Dome build: Half finished or still only half way there?

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As I mentioned yesterday I have duly drilled the ribs for the completed horizontal struts. Then pre-drilled the ends of the struts. After that I could clamp together all the finished segments.

Voila! Eight segments make a half dome. [If you have the imagination.]

The segments are sagging inwards due to a lack of support. So the final structure will be slightly taller but cover not quite such a large circle. [Nope!See below!] The squared off base of the ribs is a good indicator of whether a rib is actually level or drooping.

I suppose it is a landmark of sorts on my long journey to completion of my dream observatory. The promised rain arrived just after I took these pictures. So I had to quickly take everything apart and stack the segments under a lightweight tarpaulin. 

I have discovered the Bessey clamps are very much weaker at clamping than the Raw-Link. So I am tempted to buy a dozen more Raw-Link clamps despite their obvious shortcomings. Two medium clamps will safely hold two segments where four of the Bessey clamps will not. That extra clamping strength is most valuable in quickly assembling segments together. It can make the difference between a safe structure and something which will easily collapse. I can't have that happening up on the platform!

With eight segments to play with it was time to be creative using the base ring arcs for support. Leading to intense worry that I had badly miscalculated! My "impoverished" dome was barely 1.4 meters high. It was supposed to be 1.5 meters [5' in Old Money] inside height!

I struggled for an hour making adjustments and remeasuring but it wouldn't grow any taller. So I reduced my mocking mock-up to only two segments and lots of spare arcs. Finally I could sigh with relief that even I could limbo dance under the arc de triumph over adversity. Despite it seeming slightly illogical the height grows with diameter. This is because the arcs are curved not straight. Think of it more like blowing up a balloon rather than stretching the ribs further apart.

The clue was in the arrow on the right in the image above. I had been trying to fit the dome into a 1.5m radius instead of 1.6m. There is a wrap of black tape on my long measuring pole to show the correct new width. I had been using my original 3m mark instead. Later I'd made the dome 3.2m outside to allow the ribs to safely overlap the octagon. Though later again I had changed to 16 segments from the original eight after deciding on a trapezium, flat paneled dome.

Now I need to saw out the remaining [14] ribs to build more segments. I'll need more 9mm [3/8"] birch plywood but have all the horizontal struts ready to go. Not long now before I have a skeleton to clad. Weather permitting.

Sunday 1st October: A fine, warm autumn day with sunshine. Cut out five more ribs before running out of ply. Bought two more sheets of 9m birch ply and one of 4mm. Plus 10 more Raw-link clamps and some more, fine toothed blades for my jigsaw. I had been using a fine tooth, metal cutting blade but it has worn out, is slow to cut and is badly fraying the edges of the cut.

Changed to Bosch's fine tooth 'Laminate' jigsaw blade. Still slightly ragged on the veneer edges but much better than the old [worn] blade. Finished cutting out the rest of the ribs. [14 to leave room for the observation slit.] Followed by a quick sand with the angle grinder using a coarse flap wheel to clean up the edges.

Brushing the splinters over the edge with my hand can allow the flap wheel to clean  most of them away. It's a battle between short sharp splinters or longer and softer ones which actually do much less harm.

I even tried a fine toothed, downward cutting blade but it suffered from vicious vibrations no matter how hard I pressed down on the jigsaw. The 'Laminate' blades may be the best option when they have a little more wear on them. Birch is very prone to splintering at the cut edge. Taping the cutting line might work but I'd need miles of tape.

Click on any image for an enlargement.

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Dome build: Your struts are joined to your rib bones.

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Despite the dust storage barrel collapsing, yet again, I left the cleaner running while I compound mitered the final 12 struts of each required length. 

The secret to getting lots done is to set a firm end stop. Then to cut only one length of strut at a time. Rolling each piece 180 degrees and reversing end to end, after the first cut, sets the strut up for the second and final cut. It took about an hour to cut all 36 struts without rushing.

Even then I was measuring each one for exact length and marking the orientation of every strut with a pencil. This surface will face outwards and will be covered with the cladding panels to hide my graffiti.

