Originally Posted by Doug Fattic

A mistake non-framebuilders make is assuming bicycle tubes are straight. They seldom are straight. In my frame building classes I have my students roll the tubes on the alignment table to find and mark where the bend in the tubes are so the miters will place the curve in the plane of the frame. It is common for them to be as much as 1/8" out. The reason they are not more accurate is because they would cost a lot more to make.