10" f/8 Caging the 10" f/8.

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After much searching I have found a nearby stockist of Birch plywood including the desired 12mm. Not cheap but excellent quality. BWP B/BB. I'm taking the trailer out tomorrow to collect a 1.52m x 1.52m sheet of the 12mm to make some more and better support rings. Their thinnest is 4mm but I'm not sure what I could use that for. Probably too thick to make knife-edge baffles for each support ring. Now the weekend weather is committed to gales and very heavy, thundery rain as a low passes over. I shall have to wait for next week to collect the new plywood. No point in it getting sopping wet, or even blowing away, before I can even use it.

Returning to collect the 12mm Birch plywood proved that it was mislabelled. Fortunately an 8'x4'  sheet of old stock was hiding underneath. As I was given a discount for cosmetic appearance, I accepted the very dusty exposed side and took it home. Birch plywood is incredibly dense and heavy at 700+kg/m^3!

I'm still practising on scrap 10mm in getting exactly the correct diameter to perfectly fit the cardboard tube. Once I had thought it through it was actually quite easy to cut a ring. First I made a slightly oversized 1/8" [actually 3.2mm] central pivot hole in the plywood. Then I cut the outer circles first from both sides of the plywood. Until I had just broken through on the second side. Using the router's stepped, depth stop safely avoided going too deep and cutting into my workbench!

I just had to be very careful with the very last bit of each full arc. Then I reset the radius jig and started with a first shallow cut on the inner circle. After repeated increases in depth from both sides this removed the inner circle from both sides of the sheet to leave a neat ring. Again, I had to take great care over the last inch, or so, as the scrap material became loose. The friction of the plywood on the working surface was just enough to keep it still while I swung the router gently around the pivot pin in the circle cutting jig. It is impossible to clamp the material without using pins or screws because the router or circle cutting jig will surely strike the fixing.

 I have now paused on the twin beams design and shall build an 8 dowel, 8 ring, skeleton tube next. Just to see how stiff and strong it feels. I shall use short 8" or 20cm lengths of cardboard tube to act as the stiffening cells for the primary and secondary mirrors.

 As it will be an altazimuth design the skeleton tube will be evenly supported from either side by the altitude bearings. Which is not remotely as locally stressful as hanging the entire OTA, from one side only, on a declination axis saddle of an equatorial mounting. If the 16mm dowels prove too weak, to hold optical collimation with changing altitude, I can always exchange them for aluminium tube. Though I doubt there'd be much difference in overall stiffness unless I seriously increased the diameter of the metal tubes. AE of the UK used a very attractive, all-aluminium skeleton tube design for some of their telescopes. Though those were really intended for their excellent equatorial mounts. Bath Astronomical Society had a 10" AE in a dome in the secretary's garden. A lovely bit of kit but well beyond my means, then and now.

I have now removed my DIY plywood circle cutting jig and now use the original router fence bars with a piece of alloy angle to hold the centre pin. Again, I used a T-nut with the spikes cut off to allow plenty of thread length for holding the centre pin really firmly without increasing the height of the pin assembly below the alloy angle. Once I had the exact radius settings for the circle cutter on the router I was able to produce 6 rings quite quickly. The secret was to cut the outer circles first from both sides until the disk parted from the sawn square. Then I cut the inner circles [again routing in increasingly deeper steps from both sides] to cut out the centre circle to finish the rings. I was using an 8mm bit because my 6mm had broken on an earlier project and I had lost its replacement. Sawing out squares from the 8'x4' sheet ensured the maximum ring size without cutting into each other where they joined. The router bit would overlap the intended circles if I had routed the rings straight from the whole sheet.

Now I will need to fix the rings together with register pins. Then drill them precisely for the eight dowels to the pattern I drew on the first ring before it was cut out. I made the rings as large as I could within the 4' width of the sheet of plywood. Which made them just under 16" in diameter. I need not have worried about slipping the mirror cover between the dowels of the cage. There is plenty of room to fit a hand between the dowels.

Drilling the rings with an initial 3mm-1/8" drill went smoothly thanks to the twin coach bolts I set up on the cheapo drill table to help centre the rings. With each new hole bored through the master ring pattern I dropped a galvanised nail into the drilled hole. This kept the position of all the holes identical in the rings lying underneath. In fact it went remarkably smoothly as I numbered and edge marked each drilled circle to ensure they would all match. After that I cold bore the 16mm holes, with a pointed, spiral bit, with full confidence that they would all lie exactly over each other.

I was just going to trial build the cage, for a photograph, when I discovered that the dowels varied slightly in diameter! They were all bought at the same time and from the same DIY superstore. They have also been standing vertically in a tied bundle in the workshop for literally ages. So they must all have equal [and low] moisture content.

I will now have to measure their individual diameters with a vernier calliper and sand them to a suitable size to slip easily through the rings. Gluing will hold them safely even if there is some slight slop. This will help get all the rings more easily onto the dowel cage. It's a shame because the 16mm spiral bit was rather expensive! It was purchased for this one project just to ensure greater accuracy than using a normal twist drill, or even a spade bit. Trying to build the cage with such tight fits is all but impossible! However tempting it might be, I am not driving the 20 miles to check their present dowel sizing to find replacements!

The dowels proved to be slightly oval and sanding had no effect. By sheer coincidence I had a ball shaped bur which would produce a nice hole provided it was used at very low speed. Assembly went rapidly after that. As can be seen in the image above. Now I need to provide a primary mirror cell and fit the focuser, spider and finder to get some idea of the longitudinal balance point. Weight, as seen, is 5.55 kg or 12.3 lbs. The bare cardboard tube had weighed 22lbs or 12.5kg! So nearly a halving in weight for the dowel and ring design. Gluing the thing together is going to be fun!

The problem is that gluing is unlikely to increase stiffness by very much. Just glancing along the leaning OTA showed significant droop. So I spanned the distance between two workbenches to see the "tube" sag even more! I took the image from some way off to counteract any barrel distortion in the camera. Supporting the OTA from somewhere near the middle would probably halve the sag visible here. Higher altitude angles would reduce it even further. I still don't think it would help much. This long focus design is a complete non-starter with only 16mm dowels. This is without the extra weight and moments of the optics and their supports at each end. Alloy tubes might be stiffer but not by much in this relatively small diameter.

Lesson learned, so now it's back to the alloy beams. With only 1/3rd of my sheet of 12mm Birch plywood wasted on the pointless rings. I don't think they will serve in the simple two beams approach because of all the bored holes. I will have to look into this in case they can save some time and recycle the wasted materials. Short bridging dowels may offer increased resistance to the rings twisting despite the cardboard tube reinforcing the cells.

Click on any image for an enlargement.

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