The next step is to spot through an accurate rib [pattern] into all the others with a drill. Dome assembly should then go quite quickly because the screw holes will already be countersunk.   

The vertical struts will also need to be mitered. Though not compound mitered. I thought I might cut these battens into triangular cross sections to halve their weight.

Only the outer face needs to be full width to support the panels and accept the panel fixing screws. These outer faces will also need to be beveled to allow the panels to lie flat. See image alongside. [Not to scale and some angles have been exaggerated for clarity.] The angle of the required bevel is easily found by laying a straight edge horizontally across the ribs.

I was just watching more YT videos on workshop dust extraction. One very experienced chap used a box made from scrap, sheet materials as a collection bin. I would have thought it would collapse under external pressure but it seemed not. One could build a really solid, plywood box for the price I paid for this flimsy plastic barrel. Perhaps this is the way forwards?

Just drawing on the tiny pipe of the DeWalt miter saw was enough to collapse the barrel today. This was on the lowest speed and therefore lowest suction on the Shop-Vac Ultra. I was going to add a batten 'tree' on a length of threaded rod to support the barrel from the inside but the effort seems wasted. I'd have to extract the tree just to empty the dust. A square section box will hold far more dust than a barrel of the same width. Downside is the weight of a sturdy plywood box. Castors would probably be essential.

Click on any image for an enlargement.

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Another detour: Cyclone dust separation and collection.

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The mountains of dust I have made on this project has simply accumulated in cardboard boxes, all over the materials and on the floor. My cheapo, all stainless steel, workshop vacuum had expired years ago. Leaving me with no sensible way to collect the dust. Using a dustpan and brush was a disaster. and the fine dust was dangerous to [my] health!

So I searched online, read the reviews and watched the YT videos for a suitable shop vac and settled on a Shop-Vac Ultra. This duly arrived with a silly, narrow, 'domestic' hose and matching tools. Not much use for serious sawdust extraction.

This particular model has adjustable speed/suction and automatic vacuum switching when a mains operated tool is switched on. A mains socket is provided under a flap on the cleaner lid for any separate power tool.

There followed days of searching for a European source of the 64mm [2½"] hoses and fittings. Eventually I ordered the hose, some straight pipes and a suction head from Shop-Vac Austria at a modest price.

Then the postman arrived with the cyclone dust separator I had ordered on eBay[UK] So I quickly cut out a plywood disk to fit my airtight, 60 liter, plastic barrel. I put the disk in the lathe to cut the tapered 75mm [3"] hole for the cyclone base. The idea being to support the cyclone as evenly as possible to extend its life when dragged about or fitted with large hoses. The disk looks a bit rough at this point because it hasn't been smoothed with sandpaper.

The cyclone model I bought was a later model Chinese version without the usual bottom flange. Four self-tapping screws travel upwards, through the supporting plate/barrel lid into moulded lugs on a 90mm Ø circle. The base is cut off at an angle and is 75mm wide where it sits on its supporting surface. Plywood is a good way to reinforce the base area to stop it rocking when attaching hoses, etc. Though the barrel lid was surprising thick and stiff when ti came to cutting a hole.

I'll have to wait for the larger hose to arrive before I can set up the dust extraction system in earnest. There is so much dust lying about that I didn't dare use the vacuum with a normal dust bag. Hence the investment in a bagless, dust separation and collection system.

The cost of the cyclone over here [via eBay UK with European postage] was about 1/3 that of the popular, US patented Dust Deputy bought through eBay. The Dust Deputy is a much simpler design than the"turbo" Chinese version. I also hear the Chinese cyclone has much more useful, standard inlet and exhaust port sizes. 50mm [or 2"] is easy to match off-the-shelf plumbing or flexible hose. As I said above I have ordered a 64mm/ 2.5" workshop hose kit to allow far greater airflow through the system. The cyclone's own cost roughly matches a single Dust-Vac, dust bag at retail prices. So it should quickly pay for itself in unneeded bags.

