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Michele Scotti

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About Michele Scotti

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  • Interests
    Telescope making - Instagram #800mm_telescope
  • Location
    Woking, Surrey
  1. Time for the spider! The secondary is quite a large mirror -around 175/180mm on its short axis- so the spider comes with a generous central hub around 120mm in diameter. Vanes are cut from a fairly affordable 500x400mm carbon fiber sheet 2mm thick -around £50. The inward ends are bonded to the CF hub with the help of a reinforcement specifically made with a angle slightly bigger than 90deg. The 4 straight vanes attaches to the upper telescope assembly thought 3 coloums and the focuser board. The attachment consists in pairs of long M4 screws that poke through the columns and put tension on vanes once their nut are tightened. A small Aluminium U-shaped trough -kindly provided by Ikea- is used to clamp the vanes ends and connect it to the M4 screws. This try-out of the incomplete end might give a better idea of the clamping? All bits were assembled on a 'jig' for a decent alighment... This subassembly weighs in at 1,553g. In hindsight given the tension in the structure I think vanes thickness could have gone down to 1mm or 0.8mm with a potential saving of few hundred. This is the final result: Clear skies, Michele
  2. Following up on the 12 Sep post on the focuser board: The blank board was 2kg and with all cut-outs it got down to 1200g - still too heavy for our roadmap to a UTA within 15kg. Hence the work with the router to hollow out the inner layers of the cut-outs while leaving the external side untouched. This is just one of the way I came out to 'add lightness' whilst keeping the robustness. Final result is ca. 900g. After that a coat of clear epoxy to seal off the trimmed CF layers and protect the exposed wood. Clear skies, Michele
  3. Here is a picture of the azimuth table with the central journal and the 'tripod' - this assembly is upside-down compared to its operating position. The rollers are placed into a slot at the end of each leg where the azimuth stainless steel track is. You won't see them from the previous picture as they are on the upper side of the leg, facing the azimuth table. The four holes in the leg are going to accomodate some M5 aluminium rivnuts I hope is clear enough and you are enjoying the build with me. Any questions? Don't hesitate! Clear skies, Michele
  4. Epoxies - a huge family of bonding agents actually - have developed quickly in the past few decades in terms of performance, durability and reliability - still they might be quite controversial. I use a variety of epoxies that can't be found in a diy warehouse. yet commercially available. Here's an example that comes to my mind to summarize what a proper application of epoxy can do. And it's a very British example, almost 25y dated. So far there are no reported cases of failure. Only one during development as far as I know Nowadays the applications of epoxy are everywhere and in industries that run intensive and thorough testings. To be precise I am NOT an expert in the field.
  5. Following on the 'tripod': Cutting hole in the of the middle section of the central hub and how the 'legs' slot in: This is the machining of the flange that goes onto the azimuth table - this is the ID for a 80x50x10 bearing to match the go-kart shaft Here is the complete journal. The bearing is concealed by a piece of tape to protect from dust. The view is up-side-down compared to its actual positioning. The machining of the Mg flange was pretty uneventful - no spark, no smoke, nothing! Phew. Just few machinings on the mill to fit on the upper section of central hub. The central hub with the fix part of the journal - but missing the shaft Lastly, another application for aluminium bonding....the three legs are put together with two 80x40 sections to generate a 160x40. The latter cross-section is somehow commercially available but pretty rare and expensive whereas the 80x40 is much easier to get hold of and the joining generates a nice reinforcing ridge in the middle of the section. A 120x40 section-from two 60x40- would do too. There's actually little force going through this structure once the feet are placed close to the rollers. Clear skies! Michele
  6. Well it was well concealed. Here is a better picture before the bonding. For the sake of discussion - the sections joining the lateral element of the H are 10 in total - per side there are 2 transversal elements with 3x 45deg reinforcements. The transverse joints (being 60x30x1.5 sections) sums up to 4x261mm^2. The other 6- due to the angle are 6x369mm^2. Total sum is 3200+mm^2. - and it's just the pure geometrical one. Now take your wallet and pull a credit card out - that's pretty much the surface to picture. Bonding can do wonder The mirror is in the ballpark of 50kg. So gussets are there to make the assembly much easier and for peace of mind. But do I really need them?
