# Geometry question

I'm trying to figure how to lay out a sheet of metal that I'm going to roll into a cylinder and then weld a bent piece of metal to one end.
Imagine you were to put a metal chimey right through the peak of a roof and were building the flashing. I'm not going to specify diameter or pitch since I'm looking for a general method.
Thanks,
Wes S
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Similar to the "How can I make a pattern to cut a metal pipe at 45 degrees?" thread answer by Randy Zimmerman.
http://jwilson.coe.uga.edu/EMAT6680/Parsons/MVP6690/Essay2/round.html
Surface development using descriptive geometry...
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pitch since I'm looking for a general method. ^^^^^^^^^^^^^^^ The general method is to make a working drawing. In the view showing the end of the round pipe, put in equally spaced radii. Project these into a side view, where you can see their length. Transfer each of these lengths, properly spaced onto a view of the "unrolled" pipe, and you will have a "development" of the piece you are looking for. It's a standard drafting procedure.
The method is general enough that the part the pipe fits to does not have to be a certain shape. It can be another pipe, any size, at some angle if you want, or even a sketched-in shape.
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Evidently Leo took descriptive geometry too! :-)
Worse still, since he remembers the correct terminology, he likely teaches a course in the 'dreaded' descriptive geometry. :-)
Were I you, I'd pay very careful attention to anything the Leo tells you.
Harry C.
Leo Lichtman wrote:

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wrote: (clip) I'm not going to specify diameter or

For extra credit develop the end caps...
I'm sure the AutoCad weenies will demand equal time :)
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Divide the problem into two problems. How to lay out a cylinder that goes through the roof on one side of the peak. And how to do the same on the other side.
Dan snipped-for-privacy@lycos.com wrote:

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The class in engineering school that I most hated was called "descriptive geometry".
Part of that course was learning how to graphically construct flat layout drawings of 3-dimensional objects, precisely what you are trying to do.
The required tools here a a texbook on "descriptive geometry" and a roll of grid paper, plus the time to read a chapter or two in the book. The methods are entirely graphical, and require no math.
Damn, how I hated that course. :-) Still, every 10 years or so I find something that I was taught in the course to be useful.
Harry C.
snipped-for-privacy@lycos.com wrote:

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The hole through a flat piece of flashing or one creased at an equal angle on both sides across the center of the pipe will be an ellipse with a simi-minor axis (r) of the pipe diameter and a simi-major axis of r/Cos(intercept angle).
If the angles are unequal you have to solve for both angles and lay it out as two half ellipses.
--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
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Thank you Rick. I can use the example you gave and life should be fine. Descriptive geometry was one of those classes I wanted to take but at the time I drifted off to the world of computers.
Here is my first attempt to draw it: http://wess.freeshell.org/usenet/roof%20flashing.pdf
Wes S
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Twenty years ago, I'd have done that with out asking anyone. When I saw Rick's link I was somewhat chagrined to find how much I have forgotten from when I took drafting.
Once I could 'see' the problem it wasn't too hard to make a spreadsheet to do the calculations either.
Thanks to all that replied to my question,
Wes S