On Sunday 23 August 2020 14:59:11 Chris Albertson wrote: > You are pretty much confirming my opinion that you know more about > these than the guy who designed the one you are trying to build. > > Some of my first projects were a box to house a PCB, A plastic foot to > a stand mixer. and a rear cap for an SLR lens. And other things that > don't matter if the tolerance is off by 0.5mm Then you work up to > more complex stuff. > > The key is to develop CAD skills or you are at the mercy of random > people who post to Thingverse and the quality there is all over the > map. > > The key to printing machine parts is to never depend on precision. > I'll place a screw adjustment on a shaft to shaft distance and then I > can adjust the tightness of the gears after assembly. Precision > holes are printed undersize then bored or reamed. One way is to > treat printed parts as if they were raw castings. > > I'm starting another project soon, it will be an open-source robot > that is modular, lego-like with wheels about 200mm diameter. We are > designing this collaboratively with the idea that it by easy to print, > even on low-end out of spec printers. We'll use 8mm drill rod axels > and skate bearings. > > Long terms is a four-leg robot that is sized large enough to do normal > stairs and have an eye-level high enough off the floor to navigate a > normal house. Perhaps like a small dog. The key is being > buildable by amateurs with limited tools and experience. The wheeled > version goes first. The legged version needs 12 reduction drives. > You can see my interest in reading about your problems with this.
Yes, I got that from reading between the lines, Chris. :) But I just destroyed two more flexgears quicker than I can make them, took about 3 hours running at 5 or 7 rpms out to break the last one and it had plenty of hop room. So while the white pla might feel a hair more flexible, over the long haul it fatigue cracks just as fast as the black stuff. So far I've got one that has run 24 hours, but it also has a degree of backlash. Thats not good enough for a Stellarium drive. So after the one on the build plate now has self destructed, I think I'll bag up all the bearings and go back to making a 100/1 worm drive work on the BS-1 clone. Its been a learning experience, but I've also wasted at least $1000+ and 2 months. The TPU hasn't arrived, and its way too flexible for this from what I've found on you-tube. Wait for a different design to show up. From what I've learned, the cup of the flexgear could be doubled in depth aspect ratio. Or we need a plastic with more flexability, and tpu isn't it. ABS maybe, its pretty tough stuff. And higher priced too. And I need a way to calibrate the extruder on the micro-swiss hot end kit. They recommend a starter setting of 130 steps/mm, and I am getting close at 320/mm. > On Sat, Aug 22, 2020 at 9:26 PM Gene Heskett <ghesk...@shentel.net> wrote: > > On Saturday 22 August 2020 19:25:25 Chris Albertson wrote: > > > Then go into the CAD software and fill the hex pocket and enlarge > > > the holes to accept the brass inserts or make models threads. > > > > > > At some point to you have to decide that the person who designed > > > this thing only made a first level prototype and more development > > > is required. I'm following your story and I would have made that > > > determination weeks ago. It is a good fist try, to prove it might > > > work but they break quickly and seem to be very hard to print. > > > My guess is that a few more design iterations are needed. To make > > > them easier to print and stronger. > > > > > > I think there might be trade-offs reduction ratio and backlash and > > > strength and "printability" > > > > > > I had a professor once who said you can always tell a new or > > > amateur engineer because the stuff he designs only works if you > > > use non-standard high precision parts. Better designs don't need > > > any of that. Designing things that work and are printable is > > > hard. Easier to design stuff in metal with 0.0001 level specs. > > > This designer was tinking in metal but using plastic. > > > > Tinking, could be thinking or tinkling. I'm in favor of calling it > > the latter. > > > > "gear ratio" in the case of a harmonic drive, is a huge compromise, > > tradeoff if you will, between the ratio and the amount of flex > > imparted to the flexible member, lower ratios mean taller, wider > > "teeth" and more flex. Higher ratios need smaller "teeth" and > > correspondingly less flexing. And this one has about the maximum > > flex the plastic will allow without rapid fatigue cracking and > > failure. Go for too small and you are going to exceed the tolerance > > of your machine to lay up an accurate representation of what the > > optimum design needs. And this design may be right on the ragged > > edge. So the exact size of each of the three working parts must meet > > the following criteria: > > > > 1. The max push needs to be far enough out of round that the "teeth" > > of the flexgear spline must, by the shrinkage between the rollers, > > clear the teeth of the internal spline at the mid-point between the > > rollers, otherwise the teeth are forced to snap up and over each > > other as they're forced to pass. The 3 rollers on each end of the > > rotating wave plate, seem to force tooth contact over a wider > > section, in turn needing a more precise pull-in tension to clear > > this hop. > > > > 2. The max push needs to drive the teeth to as close to full > > engagement as it