The present invention relates to a mounting structure for securely and accurately positioning an object on a rotatable shaft. In the preferred embodiment, the mounting structure is utilized to position a rotary bell on a rotatable shaft of a rotary atomizer coater. More specifically, the mounting structure includes a rotatable shaft member having an axis and adapted to be connected to a rotating drive mechanism. The shaft member is provided with a first tapered mounting position. An object is adapted to be securely mounted on the shaft member for rotation therewith, and is provided with an internal bore having a second tapered mounting portion engageable with the first tapered mounting portion. The first and second tapered mounting portions cooperate to accurately align the object with respect to the shaft member. In accordance with the present invention, one of the first and second tapered mounting portions includes a plurality of axially spaced apart and generally annular grooves defining tapered land surface portions therebetween. The other one of the first and second tapered mounting portions defines a tapered surface engageable with the tapered land surface portions for securely and accurately positioning the object on the shaft member. In the preferred embodiment of the invention, the grooves are uniformly spaced apart and have a predetermined width which is substantially equal to the width of each of the tapered land surface portions. Also, one of the first and second tapered mounting portions which includes the grooves is formed of a material such as aluminum, while the other one of the first and second tapered mounting portions is formed of a harder material such as steel.