#1




Calculating dimensions for cutting a cone.
I was hoping that someone might have a better sense of geometry than I.
The plan is to cut a sheet of steel to form a truncated cone, rather like an Ice Cream Cone with the bottom cut off, then to roll a cone to form the aft part of the exhaust manifold below, but the mathematics are causing me some difficulty. What is the best method for working out the shape to cut from a sheet. I assume there must be some basic set of calculus to work it out? If it helps, it seems to need to be 20 inches long, 4 inch dia at the widest end and 1 1/3 at the thin end. All rather embarrassing. Am I going Senile???
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Rob Thomas, Cardiff, Wales, UK 
#2




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Kerry Pinkerton 
#3




Personally I'm not very good with formulas so that page didn't do me a lot of good. Here's how I calculate it. 20" is the height of the cone or the length of the long edge. then you figure the circumference of the small and large diameters. Pi times the diameters. So the top edge would be pi times the inch and a third and the bottom edge would be pi times 4 inches. The problem with this is the top and bottom edges have a slight curve to them. Personally I would use these dimensions to get me in the ball park and then I'd form it out of paper with a little extra material for trim. then form the paper cone to the size I needed and trim the edge so it was flat. Then I'd unroll it and trace the pattern onto sheet metal. I realize this is not very scientific. If you can get the formula Kerry gave to work for you that would be a better method as it figures out the curve to the base and top.
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Have a good one, Jawno 
#4




Per's method for taper layout
Hi
take a threaded rod the length of the cone or a little longer. cut two circles the sizes of each diameter. put a hole in each to mount on rod. use nuts & washers and mount disks on rod at desired length for cone. lay on sheet & mark the contact points of the disks. Now mark the start points on the metal. roll the tool a little and mark the end points of the cone. Continue this process until the tool has gone one revolution, then mark the stop point, now connect the dots. Two curves one at each end...... two straight lines at start and stop from end to end. Perfect every time no math required..... even I can do it. Steve
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Steve Hamilton Hamilton Classics Auto Restoration & Metalshaping 
#5




Thanks, guys.
I'd seen that site before when googling but it didn't do anything other than confuse me further. I can't work out what length C1 would be for my planned cone. Jawno. Maybe it is best to take the Pragmatic Aussie view and aim for 'sensible ballpark figure' rather than going for total accuracy.
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Rob Thomas, Cardiff, Wales, UK 
#6




Steve. Our posts must have crossed. Fantastic idea....practical and simple. I like it!
PS Passed through Fond Du Lac in 1989 (aged 19) when delivering a single engine aircraft from Southampton (UK) to Oshkosh.
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Rob Thomas, Cardiff, Wales, UK Last edited by Cardiffrob; 11132013 at 12:43 PM. 
#7




Hi Rob.
Google "Cone Layout" and make a choice. I have bin able to just type in two different length of cone and Di at both ends and it shows you the layout for cones with angles on both ends. You can print it out and lay it on the sheet. Hope this helps. Jere
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Jere Kirkpatrick Valley Forge & Welding HENROB Torch Dealer. Teaching The Fundamentals of Metal Shaping www.jerekirkpatrick.com All tools are a hammer except the chisel.....That's a screwdriver. 
#8




http://books.google.com/books?id=qCm...urnier&f=false
Look at page 36. Ron Fournier's book. I just make a small cone the other day using this method and it came out exactly how I wanted.
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Sean 
#9




Big sheet of paper, calculator, pencil etc etc.
After thinking about the idea that Steve had I realised that the angle of the cone remains constant so I could work out "X" from the distance between the 2 ends and the difference in their radius. Simple, after that. After all the farting about I came to a solution that shows a certain simplicity to the design. The cone is 33 inches long, has 1/3 of it lopped off of the top and then 22.5 degrees of the resultant 33 inch radius cone is used (1/2 of a 45 degree section). The scaling of the photo was probably a little bit off but these new dimensions look close whilst having some 'order and symmetry' to them. So, draw a 33 inch radius circle with an 11 inch circle inside, slice 22.5 degrees from it (1/16th) and add tabs to it as necessary. Transferred the shape to a roll of wallpaper as a template for the cutting of the sheet. Now comes the fun part of rolling it into a cone! Thanks, guys. Rob
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Rob Thomas, Cardiff, Wales, UK 
#10




Cone calcs
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Kym 
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