The basic idea is that a vacuum cleaner hose is applied to the top spigot of the cyclone moulding. [Blue arrow] Dust is sucked up as normal but must travel back to the cleaner via the cyclone. Whose a second, 2" horizontal inlet port is provided for the dust hose proper.

Dust and debris rush into the cyclone housing at a tangent to the cone. Where it flung out to the edges at high speed by the airflow. It all spirals gently downwards until it falls clear into the sealed, dust collection bin sitting directly under the cyclone. The dust is physically separated by the cyclone so that it cannot take the short cut to pass straight back up the inlet pipe.

Air tightness is essential throughout the suction system for [a claimed] 95-99% efficiency in separating the dust and wood chips from the air passing through the dust extraction system.

Rubber O-rings or gaskets are advisable on plate to plate connections and clamps on hoses. The dust collection bin must not collapse under the applied vacuum nor leak. The collection bin should also be regularly monitored to avoid it becoming overfull. Which can lower the separation efficiency by affecting the cyclone's behaviour due to it being too close to the sawdust's surface. Presumably the contents of the collection bin can start swirling too. Or block the cyclone's outlet with dust.

The vacuum cleaner's dust bag, pleated filter and collection bin, in the cleaner body, remain almost perfectly clean. Greatly extending the life of the vacuum cleaner. Largely because the motor never overheats due to dust clogging the internal bags and filters. Which was presumably the reason for my old wet/dry vac dying on me some years ago.

Some users of dust cyclones don't bother with either bags or filters at all. Though the small residual quantities of dust, missed by the cyclone system, may eventually clog the motor itself. It may also be distributed in the air within the workshop with serious health consequences! Retaining the pleated filter would seem a very good idea even if a dust collection bag is not to be used. Re-usable pleated filters are available at higher cost than the standard paper type. Even the very finest dust can be stopped by special HEPA pleated filters.

An image of my new cyclone sitting on its turned plywood disk sealed to the lid of a 60 liter, 16 US gallon, screw-top plastic barrel. Picture taken at dusk with flash and brightened.

More advanced workshop owners sometimes apply a stand-alone motor/impeller system. This sucks larger quantities of air through the cyclone instead of using a commercial, workshop, vacuum cleaner. Piping is run throughout the workshop so that each woodworking machine has its own extraction hose. Blast gates [slide valves] are used on the extraction pipes of the machines not actively in use. Cyclones come in various designs and sizes depending on the required throughput and quantity of sawdust to be collected. Efficiency varies with each design.

Today I found some 2" corrugated flexible hose in a DIY store. It had a nicely smooth bore for low air drag. So I bought 5' and some matching worm drive, hose clamps. Using my lathe I tapered both ends of some recycled, thick wall, 2" PVC drain pipe. A temporary spigot to fit the vacuum cleaner socket was another 2" recycled plumbing piece with a joint turned down to a taper with a 2" newly tapered other end.

My first trial of the dust separation system was in removing the accumulated dust from the miter saw and this went splendidly.  I could see the sawdust spiraling rapidly down through the translucent plastic cyclone, back-lit only by the open shed door. The storage barrel began to fill with mixed sawdust while the cleaner body remained clean.

All very satisfactory and I can now connect directly to each of the saws as I am actually using them. This will avoid spraying sawdust from on high down into an open cardboard box via a plumbing elbow. With the result, until now, that much of it ended up anywhere but in the box!

Whoops! I turned up the suction on the vacuum cleaner just as a small piece of wood blocked the tiny hose inlet. In an instant the barrel turned itself inside out with a bang!  Oh, Dear.

I'm thinking of arranging a stars shape of crossed battens inside the barrel to stop it collapsing while I seek a stronger alternative. Metal drum?

It should be emphasized that the cyclone and pleated paper filter must NOT be used for wet suction jobs. This requires the supplied foam sleeve and an upward facing elbow inside the empty vacuum cleaner. The paper bag and pleated filter are removed.

