An outer member (160) of a surgical instrument (10) defining a cutting window (166). A cutting member (150) being at least partially positioned within the outer member (160). The drive member (130) being coupled to the cutting member (150) such that in response to only a rotational force applied to the drive member (130) in a single direction the cutting member (150) rotates and moves linearly in a back and forth motion. The drive member (130) having a continuous helical groove including a first helical channel (136) and a second helical channel (138) that are blended at their ends. The first helical channel (136) having a first helical angle that causes the cutting member (150) to move linearly in a first direction at a first linear speed and the second helical channel (138) having a second helical angle that is smaller than the first helical angle that causes the cutting member (150) to move linearly in a second opposing direction at a second linear speed that is slower than the first linear speed.
