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[5]
Despite a temperature of only 33F, +1C, in the shed I cut out six 12" circles from the waste 15" plywood cut-outs. I then ran the circles against the router with a central pin just to tidy up the jigsaw cuts. The saw cuts weren't bad but had a few lumps and bumps to tidy up. The blade also tends to lean over with added pressure. This is largely unavoidable. Since the 3/4", 18mm plywood produces different levels of blade resistance at different angles to the veneer grain. Even taking it very easy with the forward pressure on the jigsaw the resistance changes constantly.[5]
These circles are only to round out the very oval Biltema cardboard tubing. I pushed one circle to the centre of each tube and then pressed one each at about 4" from the ends. They were easy to fit by sliding them in on edge. More difficult turning them square to the tube to fill the inside tightly. I'll leave them in place for a few days to see if they improve the roundness of the tubes. The circles offer more far resistance to returning to oval than much thinner outer rings are likely to offer. The alternative tubing suppliers website insists that the tubing be stacked vertically before use.
If I was doing it all over again I would cut further outside the beam compass line. Then use the router to produce much more accurate circles. Not that it is at all critical in this context. It would just be nice to have accurate circles should they ever be needed for another project. I sanded and chamfered the sawn edges slightly to avoid damaging the inside of the tube as the circles were forced square to the tube.
The cost of the cardboard tubing, from the other dealer, is prohibitive despite the much more robust 9mm wall thickness. Though it does include "free" delivery. If I can arrange personal collection, at lower cost, it would be quite tempting. I could then ensure the tubing was round and straight before purchase. They also offer a 350mm /13.75") size. Which would ensure improved clearance between the light path and inner tubing wall. The weight increase is not dramatic.
If I stay with the existing Biltema tubing I shall have to use an exo-skeleton of ribs and formers to join the two halves. The external framework will help to stiffen the 4.5mm tubing into the bargain. As well as holding it round over time.
It has been suggested by a very experienced contact that I paint epoxy resin onto the tubes. It wouldn't be a bad idea to coat the exo-skeleton at the same time. To help to bond everything into one solid unit.
Royce recommends painting the entire tube with polyester resin but this isn't so tough as epoxy. Neither is likely to harden properly without a reasonably warm sunny day. Which is probably several months away in the present Danish climate!
The option still exists to wrap and glue another pair of tubes over the ones I have now. They would need to be neatly slit lengthways first. The round formers would maintain roundness. The joints between the tubes could be staggered to achieve high strength and stiffness. The long edges of the second layer would not meet but could be filled with strips cut from the waste lengths of tube. Epoxy might be a good choice of adhesive to ensure a solid tube.
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Next day I removed one end circle and remeasured the tube. With two circles still in place the open end measured perfectly round across several diameters. Without removing all three circles it is difficult to say how soon the tube will assume permanent roundness. If ever.
I have found another cardboard tube stockist but it isn't remotely local and only offers 1.5 metre lengths. Which is only about 5' long. Still well short of the 6'6" I need in one length.
There is also a stockist offering 1.5m x 1.5m of thin aircraft plywood. I have previous experience of rolling a 6" tube out of 2mm birch aircraft ply. Though the meeting edges were very difficult to curve to match the remainder of the tube. I placed a batten along the join as I added dozens of car inner tube, cut-off rubber bands. While the glue dried between two sheets of the ply bent round circular formers. The tube still remained rather egg-shaped in cross section despite my best efforts.
Cutting the long meeting edges of the plywood tube after gluing is fraught with difficulty. Almost as difficult as cutting the width of the sheet accurately prior to wrapping and gluing. One simply cannot judge reliably where the edges will meet when the pressure is applied to the glued layers by tightening bands or straps. Too short and there will be a gap between the edges for the entire length of the tube. Too long and the joint will rise against the butting pressure of the edges. With two plywood layers each sheet needs to be quite a different width.
I have seen some suggestions, online, to soak or even to steam the plywood prior to bending. Not an easy task to steam a 5' square sheet. Though it would be easy enough to soak it. Even pour boiling water from a kettle over the sheet if it helped. It would be a hell of a waste if this damaged a very expensive sheet of plywood! I am loathe to get involved unless I can find a sheet of ply large enough and thin enough to form a full length tube around the cardboard core.
