Abstract: The present invention relates to an adaptor for converting a rotary power tool with a driving bit to a rotary cutter. The adaptor is particularly useful for making a conventional rotary screw gun into a gypsum wall board cutter. On one end of the adaptor is a rotary cutting bit, while a flexible open-ended bulb is fitted on the other end of the adaptor to receive the front end of the screw gun from which the screw driving bit extends. Between the bulb and the rotary cutting bit is a casing for housing a power transmission mechanism which transfers torque from the screw driving bit to the rotary cutting bit. The power transmission mechanism includes a plurality of rotatable gears and shafts to enable the cutting bit to turn at a speed preferably six times greater than that of the screw driving bit of the screw gun. The adaptor is operatively engaged with and disengaged from the screw gun without clamping or unclamping the adaptor to the screw gun.