When Does a Plane Sole Need Flattening?
I have several old cast-iron planes. I flattened their soles when I first got them, but wonder if they will ever need to be reflattened? If so, how often?
—Elijah Evans, Columbia, SC
The only way you know a sole needs to be reflattened is to check it with a straightedge. Lay a steel ruler across the sole heel to toe, then side to side, and check for gaps. How flat the sole needs to be depends on the accuracy you expect from the plane. And where the sole is flat depends on what the plane is used for.
From Fine Woodworking #201
Drawing by Kelly Dunton.