Probably using epoxy to bond it directly to the bare cardboard while lined with several circle formers to resist the crushing loads of the tightening straps. Where several laminations are desired thin aircraft quality plywood is so expensive that it would be cheaper to make a fibreglass tube. Or even a carbon fibre laminated tube. Though neither is as respectable for thermal neutrality as cardboard or plywood laminations.
Cutting the long meeting edges of the plywood tube after gluing is fraught with difficulty. Almost as difficult as cutting the width of the sheet accurately prior to wrapping and gluing. One simply cannot judge reliably where the edges will meet when the pressure is applied to the glued layers by tightening bands or straps. Too short and there will be a gap between the edges for the entire length of the tube. Too long and the joint will rise against the butting pressure of the edges. With two plywood layers each sheet needs to be quite a different width.
I have seen some suggestions, online, to soak or even to steam the plywood prior to bending. Not an easy task to steam a 5' square sheet. Though it would be easy enough to soak it. Even pour boiling water from a kettle over the sheet if it helped. It would be a hell of a waste if this damaged a very expensive sheet of plywood! I am loathe to get involved unless I can find a sheet of ply large enough and thin enough to form a full length tube around the cardboard core.
Probably using epoxy to bond it directly to the bare cardboard while lined with several circle formers to resist the crushing loads of the tightening straps. Where several laminations are desired thin aircraft quality plywood is so expensive that it would be cheaper to make a fibreglass tube. Or even a carbon fibre laminated tube. Though neither is as respectable for thermal neutrality as cardboard or plywood laminations.
It might be possible to make a rotating head to allow the eyepiece to be always brought to a comfortable position. Rotating the entire tube in its mounting rings is never easy in larger diameters due to weight and friction.
I have now removed the stiffening circles from one tube and left it standing upright to check for any subsequent change in form. It now seems perfectly straight and round. I wonder whether it will remain that way? l'll leave the other tube with the plywood circles in place for a little longer to see if that helps.
It would be interesting to understand why these tubes do become so oval when laid flat. I think one can assume that the atmosphere in Biltema's warehouse storage is warm and dry. So moisture is not to blame. Yet the stock tubes were very oval indeed. While at the same time they resist hand pressure when trying to force them to become round.
Only the considerable pressure exerted by the plywood circles was able to force them back into roundness. I had to give the circles in the very oval tube a few good wallops with a hammer, via a roofing batten, to get them to turn squarely across the tube.
The rapid improvement in roundness suggests that the tubes will respond just as well to external plywood rings. Even more so with dowel stringers to help to keep everything in shape and straight. A seal with the recommended epoxy should do wonders in providing a relatively cheap, lightweight thermally neutral and stable telescope tube.
Plastic tubes do not change shape noticeably despite lying for years out of doors. I had unused lengths of 14" and 18" PVC ventilation tube from unfinished Dobsonian projects. Which became rather brittle and bleached, after ten or more years in sunshine and weather, but never obviously changed shape.
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Another tubing update: The tube, from which I removed the plywood circles, has retained perfect roundness. The trick seems to be to leave it standing vertically. Had I laid it down flat then I am fairly sure it would soon become oval again. Which suggests that, for a telescope tube to stay perfectly round, it will require mounting rings. Or external plywood stiffening rings.
And another update: The tubes now seem stable provided they are standing up. I have failed to obtain flexible plywood to laminate over the cardboard tube. It is only available in 9mm which is far too heavy. It could be months in arriving if I special order 3mm.
I weighed the cardboard tubes this morning. 120cm/47" lengths of 4.5mm thickness = 6.7lbs each. So a full length 2m tube of single thickness will turn out at about 12 lbs. If I laminate one tube over the other it looks more like 24lbs! Plus mirror, plywood rings, dowels, spider and secondary. Lumpen!
I have finally found a stockist of thin, flexible plywood online . (DK= Bøjelig krydsfiner) Supposedly available in oversized sheets with a choice of wide or tall flexibility. With considerable effort I finally managed to get the local builder's merchants to make enquiries about pricing. I thought we were in the middle of a recession with a complete stop on house sales. Since the majority of building work follows a house sale builders are going bankrupt left right and centre.
You'd think the builder's merchant staff would be falling over themselves to supply a willing potential customer. But, no, I had to hand feed him the information and insist he look them up online so that he could make enquiries on my behalf! The stockists have the builder's merchant's own company logo right at the top of their website!!! Why else would I bother to go to him? Because the suppliers are wholesalers and won't deal with the public. So I am forced to use the builder's merchant. Arrgghh!
