FrenchyArnaud

No, the electronic bits you saw on the desk are actually the prototype for the next instalment (I am building a much heavier mount based on OnStep).

Typical of the GF. Vic is as brilliant with gifts as I am inept 🤣 Got me to pilot a WWII warbirds, took me to the World Final of snooker, brought me original copies of Jules Verne (I am french and a vintage scifi enthusiast) , offered me a deluxe full scale print of DaVinci Leicester Codex (he is one of my heroes) etc - she always knows exactly what to do. And she pulls up with my antics and lets me invade the flat with my electronics, bits of telescopes, camera stuff etc. She's brilliant.

Thanks. I was given the track a few weeks ago and of course I recorded the tv 🙂 Well, that was my 5mins of fame and it's now gone 🤭

I shared a cheeky meme on a facebook group comparing my 130pds to hubble. (a very sound, credible and totally justified comparison) That attracted a local journalist, then other journalists, then someone from the bbc saw the newspapers and messaged again on FB...

I don't know... I am famous now.... Maybe I should protect my brand 🤭🤭🤭

TBH... I heard about it for the very first time the day they called me 🤣 I don't watch much TV. Still, it's a funny thing to happen and it makes my kids proud so... 🤭

Airing tomorrow 23/05, 7pm, BBC1 "the One Show" 😁

I discussed it with a buddy of mine who does a lot of 3D print and he thinks it's simply too delicate a part to sustain any chance of it taking on the forces. In the meantime I have found job lots of similar cogs used for RC models and printers and there is a good chance one within the job lots would do the job as it is apparently a common model (24-12, 2.5mm bore) However, as I have the electronics and motors ready to go for the mount I am building, and this gear box is only 50:1, I am ore and more interested in simply shoving my own motors and PCB and converting the mount to a system I trust more and would give better precision. I can symply clone my pcb/motors when build time comes. I am undecided yet.

I contacted them - twice- MONTHS ago as I was interested in having this very mount serviced and put back to specs. They never even answered. So tbh I will not even bother My custom made GEM will be in serice before they just respond to the messages

Ok so after very careful reassembly... well not careful enough apparently. Turns out the vynil/plastic primary cog of the gearbox was damaged in the process - it's missing 1 tooth. I am very confident i can put the mount back in service if I can find a replacement. Any ideas where to find that?

That's a valid point. This particular mount was supplied with a C8N intented mostly for visual use and even though it's obviously much heavier than the 130pds i use, it is probably marginally lighter than the 130 geared up for AP; crucially the 130 is outdoors on the pier 24/7 under a cover so it's possible that the wind has put a lot of stress on the assembly day after day. The screws I have fitted are much sturdier though, so if the thing restarts and runs more or less smoothly I think it will hold. Plan B is to do an onstep conversion. Like an upgrade / intermediate step towards the GEM i am building anyway.

Yesterday was dry so I took everything down and performed the autopsy. The CG5 drive train is : Stepper > integrated cogs gearbox > braket > cogs > worm > axis. As soon as I took the mount off the pier I knew something was physically broken inside, it was obvious a heavy part was moving freely and knocking and I could hear smaller parts dancing around. It turns out that the gearbox on the stepper had sheard off. It is fixed to the stepper axis by two MINUSCULE screws, about 1.5mm x 5mm. One screw broke clean (i suspect metal fatigue as in effect, these 2 tiny, tiny screws must absorb all the effort of the mount forever) That in turn freed the bottom plate of the gearbox on one side, warping it; the second screw followed, the motor fell in the casing, the outer case of the gearbox sperated from the bottom plate and all the cogs, bushes etc went flying in the plastic outer case of the mount. The caustiously optimistic good news is that this catastrophic failure MIGHT not be terminal. I was able miraculously to collect all of the bits of the gearbox and extract the broken screw bits from the bottom plate. I was also able to unwarp it (more or less) I very cautiously drilled 2 new holes and reassembled as tightly as I could. By eye it LOOKS ok. I will clean everything of and check/service the RA assembly as well in case it's having the same impending failure and try if it works once reassembled. I am not sure at all my gearbox work comes anywhere near expected tolerances and it will work again, but how much more dead than dead can I make it? If you ask me, having all of the torque transmitted to these 2 ridiculous screws inside the gearbox is a major design flaw. All the rest is pretty sturdy, it's obvious that this is a really weak point in the drive.

