Materials to use for a hollow hemisphere

Dennis B

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Joined
Mar 17, 2019
Location
Dixon, CA
I've been asked to design a rolling set piece that contains a 4' half-sphere, part of a fairy throne (imagine a hollow ball, sliced in half vertically, with the front face removed). The "shell" needs to be fairly thin, say 1-2 inches, and it rests on a wagon. I'm stumped figuring out the best materials to use. This is a low budget ballet production, so I can't go crazy. My favorite idea so far is fiberglass, if I can figure out what to use as a mold (hard to find 4' EPS spheres!). Or maybe a foam block, carved out and surfaced with AquaResin or plaster, but that's a lot of foam to carve.

Other ideas: EVA foam cut into strips and glued seems too flimsy. Chicken wire and papier (or cloth) mache could work, but what to use to stiffen the opening?

Thanks for any suggestions...
 
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I don't remember the last time I had to build a hemisphere like you're describing (I'm sure at some point it's happened, but certainly not often) so I'm just spitballing-
The fiberglass idea is good. I would start with a giant balloon (just look up giant balloons on amazon or another online retailer- they're readily available up to 72" or so), then go over that with a layer or two of papier mache so that you aren't relying on just the sturdiness of the balloon for too long. Wait for that to dry, then go over that with fiberglass. I like to use the bondo fiberglass cloth and fiberglass resin. I would just layer it until you feel confident in its rigidity. If that would take too much fiberglass for your budget, you could do one good layer of fiberglass, then a layer of chicken wire, then more papier mache. The whole thing should be more than sturdy enough long before you get to anything like 2" thickness.
 

bobgaggle

Well-Known Member
Joined
Nov 19, 2007
Location
Philadelphia, PA
1. Build a plywood form. 4 or 5 ribs routed to the radius you want. Screw it together with screws accessible from the inside.
2. Cover in chicken wire, use staples sparingly.
3. Hard coat the exterior with your choice of material, fiberglass is fine.
4. Unscrew and remove plywood form
5. Hard coat interior
6. Finish as specd by designer...

ALTERNATIVELY:
2b. spray foam chicken wire to build thickness (add masking, don't glue to form with the foam)
2c. carve to shape.


Also, welcome to CB! lots of good info here. Stop by the new members board and introduce yourself.
 
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Chase P.

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Feb 3, 2017
Location
San Francisco
Ram Board could work. It's a cardboard roll product, used to protect floors during construction. Cutting ellipses out of it and gluing them together would be pretty strong. Would require some calculating to make the proper template to cut the pieces. Might also require a coat of fiberglass to stiffen it up. Would be smoother than chicken wire, probably also more expensive.

Just googled '48" fiberglass sphere". Wow, nearly two grand to purchase. The concrete sphere parking bollard is less than half that, but somehow I doubt that fits your needs.
 
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Amiers

Renting to Corporate One Fixture at a Time.
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cdiamondz

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Jun 29, 2016
Location
Michigan
I've personally done 1/2" plywood ribs, chicken wire stapled to them, then spray foamed. It works alright as long as you're willing to go through a good number of cans of expanding foam and carve afterwards. In all honesty it isn't the prettiest of methods however it does work, you will need to finish the surface though. I always enjoyed the repurposing game, if you manage to find something already close to what you need (even better secondhand!) then don't be scared to do a little work to make it become what you need!
 

Butch!

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Dec 16, 2018
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Under there
You can take 4 by 8 stryofoam sheets (not beadfoam), preferably 2" thick, (thicker if you can find it), ideally without the prescored snap lines and cut them into rings of diminishing diameters from 48" and down to rough out the shape of the piece. Then hang a router from a cable anchored to a point at the theoretical center of the sphere and use it to carve out the inside to make a concave sphere. The length of the cable forces the router to cut a near perfect sphere inside. Then mount the router to a fixed point and fashion a holder for the sphere that will carve the outside of the sphere. Kind of hard to explain without a model, a little tricky to set up. But it does a perfect job. It also makes a heck of a mess so wear a full face respirator to protect your lungs from the dust and your eyes from the flying debris.
You could have the outside pedestal/support on the back be part of the piece if you did some extra figuring and didn't use rings and instead cut the pieces more like those donuts with handles on them.
 

bobgaggle

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Joined
Nov 19, 2007
Location
Philadelphia, PA
Then hang a router from a cable anchored to a point at the theoretical center of the sphere and use it to carve out the inside to make a concave sphere. The length of the cable forces the router to cut a near perfect sphere inside.
This would never fly in my shop. Free swinging router tied to a string, what could go wrong? If you really want to go the pendulum route, there are lots of plans out there, like this one. Constrain the tool, keep your fingers intact while keeping the scenery free from blood. Make a rigid armature that keeps the tool where you want it to be, especially when you're awkwardly leaning over/inside a 4' hemisphere.

