A powered medical handpiece including an oscillatingly driveable chuck, and a blade fixable on the chuck and oscillating driveable thereby at a small amplitude for cutting. The chuck comprises a platform, a holddown screw having an enlarged head surrounded by a frictionally engaged cup washer and a shank structure threaded down into a central portion of the platform. A set screw limits the extent of unthreading of the screw from the platform. The blade is plate-like and has a forward cutting portion and a rearward mounting portion. The rearward mounting portion comprises a key hole having locking structure at the small diameter end thereof for resisting unthreading of the screw from a tightened position clamping the blade atop the platform. The blade further has drive openings flanking the narrow portion of the key hole for receiving upstanding drive pins and guide pins fixed on the platform. A cap preferably overlies the chuck and rear portion of a blade fixed thereon, for example to act as a vacuum hood for suctioning of chips created by a cutting operation.