@windjammer Well Simon this is a LOT to process 🤩I will be back to you shortly with about 624 questions (+/-25%), but one thing that immediately stroke me is this : The design speaks to me for a lot of reasons (and mostly because I drew something very similar before opting for 100% belts throughout) but I fell of my chair when I realized both motors are actually geared to PLASTIC cogs ??? Are they some special vynil or specific materal, because I discarded the idea of plastic cogs with no second thought, absolutely certain that there was no way in this universe that 4mm thick plastic cogs could transmit any sort of torque without stripping the teeth clean in seconds. And clearly I was wrong. Don't they wear at all, warp, fracture?

Yes it's in the books, just waiting to have dry weather now to uncover it and take everything indoors which is a bit of an undertaking. I'll do it "just in case" but not holding my breath. As I already said elsewhere, i see it as an autopsy, not a rescucitation attempt, i really do not hold any real hope.

A LOT to take on from this and more good ideas I will shamelessly steal than I dare to admit 🤭 Really very useful and very much appreciated. My first axis was delivered today (front wheel bearing assembly bmw x5) and immediately several remarks. First : it's an absolute BEAST. It scales at 4.4kg - and I'll need 2, so with bolts etc, that's 10kg right there (22 pounds) 2) There is way more friction/stiction than I envisionned. It takes about 550g of force to get it moving. As the part where I will apply the drive is about 10cm in diameter, so 5cm away from the center, that is about 2.5Ncm required just to get it moving - I'll come back to this in a moment. 3) There is ZERO play. At all. The thing is absurdly stiff on all axis and ONLY rotates. I am 101% confident it can be loaded as a cantilever with 100kg of telescope and never ever flex by the width of a hair. I am absolutely certain that any flex will come from the supporting frame and never from the bearing in a million year. You have to hold it to realize how stupidly heavy duty it is by our standards. 4) the middle part, that rotates (in between the plates) has a perfectly cylindrical section of about 15mm and a diameter of 98.5mm. So i think I'll install a reversed 11mm gt2. I could have done the maths but i just sacrified on of my gt2 belts and, put inside out, that's exactly 160teeth with a neat seam! If the last pulley is a 12teeth that's 13.3:1 on top of the gearbox, or 10:1 with a 16teeth. I might go for gt3 but i don't have one here to test (the ratios obviously would stay the same) 5) the bearing is PERFECTLY smooth. It is impossible to detect any ball wobble, sticky point etc... 6) The torque issue! Basic maths problem... Given a Torque T at the motor, a loss of torque L due to microstepping, a total distance D between axis and an efficiency E due to mechanical imperfections, what is the force ACTUALLY applied to this axis? T: 44Ncm at full step, 6Ncm step32 and 3Ncm step64. 😧D : estimated 50cm E: say 50% for safety Gearbox : hypothesis 720:1 That gives for step 32: (6*720*50%/50cm) - 2.5Ncm of known friction = 41Ncm. At step 64 that's 20Ncm - 0.2Nm. Apparently you need a minimum of about 10% of the inertia moment to induce smooth motion of the bearing which means that at step 64 the heaviest load is around 40kg with a scope 1m long. (half of the length for the inertia moment) but 50% is the goal. So the system will be, even with a very low efficiency, vastly sufficient even for massive OTAs but step32 would eliminate any limit (given that I cannot conceive of have ever heard of an 80kg ota) and would give an upper weight limit at 50% torque (rather than 10) of 16kg - and a bit more if the gearbox can have an efficiency of more than 50%. At 25% torque vs moment, that would be 30kg+ and should be enough for any conceivable "normal" OTA. So I am now looking at 925:1 @32usteps because that would give the torque reserve i want with a theoretical resolution of 0.11"/actuation and a payload capacity of about 30kg and up to 50kg. 720:1 @64 steps gives 0.07" but then I'd be limited to about 12kg for ideal torque reserve and 25kg at a push. Besides, I can flash the microcontroller later for 64ustep even with the 925:1 gearbox which would give 0.05" if it proves needed. But 925:1+64usteps would give ridiculously low slew speeds in the region of .25deg per sec (0.5deg per sec is already stupidly long horizon to horizon, about 6 minutes) Anyway! Having the actual part in hand has answered a lot of questions and dramatically narrowed down the gear options. And crucially it confirmed it defo will be a mechanically sound concept.

Well, that's it. It had to happen one day: my trusty CG5GT, circa 2006, died tonight. In the middle of imaging, PHD2 suddenly screamed at me that it was not able to do enough corrections in dec anymore and a rapid inspection revealed that the dec axis has gone for good; the motor makes a weak whine but nothing moves, so either the motor is dead or, much more likely, the mechanical drive has failed. Either ways, it's almost certainely the end of the road for my first "real" mount, that will have served for a solid 15 years inc. about 7 at my service. I have begun the built of a custom high specs GEM a few weeks ago but it will not be in service before several months (possibly just in time for the next clear night, forecasted in november 2024) and I really don't have the funds to replace the mount or hurry the build, so for now it's lights out for me... Bit gutted really, but it's less serious than a heart attack I guess. I will strip it down (again) to see if it is salvageable but my instinct says this time, it's RIP CG5.

