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Metal for dome, skin or not?

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24 replies to this topic

#1 Spoonsize

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Posted 18 December 2018 - 06:26 AM

I am trying to figure the optimal plan for a dome. Is it best to skin a dome made of a wood base? How thick and what metal to use for a skin, galvanized or aluminum? Or, best to increase the number of gores and assemble them with lap or standing rib joints? Again, galanized or aluminum and how thick?

#2 wa5dxp

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Posted 18 December 2018 - 06:43 AM

How about Fiberglass over plywood?  or Fiberglass Reinfoced Plastic (FRP) sheets over plywood?  Epoxy paint? Titanium Dioxide paint?



#3 mark77

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Posted 18 December 2018 - 07:06 AM

I was going to do fiberglass over plywood but the cost of the resin alone was going to be $10,000 (for a 15 ft dome).

 

I looked into using truck bed liner coating.  It was recommended by a fiberglass expert.  It looked like a good alternative but I ended up going with aluminum flashing. This worked out very well

https://www.homedepo...24182/202091135

 

you can see my build at http://www.skychariot.com/dome/

 

Mark



#4 bmwscopeguy

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Posted 18 December 2018 - 07:08 AM

I think I'd be against a metal skin as it might promote condensation on the inner surface during temperature fluctuations. 18 or 22 gage aluminum would likely be more than enough depending on how much distance you are spanning. This is mitigated by the fact that there is little force acting on the panels where they are widest - ie: the bottom.

 

I once built a slab sided 'dome' that had plywood ribs and veneered in thin 1/4" plywood. I then gave it a thick coating of rubberized paint. It never leaked, but it was on the heavy side....

 

I've wondered since then if I could have gone with some sort of doped canvas stretched over the frame...



#5 mark77

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Posted 18 December 2018 - 08:00 AM

So far I have not had any noticeable condensation on the inside of the aluminum skin.  I have had condensation on the scope mount.

 

The flashing is 0.020 thick, I am not sure what gauge that is.  My rib spacing is approximately 16" centers at the base.  I do still have some leaks but my total weight of the dome is only 1500 pounds.  That may sound heavy but it is 15 feet in diameter and the door is very solid.

Mark



#6 TOMDEY

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Posted 18 December 2018 - 08:19 AM

I have used them all... and (now) highly promote aluminum as the outermost layer. Nothing else seems to last forever. If the panels are flat of curved in only one dimension (e.g. gores), the aluminum can be pretty thin, like flashing, provided it is entirely backed from behind. Other than that, the joints and laps are critical. And for that, RTV adhesive sealant seems to last forever, provided it is the "good stuff" 100% Silicone RTV, the kind that smells strongly of acetic acid, out of the tube. Be sure to entirely degrease the aluminum with acetone.

 

The worst possible material seems to be Masonite... everyone I know, who has used that... has come to regret it.  That stuff is just one tiny step up from cardboard...  Tom


Edited by TOMDEY, 18 December 2018 - 08:20 AM.


#7 bmwscopeguy

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Posted 18 December 2018 - 08:41 AM

Hi Mark,

 

That's a gorgeous looking dome - also much more ambitious than anything I've attempted.

 

The other material I've had some experience with is thin fibreglass sheeting. You can get it in 1/32 or 1/16" up to 4x8.  I only used it for the dome shutters though..



#8 rcdk

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Posted 18 December 2018 - 11:33 AM

I did a 12' dome in fiberglass over plywood and the whole project was far less than that. I don't know the final number off the top of my head, but the resin itself was the most expensive part and it was less than $1000. I did some judicious shopping around and used polyester resin rather than epoxy. I put around 15 gallons of resin on it and averaged $40 to $50 a gallon.

Gelcoat is more expensive, but I only needed a couple of gallons. Fiberglass cloth is pretty cheap.

#9 Spoonsize

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Posted 18 December 2018 - 01:44 PM

Thank you all for the comments. If I decide to build I will be sure to post

#10 starcanoe

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Posted 18 December 2018 - 04:21 PM

An observation. If you build a "dome" that is cylinder laying on its side....your life will be much easier in pretty much every way.

