I thought I might seek out a square tubular metal option to compress between the flange bearings but have drawn a complete blank. Size would be critical but I can't find anything remotely suitable! My usual metal outlets had only flat plate or small profiles in aluminium. No combination of my larger and heavier alloy profiles suits the lateral spacing of the studs nor the size of the bearing flanges.
These would suit an [almost empty] 8" diameter round PVC pipe but a 180mmx189mm [7"x7"] square [structural] PA tube makes far more sense. A square, steel tube of that size, in any reasonable wall thickness, would probably need a crane to lift it.
Another alternative for the bearing housings is to use laminated kitchen worktop. I have a complete length of unused hardwood 60cm/24" wide x ~30mm thick which could be routed with longitudinal slots to house the threaded rods. [Studs.]
Applied compression forces by the studs should be much better resisted by the hardwood end grain than the weaker edges of the very thin laminations of the birch plywood which I had originally intended to use.
This very rough drawing shows how a 30mm thickness of board fits the flange bearings with a suitable overlap. The step between them will be disguised by the presence of alloy end plates sandwiched between the flange bearings and their [square] tubular wooden housings.
The inner edges of the 45° mitre joints need a pocket to clear the studs. This narrows the width of the mitred joints but probably leaves enough material for a strong joint. A second square tube of 30mm thickness could be fitted inside the outer tube to reinforce both and enlarge the compression area. Slight clearance issues exist with the axis shaft, inner bearing race and the rubber seals. Slight relief is easily managed with the router at both ends of the inner tube once glued together. While slight shaft clearance should be routed out prior to assembly. Making both tubes larger [but still nested] solves clearance problems without any bearing protrusion, clearance routing being required.
The worktop laminations should make stable but unusual and attractive bearing housings. Probably with far better cosmetic damp resistance than the birch plywood. The worktop has been standing on end for a couple of years in the workshop so should be well seasoned by now. The only real problem I foresee is deciding on corner treatments where each of the four boards meets on edge. A 45 degree edge [miter] would work best but would be critical of exact fit and finish for neatness. The overall dimensions of the wooden strip tubes have yet to be decided but ~180mm looks about optimal. Ignore the outer dimensions on the drawing.
Provided I get the sizing and grooving exactly right the studs and axis shaft can be easily fed endways through the assembled hardwood strip [tubular] monolithic housing. Then the flange bearings can be added and the washers and nuts tightened down to build a really rigid unit. The design of the bearing housings has been on the back burner while I considered alternatives and waited two months for the wormwheels to arrive. The plan is to bolt the declination housing on top of the axis joint cylinder. Major sinking of the cylinder into the Dec housing [to reduce overhang] is NOT easily achieved due to the enclosed compression studs. Though a circle could be relieved in the outer tube material with the router to reduce the degree of cantilevering slightly and perhaps to improve the appearance. This might weaken the joint between the alloy cylinder and the tubular axis housing.
It may seem odd but I have absolutely no experience of using mitre joints between the edges of boards. Edge butt joints are my usual skill level. Only the odd picture frame has enjoyed the manual mitre saw and that can't handle longer lengths. So it will have to be the circular saw or the bandsaw. Perhaps I should have a practice on cheap materials before wasting the kitchen top! I don't much fancy my chances of planing a perfectly straight edge at 45° on hardwood.
In the end I decided to cut strips of worktop 36cm x 95mm wide long to just fit between the studs. Careful balancing of the strips on each other showed they should be mitred to allow them to sink nearer to the center of the housing. A 6mm, 1/4" flat brought the strips to flush with the outside of the bearing flanges. After cleaning up the strips with a wood plane I made the first bevel. Then it occurred to me that I had a 45° router cutter with an edge support bearing. After careful depthing of the cut I turned the router upside down on the workbench. This proved far easier to work with by feeding the strips by hand than trying to balance the heavy router on the the narrow edge of the strip.
The rather poor image shows the strips are now flush but could be mitered further to sink them below the edges of the flanges. This would allow room for a second layer of mitered wood strips to be glued on on top. I took a second cut with the router's depth turret rotated by one more stop. The image alongside shows the result. I can keep on taking shallow cuts with the 45° cutter until I reach a suitable depth. Balancing the assembly on its nose allowed fine adjustment of the depth of each strip. The miters are still too narrow to be self-seating against each other. A light tap with soft hammer on each wood strip allowed them to be joggled into place. Once the nuts were tightened the wood made a much more solid feeling unit compared with just the studs and well tightened nuts. I'm rather pleased with how well it went. It is so long since I used my cheapo circular saw that the blade was rusty! As was my technique.
My 3mm PTFE has arrived from http://www.ebay.co.uk/usr/gfgplasticfabricationsltd via eBay[UK] Ordered on Friday afternoon and arrived in the post today [Monday.] Absolutely amazing service!