A biopsy device comprises a body, a needle, and a cutter. A self-reversing cutter translation mechanism translates the cutter proximally and distally in response to rotation of the cutter in a single rotational direction. The cutter translation mechanism comprises a lead screw with discretely formed threads and a rotatable pawl. The cutter is translated proximally when the pawl is in a first rotational orientation; and distally when the pawl is in a second rotational orientation. Resilient members cause the pawl to be rotated between the first and second rotational orientations when the cutter reaches a distal position and a proximal position. A vacuum draws severed tissue samples through the interior of the cutter to deposit the samples in a holder. The translation mechanism engages a valve mechanism to provide simultaneous venting to part of the needle and reversal of the pawl orientation when the cutter reaches a distal position.