on the wiring and electronics.