can, without trapping the teeth between the rollers > > and the internal gear. Bottoming of the tooth engagement, trapping > > the flexible spline against the internal gear is semi-desirable > > because that also corresponds to the zero backlash condition, but > > that flexes the cup at the base of the spline, forcing a quick > > breakoff at the bottom of the spline. So there must be a teeny bit > > of backlash in the plastic version. > > > > So you chose, by diddling the printers x and y scales, a size of > > wave plate that allows those 2 conditions. And the tolerance would > > appear to be about .1mm. Next you build the flexgear, again by > > diddling the printers scale, such that the built cup, placed over > > the running wave gear, can be seen to be flexing about 1.5 times the > > height of the teeth. > > > > Then you measure the teeth at the maximum push, and make the > > internal ring, about .15mm bigger to preclude the entrapment between > > the wave gear and the stationary internal gear, and when you then > > drop that ring over the rest of it, you see, on both sides, that > > they do clear enough to do this hop-over, without catching and > > causing a click. If that ring gear doesn't fit the body's you've > > already made, then you adjust the wave bearing size by say .2mm and > > start over. But you can't go very far without exceeding the amount > > of slop in the 4mm bolt holes that put it all together. So the size > > tolerance for each of those 3 parts is around .1mm. > > > > In plastic? Keep your rabbits foot VERY handy. Oh, and the flexgear > > and the output shaft/flange should be a press fit in the 35mm bore > > of the main bearing, so in the case of the flexgear, you can only > > adjust it by about .1mm undersized at the projection into the bore > > of that main bearing where they are bolted, up to what will still be > > drawn together by those 6 bolts without making the bearing too far > > out of round and lumpy turning. > > > > In short, this was NOT a good first project for a new printer that > > was way the hell out of calibration OOTB. I'm not convinced it was a > > readily achieveable project. At $10/hr for my time, I should have > > bought 3 new small ones on ebay and been ahead of this game. > > > > > On Sat, Aug 22, 2020 at 3:42 PM Gene Heskett > > > <ghesk...@shentel.net> > > > > wrote: > > > > On Saturday 22 August 2020 16:30:56 Chris Albertson wrote: > > > > > The solution is not to use captive hex nuts. > > > > > > > > Thats what this thing is designed for, if indeed it has pockets > > > > for the nuts. The m4's generally have a pocket, but the m3's in > > > > the wave bearing carrier are plumb bare. > > > > > > > > > You can simply print M4 > > > > > size threads inside a hole and screw the cap screw into the > > > > > threaded hole. This works if you don't go through so many > > > > > assembly/disassembly cycles. The best is to use these > > > > > heat-set brass thread inserts. Then your screw goes into brass > > > > > threads. > > > > > > > > > > In all cases, blue (medium) Locktite works well. > > > > > > > > And of coarse I have red and green. :( > > > > > > > > Got the second one running, fairly quietly, but I do need to > > > > split it and molycoat the wear areas. 3rd one has a slightly > > > > smaller flexgear about 5% built, hopefully it will work with one > > > > of the ring gears I have a pile of at various sizes, flow > > > > looking good at 100%, even on the brim. Running at 210 and 55 > > > > once past the startup layer, I read someplace where strength was > > > > enhanced with less cooling under the nozzle, more time to weld > > > > itself I guess, so its running that fan at 55%. And I've > > > > resliced with some elephants foot comp ( 0.5mm) enabled. Thats > > > > why I had to take the first white one to the lathe and shave > > > > that off. That was not a problem with the black, but that stuff > > > > is more brittle than the white. And I'm outta m4x10 ss cap > > > > screws and m4 hex nuts, miss-counted badly. Monsterbolts and my > > > > banks MC is my friend. :) > > > > > > > > > amazon.com/Threaded-Heat-Set-Inserts... > > > > > <https://www.amazon.com/Threaded-Heat-Set-Inserts-connecting-i > > > > >njec tion > > > > > /dp/B07BH5X252/ref=sr_1_18?dchild=1&keywords=m4+brass+insert+3 > > > > >d+pr int&q id=1598128018&s=industrial&sr=1-18> > > > > > > > > That would need the source .stl's modified, and I don't know > > > > enough about freecad to attempt that. So I'm stuck using the > > > > designers fasteners. > > > > > > > > Thanks Chris. > > > > > > > > Cheers, Gene Heskett > > > > -- > > > > "There are four boxes to be used in defense of liberty: > > > > soap, ballot, jury, and ammo. Please use in that order." > > > > -Ed Howdershelt (Author) > > > > If we desire respect for the law, we must first make the law > > > > respectable. - Louis D. Brandeis > > > > Genes Web page <http://geneslinuxbox.net:6309/gene> > > > > > > > > > > > > _______________________________________________ > > > > Emc-users mailing list > > > > Emc-users@lists.sourceforge.net > > > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > Cheers, Gene Heskett > > -- > > "There are four boxes to be used in defense of liberty: > > soap, ballot, jury, and ammo. Please use in that order." > > -Ed Howdershelt (Author) > > If we desire respect for the law, we must first make the law > > respectable. - Louis D. Brandeis > > Genes Web page <http://geneslinuxbox.net:6309/gene> > > > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
