Lay Out a Pipe Saddle Cut With a String Compass

The Problem

Joining round pipe to round pipe means the end of one pipe has to be scalloped — a saddle, or fishmouth — to wrap the other. A notching tool does it in seconds, but without one, most people cut the end square and try to weld across a gap that opens to an inch at the sides. That's a weak joint and a lot of filler.

The Technique

This fabricator lays the curve out with nothing but a measurement and a string:

1. Mark the centerline of the joint on the pipe being coped — where the center of the crossing pipe will land.
2. From that center mark, measure down the pipe a distance equal to the full diameter of the crossing pipe, and mark it.
3. Cut a piece of string to that length. Anchor one end at the center mark, tie the other to a marker, and swing an arc — the string works as a compass, tracing the saddle curve around the pipe.
4. Cut along the traced line with a grinder and clean up the edges.

The coped end now nests over the crossing pipe with a tight, consistent gap all the way around — a joint you can weld properly instead of bridging open space.