Well, it's on its way so now waiting... As this will be the most defining part of the mechanical asselmbly and might even be part of the gearbox, I am pausing design and waiting for it to arrive so as to work from actual, measured dimensions.

This is MORE than useful, there is more to take on from these 3 pics and associated description than in all of my mechanical research so far... If I dared I'd ask for an extra picture of the front of the RA assembly - basically the mechanics on the top part of the wedge to have a better view of how you geared/aligned motor, gearbox and shaft to the dec assembly. And if I was a real pickle i'd ask for a close up of your caliper brake... But as I am not a drag I will not ask 🤭 Sidenote: that's a very impressive bit of kit, seriously. How long to put it together? Your day job is in mechanics or engineering?

@windjammer Wow that is one serious piece of kit with a few interesting points for me. The most obvious one is that you have set your RA shaft 90deg of how i envisioned it. I mean that your assembly is parallel to the wedge, not at a right angle to it and now that i have seen it it makes perfect sense... Also I note that you welded jack on that..Everything is bolted. That's an awful lot of bolts, some pretty complex. So 3 questions : Aren't you plagued by loosening bolts? Does it have any effect on the overall rigidity? (the bores have to be bigger than the bolts!) Where did you find these long bolts that you can bolt into at a right angle, I never saw these before. Also, I considered these pillowed bearings, how to they fare play-wise? Also, most of it seems to be aluminium rather than steel, any particular reason? It sounds much more expensive 🙂 Is it an OnStep system too?

Here is a very crude free hand representation of what I am thinking of at that stage (just did that in 45secs right now - i KNOW it's nowhere near to scale!) But that might make what I said clearer. Basically, 2 welded "cages" assembled by one of these bearings, nemas and gearbox within these cages, and the top one carrying the second bearing carrying the dovetail. So, a bit like you said, "just bolt them together" - not far from what I was seeing! I am trying to figure a simpler way pier > RA assembly because I think there should be enough space to put a lot in the opening of the wedge and simplify a lot the part wedge+RA, doing it a single piece. Still unclear how to set az for PA, I am thinking maybe a second brake disk freespining on top of the one already existing on top of the pier. In any case, this sort of "open core" assembly would take like an hour to build perfectly square to someone who knows about welding and grinding. It looks complex to me because I never did it but it seems to me the design is intresinquely very simple with very limited flex point and potential failure points. And, with proper gearboxes and 2" square tubes, could easily carry any conceivable OTA.

I will look into this. Right now the only three things that are pretty well established as for the mechanical design are : 1) no turret but a wedge (much easier to draw) 2) axis based on car bearings. 3) no shafts (except for the weights but they are not really part of the mechanics) And yes, those are the kind of thing I am thinking about. Specifically, these have a very interesting shape and price point : For BMW X5 E70, F15, F85 2006-2018 Front Wheel Bearing Kit The fact that they are almost flush, with a very symetrical drilling design, and allow to be used as an axis for the counterweight shaft really speaks to me. Also, because the ABS support is almost straight I am thinking I might be able to glue a belt around it, dips on the outside to convert it de facto in the last stage of pulley. A belt glued there would not be sujected to any traction but only to lateral forces and due to the pressure of the actual driving belt would very likely never move even with just a tap of double face tape! Let alone proper gluing with cyano or CT10. simple spacer on the visible part of the picture will make sure there is no friction across the support plate. And OBVIOUSLY these things are designed for non-existant play and immense load tolerance. So... yes, that's my idea. I don't intend to use shafts at all. I am thinking along the lines of a welded (with my about to be acquired welding skills 😁) square tubes squeleton on each of these bearing, one fixed to the wedge and carying electronics + stepper/gearbox on one side and carrying the other one with the second stepper/gearbox on the other side, itself carrying the second bearing, which will in turn carry the dovetail. So, no need for shafts at all. These will provide all the rigidity and articulations needed. Again, if they can carry a SUV at 120mph I doubt they would need help to carry an OTA at 15deg/hour... I did not go that far yet On the bench, the firmware nicely goes to home and pauses before flipping which makes for very smooth operation so I was happy with that but now that you have mentioned it... I need to think about it

