# developing true length on compound curve?

hi all, Im trying to develop a shape thats a bit complex to me, for fabrication at work. What we have is a large copper furnace, its a cylinder lying on its
side. The cylinder is about 20 feet in diameter. I have to build a mouth to sit on top of it, you could imagine the mouth as being a second piece of pipe, meeting the cyllinder at a 90degree angle. The mouth itself is actually a shape with two straight sides (parallel to the ends of the cyllinder) and two curved sides running parallel to the cyllinder, each curved side with a different radius.
Ive drawn out the cyllinder and the mouth, in plan and in section and the results are showing me what i want, but its not really what i want. The purpose of this drawing is to allow me to make dimensions for making the two curved pieces. What i really want, is to be able to 'unroll' the curved piece so its flat, then i can take dimensions off it at will. Imagine, more simply if you like that im just joining two pipes together, and the second pipe is joining into the middle of the first one. what i really want is to be able to slit that second pipe down the middle and unroll it so its now a piece of flat sheet. any clues how to do this? Im sure there is something simple i can do here that im missing.
the alternative, is looking at the overhead view, taking lines every foot or so and drawing them out onto the front and side views then taking individual measurements.... but this really only gives me true lengths while the piece is *curved*. Which means the sheet will need to be rolled first, then measurements projected onto it from a hypotehtical flat line, then the ellipse drawn, then cut. In an ideal world i would be able to calculate that ellipse first, mark it out onto a flat sheet, cut it, then put it through a roller and it would all fit up nicely. The sheet of steel is about 2" thick which is why it would be easier to cut ot before rolling.
Any and all advice welcome! Shaun Australia
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You need something like:
http://www.plate-n-sheet.com /
http://www.tyharness.co.uk/sheetmetal/#100
http://www.litio3d.com.ar/index.html
snipped-for-privacy@bigpond.com (Shaun Van Poecke) wrote:

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wrote:

A while back I used a free utility called unwrap that, although crude, did work for this kind of thing. Maybe it's still out there somewhere.
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Here's a page taken from a technical manual explaining how to do the basics. It's in french, but I'm sure you can figure it out from the graphics.
http://www.box.net/files#0:t:104368
BTW, I've never tried to post a link from there, so it might not work. I'll check back to see how it turns out.
Dr Fleau
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Ok. Need to register to Box.net for it to work. Sorry. I'll try to figure something out.
Dr Fleau
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Take 2
http://www.box.net/shared/tccvlpxs98
Dr Fleau
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Thanks for the reply, Looking at it, this looks pretty much like what i ended up doing, which is the same way i would have approached it if i were drawing it manually. It ended up taking about half a day though. This way of doing things, while accurate is hardly quick, or elegant though :-(
I drew out a top and side view, then a front and back view to the left and right of the side view. i then took a series of xlines at 100mm intervals horizontally from the top view and took them across and down to the front back views, then created vertical xlines down to the sideview and across again to the front and back views. than i could draw a manual line at each 'crossing point' on the xlines. this gave me height at 100mm intervals, but still not a true length as the pipe was still 'curved', if you like.
Next i took the top vew and broke the curve at every xline.... by then measuring the arc length of each of the curved sections i could set up a spline line and bring the vertical lengths across to it. What it means in reality though was that i had to measure each one manually, then re-create this arc length on-the-flat with an offset command. Sound tedious enough? yeah, it was. at least i only had to do half the curve, because i could mirror the other half. Given that it was about 3 metres wide, this ammounted to 15 measurements. I had to work out two developments since the object is straight on the sides with a different radius at the front and the back, so i did one development for the front and one for the back. One less than pretty thing about the final development is that the reference lines ended up being uneven once it was 'rolled out'.... sincei took them at even 100mm intervals on the curved section, when on the flat they started out at about 110mm aprt at the outside (but all odd numbers, like 109.7584) and ended up at 100mm in the center.
If you're interested, i can send you a copy of the drawing, i also have a self executing solidworks 3D model of the object with an inbuilt viewer if you'd like to take a look, but its biggish (about 2.5meg).
I'd be interested in hearing any improvements on the way i do this, even if it is just a small improvement over the xlines, then cut the arc and manually measure, then offsets technique. While i dont have to do developments like this very often, it would be nice to find a more automatic (and hopefeully less prone to human error) way of doing it.
Thanks, Shaun

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