making large domes- simple template file

[Kenny]

Does any one have any tricks to make smooth domes from sheet styrene? Perhaps a template program like the cone maker?

Looking at 30-40cm (12-16inches)

[karim]

Not styrene, but how about this:

Get a spherical balloon and inflate it to the desired diameter.

Mask off a circle around the balloon to mark the desired bottom edge of the dome.

Pour resin over the balloon until you get a thin, even coat.

Allow to harden.

Repeat until desired thickness is achieved.

Pop Balloon.

Cut resin at Masking tape.

[Kenny]

Not styrene, but how about this:
Pour resin over the balloon until you get a thin, even coat.

Not a bad idea, I thought of a paper mache form of this, and I will keep in mind but I would prefer the styrene for lower cost

[Andrew Gorman]

For flattish domes, check out clock "crystals" at the office supply store, etc. You'll have to buy the whole clock, but they're cheap. You can also take thin plexiglas/acrylic and put it in a frame like you would for plug molding. Blast it with a heat gun( or prop it up in an oven) and just let it sag into shape. Hemispheres that diameter are pretty widely available for covering surveillance cameras. What are you making?
Andrew

[rocketrider]

one other source

http://www.plexiglasdomes.com
Glen

[DX-SFX]

You can't make a dome (hemisphere or a part thereof) from a flat sheet. It curves in two directions. The sheet would have to be heat formed or some other moulding process used.

[Kenny]

For flattish domes, check out clock "crystals" at the office supply store, etc. You'll have to buy the whole clock, but they're cheap. You can also take thin plexiglas/acrylic and put it in a frame like you would for plug molding. Blast it with a heat gun( or prop it up in an oven) and just let it sag into shape. Hemispheres that diameter are pretty widely available for covering surveillance cameras. What are you making?
Andrew

Cool, more good ideas. That is probably the easiest way, though my rig probably won't be spherical enough. But using a balloon as a plug might work...

Can't tell what as it's for the wrecks competition.

[Kenny]

You can't make a dome (hemisphere or a part thereof) from a flat sheet. It curves in two directions. The sheet would have to be heat formed or some other moulding process used.

Yeah, I was sure I had seen a template like someone would use for a paper model and then with some putty and sanding get a pretty close smooth curve.

thanks rocketrider for the link, but too much out of my price range with international postage.


this is kind of what I was looking for, but it doesn't allow me any control of the curve

http://www.korthalsaltes.com/globe.html

Anyone seen anything similar like the cone calculator?[/img]

[Wug]

Hi,

With a heat source and some plywood , you can blow-mold sheet plastic into domes.

Here's how Darryl Starbird does it in 1/1 scale

http://aeclassic.com/bubbletop/index.html

HTH

Mike

[DX-SFX]

Why don't use press mould one. Carve the shape in balsa of something similarly easy to carve. Pin a sheet of styrene to a piece of plywood with a suitable hole cut in it and then heat the sheet over a cooker. When the plastic softens, push your balsa pattern into the plastic and through the hole from the plastic side.

[Chacal]

With thermoforming methods, you can also do straight-up blow molding. get yout styrene sheet between two frames with round holes (diameter of the holes = diameter of the dome), heat it up until if softens and, like vacuforming, but the other way around, blow and inflate the dome out (you'll need the equivalent to a low-tech vacuformer: just a box with an open top, a seal to keep the air from escaping between the frame and the box and an air entrance for you to blow into – a thin hose will do fine).

You have to keep a steady pressure until the plastic cools, so the dome will be the shape you want. Ideally, the whole shebang should be assembled upside-down, so gravity would assist in the dome-forming, but that would be more trouble than its worth.

[Kenny]

With thermoforming methods, you can also do straight-up blow molding. get yout styrene sheet between two frames with round holes (diameter of the holes = diameter of the dome), heat it up until if softens and, like vacuforming, but the other way around, blow and inflate the dome out (you'll need the equivalent to a low-tech vacuformer: just a box with an open top, a seal to keep the air from escaping between the frame and the box and an air entrance for you to blow into – a thin hose will do fine).

You have to keep a steady pressure until the plastic cools, so the dome will be the shape you want. Ideally, the whole shebang should be assembled upside-down, so gravity would assist in the dome-forming, but that would be more trouble than its worth.

That sounds good sense. I have also managed to figure out how to make a template and will post it soon (maybe tomorrow). Will see how that works and then if it doesn't, blow forming time

[irishtrek]

You could just go to Home Depot or some place that sells lighting supplies and get a dome already made the correct size that you want.

[Kenny]

As I always try to build on a budget I present to you guys plan A (draft form)

And welcome comments and edits.