 

A true dome dome might look a little more "right" and give you a modest advantage in winds high enough that stuff going flying is a worry...but other than that IMO it looks to my inexperienced eye to be a lot of pain for very little gain. And for that matter a cylinder dome actually has some serious practical advantages.


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#11 TOMDEY

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Posted 18 December 2018 - 09:29 PM

An observation. If you build a "dome" that is cylinder laying on its side....your life will be much easier in pretty much every way.

 

A true dome dome might look a little more "right" and give you a modest advantage in winds high enough that stuff going flying is a worry...but other than that IMO it looks to my inexperienced eye to be a lot of pain for very little gain. And for that matter a cylinder dome actually has some serious practical advantages.

That's interesting! I have never considered or seen that before. Are there any examples of this being done. Harder question: Are there any exapmpes of this being done... successfully?!  Or is it just a concept, at this point?  Tom



#12 bmwscopeguy

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Posted 19 December 2018 - 07:35 AM

Has anyone ever built a geodesic dome observatory? Thoughts? 

 

I was thinking of cutting the panels out of rigid Styrofoam SM (pink, blue stuff) with a hot wire jig and gluing them together using expanding foam. Apart from the obvious question of wether it would work, it has the advantage in that all the components can be made up ahead of time, in the comfort of my basement, and then assembled on the base structure this coming spring.



#13 starcanoe

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Posted 19 December 2018 - 07:37 AM

Tom

 

 

I'm pretty sure I've seen one somewhere...but no idea where...your internet search is as likely to find one as mine would be.

 

And as I mentioned...besides not looking quite as fancy...and being a big more draggy in high winds...pretty much everything else would be better and easier doing a cylinder.  


Edited by starcanoe, 19 December 2018 - 07:38 AM.


#14 radial195

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Posted 19 December 2018 - 12:51 PM

Before I happened to find a used Exploradome for sale I was thinking of using a child's 10' diameter playground geodesic climbing toy for the framework of a dome. I even went so far as to purchase the toy from Costco and partially assemble it. I had 2 major concerns with going down this avenue:

 

1) How to incorporate a slot into the dome?

 

2) How to seal the many seams in the dome?

 

I never came to a solution for #1 that satisfied me. Problem #2 turned out to not be a problem at all IF you use a silicone based product called "Gaco-Roof". It is manufactured in Seattle. It's one drawback is it is EXPENSIVE!, (it cost me about $350 to paint my 12'D walls and roof) but, it works better than anything else on the market. I've had it on my observatory walls and roof here in Arizona for 7 years with 'nary a leak, and no maintenance. Gaco-Roof expands and contracts with heating and cooling. We have a fairly wide temperature range where I live, although not as wide as some other places. I've seen temps from +9 degrees (I'm sure it can get colder than that here!) to about +108 degrees. I did not tape the seams in my walls or roof prior to painting this stuff on, but I did put more coats on the seams. As far as the complaint of how expensive it is- - -how expensive is your astro gear, computer, books, or everything else in your observatory? If it gets wet and ruined because you were too cheap to use the best stuff out there, that's your fault!

 

RLD Observatory


#15 mark77

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Posted 19 December 2018 - 01:21 PM

go to google and type  "home built observatory geodesic"



#16 PETER DREW

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Posted 19 December 2018 - 01:41 PM

That's interesting! I have never considered or seen that before. Are there any examples of this being done. Harder question: Are there any exapmpes of this being done... successfully?!  Or is it just a concept, at this point?  Tom
Ive built several. Much easier and cheaper to build than domes.

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#17 starcanoe

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Posted 19 December 2018 - 01:46 PM

Looks good and much simpler....form follows function...or is the other way around here?



#18 TOMDEY

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Posted 19 December 2018 - 02:10 PM

Has anyone ever built a geodesic dome observatory? Thoughts? 

 

I was thinking of cutting the panels out of rigid Styrofoam SM (pink, blue stuff) with a hot wire jig and gluing them together using expanding foam. Apart from the obvious question of wether it would work, it has the advantage in that all the components can be made up ahead of time, in the comfort of my basement, and then assembled on the base structure this coming spring.