We once bought a secondhand Vax from a garage sale. The machine looked brand new and we were keen to replace our own, tired and very elderly Vax. Alas, it seemed somebody had done some serious wet suction with the dry suction bag in place and the entire machine, including the motor, was full of horrible wet mud and totally inoperable! Sadly, it had to go back for a refund.

 

Click on any image for an enlargement.

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Detour: Spring [glue] clamps comparison and more DeWalt junk!

As can be seen in the image my Bessey spring clamps have finally arrived. Though made in the PRC, like most others, these are actually very pleasant to use. The grippy rubber on the straight sided handles makes them very easy to grip and press. Far superior to the polished plastic of Rawlink clamps the Besseys open and close easily without hand cramps setting in.  Nor one's hands sliding automatically and inexorably towards the pivots where no leverage is possible!

The jaws are slightly shorter than the handles on the Bessey Clippix. Which provides some extra leverage. I can't be certain of the pressured applied at the jaws without proper testing. Small, thin digital scales? I don't have any.

None of the Bessey jaws have fallen off yet. Unlike the crappy green and black Rawlink ones which I bought first. Which lose their bright green, pivoting jaws at the slightest whim. I have wasted quite some time searching in grass for these after discovering a jaw was missing.  Leading to the likelihood of seriously damaged surfaces when clamping without a jaw. Don't waste your money on the green and black Rawlink clamps. I have large, slim hands but still find the Rawlink clamps very difficult to use due to the reasons mentioned above.

I suppose I could slip some narrow, bicycle inner tube off-cuts over the handles for more grip. Though that wouldn't keep the jaws in place. These rely on the precision of the simple moulded clips in the jaws. Which are badly formed on some of the clamps I bought. Probably due to pushing the moulding dies well beyond their natural working life to save money on tooling. Followed by poor inspection routines by dissatisfied or bored production workers.

I'm guessing, of course, but that's exactly what it looks like to me. Why even bother to make something in the first place if the design and production qualities are as crappy as this?

Talking of which: The expensive, DeWalt reversible chuck with pilot drills, countersinks and drivers is proving quite a problem. One holder has such a wobble that it is virtually unusable! The tip of one pilot drill has been ground so wrongly that it doesn't even drill softwood!

Just getting the kit out of the case is very hard work. I've tried sliding and I've tried levering and I've tried both. Keep taking the money and churning out Chinese crap, DeWalt. It's only your reputation at stake for overpriced junk, after all ! 

Click on any image for an enlargement. 

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Dome build: A better pair of segments.

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After much sawing and checking yesterday I made another pair of segments today to my own dimensions and angles. It is vital that the dihedral angle between segments causes each segment base strut to follow the base ring. No problem there.

The adjoining ribs between each segment must also lie perfectly flat against each other without any distortion. Any gaps between the ribs in the image are due to protruding screw heads. I did not bother to countersink the screw holes just for this quick assembly check.



 As I was checking the angles for the final time, on the second segment, it occurred to me that I had moved the goalposts. By reducing the width of the segment, the squareness of the lowest sheathing panels was lost. So I decided to make them square again. This required a reduction in strut spacing.

The result can be seen alongside. Where I have added new struts lower down without removing the originals. This ensured the successful geometry of the segment was not changed. The effect can be seen in the other image once the original struts were removed.

And then it all went completely awry!😖 I had my drawing with angles and lengths of strut beside the saw.  So I busied myself sawing out enough pairs of struts for two segments. The problem was using a pre-drilled rib as a pattern but using the wrong holes. Grr?

Now I need to use a new pattern with only active guide holes or just mark the active holes. It only means the loss of a few minutes work but I was hoping I was setting up a routine for churning out segments. Making lots of struts the same length would be far more sensible than changing the saw settings for for each one. I just need to make sure I have the correct settings and strut orientation.

With the four segments to play with I placed two of my builder's straight edges against the outside ribs to the center to check where they crossed. About 155-160cm radius as near as I can judge. This is with the segments standing on uneven ground and held only by clamps. But it seems close enough not to worry about the dome being far too small or much too large. It is odd how small the dome seems when viewing from outside with it down on the ground. I wouldn't want to carry it far but I can manage to lift the four segments.

