Welding Tips and Tricks

Hello everyone, I'm trying to wrap my head around what mistake I made while trying to do a 45 degree miter on a piece of 3" sch 40 pipe. (3.5" actual O.D.). I would usually throw a piece of pipe like this in a band saw but do not have one at my disposal, so I opened up my blue book and used the chart found in the book to layout my measurements.

I indexed the pipe in quarters as recommended for 3" pipe in the book and on the 1's I made a mark at 1.75". Everything looked good albeit curved, this is not my wheelhouse so I went with it. I got the cutoff wheel out and followed my lines as best I could. When all was said and done the pipe did not fit together even close. I could have done a better job eyeballing it, but I wanted to be as precise as possible.

My questions is, did I do something wrong? I used a wrap to connect the quadrants and thought that was my problem, then I grabbed a straight edge and connected another mocked up pipe and found the same curves as when I used the flexible wrap. I went through the formula and plugged in my O.D. and got the same measurement provided by the chart in my book so I don't think that was the problem either. Any pipe fitters here that can offer me a bit of advice? I will attach some pictures to help better illustrate my dilemma. Thanks in advance for any advice guys, stay safe.

Looks like you used the measurements for a saddle but were off by 180 deg.

If you want a 90 degree miter, I think you need to remove a 90 degree triangle which is 45 degrees on each side. IOW, it is a 90 degree triangle, but needs to be turned with the 90 degree corner facing perpendicular to the side of the pipe so that the 45 degree sides are equal. Someone correct me if I'm wrong.

How do your cuts get so rounded when using a cut off wheel on a grinder ?

Stevie wrote:so I opened up my blue book and used the chart found in the book to layout my measurements.

I love analyzing geometrical problems, but I don't have access to the procedure you mentioned. Do you know of that procedure is posted somewhere so I may look at it? I mean I can construct my own procedure in my head since I already know what the cross section shape is, but I don't want to complicate things.

I would like to see the document you were reading but just thinking about it, I picture wrapping a piece of paper around the pipe and if I had a perfect line on the piece of paper and then I unrolled it starting from the Midway point halfway between the top or long Edge and the short Edge it would be two reverse ellipses, almost like a sine wave but different.
Sorry for the grammar I'm talking to text

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I appreciate all the responses. After sleeping on it and doing some thinking I have it figured out. I am going to take some pictures and expand on this tomorrow.

The short version is that I was connecting my layout lines incorrectly.

The blue book procedure is basically this:
1. Make perpendicular wrap around line on pipe
2. index pipe in quarters
3. top and bottom are 0, front and rear are 1.
4. measure 1.75" out each way on the 1 lines
5. Connect the dots.

Step 5 is where I messed up, I put the wrap around flat against the pipe and that's where I got the S curve from. I picked it up off the pipe to make a smoother transition across the line instead of into it and when eyeballing it it has that straight across miter line that it should have. Now I just need to cut it tomorrow with the cutoff and see how well she lines up. I'm making a rocket stove.

I have to do my layout two or three times for a saddle or an offset before I remember the correct way, finally get it dialed in then I’m good for about 30 days before I forget.