Rusted

Thanks. Until last week the mounting held a 7" f/12 refractor, a 6" f/8, modified PST, H-alpha refactor and a 90mm f/11 Vixen. [With a 9x50 finder. Just in case.] I am presently building a completely new 6" f/10 iStar, H-alpha, modified PST refractor. Which will join the 7" f/12 [for solar white light.] The Vixen will be rested as somewhat superfluous.

Episode 11. Correcting the most obvious Fullerscope MkIV design flaws: PA support fork: In the last [rather wordy] episode I discussed the weak "ears" of the PA support fork of the MkIV Also, how the short screws were very limited in their ability to tighten the fork tines firmly onto the Polar Axis casting. So my big mounting went totally overboard in fixing these perceived design flaws. Once again the scrap metal gods smiled on me and provided a strip of 20mm thick aluminium at precisely the time I needed it. After making a rough, cardboard template, to check the dimensions needed, I miter sawed it into three pieces. As in all, low budget projects the available materials define the build. So I kept one rectangular piece for the base plate and made the fork tines as tall as possible. This is important for matching the longest possible PA housing while allowing adequate clearance for the 11" RA drive wormwheel. The 12" compound miter saw was actually bought for building the dome but I was advised that it would also cut aluminium. It does! But makes one hell of a mess of the lawn! Paraffin [heating or odour free, lamp oil] makes an excellent cutting fluid. I just brushed it generously along the marked cutting line each time and this really helped to avoid aluminium welding itself to the saw's teeth. These heavy fork tines were then drilled and tapped on their lower edges and fixed to the base plate through recessed holes. This heavy fork is to support the weight of the rest of the mounting, telescopes and counterweights while allowing altitude adjustment. The altitude pivot is a 16mm stud with pretty brass domed nuts for a bit of character. It is placed to just miss the Polar Axis shaft. So the stud can be tightened to the breaking limit of the nuts if so desired. Though it isn't remotely necessary, of course. The very solid, Polar Axis, bearing housing is clamped very firmly between the support fork tines. I actually used the PA bearing housing as a spacer for the fork tines as I fixed their exact position on the base plate. The hope is that this heavy clamping effect will further reinforce the already massive fork against any chance of twisting. A turnbuckle is hidden inside the support fork to allow fine adjustment of PA altitude. I'll show that in a later episode. The northern plate of the fork [with the large hole for hand insertion] provides a further clamping surface. I put my hand through the hole to adjust the turnbuckle for PA altitude. This large hole was cut with a hardened tooth, hole saw in the bench drill with more lamp oil to aid cutting.

Thanks. Shall I get some paper towels?

That sounds sensible.

Excellent video! It was just like being there! Have you tried loading it with the expected weight of the completed structure? Roller bearings can completely change character when subject to loads. As I learnt to my cost with a 24" cast iron, face plate. Too heavy and too much friction! It was supported on 4" rollers with journal bearings on the rim. While tilted up at my latitude on a short shaft with a thrust bearing. It was supposed to be my [98" Isaac Newton stye] equatorial fork for my 16" f/5.

I agree entirely. I use the idea of an invisible audience to spur me on to project completion in my rural isolation. Whether they exist in large numbers, or are purely imaginary, you still feel you don't want to let them down. Well, it works for me. I make the effort to produce an acceptable outcome without anybody physically looking over my shoulder. In a forum situation you'd be amazed how new ideas just pop out of the woodwork. Nobody can be an expert on everything. The hive mind works, but you must be able to accept constructive criticism or corrections to firmly held beliefs. There is the excitement of walking the high wire but the gains can easily outway the fear of failing.

