Abstract: A pneumatic actuator having a cap-shaped diaphragm cam rigidly attached to a resilient diaphragm retained in an actuator housing. The diaphragm cam is provided with opposed drive slots cut through the wall thereof, and with at least one correction slot cut through the wall at a bottom edge thereof. A drive pin coupled to a drive shaft is rotatably mounted with the drive slot such that a vertical movement of the diaphragm cam produces a rotational movement of the drive pin and the drive shaft. A correction cam having a shape corresponding to that of the correction slot is formed in the actuator housing beneath the correction slot and is firmly seated within the correction slot upon descent of the diaphragm cam to correct any rotational movement of the diaphragm cam.