Click on any image for an enlargement.

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Dome build: Treating the segment as a gore.

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Friday is a still, sunny day with 60F, so I am cutting out more ribs. A job I can't do in the shed for lack of room to handle full sheets of plywood. Reached 11 ribs in total. Both edges of the rib have to be cut twice because of the difference in radius. Leaving a long thin and curved, tapered piece of waste.

Saturday is another fine, still day and I just finished the last 5 of the 16 ribs required before morning coffee. Which makes me realise that I had better start building something soon before it starts snowing!

Sunday: I rebuilt the first segment using two ribs and new, shorter strut lengths. It was immediately obvious that the angles and lengths were all screwed up. As were the ribs which had become corkscrews. Something obviously needed to be done.

I decided to build a complete segment [or gore] reaching to the dome's pole or zenith. Easy enough with lots of spare plywood ribs and plenty of spring clamps.

I set up a vertical pole and adjusted the rib overlaps until the ground radius was correct at 1.52 meters. The dome height obviously had to be the same too. So I clamped a bit of scrap plywood to the vertical pole to support the dome ribs at the correct height.

 I have been finding a 'speed' square very useful for transferring the scribed miter angles to the very ends of the struts for trimming on the compound miter saw. It is much easier to use than a clear, drawing protractor.

These images shows the full size, complete dome gore mock-up. The long straight edge [image top right] is checking the angle of the rib at each horizontal strut. With the nearest edge centered on the upright pole I could easily see if the rib matched the angle of the straight edge. It didn't.

There followed some pencil marking and trimming of the struts to get the ribs flush with the straight edge. The ribs are certainly much straighter than before. Though still not perfect. This suggest that the ribs are no longer the correct length.

The answer here is to fit just the lowest horizontal strut and clamp the ribs at the pole. Just like a hemispherical gore. Then the intermediate struts can be measured for correct length by measuring between the ribs and newly trimmed struts tried for fit.

The ribs must be relaxed, perfectly straight and flat for this fitting. Or each segment won't lie flat against the next. Symmetry is also important. Sandwiching the 'real' ribs with more spare ribs and clamping them all together should ensure the ribs are nicely straight and flat.

The image [above left] shows the reinforced ribs and the carefully spaced extension ribs meeting the upright pole. I used the 4' level to ensure the 'real' ribs were perfectly straight and properly spaced. Spare, 15mm thick, dome ring arcs ensured the ribs were stiff enough not to flex sideways.

The final image [right] shows the complete segment with the struts fitted. The segment now lies evenly on either rib against a flat surface. I have noted the angles I cut on the struts and their length ready for building the next segment tomorrow.

Later I dragged the segment up onto the platform and propped it on one of the octagon's posts. The dome curve is slightly too low to clear the telescope but this is without any base rings or support wheels. It was now too dark for photography so I shall try again tomorrow.

Click on any image for an enlargement.

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Dome Build: Compound miter angles.

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My continuing thanks to Paul Robinson of Geo-Dome.Co.Uk:   

Trapezium Dome calculation tools

I was able to find a drawing on his website of a trapezium dome 'gore' with the internal angles shown. I then subtracted these angles from 90 degrees and printed these in red.

These are the vertical angles I need to set on my compound miter saw for cutting the horizontal struts.

These numbers certainly explain the difficulties I had in getting the correct compound miter angles for each horizontal strut. I had falsely assumed that the angles would all be the same. Then found each rib was pulled in or out as I tightened the screws. I noticed that the lower struts needed hardly any vertical miter at all. While the upper struts obviously needed more. Though my choice of 5-6° was still more than required.

Tomorrow is wet again but I can still use the miter saw in the shed. This time I shall set the horizontal miter angles to 11.5°. Then cut new struts to the correct lengths and vertical miter angles.

I am most grateful for the continuing advice given on Cloudy Nights DIY Observatory  discussion forum. Without which I would not have made so much progress. Nor made anything to a reasonable accuracy. Those who make sawdust for a living develop experience and tricks of the trade to succeed.

