What is a shoulder plane?
What it's used for
The shoulder plane is a quick and convenient aid to trimming and "tidying up" the parts of some woodworking joints.
It is designed mainly for trimming the "shoulders" and faces of the tenons in mortise and tenon joints.
What is a mortise and tenon joint?
A mortise and tenon joint comprises two components: the mortise (a rectangular hole or recess) in one piece of wood and the tenon (the tongue) on a second piece.
The tenon, formed on the end of a length of wood generally referred to as the rail, is inserted into the mortise of the other piece.
The tenon is narrower than the rest of the rail, and has "shoulders" that seat when it fully enters the mortise. The shoulders need to be perfectly flat and "true" to butt up perfectly to the second piece.
A shoulder plane is used when it is necessary to trim right into the internal corners of these shoulders, and other situations where two surfaces of the same piece of wood meet at 90°.
The iron of shoulder plane is usually bedded at a low angle, which is ideal for cutting end grain.
Like a rebate plane, the shoulder plane's blade extends to the full width of the tool so it can cut right into the corners formed by the shoulder and face of a tenon.
Metal versions of the shoulder plane made by Stanley were allocated Stanley model numbers, some of which are still used today when cataloguing, advertising and ordering planes.
Planes based on the Stanley No. 92 are still quite widely available.