How to cut large and small round pipes when the saddle mouth is not 45 degrees?

Large and small circular tubes, such as the saddle mouth of oblique mouth, are not 45 degrees. First, you need to draw the center lines of the two pipes.

Weld a point on the center line.

With the center point as the center, draw a square and a diagonal line with the pipe diameter as the side length at the pipe to be cut.

Make the diagonal line consistent with the direction you want to cut, and cut according to this diagonal line.