It is not simply a matter of saving time and materials but of simplifying seemingly difficult tasks and procedures. There are checks and working methods which ensure accuracy which are very unlikely to occur to the average hobbyist.

If carpenters had to reinvent their work practices every single morning then mankind would still be living in caves! Their willingness to offer free advice based on their long experience is an invaluable gift to a bumbling amateur like myself.

In that light I have been advised to mark the base ring to avoid nasty surprises from cumulative errors. Multiples of 16 soon add up. Using simple measuring jigs and sticks is another vital piece of advice. This will depend on my having dry weather so I can set out the complete ring, of course but that day must come eventually. Then I can use their practical advice to help ensure success.

And then there were 11 [ribs.]

Sooner said than done. I pinned the half lap joints of the base ring with screws and proceeded to scribe 16 segments using a batten and two nails as a divider. A couple of hours later I managed to find the exact division after repeatedly under and overshooting my start point. 60.5cm was the final measurement obtained by gently bending the nails in and out to change the divider's radius. It is easy to imagine one is marking off the circumference of a circle when in fact these are flat sided segments of the circle. A straight line [a chord] is shorter than the arc it subtends.

A forum member suggested using a long tape measure and I realised that it could be held in place by clothes pegs without needing to pin through it. 

Friday is a still, sunny day with 60F, so I am cutting out more ribs. I job I can't do in the shed for lack of room to handle full sheets of plywood. Reached 11 ribs in total. Both sides of the rib have to be cut twice because of the difference in radius. Leaving a long thin ans curved, tapered piece of waste.

Saturday is another fine, still day and I just finished the last 5 of the 16 ribs required before morning coffee. Which makes me realise that I had better start building something soon before it starts snowing!

Click on any image for an enlargement.

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Dome build: Trial Segments.

Now I need to cut the miters on the cross struts to make up my first dome segment. If it proves satisfactory I can start mass producing struts to length with the correct, miter angles on each end.

The ends needed to be compound mitered. Not only do the gores shrink in width as they rise but the ribs are splayed. They should all lie on radii from the center of the dome. The compound angles I used were [incorrectly] between 5 and 6 degrees both ways. I pre-drilled the plywood and then screwed into the end grain of the struts.

Being a fairly sensible chap I decided to test the segment by laying it on each side on a flat surface. The ribs wouldn't lie flat!  So I trimmed the middle strut and all looked well. Satisfied I was on the right track I made another segment using the new strut measurements. Whoops! Oh, deary, deary me! They certainly fitted together but wouldn't stand evenly on a flat floor.  There was a difference of 1" in level on both outside ribs!

Image of the two segments inverted and tested against the base ring. Wrong! There is a four inch error in the horizontal plane and a 1" error in the vertical plane.

My choice of 5 degrees for the compound angles must have been half that required. Watch this space. I shall certainly have to increase the horizontal miter but the other should not need to be increased. The middle struts will also have to be lengthened to match. Or, rather, a new strut cut to the correct length as suggested by Geo-Dome's trapezium dome calculator.



Later, I set the miter saw to 11° and re-cut all the strut ends. This sharpened the dihedral angle of the battens to much better match the base ring. Though the top struts now seemed to be too tight an angle and this put a slight twist in the ribs. That  said, the strut lengths were now far from accurate in length having been cut and re-cut numerous times.

Having confirmed the dihedral angle is much closer I now need to make up new struts to the calculated length. Just to check the full sized model is correct. The exact miter angles are still an unknown so some adjustment will be necessary. Though these angles may be obtainable from the Geo-Dome trapezium dome articles if I do a bit of simple maths.

At this point I am really just building a normal hemispherical dome with gores. Only when I cut away the flats and apply panels will the form be changed to a tiered and faceted design.Which means I can use a dome design calculator to check my strut lengths and miter angles.

Click on any image for an enlargement.

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Dome build: Getting serious.

