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Michele Scotti

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Everything posted by Michele Scotti

  1. UTA columns connect the hexagons and provide anchoring to the spider's vanes. Their weight is crucial and the FEA assumed being Aluminum with a 50x50mm cross-section and a 1.5mm wall thickness. System overview: For our application we wanted to toy with CF so why not vacuum bagging5 layers around a 'mandrel' - represented by an Aluminium tubing?!? Sleeving it out was quite a painful process. though... Coloumns are internally reinforced in the area where spider's screws poke through them to create tension on the vanes - this is achieved by sleeving 2 pieces of small tube - 20mm in diameter- to help to transfer the compression forces across. You can see them in this CAD here below, running horizontally – you can see also that the ends are closed with 2mm plates to evenly distribute the pressure and help with the placement durinf the assembly. A 400mm long M4 rod is used to clamp the hexagons and the together. An M4 can withstand up to 250kg of pull but if used at yielding it climbs up to 430kg - we reckon this is enough clamping force. Each M4 rod weighs in at 60g and helps to load the structure with compression forces in a uniform way. It also represents a weight opportunity compared to few M6 bolts I would have chosen to use instead. Let alone that is dead easy to implement.
  2. Here is a quick and dirt first assembly of the UTA - this is more to have a glimpse of the assembly method and whether everythign was falling in place. Pretty happy witht the weight as it's in line with the projected 15Kg target for the whole UTA Say hi to Marco! Fellow astro club member and 33% of the workforce on this project.
  3. Some progress on the secondary holder. It's still pretty rough and I overused sandpaper in some spots. Needs some finessing... The small fan to help the thermal stabilization is loosely housed in the upper part - I still have to figure out the best pattern for the outlet holes. You can also see the tube inserted inside the secondary. The idea here is to have the pivot as close as possible to the centre of the sec so that adjustments are not throwing the light path too much around during alignment. Still not sure if it's worthwhile though.... I attach a rudimental sketch of the whole system - pinch of salt needed as it's nor an engineering drawing or a fully functional schematics. Just to give a gist of it....
  4. In the meantime a hint of the sec mirror holder. Mold is soft wood not meant to be permanent... It looks shi....unrefined as of now. I'm collecting some sketches fo rthe whole system as it is not detailed very much on CAD - need to do that at a later stage
  5. Central hub secondary mirror colimation mechanism The secondary hub mechanism is nested inside the spider central hub. The idea here is to provide a movable platform that offers smooth collimating operation as well as a wide contact area for the secondary mirror collimation screws. I have the feeling we have a bit of over engineering here..... Best thing we came up with was to re-purpose a piston and its cylinder. They are Aluminium bits duly gutted and lightneded. To complete the system 2 flanges connect the cylinder to the spider hub whilst the top one accomodates 3 knobs for collimation and ease of use - potentially they could be motorized if we'll fill the need for that. The target for the whole system including the mirror holder but without the mirror itself is 725g.
  6. Are they still active? HAve they ever posted on SGL? Any chance to get in contact? I have a couple of questions for them
  7. In retrospect - the altitube bearing bracket. Maybe not the most exiting part (?). Main thing here was to avoid soldering which would have most likely involved a following machining of the piece. Hence the interlocking and bonding approach which concentrate most of the work in machining different sections of a flat aluminium bar. Here's some details: As the aim of the bracket is to carry the load but also to place with (some) accuracy the altitube rolling bars, we machined the side of the bearings to a specific lenght. You can see the ends being trimmed on the milling machine - this involved placing them with the bar. A ground pin is placed on the bracket. Bearings are standard heavy duty pillow block bearing - pretty inexpensive at les than £10 each. The ground bars are 20mm OD - here in the picture a smaller section used to align the bearings for "trimming" The set ready for bonding (upside-down and withouth the dowel pin):
  8. Hi Nigel, thanks for the contribution. Would you be able to elaborate on that? Whilst is clear that thre's no reason for oxide to wait conversely I wonder how long does it take to reach its final layer thickness. I think I rememebr that to complete the oxidation it's a matter or hours although I can;t rememebr that paper and it might not have been linear. If you think about rust on iron that takes time and it's dependant on other factors on top of O2 in the air. Althuogh I prefer bonding - for personal reason- the oxide layer in very relevant
  9. Yep, exactly. And that's what is in my mind every time I was thinkig about giving it a try.... 😁
  10. Bit by bit....so here is the crossing bar in the back of the lower assembly. From a construction point of view it’s a 40x80mm with a 1.5mm wall thickness. Not been able to procure such a section we opted to join two 20x80 section with the benefit of having a stiffer 3mm wall running across the bar. All the holes are to keep it together during bonding with some bolts. The bracket is made of CF once again just because we had a suitable section ready available. Connection is via rivnuts hence the reliefs on the bottom of the bracket. Modal analysis suggested a residual weakness that makes the bar sag - as a consequence 2 supporting bars will bee added to help transferring the load to the pac-men
  11. I tried to stay away from soldering for few reasons although I grazed the idea of using brazing for the H frame. Anyway, reasons in random order are: 1 - welding may introduce distortions with the subsequent need to maching surfaces 2 - welding is not ready available to all ATMers, requires skills and - this project has to be as easy and affordable as it can be 3 - most of the construction is not involving highly stressed parts 4 - modern bonding epoxies can be as strong as a good weld Here below some details of the Alt bearing bracket before bonding:
  12. I almost missed this! Humbly honoured the project is cited in this list although stil in the making. The target is to build a long exposure 'attainable' imager that can be casually used as visual - finger crossed....
  13. Hi, I assume you are looking both at the piece at the end of tubes and the bracket to connect it to the UTA and the lower part of the scope. Is the scope intended for visual or imaging? Were you unhappy with the system being too woobly? Are you re-using the beams - is the section round? what's the ID / OD? Clear skies, Michele
  14. Placing the 4 brackets (those clamped parts) for the altitude bearings on the azimuth table using the mirror box as a jig. Here below a close-up of the bracket
  15. There are 3 guys close to the Liverpool Astronomy Society who built a 30" and then another 30" with shorter focal lenght (??). Unfortunately their webpages have been taken down so there's little around. The 2 telescopes were named TRO and TROK - I reckon one of this is actaully the one in Bulgaria. I saved few pics of the project....
  16. I have a similar design - I'll post it soon. You used a spherical joint didn't you?
  17. Peter, Michael, thanks for the info. To be honest this is a part of the project that I haven't really paid too much of details into. I know glueing was tricky though but also the desing of the holder can be modified to hook the mirror in a more traditional way. May I ask what glue did you use?
  18. I put the density in the model and ran the analysis - I actually didn't know the wieght till you asked. However with some rough calculation your secondary should be around 850g whereas mine that has almost double the area but 12mm thickness id is ca. 1.2Kg. It's going to be 'glued' on its back rather than hung from the perimeter.
  19. Can I see any pictures of your scope here on SGL, specifically the spider you are talking about - I'm curious to have a look at that for similaritieswith our design. As of now I have an FEA that tells me that I'm ok both for modal and deflection. I might well have a look at that again.
  20. 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
  21. 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
  22. 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
  23. 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.
  24. 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
  25. 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?
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