THE INFLUENCE OF DESIGN FACTORS ON THE PERFORMANCE OF THE INVERTED CONE BUR

¹ School of Dentistry, University of Michigan, Ann Arbor, Mich.

This report has presented briefly some results obtained during a testing program designed to show the effects of different design parameters on the performance of the inverted cone bur. These parameters are (1) flute finish, (2) heat treatment, (3) end flutes, (4) diameter of bur, (5) length of engagement or cutting depth, and (6) the number of flutes in the bur. The results have been expressed as graphs showing the average linear displacement per flute revolution as a function of the peripheral velocity. The average linear displacement per flute revolution is equivalent to the average distance that the bur advances into the cut per flute revolution. It has been found that this manner of data presentation demonstrates changes in edge-cutting ability very effectively.

It has been found that only the last one-thousandth of an inch of flute edge is effective for cutting a soft material. This distance decreases as the material hardness increases until only a few micro-inches of blade edge are effective in some cases. The largest manufacturing difficulty lies in producing equally sharp edges consistently; i.e., bur flutes that are identical within the outer one-thousandth of an inch.

The flutes of commercial burs usually have some fins or excess material. It may be removed by subsequent cuttings of the bur flutes; however, it has been found that the small increase in the effectiveness of the bur does not warrant the extra cost. Heat treatment must retain the sharp edge placed on the flute by the cutter as well as harden the bur and increase its cutting life.

During peripheral cutting it is felt that the end flutes are inoperative. Test results indicate that the "Revelation" type flute is superior to the "Star" type end flute and the bur without end flutes. However, the results are not conclusive.

It has been found experimentally, as well as theoretically, that the average linear displacement per flute revolution does not depend markedly on velocity, indicating that the length of cut is nearly constant irrespective of reasonable changes in bur diameter. It is assumed in this statement that the edge conditions at the flutes as well as the test conditions are the same.

Changing the length of engagement as well as the number of flutes changes the manner in which the external load is distributed among the flutes of the burs. If the force on the flute is increased, the average displacement per flute revolution is increased. This occurs when either the length of engagement or the number of flutes is decreased.