Episode 10: What vital things can I/we learn from the Fullerscopes MkIV mounting? Some users hung 15" reflectors on them! Not stumpy, short focus reflectors either. So it must be good? Right? The [hollow] cast, conical bearing housings are very good. They put serious "meat" [thickness] where the highest loads are concentrated. Moreover the flat tops [plates] of the cone shaped, castings provided local resistance which is perpendicular to the bearing loads. Great! The combination of shaft and 6" diameter plate bearings provides some very useful "dynamic" triangulation. Don't sneer! The heftier, 1.25" shaft size of the MkIV offers greater stiffness than the 1" of the smaller MkIII and competitors. The overall size [and inevitable weight] provided serious stability compared with other mountings of the time. 1960s-70s. However, the MkIV had some very serious design flaws. Which are very hard to overcome without damaging the structural integrity. Or completely spoiling the rather "pretty" originality. The worst part is the PA support fork. It looks sturdy but isn't. It's all an illusion! The supporting "ears" are thin and hollow cast. Far too thin to be called "tines." Which makes them far weaker than desirable. The PA, altitude pivot screws must bite into the very soft, casting of the PA cone where there is far too little material. These screws are seriously undersized for the loads! They used a coarse thread on the pivot screws and in the casting but it was still a very poor arrangement. Over-tighten the altitude pivot screws and you strip the threads. Or literally crush the fork "ears" causing cracking. Now what? The altitude locking screws are absolutely pitiful! No excuses! They have no holding power and soon strip their threads. Eventually I became sick and tired of the sagging PA and fitted a turnbuckle between the PA casting and the pier. That was once a brand new exhaust clamp! Finally the MkIV had the fine adjustment and locking in PA altitude it deserved. It never had one before. It also stayed put instead of constantly sagging. My restored MkIV had all the rust-prone, original screws replaced with larger, stainless steel ones. It didn't help! What should have happened, right back at the design stage, was passing the altitude pivot right through the ears and PA cone in the form of a sturdy stud. [screwed rod] That would have required an offset to clear the polar axis shaft but could have been done. The "ears" should have been cast solid at full thickness and fitted with very large and very thick, load spreading washers. Without modification the MkIV couldn't be taken seriously, in my opinion, as a constantly frustrated owner. There's more to follow on the MkIV's handicaps. Vital lessons in mounting design.

Episode 9: Basic theory: The sheer scale of my mounting was because I intended to carry long and heavy refractors. I already had a 7" f/12, 6" f/8 and 90mm f/11 and had plans for my 10" F/8 Newtonian, if I ever finished it. Long telescopes have a high moment = Mass x Distance from the pivot. [or fulcrum] Telescopes have heavy bits at each end. Even higher moment! M x D!!! Most importantly I have learned the lesson of totally avoiding portability the hard way. When I welded this pier/stand together I literally couldn't lift it onto my builder's trailer to take it home!! I had to get help. All the sections were the heaviest I could find at the time. The pillar is 180mm or 7" diameter, thick wall tube. The legs are massive rectangular sections with triangulating braces. That's my old, secondhand, 6" f/8 on top of the Fullerscopes MkIV. An interesting mounting but with a serious handicap. The MkIV met its limit with my 7" f/12 DIY iStar refractor. I learned a lot about mountings from the MkIV. I could take afocal snaps of the moon or sun at the eyepiece without any wobble! Despite my "make it up as I go along" design and build methods I do have half a century of absorbing ideas and making lots of things. The multiple disciplines of telescope making, reading, countless hobbies but [fewer] jobs provide plenty of experience in what works and what simply doesn't. Triangulation is a good form. A triangle is incredibly stiff in its own plane. Bicycles are made from triangles. You can separate your bearings for greater stiffness. Or you can expand them into large plates. Or do both. Heavy, localized loads, cause distortion in the supporting structure unless you have plenty of thickness in your bearing supports. Or design it to have great depth in resistance like a floor joist. Or triangulated roof truss. Space your bearings wide apart and the bearing support or shaft can easily bend if either is too flimsy. The more typical DIY habit of fixing pillow block bearings to a flat plate, or even plywood, is fraught with danger. A flat plate, or even an inverted U-beam, are the weakest forms in twisting. Trying twisting a roofing gutter if you don't believe me. The cheapest answer to twisting is to massively increase the depth of the supporting plate or beam. Scrap metal is cheap but very heavy. Adding ribs helps, though a square or rectangular tube would be much better. The idea is to put depth of material in the plane of the applied forces . A sheet of plywood, or even thin, doped canvas can stop a structure from distorting. It is called "stressed skin" when fixed onto something with depth. The flat material has such depth in one plane that it cannot possibly tear apart. The problem is supporting the thin material to avoid twisting. Thin sheets have no resistance to twisting. Even carbon fiber needs some structure to maximize its great strengths. Nobody laser cuts bicycles out of a single sheet of carbon fiber. It would be hopelessly weak and/or ridiculously heavy! The trick is to use boxes and/or tubes. The bigger the better. [Within practical limits!] Otherwise you just waste expensive materials trying to make stiffness where it simply doesn't exist!

