Abstract: A dental tool assembly having a driving mechanism with interengaging camming surfaces shaped for continuous contact during use. The mechanism includes a drive shaft having a drive surface and a driven shaft having a driven surface. The cam surfaces are shaped and the shafts are transversely juxtaposed with respect to each other such that the drive surface and driven surface are in continuous contact during rotation of the drive shaft to impart oscillatory rotation to the drive shaft. Specifically, the drive surface is conical and preferably has a cone axis that is at an approximately 45.degree. angle with respect to the rotation axis of the drive shaft. The driven surface has alternating hills and valleys that are shaped for continuous engagement with the drive surface as the drive shaft rotates. Preferably a pair of juxtaposed valleys are provided with juxtaposed hills formed therebetween.