  7. Following up on Posted September 19 regarding the scope bottom part. Under the azimuth table, there's a sub-assembly I'd call a flattened 'tripod'. CAD pic: Feet are not shown - along with a few other details... What I call tripod is a 3-leg structure that houses a central journal and secures in position the 3 sets of azimuth rollers found at the end of each leg. The central "hub" is in 3 pieces: a lower hexagon, a middle piece to slot the 3 legs in and an upper hexagon. The latter sustains the highest stress and can't be too thick. The middle piece is shown here: The central journal below in the schematics is made of a flange+bearing attached to the azimuth table and another flange with a fixed tube that matches the bearing. We want a bearing to achieve a robust radial constraint during the azimuth rotation so that the rollers are purely taking vertical forces. Also, we need a hollow axis to manage all cables coming from the mirror box and the UTA. After some considerations, the rear axle of a go-kart proved to tick all boxes. Its shaft is precise enough to match a bearing, it's hollow and we could make use of a matching sprocket flange. It's also dirt cheap. Here is the flange, made out of magnesium - more for the fun of machining Mg for the first time...been told it's sparky £18 on eBay. The shaft is a section of a warped one - free. In this picture the material for the 'legs' cut to length: Last but not least a pic of an inspirational design - don't have a clue where it's coming from, saved on my laptop for quite a while... Clear skies, Michele
  8. Once we started to deal with the main elements we were surprised - in a good way- by the dimension. Thanks for following!
  9. Follow-up on the azimuth table from #90 Surface preparation for bond adhesion - the strip needs to be 'normalized' as the CF layer was pretty heavy - 800gr/m^2- and left some pattern that we are concerned can cause a patchwork lack of bonding. So we poured some additional epoxy. Extreme flatness is not a concern/target in this phase. No fancier way to apply some load to the 3 stainless steel arches that make the ca.1200mm diameter - we want to get the best out of this but it's not the final surface. Removing some excess of epoxy - btw the one used is loaded with filler in order to control the flow and the fill under the metal strips. Semi-finished part under some sun - not (just) for a nice pic but rather for the final "high-temp" bake - a compulsory phase for most of the epoxy formulations. Some epoxies reach their peak performance after several days/weeks depending of environmental conditions. Next step: grinding the track
  10. The structure is bonded with epoxy. Welding would introduce distortion unless done very carefully. Also welding tends to require bigger wall thickness whereas all elements here are 1.5mm thick. Some gussets are riveted in critical joint. We used small L-shaped brackets to place the elements in a fairly precise way - the 2 main diagonals (corner to corner) were checked with a physical 'jig' to make sure the whole thing was 'squared'. The finished part needing only the holes for the mirror cell which dictated the position of the crossing elements. During the execution we added some six 45deg reinforcements - they were not part of the original FEA hence providing an additional safety margin to the results used so far to validate the structure. Given the small thickness some big aluminium 'washers' will be adopted to spread the pressure of the fixings on the pac-men.
  11. Material gathered from warehouse for the 'double H' mirror cell bearing structure that is an integrated part of the mirror box - the long and thin one is an inexpensive levelling bar (the one for concrete work) - amazingly straight, the beauty of extrusion All cut in a nearby shop - all 'paired' parts needs to be exact same length For the same reason holes are drilled together. This CAD screenshot is highlighting (maybe???) the structure for which we added some 45deg reinforcements during the execution - which were not part of the original FEA hence providing an additional safety margin to the results used so far to validate the structure. Assembly in place using the azimuth table just after the drum sanding so hopefully the flattest thing we had laying around. The diagonals dimension is checked to have everything squared and small L shaped brackets are used to keep everything in position before the final assembly Cheers, Michele
  12. Limit dripping of epoxy so there's a chance to have a sound bond between the stainless steel strip and the perimeter video.mov
  13. Next step on the pac-men build. Some 2mm stainless steel was laser cut at a nearby shop. Total for two cut and rolled 1800mm strips + 3 arches is 125Euros - I felt it wasn't much of a rip-off... still one of the highest amount of money spent on parts so far. . My line of thought is the following - the joining needs to be stay in place for as long as I can think of and should have a minimal settling along with not creating bumps. hence no way I'm screwing this to the pac-men.Also tensioning by the ends with springs or other mechanism - which look cool- I feel they don't provide the stability we need. We resorted once again to bonding with epoxy. Tricky bit is always exerting some sort of clamping pressure - truck-style ratchet band 50mm wide can do the trick. The overall operation was remarkably straightforward after sanding the surfaces and loading the epoxy with some filler to achieve higher viscosity.
  14. The casing for the 18650 is a great alternative to soldering. I'm in the process to convert an hand drill - this is an fairly informative YT https://www.youtube.com/watch?v=flNBEeG5KmQ&t=5s It adds a buzzer on top of the BMS. I'm planning to use a laptop charger instead of a 12v+step-up (who doesn't have a spare charger now!)
  15. Back from the shop for a final retouch with the belt sanding machine to have the face as parallel as possible Wrap of carbon fiber - no need for vac-bagging, only wet lay-up. More for protection than anything else - could be glass fiber Finding the CNC holes under the carbon skin and inserting rivnuts bonded in place. Holes are for the altitude bearing brackets
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