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Rostok 3D Printer Build

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.

Assembled Rostok Mini frame

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).

B8gNylwCAAAXUCqThis 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.

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