Now the price will probably be set unrealistically high! If it is cheap enough I can buy two sheets and laminate them together to make a really strong, but rather heavy tube 8mm thick. If it is just affordable I might laminate one sheet over the cardboard tubes to save weight. I tried lifting a sheet of 4mm ply while I was there and they are a bit heavy. Far heavier than my cardboard tubes! A standard metric sheet of plywood would be 2440mm x 1220mm. A full telescope tube would only need 1981mm x 965mm (or ~78" x 38") Roughly 64% of a full sheet. Or 128% of one sheet with two layers. From searching online, an 8' x 4' sheet of 4mm (about 3/16") weighs from 20 lbs upwards depending on the timbers used.
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I have now removed the stiffening circles from one tube and left it standing upright to check for any subsequent change in form. It now seems perfectly straight and round. I wonder whether it will remain that way? l'll leave the other tube with the plywood circles in place for a little longer to see if that helps.
It would be interesting to understand why these tubes do become so oval when laid flat. I think one can assume that the atmosphere in Biltema's warehouse storage is warm and dry. So moisture is not to blame. Yet the stock tubes were very oval indeed. While at the same time they resist hand pressure when trying to force them to become round.
Only the considerable pressure exerted by the plywood circles was able to force them back into roundness. I had to give the circles in the very oval tube a few good wallops with a hammer, via a roofing batten, to get them to turn squarely across the tube.
The rapid improvement in roundness suggests that the tubes will respond just as well to external plywood rings. Even more so with dowel stringers to help to keep everything in shape and straight. A seal with the recommended epoxy should do wonders in providing a relatively cheap, lightweight thermally neutral and stable telescope tube.
Plastic tubes do not change shape noticeably despite lying for years out of doors. I had unused lengths of 14" and 18" PVC ventilation tube from unfinished Dobsonian projects. Which became rather brittle and bleached, after ten or more years in sunshine and weather, but never obviously changed shape.
________________________________________________
Another tubing update: The tube, from which I removed the plywood circles, has retained perfect roundness. The trick seems to be to leave it standing vertically. Had I laid it down flat then I am fairly sure it would soon become oval again. Which suggests that, for a telescope tube to stay perfectly round, it will require mounting rings. Or external plywood stiffening rings.
And another update: The tubes now seem stable provided they are standing up. I have failed to obtain flexible plywood to laminate over the cardboard tube. It is only available in 9mm which is far too heavy. It could be months in arriving if I special order 3mm.
I weighed the cardboard tubes this morning. 120cm/47" lengths of 4.5mm thickness = 6.7lbs each. So a full length 2m tube of single thickness will turn out at about 12 lbs. If I laminate one tube over the other it looks more like 24lbs! Plus mirror, plywood rings, dowels, spider and secondary. Lumpen!
I have finally found a stockist of thin, flexible plywood online . (DK= Bøjelig krydsfiner) Supposedly available in oversized sheets with a choice of wide or tall flexibility. With considerable effort I finally managed to get the local builder's merchants to make enquiries about pricing. I thought we were in the middle of a recession with a complete stop on house sales. Since the majority of building work follows a house sale builders are going bankrupt left right and centre.
You'd think the builder's merchant staff would be falling over themselves to supply a willing potential customer. But, no, I had to hand feed him the information and insist he look them up online so that he could make enquiries on my behalf! The stockists have the builder's merchant's own company logo right at the top of their website!!! Why else would I bother to go to him? Because the suppliers are wholesalers and won't deal with the public. So I am forced to use the builder's merchant. Arrgghh!
Now the price will probably be set unrealistically high! If it is cheap enough I can buy two sheets and laminate them together to make a really strong, but rather heavy tube 8mm thick. If it is just affordable I might laminate one sheet over the cardboard tubes to save weight. I tried lifting a sheet of 4mm ply while I was there and they are a bit heavy. Far heavier than my cardboard tubes! A standard metric sheet of plywood would be 2440mm x 1220mm. A full telescope tube would only need 1981mm x 965mm (or ~78" x 38") Roughly 64% of a full sheet. Or 128% of one sheet with two layers. From searching online, an 8' x 4' sheet of 4mm (about 3/16") weighs from 20 lbs upwards depending on the timbers used.
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