More haste less speed? See next week's exciting episode! I was just trying to avoid too much text! Having already talked it to death during the design and build phase I had no real idea how to present it here.

Hi Marcus, Thanks for the kind words. That sounds like you have a positive philosophy for success. May we see an image of your creation? I'm sure you are being far too modest. My mounting has been working for quite a while now. So is well proven. I am still finding details to improve so it will probably never quite be finished. Another visit to the scrapyard, or something I see online, might take me in another direction. I like that about projects. It keeps me thinking and active. Nothing is fixed in stone.

Episode 8: The goal: I haven't shown an image of the completed mounting yet so here it is: More episodes tomorrow.

Another image showing the woodworking route to bearing support. This might be copied using tough and stable materials like laminated, birch plywood. Though drilling through from the sides for the cross studs might be a trial without a drill press. Some serious glue might make a solid block without the need for the cross studs.

Thanks. My computer keeps forgetting where I put my resized images so it's taking much longer than expected.

Hi Marcus, I hope you can glean something from my thread. I hoped to inspire others to try. The aluminium strip, I used for the bearing boxes, might be too expensive from normal metal stockists. Mine came from a scrap yard which had lots of 2m lengths. Cheap! If you were to copy the design using very thick, birch, multiply ply for the bearing boxes it might still work. Though I'd use the big, through and cross studs like I did with the aluminium plates to hold it all together. I haven't tried this, except with kitchen working surface board, so you'd be forging your own path. I'd hate for you to waste your time and money on a complete dead end. Nor would I want somebody to injure themselves if it all came apart!

Episode 7: Wooden bearing housings. Fail! Before I became involved in the aluminium bearing boxes I tried to use solid wood. Despite countless hours hand planing miters and cutting around the 16mm studs it didn't work. The wood was used in two thick layers but warped and the joints opened out. I had hoped end pressure and the stud containment slots would work. It didn't. Quality plywood [birch] might work if you copy the aluminium box design but you are on your own. I'd use much thicker board than 10mm! Even laminated sections to achieve a solid mass of material.

Episode 6: Enlarged PA to Dec. Plate bearing: The turned PA cylinder, containing the Tollok bush, is shown here with a PTFE sandwich underneath the 11" diameter, RA wormwheel. I had the RA wormwheel at the top of the PA for a long time before moving it down to the bottom. The thickness of the wormwheel boss lifted the PA more than necessary. So I moved it down later on. The plate bearing is supposed to stiffen the joint because it has great resistance to lifting one edge. The massive weight of the telescopes, Declination housing and its shaft plus counterweights all rest on the plate.

Episode 5: Belt and braces: As hinted by those who responded to my earlier post the Tollok bush needs firm support from the outside. Rather than rely on the supplied expansion rings I decided to beef up the PA to Declination joint. Some mountings use a combination of shaft and plate to stiffen the PA to Dec joint. The Fullerscopes MkIV is one example. I decided to add a big cylinder to act add a local thickening of the PA and to form a plate bearing. Lady luck had provided me with a length of 7" or 180mm scrap aluminium bar. It wasn't strictly necessary to the design philosophy but I went ahead anyway. I turned a matching recess in a slice of the big cylinder the length of the Tollok bush. The first image is of my old S&B Sabel lathe rough turning the recess. Paraffin was used as a cutting fluid to obtain the pretty final finish.

