hi all, I'm trying to make a 3d funnel shape out of 2d lexan, and I'm having trouble figuring out the flat shapes I need to cut in order to have them mate correctly in a funnel shape. Anyone know a formula or any advice? I have the inside and outside diameters and the height to work with...
you wont without heating the lexan, unless you segment it. build a cone and a cylinder and chop the tip so that it sits on the other or circle and heat the centre, then push it down forming the funnel. http://mathworld.wolfram.com/Funnel.html probably wont help you.
Can you draw up a quick sketch of exactly what you want? A normal cone is very easy to do, you just have to match up the parts of the 2D figure with the 3D model. Your 2D figure is going to be a circle segment, the radius of which will the the hypotenuse of the the cone and the arc length of which will become the circumference of the base of the cone. Once you make this association you can get all the rest is real easy. You can work the height into this using the Pythagorean theorem (leg^2+leg^2=hypot^2) and the radius with circumference=pi*diameter. Hope this helps:
Aaah, M$ Paint skillz to teh res-Q! Hopefully this will help elaborate on what zackbass wrote. I'm a very visual learner (read: story problems were never my strong suit) Ok, to expand on what zack said: 1. Figure out what the radius of both ends of the funnel will be (r3 and r4). Calculate the circumference of each of these: circumference = 2 * radius * pi 2. Figure out the height of the overall cone and the subtracted cone from your own drawings. Use the pythagorean theorum to get the length of the hypotenuse like zack said. The longer hyp (r1) will be the radius of the larger circle and the hyp of the smaller (r2) will be the radius of the inner circle. 3. Calculate the angle of the arc (theta: θ) (arc 1 or arc 2) using: length = θ * radius * (pi/180). Solve for θ. 4. Draft a circle with radius r1 and r2 from the same origin. Mark off a radial line from center to edge. Measure the angle θ and mark another radial line. That will give you the shape you need to get your funnel. I would seriously recommend getting with a wood shop or someone with a wood turning lathe and have them make you a blank to wrap the funnel around while forming the plexi. Go to an ice cream shop that makes waffle cones and watch the procedure to see what I mean, or check out the vesper-deco HTPC that was featured here last year. Any wood that isn't covered with pitch/tar will work. Just make sure that the diameters of the blank take into account the thickness of the plexi/acrylic you're using to get the right final size. Putting a handle on it will prolly help too as the plexi will be fairly hot. If you don't need a perfect seam where the ends meet, you can cut a slot/kerf in the side of the blank and cut the arc a little bit longer than needed. If it has to be a perfect seam, then just clamp a strip of wood over the starting end of the plexi.
Ahh, cones are easy! I need an inverted funnel, or curved cone, with concave sides. The smaller (top) end is 6.5" in diam., the outer rim is 18.125" in diam, and the sides need to have a sweeping curve up to the top. I'm actually redoing the saucer section of my USS Reliant mod, this is the "bulge" on the saucer. And the lexan i'm using is thin enough to bend to pretty much any curve I want, so segmenting is not necessary. Thanks guys.
Ahh, just starting to read the link that Cabe left. LOL been a while since I flunked college calc... I need to figure out how to calculate the shape of each segment so that when I bend them and glue them together, the shape comes out correctly...
Perhaps instead of using lexan, you could make a mold and create the section out of clear resin? It would be allot easier and come out as clear as lexan.
Although no where near as strong (assuming you meant a clear epoxy casting resin, anything better isn't readily available) - I'm not sure what exactly he's using it for, but it could be supporting some weight.
I'm using it for this: http://forums.bit-tech.net/showthread.php?t=87526&highlight=uss+reliant It will be made from thin, flexible lexan pie-wedge-shaped bits. 16 segments per circle. It just has to be strong enough to take a few knocks, like any other case, but the dvd drive and harddrive will be inside it, so it has to be hollow. The first one I made wasn't right, due to my incorrect shaping of the pieces. Just trying to get the math right, so that the edges of each segment line up in the proper curve, like making a map projection.
I'd think you'd want to form that out of a circular blank of plastic. First I'd form over a large sphere to create the top dome and then form it over a less steep cone to form the variable taper. Assuming Lexan forms like sheetmetal, its going to be hard to maintain a consistent thickness in the material and the optics of it may get wacky.
