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Author Topic: Can't find the exact center of a styrofoam hemisphere (problem)  (Read 47 times)

Offline homyakchik

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Hey, all

 While browsing Hobby Lobby for something else, I lit on some Styrofoam shapes that cried out "make a model of me!" to me. Weak-willed fool that I am, I snagged some stuff.

 Plans right now are to make the first ship either a 1/72 (or close to that) version of the Good Hope, the first starship that came into Perry Rhodan's hands. Alternatively, I make construct an early-generation Torchship from the Heinleinverse. Either would be good (whichever one I don't do this time I'll do next). I already figured out how to work with them without melting them; a coat or two of Elmer's Glue seals (and smooths) the outside, and I can even use Bondo for a coat over that (to my amazement, it doesn't melt anything through that glue layer) and sand it. All well and good.

 Now I'm looking at a way to attach styrene to it correctly. Fitting two of the half-globes together gives me my sphere--but I cannot for the life of me figure out the exact pole (North or South) from which I could center and place things.

 What I need: a method of determining the pole on a Styrofoam hemisphere (or whole sphere). If I place something exactly circular (say, half of a Pegasus UFO), I don't know whether I've centered it or not. I thought about measuring on the inside from any of the fitting parts (and just pushing a pin through), but the string I use to measure with wanders a bit no matter what I do. Same for trying it on the outside; if I draw my measuring string too tightly, it has a tendency to wander down the curved slope, leaving me with a suspect mark when I cross a couple of them.

 If anyone else has ever worked with a sphere (of anything) in a scratchbuild, maybe you have a technique that you used to do this. Anyone have any notions about how I can measure to the exact center/pole of a half-sphere (or the whole thing) without having measures go awry? I'd really appreciate any suggestions.

 Thanks!

 Davey

Offline starsiegeplayer

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Re: Can't find the exact center of a styrofoam hemisphere (problem)
« Reply #1 on: August 02, 2018, 12:08:39 am »
Davey, do you have a caliper?  If not, they are not too expensive.

Get the diameter of your hemisphere.  divide that by the square root of two.  Set the caliper to that new number. 

The true pole of your hemisphere will be the point that you can lay the tip of one jaw of the caliper on and have the other jaw tip just touch the equator for multiple points on the equator.
« Last Edit: August 02, 2018, 12:10:14 am by starsiegeplayer »

Offline starsiegeplayer

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Re: Can't find the exact center of a styrofoam hemisphere (problem)
« Reply #2 on: August 02, 2018, 12:22:40 am »
You could also cut a couple strings to length

 π * (diameter) / 2

These strings will cross the pole if laid from one side of the equator, over the top, and back down to the opposite side of the equator ( exactly, with no slack)
« Last Edit: August 02, 2018, 12:28:34 am by starsiegeplayer »

 




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