Kind of a moot point, because after you get your ring glue up routed into a bowl shape, you still need to scrape/rasp/sand out all the tool marks. Would probably take more time to build the framework just to locate and mount a center point for the router than it would to just rasp out the shape by hand.
 
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lwinters630

Well-Known Member
Joined
May 12, 2011
Location
west of Chicago
I've been asked to design a rolling set piece that contains a 4' half-sphere, part of a fairy throne (imagine a hollow ball, sliced in half vertically, with the front face removed). The "shell" needs to be fairly thin, say 1-2 inches, and it rests on a wagon. I'm stumped figuring out the best materials to use. This is a low budget ballet production, so I can't go crazy. My favorite idea so far is fiberglass, if I can figure out what to use as a mold (hard to find 4' EPS spheres!). Or maybe a foam block, carved out and surfaced with AquaResin or plaster, but that's a lot of foam to carve.

Other ideas: EVA foam cut into strips and glued .seems too flimsy. Chicken wire and papier (or cloth) mache could work, but what to use to stiffen the opening?

Thanks for any suggestions...
I have worked with fiberglass quite a bit. Let's start with a 48" beachball $18. Then coat 1/2 of it with vaseline thin coat. From a boat store, get fiber glass resin slow cure. I like West Systems. A roll of glass matting. A roll of glass cloth.
cut into strips like you would for paper mache. First layer cloth. Next layer matting. Now you can add layer of cardboard for thickness. Then finish with 2 coats of glass cloth.

This should release from the ball and be smooth on inside and out. Don't leave air bubbles when applying. They make rollers and bonding materials also, Google West Systems.
 

Dionysus

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Joined
Feb 9, 2009
Location
London, Ontario, Canada
+1 for West System!!!
My go-to resin for sure! Especially from back when my family had two boats that needed fiberglass work.

Yeah find whatever is the right size and shape and use it to form your shape. Use your imagination. There is little limit to what you can make if you have the time and some resources.
 

Ancient Engineer

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Sep 21, 2017
Location
Sandusky, Ohio
I have built two airplanes with the West System. If you have allergies to "regular" resin products, West System is a lot less angry to your skin/nose.

I'd make my mold with plywood sheets and clark foam (or your fave EPS foam).

Cut the sheets to the diameter of the ball and slot two together at 90 deg and fill the edge areas with a few inches of foam.

I'd then machete to the rough shape and then cheese-grater to the final.

I'd go with cloth/matting/cloth/cloth 45deg out for layers.

If it has to be smooth I'd forgoe finish-sanding the fiberglass (a loooong process).

I'd scuff the surface all over with 200grit paper then skim with bondo as it finishes waaay faster.
 

lwinters630

Well-Known Member
Joined
May 12, 2011
Location
west of Chicago
I have worked with fiberglass quite a bit. Let's start with a 48" beachball $18. Then coat 1/2 of it with vaseline thin coat. From a boat store, get fiber glass resin slow cure. I like West Systems. A roll of glass matting. A roll of glass cloth.
cut into strips like you would for paper mache. First layer cloth. Next layer matting. Now you can add layer of cardboard for thickness. Then finish with 2 coats of glass cloth.

This should release from the ball and be smooth on inside and out. Don't leave air bubbles when applying. They make rollers and bonding materials also, Google West Systems.
Another thought is use the ball, vaseline and plaster cast strips like for a broken arm. They come rolled up. Just soak a piece and put on like paper mache. Not as strong as fiberglass. Just thinking outloud.
 

Dennis B

Member
Joined
Mar 17, 2019
Location
Dixon, CA
Thanks for all the great suggestions! (I was looking for email alerts of responses before I came back...just figured out that I'm not getting them. Oops!) I'll look into the West System. I have never priced out fiberglass, so will have to see how that fits into budget. I had another suggestion to heat and shape PVC pipe into rings, then band them together and cover with chicken wire. I like the idea of a foam shell, but it seems like a lot of foam and carving to get the desired shape.

I looked at 4' beachballs on Amazon, but from the reviews it seemed like most of them were "round-ish", meaning they are flatish on the panels and bulge at the seams. Not a problem if you're batting it around a pool, but less ideal when the final product is to be, well, round. I'll post some updates as the project progresses.
 

StradivariusBone

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Space Coast, FL
This would never fly in my shop. Free swinging router tied to a string, what could go wrong?
There's a reason helicopters have tail rotors. Sir Isaac Newton would like to have a word with the head carp.
 
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bobgaggle

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Joined
Nov 19, 2007
Location
Philadelphia, PA
Instead of a free swinging rotor, how about putting it on a lathe and carving it like a bowl? !! (Care to wager on the velocity of the outer edge?)
Or make a lathe. we made this one for carving a big cauldron. Would need some mods to carve out the inside...