4 foot diameter translucent half-sphere?

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Dork
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4 foot diameter translucent half-sphere?

Post by Dork » Wed Apr 14, 2004 1:34 pm

Any ideas on how to make a 4 foot diameter half-sphere that can be evenly lit from behind? I could get something approaching a sphere with a series of metal loops covered with fabric, but it will be dark where the metal is against the fabric.

Is there a better method? Maybe chicken wire instead of loops? I thought I had found a source for pre-made spheres in this size range but I can't find it again and would guess they're rather expensive.

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Post by Menehune » Wed Apr 14, 2004 2:10 pm

If you have Union 76 gas stations in your area I would contact their regional office and inquire about surplus or damage signs. I have seen damaged ones here on Oahu in a storage yard and it is my understanding that these spheres are painted and can be sanded to a clear sphere.
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DangerMouse
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Post by DangerMouse » Wed Apr 14, 2004 2:19 pm

Use clear plexiglass or Lexan strips instead of wire perhaps?

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Post by Jon Schull » Sat Apr 17, 2004 9:42 pm


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LostMachine
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Fiberglass

Post by LostMachine » Sun May 23, 2004 2:34 pm

If you only need it to be translucent then Fiberglass works great. I’ve done a few backlit projects this way. I did a sphere by stacking rings of Styrofoam insulation so when stacked they made a stair step sphere. I cut the rings on the band saw with close to the right angle so that there was little shaping to be done. Peel the foil backing off of them before gluing the rings together (3M super 77 works). Then with a rough rasp do the final shaping using a cardboard template to check the curvature.

The resin will eat the Styrofoam. I covered my sphere with duct tape to protect it, which worked fine.

After that, cut the fiberglass panels since one piece will not fold over the whole thing. Make sure the pieces overlap a good inch or two. (If you need template equations for a good panel size let me know)

Apply a mold release to the sphere/Dome, I used lumber sealing wax just because I had it on hand, but the real stuff is only $5 for a can. You can then pin the panels onto the sphere, Sewing pins work great. Mix up your resin/hardener and apply with soft plastic squeegee. After it dries sand the whole thing down. Sand more on the edges where the panels overlap. Apply another coat and sand again. With enough sanding you can get the seams out and have a uniform thickness to it. Also you can make it nearly transparent by sanding down to 400-600 grit then use 0000 steel wool, then a marine varnish.

On my three foot sphere it took me a whole day to build the sphere and get one coat on. A few hours to sand and get the second coat on. It took a whole day to carve away at the foam through a 10” circle to hollow it out and sand some more. This is defiantly a time consuming project.

Okay, I think that makes sense. Anyway good luck I hope this helped a little.
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Alpha
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Post by Alpha » Sun May 23, 2004 3:07 pm

You certainly helped me, thanks for the education!

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Post by LostMachine » Sun May 23, 2004 9:52 pm

Cool, I'm glad that helps ; )

The same thing works for all kinds of shapes, if you can cut it out of a block of foam you can make it into a fiberglass shell.
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Alpha
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Post by Alpha » Mon May 24, 2004 7:25 am

Where do you recommend getting large blocks of foam? As I recall my local crafts store doesn't have anything larger than maybe 2' x 1' x 6" !

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Post by robotland » Mon May 24, 2004 11:51 am

Ask around at furniture and appliance stores for styro packing material....flat sheets and angular chunks can be glued together to get larger pieces, and reutilize a dumpster scourge at the same time! Or you can buy sheets of high-density foam at Bigbox Builderland. Often there's a "scratch and dent" pile of damaged sheets that they'll either thank you for hauling away or charge you minimally for the privelige.....

....next project: Googling "homemade hot-wire cutter".....
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Post by robotland » Mon May 24, 2004 11:59 am

ahh, here it is!

http://www.dansworkshop.com/Hot%20wire% ... ting.shtml

There's stuff about aluminum casting and other fun stuff, too.
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Alpha
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Post by Alpha » Mon May 24, 2004 12:10 pm

What kind of glue adheres to styrofoam without melting it?

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LostMachine
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Post by LostMachine » Mon May 24, 2004 7:25 pm

For Foam I would use the 4’X8’ home insulation foam panels. They come up to 3” thick. I learned the hard way that you need to peal off the foil backing before gluing them up. The rasp grabs on the foil and makes shaping a nightmare.


For Glue I used 3M super 77 I like to spray a coat on both pieces and then let them sit for 45-60 seconds then put the two pieces together.

Don’t use Cyanoacrylates they will burn through foam like Acid (It is fun to watch).

For gluing I like this page if I got something funky to stick.

http://www.thistothat.com/
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Alpha
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Post by Alpha » Tue May 25, 2004 7:43 am

thistothat.com -- what a great site! Thanks for the link.

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