With a lot of looking and a bit of thinking I have devised this template for the production of paper/styrene domes without the need of any equipment

Please take a look at:

http://i107.photobucket.com/albums/m283 ... mplate.jpg

I have managed to leave off a few words and as I had to do this at work will not reedit it first.

How does this look and anyone outhere really good at maths that can confirm my theory

[Chacal]

That'll give you an ellipsoidal dome (a hemi-ellipsoidal dome actually). For a true spheroidal dome (a section of a sphere) you must cut the lenght of each "slice" (keeping the tip) to the desired size, then attach them by their (bases) making a cone pattern-like "fan".

OR

you can do it the other way around, having a flat center circle and the "slices" radiating outward. This way, you just have to pare them to size.

[Kenny]

That'll give you an ellipsoidal dome (a hemi-ellipsoidal dome actually). For a true spheroidal dome (a section of a sphere) you must cut the lenght of each "slice" (keeping the tip) to the desired size, then attach them by their (bases) making a cone pattern-like "fan".

Giving me a headache now. How will it be ellipsoidal if all the members are the same length?

Do you suggest more along the lines of cutting off extra latitudinal sections? (the ones marked for score lines)

[Chacal]

If you shorten each of the "slices", you are effectively shortening the height of the dome, while keeping the diameter. That's one of the definitions of an ellipsoid: take a sphere and shorten it in one (or more) directions. If you shorten it in one direction you'll have a "pill" type of ellipsoid (think plain M&Ms). That's what you're going to get.

Of course, if you shorten each of the slices too much, you'll end up with something that won't even close up on the center.

When I have some extra time, I'll post a pic of some of the ways to build a dome with flat sections.

[Kenny]

If you shorten each of the "slices", you are effectively shortening the height of the dome, while keeping the diameter. That's one of the definitions of an ellipsoid: take a sphere and shorten it in one (or more) directions. If you shorten it in one direction you'll have a "pill" type of ellipsoid (think plain M&Ms). That's what you're going to get.

Hmmm, if I understand your better visualization of the maths, that is kind of what I wanted to go for, a half M&M. So it looks like a slice of the top of a globe

Of course, if you shorten each of the slices too much, you'll end up with something that won't even close up on the center.

Trying to figure out how much was too short gave me the most headache

When I have some extra time, I'll post a pic of some of the ways to build a dome with flat sections.

Thank you. I have put my template on to what would have other wise been a waste scrap of styrene so test time soon to see what it does. I look forward to your pics and thank you.

[Darth Humorous]

A plain "m&m" is in a shape that can be regarded as an oblate spheroid. It is NOT spherical at all. True, there are elliptical domes, but I get the idea that is not really what you are after. What you are after is sometimes called a "spherical cap", or more traditionally a "spherical segment". In your case, it will not be a complete hemisphere, but it would be spherical, nonetheless. The heat-and-sag method or "ballooning" method variant does not lend itself well to the formation of a spherical cap that is less than a full hemisphere. You could make a full hemisphere, and later cut the desired spherical cap from it. The drawback to doing that is you will need a larger sheet of plastic, a (possibly) larger heat source…in effect, larger everything. If your spherical cap is not much smaller than a full hemisphere, then no probs. Otherwise, the bigness could get out of hand.

Vacuum forming over a mold is a better way for sheet plastic, or the scree method over a mold can be used if you are willing to use something like an epoxy putty, although I don't think the latter will help if you are wanting transparent domes.

FYI, one way to viscerally see the geometry of the oblate spheroid shape is to draw an ellipse on card stock or sheet plastic, cut it out, affix an axle like a thick wire or thin rod or tube right along the short axis. Make sure this axle extends beyond the boundary of the ellipse on either side. Now drill a hole vertically in a scrap of wood about the diameter of the axle, but as a loose fit for the axle, and not as deep as the axle extension is long (per one side of the ellipse). Place one end of the axle into this hole, and upon doing, it should be standing vertically with the ellipse oriented wider than tall. If you didn't drill too deep, the ellipse edge should not rest on the wood. And if the fit of the axle is loose enough, you will now be able to grasp the opposite upper end of the axle extension between your thumb and a fingers, and twirl it around. the resulting 3-D shape you see from doing this is the oblate spheroid shape.

A complementary shape to the oblate spheroid is the prolate spheroid, wherein the ellipse is oriented taller than wide. As you might surmise, the above cinema demo can be done for this shape by simply affixing an axle right along the long axis. Everything else is the same.

Knowing what your subject matter is might allow us to know just what shape you are truly after.