Episode 4: Yes, it's very heavy! I left the mounting resting on an old B&D folding bench overnight and found this is the morning! Do not underestimate the weight involved in such a project!

Spot on Paul! The Tollok bushes comes as bare units. Whoops! I just remembered. They do come with an expansion ring in the pack. I never used them so forgot all about about them. I expected that the bush will be fitted into a matching recess in the sprocket or pulley. The fact that they are used for driving heavy machinery gave me the confidence to use them in a static situation. Anyone using these bushes would need to find a matching sleeve for the bush to expand into. No doubt these are commercially available but I never researched the issue. Steel water pipe of the right bore to fit the outside of the bush might do? Image below shows the expansion ring and bare Tollok bush.

Hi Mark, Good question! You are correct. I didn't want to confuse the story line by introducing too much information or pictures at once. There is indeed a close fitting sleeve for each of the Tollok bushes to expand into. These sleeves also have dual purposes so I left this detail for a later "episode." I have lots more information and hundreds of pictures but didn't wanted to overwhelm anybody. The entire build was deliberately fashioned to avoid great expense or using "unobtainium." Anyone can go out and buy the materials and parts and literally bolt one together for themselves. I was extraordinarily lucky and just happened upon materials, at scrap prices, at the times I needed them.

Episode 3: Tollok bushes. How do you hold the ends of the Dec shaft to the cradle? Or the Declination bearing housing to the Polar Axis? You don't have welding equipment and demand easy dismantling by one person. The answer is to use Tollok bushes. These clever devices are used by engineers to hang heavy sprockets and pulleys onto shafts. So they must be incredibly strong and grip the shaft with a force which would make a GEM look like chickenfeed. The secret to its success is a strong flange on the end of gently opposing tapers. All finely machined in quality steel. You can see the inner, tapered sleeve inside the outer in the image below. Both tapers have their own flange. When the flanges are bolted tightly together the tapers crush together and onto the shaft. This is the Tollok bush inside the massive cradle. The other bush is hidden inside the Declination bearing housing. Visible in the first episode as a ring of 10 sturdy, 10mm screws. Tollok bushes come in many designs and sizes. I chose the largest flange and longest bush in 50mm and was not disappointed. I even had to file the edges of the flange to fit inside the cradle. I also had to file the washers to fit inside the top of the cradle. The washers were carefully chosen to be thick and stainless steel. They were also just big enough to touch each other at their edges in a ring. So they acted as a sort of load spreading flange on the opposite side of the clamped material. End of episode 3.

Episode 2 The first image is of the polar axis bearing box. You can tell it's heavy because it has a chain hoist alongside! The axes shafts are both heavy enough to be a serious individual lift in their own right. Lifting the bearing box and shaft together is strictly for a hoist! You can see the cross studs which are holding the side plates together. They are all carefully sited to miss the axis shaft. As stated earlier, they also lean against the 16mm studs to provide additional location and mutual stiffness. Think of it as a "compression bearing box" because it is being squashed firmly inwards by stud tension from the end and both sides. But cannot collapse inwards because of the four 16mm studs which are under heavy under tension. End of episode 2.

