10" f/8 As you were [again]!

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Having failed to obtain sufficient tube stiffness with the dowels and rings I am back to the alloy beams:
Rather than completely waste the [dowel-drilled] birch rings I made rectangular holes for the beams in a couple of them. Sawing with an electric jigsaw and filing smooth produced a very rough finish. With ragged edges where the top veneers splintered badly. What was worse the holes had almost no sizing tolerance. The beams either went through the holes or they did not. And, if they did they were too loose on the beams. With no means to fix them other than bolting through the beams and cardboard cells. This seemed just too crude for my liking. Plus, I really wanted an infinitely adjustable, sliding/clamping fit for the cell along the beams. The clamping was the difficult part if it was to be foolproof without becoming an eyesore.

A plywood ring does not readily lend itself to a clamping action on a rectangular beam lying within its concentric depth. I imagined cutting a slot inwards, from the perimeter to the rectangular hole. This would allow some free closing movement in the otherwise rigid plywood. Cutting outwards from the ring bore to the hole would raise a struggle between the the plywood ring and the glued cardboard tube. I need that joint to be strong to ensure the stiffness of the complete OTA.

Applying pressure, perpendicular to the slot, raised a number of possibilities. "Ears" could be formed on the ring's perimeter to accept a clamping bolt. Though the plywood is really too thin to be drilled on edge. Besides, the "ears" might catch on things including cloth shrouds. Eye bolts, fixed through the plywood rings with clamping bolts through the "eyes" offered another possibility. Or, blocks of thicker plywood glued and screwed to the sides of the rings with compression bolts between pairs of blocks. That might work but might also be very ugly if not made to a suitably high standard.

The eye bolts sounded like an easy option if only I could ensure they remained rust free. The usual 'flashy' zinc coating doesn't last long once the retail bubble pack has been brought home and opened. Stainless steel eye bolts might be hard to find outside a boat chandler. Finding some brass eye bolts might be possible but they would need to be of thick enough rod to be stiff enough.

I was really hoping for a universal and cosmetically attractive solution to the beam clamping problem. One which could be imitated across a wide range of  OTAs based on beams and plywood rings. Including refractor OTAs if at all possible. Though the rings need not be circular on their perimeter the inside "hole" would he wasted if it were not also made into properly sized baffles to increase contrast. The smaller the baffle hole the heavier the ring simply because less material would be "missing" from the middle. It might be better to go for a standard 12mm plywood ring and then add thin baffles from a suitably light, sheet material. A refractor should ideally have metal baffles, anyway, just to avoid starting a fire with unfiltered sunlight!

My cutting out of the rectangular holes for the beams needs to be greatly improved. Using the router might be a much better option. It just needs a suitable jig to ensure an accurate and neat hole every time. Though the corners would need a different approach.

I have become rather fixated on the cardboard tube and two rings type of cell. I could easily change tack completely and make sliding boxes which clamp onto the beams. I saw in interesting variation on the beam/spar type of OTA on the iStar Scope Club forum. This used a wooden spar situated underneath adjustable, right-angled cells. So there is plenty of room for inventiveness around the basic beam/spar idea.

At the moment, I am gently steering myself towards shallow, three-sided plywood channels. These channels would be set-in and glued  into the opposite edges of the rings to join the two rings to make a cell. With the present 30cm cardboard tube also joining the rings for extra stiffness and reinforcement. Though this may not be necessary if the plywood channels provide enough stiffness on their own.

An overlap of the plywood channel, beyond the enclosed beam, will allow a clamping action at its outer edges. The channels will slide smoothly along the beams but still be able to clamp anywhere along them using a T-nut and long bolt. This bolt will span between the top and bottom of the channel, outboard of the beam and apply pressure on demand to the tops and bottoms of the beams.

The present arrangement, seen in the top image above, is far too difficult to slide along the beams without the holes being made unnecessarily oversized. Nor is there any easy way to fix the complete "cell" to the beams without boring holes for screws or bolts.

The lower image shows how the inset [box] channels will appear from above the ring. The colours are supposed to represent the plywood channel with the enclosed 100 x 18mm, rectangular, tubular beam in the centre. [Centred on the redundant dowel hole.] The two shallower sides of the box channel will extended slightly more towards the camera. This will allow room for the clamping bolts to safely clear the beams to avoid cosmetic damage. Though the screws could be covered in plastic tubing. The plywood channels can be easily formed and glued around the beams with polythene packing to ensure fit and squareness. Once the glue is dry and the packing is removed the sliding fit should be perfect.

The advantage of this arrangement is being able to carry it over to any other OTA. Rings could be replaced by squares or rectangles of plywood baffles in smaller OTAs and refractors. Further tubular beams could be introduced to the top and bottom of the "tube" for extra stiffening on equatorially mounted OTAs. Or even a triangular baffle arrangement, like the iStar TCR lightweight refractors, is also, easily achieved.

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