Compass plane
Also called: circular plane
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A compass plane is a plane built to smooth curved surfaces rather than flat ones: much like an ordinary smoothing plane, but with a sole, the flat underside that rides on the wood, that is curved rather than flat.
How the curve works
Nicholson (1832) describes a compass plane as similar to the smoothing plane in size and shape, but with a sole that curves along the plane's length rather than staying flat, and on that particular plane the sole was convex, bulging outward. A plane built this way is used to work a concave surface: a convex sole nests into a hollow curve where a flat sole would only touch it at one point. Later writers describe the relationship differently. According to Sato (1987), a compass plane's sole can be made either concave or convex depending on the individual tool, so which kind of curve it will smooth depends on which plane is in hand. Bird (2004) describes a different design again: a compass plane with a thin, flexible steel sole that flexes to conform to a surface's contour as it works, rather than being fixed to one curve at all.
Where it's used
Compass planes were once made of beech wood and used to shape the concave faces of ribs, curved structural or decorative members, an older practice from before metal-bodied planes became common. A soffit, the underside surface of an arch or doorhead, can be planed true to its marked lines using a compass plane or a similar tool. In stair handrail-making, a handrailer's compass plane, now rarely seen, was sometimes needed to correct handrail work that had not been cut accurately in the first place.
What it does well
A compass plane works best on true arcs or circles, and less well on free-form curves that do not hold to a constant radius. Even so, a circular plane can shape both concave and convex forms more accurately than some other method of doing the same work.