Episode 1 How do you get a really big GEM without £20-25,000+ which you don't have anyway? DIY? You rather fancy yourself at DIY and once built an IKEA shelving unit. But you have no castings nor welding equipment. How on earth do you build such a thing? The mounting was going to have 50mm SS shafts. Normally you'd use pillow block bearings for DIY GEMs. I decided to use flange bearings instead. Same price but better. Discounted that week too! About a tenner each wasn't bad. These come in hefty cast iron housings, with big, journal, ball bearings mounted in spherical grooves. So they are self-aligning. This is important because we cannot guarantee precision alignment without castings and precision machining. So far so good. We prop up our heavy ironmongery and magic the mounting finished. Abracadabra! BTW: Those are 16mm galvanized studs for scale. The image shows an early attempt to turn the bearings inside out so I could use a metal tube between them. That was a non-starter and soon forgotten. But I had some nice oak planks. So I built laminated bearing boxes with mitred joints. It took ages and was rubbish! The next chapter begins with a lucky find at the local scrap yard. Some unblemished 15cm wide x 2m long x 10mm thick aluminium strip fell into my lap. Now I had a proper start. I still had no way to fix the plates together into two complete bearing boxes. I was far too lazy to use lots of small screws along the edges. It would take ages and would need to be neat and I don't really do neat. Imagine having to tap all those threads? No thanks. Suddenly it dawned on me that I could clamp the whole lot together using friction alone. Then I had a far more sensible idea and would use friction AND screws to hold it all together. The resulting bearing boxes are incredibly stiff but very heavy. Heavy didn't matter. Heavy was good. But only If I had a crane. So I bought a cheap chain hoist and some strong lifting strops. The next image shows the important details of my bearing box construction. The loose plates are clamped together endways, between the bearing flanges, using the massive 16mm studs. Then the 8mm cross studs clamp the plates immovably together so they can't balloon out under pressure. I used stainless steel studs and flanged head, galvanized furniture nuts. Neat? I thought so. This is the Declination bearing box without its 50mm shaft for clarity. What doesn't show is the careful siting of every screwed rod [or stud.] Not only do they lean against the 16mm studs but are mutually reinforcing under tension. They must also miss the axis shaft and all the studs running at right angles between the two other plates. The plates also rest against the tensioned, 16mm studs, so cannot possibly move inwards. The plates shown at top and bottom are the outer plates. The other two are trapped between the outer ones. End of episode 1.

Agreed. Undercuts are probably the manufacturer's response to the sale of very expensive eyepieces for very expensive APOs. APOs, being refractors, tilt upwards at the front and downwards at the back. So the expensive EPs can easily fall out onto the concrete paving. This ignores the sale of expensive star diagonals to go with all of the above but not to expensive binoviewers to go with the above. A.Cynic

That would only be true if all three screws were tightened a little at a time. Which "might" produce a concentric "float" on the tips of the screws. And then only if the difference in diameters between the "plug and socket" were large. If one screw, then two are well tightened, in sequence, then the third simply provides extra resistance under heavy loads. Try it yourself with a long PST etalon/filter stack. Now add a binoviewer on the "silly" end! Good luck with that! I gave up on T2 because even three thumbscrews were never secure enough. Only 2" fittings can cope and larger still would be better. I can still see half an inch of sag even with a dinky little ZWO on the end. A "compression band" only matches its given name with three screws. Becoming a crude collet. One screw pushes the eyepiece to one side, two is better for mechanical retention but does exactly the same. Three screws can get a firmer grip but follows the same pattern of behaviour. All systems push the eyepiece against one wall of the receptacle. It cannot possibly be otherwise. The EP is always eccentric to the optical axis with thumbscrews or compression bands. Partial concentricity is only achieved with tight tolerances. Which would be difficult to fit and remove. My own preference is for the Baader Click-Lock system. It grips like a mad thing! Perfectly self centering? Who cares? How can it possibly be worse than [eccentric] thumbscrews? My next/present project is cantilevered support for the far end of the PST stack on a new 6" f/10 H-alpha OTA. Not an easy task, at all, given the need for easy hand access for tuning and focusing adjustment midway. Plus the regular changes in terminal components like camera, binoviewer or diagonal. Now add in complete system rotation about the optical axis on an equatorial mount. Somehow, I don't think three garden canes and rubber bands is going to do it!