Cutting collectors, need help

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Hello,I'm in the process of building a collector (4 into 1) like shown below:I started off with 1.5" sch10 pvc tube to make sure everything is coming out right before I move on to the stainless steel. (which its not coming out right lol).What I am doing is using a vertical band saw and a sliding mitre guage. I set the guage to 60 degrees, push the tube through the saw, rotate the tube 1/4 of a full rotation (so 90 degrees), then push the tube through again. I then cut this shape off and repeat 4 other times.When I went to test fit it, 3 of them fit together, but the 4th one did not (the space was about half as wide so it would not fit). So my angles are obviously off, but I don't understand why.If you look at the top of a collector you can see that its a cross, which led me to believe all I need to do is rotate the tube by 90 degrees and cut and all 4 should fit together just fine. Does anyone know what I am missing?Also any advice on cutting these easier would be appreaciated, e.g. somekind of a jig you guys made, templates, a process etc.EDIT:Here is how I measured so I know that I am rotating the tube 90 degrees:The outside diameter of the tube is 1.90", so I multiplied that by 3.1415 to get 5.968" (which is the circumference or perimeter).I took that number and divided it by 4 to get 1.492". So what I did was put a mark on the tube, then measure down 1.492" and put another mark. I then lined up the first mark on the mitre guage, pushed through the saw, then rotated the tube to the second mark and pushed through again. I was under the impression that this would be the same as rotating the tube 90 degrees.Last edited by Mahonroy; 09-08-2009 at 04:51 PM.
Reply:Would it be easier to cut one angle on each of the four pipes pipe and weld them together in two groups of two at the correct angle so it resembles a Y. When that's done then cut one angle on each of the  the two groups of joined pipes so you end up with two  'Y' s with the side cut out and then weld those two together to make a collector.Sorry for the funky explanation but it's the best I could come up with.....Mike
Reply:I see what you mean. Is that normally how people make them? I think I just figured out that as the angle of the collector changes (e.g. 60 degrees, 70 degrees, etc.), this changes the angle at which you are supposed to rotate the collector piece for the final cut. So when I wanted a 60 degree collector, and I rotated the piece 90 degrees and made another cut, I really ended up with something like 96 degrees.....hmmmm......
Reply:What tool did you use to measure?My name's not Jim....
Reply:If you used a caliper....try 1.3435" instead.My name's not Jim....
Reply:I used a piece of paper and measured out 1.49" on it, then wrapped it around the tube. It seemed decently accurate, since I did it 4 seperate times in a row and the lines ended up at a starting point, meaning that the 4 rotations do add up to 360 degrees.So trying to do a little math, I figured the two points: (0,180) and (90,90), meaning that the first number (x value) is the angle I want the collectors to be, and the second number is the angle I want to rotate to get the desired 90 degree angle. Rather than fitting a y=ax+b line to fit the data points, I did a sin regression (assuming it would be this since the trig), and used this instead: y=asin(bx+c) and got: y=45cos(2x)+135 (in degrees not radians) which fits those data points exactly. So to use it, lets say I want to do the same 60 degree collector angles, I do y=45*cos(2*60)+135 giving me 112.5. So instead of rotating 90 degrees to get my 90 degree, given the angle 60 degree angle I want to do 112.5 degrees instead. So in that case my last calculation was to move the line down the tube 1.492 to rotate 90 degrees... to rotate 112.5 degrees I would move the line down the tube to 5.968*(112.5/360) to get 1.865" instead. Do you think something like this works instead?
Reply:Sorry, my eyes glassed over after the first paragraph.  I'm thinking I like my CAD.Sometimes I get bored and look to try something different.Express what you want to do in very simple terms and maybe I will draw it up later.  Maybe...Angle the tubes meet at, Diameter of the small end of the collector, etc.My name's not Jim....
Reply:Yeah sorry about that, was up late doing some number crunching.The CAD thing is a good idea, I was going to try that I just haven't used autocad or anything enough to figure out how to make 3d models like this.Basically 4 tubes merging into one. And here is the thing though, the cuts are made at 60 degrees. But if you have something rotated on the X plane by 60 degrees, and on the Y plane by 60 degrees, the absolute angle is going to be something different, like 56 degrees or something, though I guess the CAD program will probably do that automatically. The tubes are 1.90" OD, and when they merge I want to get it to around 2.5" or so (but this part isn't as important, for now they can just merge into nothing, and I can just chop the bottom off)
Reply:Ok got some pictures to illustrate more of what is going on:The first pic shows the 4 collectors cut.The second pic shows that even rotating the tube exactly 90 degrees and re-cutting, does not yield a collector piece that will fit together at a 90 degree.The third pic shows how the 4 pieces fit together - angle is too large preventing the 4th collector piece from fitting.The forth pic shows me cutting the new pieces using the formula I came up with above, its closer to fitting right (in this case the angle is too small now) but still not right. Attached Images
Reply:I did another equation, this time doing a quadratic regression, which gives me y=0.01111*x^2-2x+180, which in this case for 60 degrees it would rotate 100 to get the desired 90 degrees. I'll try that out and see what I get lol