Complex sentence: A surface grinder has a "head" which is lowered to a workpiece which is moved back and forth under the grinding wheel on a table that typically has a controllable permanent magnet for use with magnetic stock but can have a vacuum chuck or other fixturing means.
Split into simpler sentences: A surface grinder has a "head" which is lowered to the workpiece which is moved back and forth under the grinding wheel on a table that typically has a controllable permanent magnet for use with magnetic stock. The table is typically magnetic, but can have a vacuum chuck or other fixturing means.