Yes, your maths stand - 200 steps at 32ustep would require 2000:1 for the necessary resolution, that I agree with. And yes, I always thought in terms of <0.1arcsec/ustep. If I could guide at 0.5" RMS i'd be over the moon (my pixel scale is currently 1.83" and the ota upgrade i am planning will make it 1.25" so 0.5"rms would be AWESOME) But my motors have 400 steps (full, natively, 0.9deg/step, not 1.8 so double the resolution for starters) and the guiding itself is pulsed at 64 or 128 so you technically only need 1/4 of that, somewhere near 500:1. I plan to go a bit more to compensate for motor imprecisions, maybe 650:1 OR 700:1 tops (I can't really say until I have tested the actual power and precision of the motors) A 160th gt3 (11mm belt) is about 6.5" wide. Yes it's big - not overly so. Also, it was just an exemple to point out that I probably would not need 5 or 6 gearbox stages, I should be able to get away with 2 or 3. I'll consider that when I am at that stage! I am confident that with tunable rollers and absolute encoders I can bring the belt backlash really, really low. The real concern is slewing time. I simulated a 1250:1 in my config.h yesterday night and it took almost 4 minutes to slew horizon to horizon... But I suppose compromises will need to be found. I am taking on board all remarks made and that will come handy when i do the actual gearbox assembly.

Yes, that is absolutely planned. On both axis, straight of the motor shaft to benefit from the gear ration in terms of precision. The firmware makes the correction in real time to eliminate motor backlash, step slipping and PE as they occur.

Actually I spent 2 weeks discussing it and calculating it with some OnStep devs... part of your reasoning is biased. First, <0.1" total RMS is not on the table. I speak only of <0.1 tracking resolution. With mechanical imperfection, electronic limitations, that hopefully will put me somewhere in the .4" to .8" RMS. Second : The tracking, guiding and slewing res are actually led by different rules for microstepping. So the gearbox is set for, say 500:1 (for the sake of argument) and it happens to be sidereal rate at say, 10RPM (figures made up here) The microstepping does not come into account : 10rpm is 10rpm. What will come into account is the need for torque (and that is influenced by usteps) Now, the guiding is different because it is influenced by microstepping (for resolution), torque (for acceleration) and gear ratio (because it's fixed, 500:1 is 500:1) so if you need more resolution for ultra thin guiding, the ustepping is changed on the fly, say from 16 to 128 (that's 8x thiner) and the the RPM are adjusted too. It boils down to : there are 1296000 seconds in a revolution. 400steps * 32 = 12800 actuations. 1296000/12800 = 101.25 sec per actualisation. To get at, say, 0.5" per actualisation that is 101.25/0.5 = a 202.5:1 gearbox. It is not that much and defo nowhere near 3000:1. Three stages of belts 12-160 (x13.33) will give much more than that straight away, about 2400:1. Two stages would put you in the ball park actually. Taking into account mechanical imperfections and counting on an actual efficiency below 50%, say, 30%! That is still only 600:1 to ensure a guiding <0.5"... even at 32ustep. And the firmware will in fact guide at 64 or 128. For reference, the classic combination with these modern drivers and 400-full-step-motors is 32ustep + 360:1 ; that competes with pretty much any sub £2000 mount, even when the 360:1 involves a worm. I was a bit confused because I could not see at first how 8us*1440:1 is different from say, 128us*90:1 but yeps, it turns out it is! So the microstepping is really not the issue, mostly because modern driver boards are able to change it on the fly, we are not at the glorious epoch of A4988's anymore... The real challenge will be to limit all mechanical imperfections. Other guys did it before me - quite a few actually, and even though I don't have their skills obviously, the precision they achieve with belt gearbox and stepper is beyond anything a mount cheaper than a small car could only dream of.

No I haven't. I jumped on this project because I loved the ease and efficiency of the DIY focuser based on an arduino nano so I stayed with something I had some familarity with. These steppers can also be stepped at high rates (256ustep >3200RPM) and I would not qualify a nema17 400steps 4.4Nm 2A of "little ones" 😁 The torque behind the gearbox will be more than enough to crush fingers beyond repair 😅 As it stands, my choice goes to SUV wheel bearing assemblies. They are not expensive and can take ridiculously high load, even excentric, with pretty much 0 lateral play. If it can take a RAV4 surely it can take a 130PDS 15arcmin is nowhere near acceptable for me and neither is 0.5arcmin in fact. I am designing the whole thing to have an actual resolution inferior to the guiding resolution (say, 20arcsec), if the drive has 1/4 of a degree of play out of the motor shaft that totally defeats the purpose, my CG5GT, which is absolutely beyond battered, does noticeably better than that EDIT : I may have not made clear that I am not trying to replicate a EQ5 or 6 : I am not limited by weight, have no requirement for transportability, the whole thing will be fixed on a pier... So I don't care if the axis assemblies are 4 kg each As long as they don't move, I am good!