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# cost estimate for ball?

cost estimate for ball? [message #95193] Sun, 03 August 2014 22:29 UTC
I'm trying to get a price estimate as part of a project to build an unusual scale-model solar system along a bike trail. This part of the project involves making a round ball with a diameter of exactly 11.35" inches that can be securely fastened to a wrought-iron fence.

It looks like white Strong & Flexible Plastic would be our best bet with Shapeways (painted reddish since it will represent the planet Mars, since only white has a big enough bounding box), but I don't see a simple way to estimate the price.

I'm guessing that to secure it to the fence, we'd need to make two hemispheres, then glue them together around the fence. But I don't know what sort of wall thickness or internal supports would be needed. It should be strong enough to survive being outdoors in wind and direct sunlight and snow for a few years, at least.

Can someone with more experience come up with some back-of-the-envelope price estimate? Or tell me if this approach won't work for some reason? (Will this kind of plastic degrade if left outdoors, for instance?) Or if you have a better way to make a sphere of that size, I'd love to hear it. Thanks!
Re: cost estimate for ball? [message #95195 is a reply to message #95193 ] Sun, 03 August 2014 23:25 UTC
 sgdoerfler wrote on Sun, 03 August 2014 22:29 I'm trying to get a price estimate as part of a project to build an unusual scale-model solar system along a bike trail. This part of the project involves making a round ball with a diameter of exactly 11.35" inches that can be securely fastened to a wrought-iron fence. It looks like white Strong & Flexible Plastic would be our best bet with Shapeways (painted reddish since it will represent the planet Mars, since only white has a big enough bounding box), but I don't see a simple way to estimate the price. I'm guessing that to secure it to the fence, we'd need to make two hemispheres, then glue them together around the fence. But I don't know what sort of wall thickness or internal supports would be needed. It should be strong enough to survive being outdoors in wind and direct sunlight and snow for a few years, at least. Can someone with more experience come up with some back-of-the-envelope price estimate? Or tell me if this approach won't work for some reason? (Will this kind of plastic degrade if left outdoors, for instance?) Or if you have a better way to make a sphere of that size, I'd love to hear it. Thanks!

A sphere of this size will run in the \$200 plus range minimum using a thin layer of plastic. But to make it rugged enough for outside use it will need to have a thicker shell and it will most likely need to be coated to protect it from UV, bird poop, hail, baseball bats, and other hazards. If this were going in a sheltered and safe indoor museum it might be cost effective since it could last for decades. For outside use you might consider a place that makes custom sized stainless steel spheres, such as http://www.1worldglobes.com/stainlesssteelspheres.html for example that a quick google search turned up, plus they have standard sizes if you can rescale your installation. Another option might be to find a woodworker that can create custom sized wooden spheres that you can coat with an appropriate sealant.

Volume for a sphere is (4/3)*PI*r^3 so a 0.04 inch thick shelled sphere (that is totally too thin to practically produce but falls near the minimum design rule spec would be about 16.07 cubic inches. That's about 263.41 cubic centimeters. As a first estimate your cost would be that times \$1.40 per cubic centimeter or about \$369. If your overall model density is above 10% you could get a price break, if your model density is below 2.6% the cost is based on bounding box size and will probably be more. And again that's much too thin to get a good result anyway. And of course you should double check my math or upload a simple test case to estimate your true cost.

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edited because I think I was off by a factor of two.

[Updated on: Sun, 03 August 2014 23:35 UTC]

Re: cost estimate for ball? [message #95200 is a reply to message #95195 ] Mon, 04 August 2014 02:19 UTC
OK, thanks very much for your help.

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