Yes, I did this 24-foot one >>> and a 12-foot one on the same principle. It's wood-frame, but I did indeed build all of the panels in the shop and assemble them Very Quickly come spring.  The 12-foot one is written up in great detail in one of the old issues of Telescope Making Magazine.  These structures are Extremely Rigid and also precise, because the panels are made precisely and identically in the shop.  Tom

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#19 bmwscopeguy

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Posted 19 December 2018 - 02:15 PM

Hi Tom,

 

I like your cylinder dome! I can see how it would be easier to make by a long shot. Even the shutters are easier. What was your base 'ring' diameter?

 

I'll have to re-evaluate the geodesic concept.

 

Cheers

Malcolm



#20 starcanoe

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Posted 19 December 2018 - 07:57 PM

Another thinking outside the box a bit. I think there was an article in S and T or Telescope Making years ago.

 

One can make a dome by cutting gores and glueing/welding/riveting them together. Typically the gores are oriented vertically such that they all meet at the top of the dome.

 

You can ALSO make a dome where you have the gores running horizontally...and the points/each end of the gore meet at points on opposite sides at/near the bottom of the dome.

 

This has the advantage of probably making dealing with the cut out for the shutter opening a bit more straightforward. It would also probably help with making the dome more waterproof as you could make the overlaps such that water would be less likely to get in there in the first place.



#21 rgpalmer

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Posted 01 January 2019 - 12:17 PM

I am thinking of a geodesic paneled dome made with 1 1/2" thick polyiso insulation board with a fiberglass skin on both sides.  Edge glue the panels and do a little fiberglass tape over the joints on the outside for structure and sealing and then coated with something.  the Gaco Roof mentioned above looks interesting.  

 

Here in the upper midwest the insulation sheets are available at Menards for about 30 bucks for a 4' x 8' sheet.  It is a Johns Manville product I have used in commercial roofing projects.



#22 bmwscopeguy

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Posted 01 January 2019 - 03:13 PM

Hi,

 

I had a similar idea and am awaiting some nichrome wire so I can set up a jig to cut the necessary angles.

 

For an 84" diameter 3V dome, you'd need 75 'triangles (2 different sizes) the longest side of which would be 17.3". I figure you'd need 4 sheets of 4x8' depending on your desired dome diameter.

 

Rather than try to fit shutters, I was planning on slicing it in half vertically, and having the two halves slide apart - sort of a 180* viewing slot.

 

The whole dome would rotate on its ring base just like any dome. I'd make the horizontal slides long enough to get the dome halves out of the way, sort of like a roll off shed roof. So you'd have a choice -Both halves open a bit for a narrow viewing slot, 1/2 closed other half open for 180* to zenith viewing, or both halves wide open for that 'whole sky' feeling.

 

I built one this way before (just not geodesic) out of framing with a plywood skin but in my typical fashion built it like a brick %*&$house which combined with flex, and a slightly irregular rotation track meant it bound up in certain positions. I ended up painting the 1/4" plywood skin with a rubberized paint.

 

But it did work and was very convenient. I called it my Clam-Roll-Dome  I'm thinking the foam geodesic approach would be a lot lighter which would ease some of the mechanical requirements.



#23 bmwscopeguy

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Posted 01 January 2019 - 03:14 PM

Oops - meant to provide the following Geodesic website. There are a lot of dome calculators on the web, but I found this one to be most helpful.

 

www.geo-dome.co.uk

 

Cheers

Malcolm



#24 rgpalmer

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Posted 01 January 2019 - 08:08 PM

Malcolm,

 

Great minds think alike.  I have been playing with a SketchUp model of almost exactly what you are doing with you Clam-Roll-Dome only my thought was to do it with a half cylinder set on side.  I was thinking to make one side larger so only one needs to move to get a 48" wide slot open.

 

Yep, geo-dome.co.uk is a great resource.

 

cheers from this 65 year old guy in Wisconsin.



#25 gregj888

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Posted 01 January 2019 - 10:14 PM

 https://www.cloudyni...buiding-a-dome/

 

this is my current favorite:

 

 https://www.youtube....h?v=FR5WsE8gGSk

 

Flat sides, good use of materials and looks a little like ESO...  Also like the extra height away from the opening should vents be needed.  




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