From The Collaborative International Dictionary of English v.0.48:
Planer \Plan"er\, n. 1. One who, or that which, planes; a planing machine; esp., a machine for planing wood or metals. [1913 Webster]
2. (Print.) A wooden block used for forcing down the type in a form, and making the surface even. --Hansard. [1913 Webster]
Planer centers. See under Center. [1913 Webster]