a transmission for changing rotary motion into angularly reciprocating motion and adapted to be mounted on the output end of a dental power unit of the type having a rotary drive output and used to drive a dental tool . the transmission is comprised of a support member , a driving resinous member supported by the support member for rotary motion , and a driven resinous member supported by the support member for reciprocating angular movement by the support member . the transmission subassembly is formed by the driving resinous member , driven resinous member and support member and adapted to be mounted on the output end of the dental power unit with a rotary drive output mechanically coupled to the driving resinous member . a driving cam surface is disposed on a portion of the driving resinous member . a driven cam surface is disposed on a portion of the driven resinous member . the driving cam surface is in contact with the driven cam surface during at least a portion of the cycle of rotation of the driving cam surface . the driven cam surface is configured and dimensioned to be driven by the driving cam surface in a positive angular direction during one part of the cycle and is driven by the driving cam surface in a negative angular direction during another part of the cycle . angularly reciprocating motion is thereby imparted to the dental tool as the driving cam surface and driven cam surface engage each other .