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After what seems a ridiculous amount of time I have finally cut out four arcs of 3/8" [9mm] birch plywood to make two ribs. Two ribs and the cross struts makes a segment. One of sixteen. The vertical struts are not vital to the segment but will be cut anyway.

As usual, I started with a beam compass made from a stiff length of box section alloy tube. The pencil was fixed with two nuts. One above and the other below the beam and simply screwed onto the pencil by hand. The compass pivot point was a long screw fixed into a spare length of timber clamped to the 5x5' sheet of birch plywood.

For cutting the arcs I used a fine toothed, metal cutting blade in a jigsaw. The fine teeth reduce ragged edges and splintering to a minimum. No doubt taping over the cut line would help but I had no suitable, clear tape to spare. After sawing outside the arc lines I sanded the edges with an angle grinder with a coarse flap wheel. This is a remarkably quick and easy way to smooth the cut line.

I then set the overlong arcs up against the shed to check I had a full quarter dome. I could then cut off the bottom of the arc to match the floor using a bit of timber packing as a guide.

A plumbline gave me a vertical where I could measure up from a stiff, level batten supporting the squared off base. For scale, the level is 4' long. The rib is 162cm wide x 162cm high.


The arcs are overlong with considerable overlaps. As can be seen by the number of spring clamps I managed to use to hold each rib together. I haven't decided whether to half lap them like I did with the dome's base ring.  The rib plywood is quite a bit thinner than the base ring.

Then I drew the flats on the ribs for the vertical struts. Each line was 63.6cm long and the included angles between them 162°. By coincidence the height and width of the full rib is 162cm.

Click on any image for an enlargement.

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Calculating the weight of the dome struts:

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A sixteen sided figure is a hexadecagon. The internal angles are 157.5°. So the miter angle to be cut on the ends of the horizontal struts is 180 - 157.5/2 = 11.25°.

The difference is divided by two because the angle is shared equally between two contacting surfaces. This is the same angle which must be double beveled on the faces of the vertical struts. The sheathing panels will be laid on these 'flats'. The vertical ribs will both reinforce the ribs and provide a substantial bed for the flat, trapezoid, 4mm plywood sheathing panels.

I have bought some 43x43mm timber for some experimental struts to make up a single dome segment. However, the continuing bad weather has stopped most work on the build. I need to be outside to have enough room to cut the 9mm plywood arcs for the dome from 5' x 5' sheets.

It is not the matter of a few moments to tidy everything up between sudden cloudbursts! September 2017 in Denmark is heading for an all-time rainfall record. There was more rain on one particular day than the average for the whole of a normal September. The weather has constantly flip-flopped between sunny and cloudbursts. The regular gales don't help.

Thanks to Paul Robinson's Geo-Dome trapezium dome calculator I can find the weight of the struts.

 http://www.geo-dome.co.uk/trap_tool.asp

 More information here:

 http://www.geo-dome.co.uk/article.asp?uname=trap_dome

By a happy/unhappy[?] coincidence 43x43mm softwood [pine] weighs close to 1kg per meter, or roughly 2.2 lbs/ per m or 39.4".

Vertical struts = 16 x 4 = 64 x 64cm  = 41m

Horizontal struts           = 16 x 64 cm  = 10.2 m
                                        16 x 59    "   =  9.5
                                        16 x 45    "    = 7.2 
                                        16 x 24    "    = 4
Total length of struts    = 71m
Weight                         = 71kg or about 156lbs. This is just the skeleton! Eek?

To which must be added the weight of the 4mm plywood sheathing, of course.

As I couldn't do anything very useful, between heavy showers, I measured up the strut lengths using the base ring arcs. Which just happen to be the same radius as the ribs, of course, thanks to the geometry of a hemispherical dome. My calculations for miter and bevel angles were close enough within my ability to measure to 1/4°. The 9mm ribs would end up 3" deep where the flats are cut away.  Plus the depth of the vertical struts laid over each pair, of course. These measurements give me confidence that it will all go together as planned when I finally see some dry weather.

Click on any image for an enlargement.

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