Here is a Y pipe all welded up ready to cut to the appropriate inlet size.

After the layout die dries, it goes to the surface plate to get marked for the cut.  This one is for a 3" system so it gets cut so the opening is about 3.40" across to make up for the ovalizing mating 3" tube.   Fitment is critical for nice even welds.


After the cut is made I clean up the face of the saw cut and inside of the tubing to get rid of the metal dust from the cutting operation.