Mark

[Ti Raven]

Try checking out the local thrift stores. I've gotten some amazing stuff there. The housewares and the hardware areas are very fertile hunting grounds. The http://www.plexiglasdomes.com/ site is very good for virgin stuff. This is if your are not wanting to do the "I made it all from Scratch" route.

[Kenny]

This is the shape I was going for and using my template.

http://i107.photobucket.com/albums/m283 ... age008.jpg

as you can see, considerable filler was needed to make the joins smooth, though that may be down to the inexact lines that were cut as the template was transfered by printing on paper and glued rather than printed direct.

how does that match up with the geometrical terms used and now seeing the shape (might have liked it to be a bit bigger circumference) any ideas on how to do that as a cutout?

[Darth Humorous]

Kenny,

Your shape looks to have a spherical contour, and I would call it a spherical cap. Hard to tell, but it looks a little shy of a full hemisphere.

Since you are using a putty, the scree or screed method would have worked nicely and accurately.

Essentially, the idea of a scree is to cut out the desired arc from a stiff sheet of material, keeping the "negative" if you will, attach the top/center of this sheet to a tube, then slide the tube over a vertically mounted rod, and rotate this stiff sheet-and-tube assembly about the rod over a large blob of soft putty, plaster, or what-have-you. By rotating the sheet, it scrapes away the putty down to the level of the scree, leaving the dome shape you want, accurately.

This can be done for a mold to vacuum form over, or for the actual dome, though for vacuum forming, it is best for the form to be solid. To make the actual dome this way, you can either do what you have already done and apply the scree method to form the exterior, or you could have screed a slightly smaller, solid dome form first, and then scree the final dome over that, using a release agent between the two such as vaseline. When the second one cures, you can pull it off the first one and you have a dome shell.

Hope this makes sense.

Mark

[Ti Raven]

Cool!

[Chacal]

You're using too few slices. By the looks of your first try, you used 12 slices, right? That's waaay too few. I'd suggest having your slices at most 3/4 of an inch wide at the base for your intended diameter (12 to 16").

Let's walk through this example: For a 12" diameter, you'll have a perimeter of 37,7" (12" X Pi) divide that by 3/4" and you'll have about 50. That means you should have about 50 slices. As that is not a very nice number to divide, go with 48, which will give you 25/32" wide slices. Each one will be (12" X Pi) / 4, or 9.42".

The overall shape of each slice will be the same, but you should plug the values calculated to give you the real size of each one.

I'll post here, on another post an image of the shape of the slice and how to calculate it. Wait up.

[Mr. Badwrench]

Screed
How it is used:
One
Two
Three
Four
Five
With a coat of paint
From the side
The pink stuff is just foam, I didn't want to fill the whole thing in with Aves.

[Chacal]

Yo, Kenny, if you are adamant about the "slice" method (for full hemispheres), here are the maths behind it:

First: get the size of dome you want – diameter = D. Your dome, as an hemisphere will then be D inches in diameter and D/2 inches high.

Second: decide on how many slices you are going to use. The more slices you use, the smoother the dome will be, but if you end up with slices 1/8" wide at the base of a 15" diameter dome, you will have A LOT of work to cut and glue them and you will compound any errors in the drafting/marking/cutting process. The width of each slice will depend on the diameter of the dome (if you are building a 10-foot dome, a slice a foot wide will work fine – if your dome is 2 inches in diameter, a slice of a quarter of an inch is just dandy). A good rule of thumb: always more than 24 and less than 60 slices.

Third: after you decide the diameter and number of slices, the math comes in. This [url=http://www.marcelo_glenadel.kit.net/images/Slice.gif]image[/url] shows the math in place. For that you need a calculator with trig functions, as you will use cosines heavily. Tha basic idea is: the width of the slice at any given point will be equal to the perimeter of the base of the dome divided by the number of slices, multiplied by the cosine of the angle. The angle is from zero to 90 degrees, with 90 being the apex of the dome. So, at a point halfway up the side of the dome (a "latitude" of 45º), the slice will have cos(45º) times the width of the base of the slice. The height of the slice will be one quarter of the circumference of the full spherical dome, or Pi times D divided by 4.

You then draft a "slice" and use it as a guide to cut the plastic sheet into as many slices as you need.

If you want a section of a dome (say from 30º "latitude" up), draft a whole slice, then cut at the 30º mark and discard the rest. Glue the "half slices" at the sides of their base (or rest them against a template of the diameter of the dome (which will be the diameter of the original spherical – or rather hemispherical – dome times cos(30º)).

It is also a good idea to use jigs to align the slices: make a few arcs out of plastic sheet with the radius of the dome minus the thickness of the plastic sheet. When you lay the slice against the arc, it will conform to its desired shape to be glued to the next slice.