Force Prediction in Drilling Incorporating an Asperity Interface Model
 

Deborah M. Haan | MS | 1996

Governmental policy and public opinion are encouraging manufacturers to reduce the amount of wastes that they generate. One of the largest waste sstreams associated with machining aluminium alloys is the spent cutting fluid. In order to eliminate or minimize the amount of cutting used during machining, the function of the cutting fluid must first be understood. The benefits of using cutting fluid can then be assessed and alternatives can be evaluated.

Experiments are designed and conducted to provide a preliminary investigation into the effects of the cutting fluid while drilling small holes in aluminium. The basic machining parameters, the drill geometry, and the cutting fluid conditions are all varied to screen for their effects on the surface fininsh, torque, and thrust, hole quality, and built up edge. The cutting fluid's effect on the torque and thrust initiated an investigation into the drilling forces.

Mechanistic models are developed for the torque and thrust during drilling based on cutting velocity, feed, and workpiece material. The forces are defined by oblique cutting models at the cutting edges and an extrusion model at the chisel edge. An empirical relationship for the specific friction force and the specific normal force is calibrated for two types of aluminium. The predicted errors of the model are evaluated for sensitivity to the cutting fluid condition and to one of the specific drill geometry features.

The effects of the cutting fluid and the drill margin on the drilling torque and thrust are incorporated into the model predictions through the asperity interfaces at the surfaces. A normal pressure on the drill margin is established from the asperities on the surface of the drill and the surface of the hole. The lubricity of the cutting fluid at these interfaces reduces the friction forces when the cutting fluid is used. The trends associated with modeling the asperity interfaces at the drill margins compensate for the inadequacies of the previous force model predictions.

One of the functions of the cutting fluid is shown to be lubrication of the drill and the workpiece. The models describing this interface consider the presence of the cutting fluid, but not the amount of the cutting fluid used. The research suggests that the amount of cutting fluid used may be reduced without diminishing its benefits. To eliminate the cutting fluid, the lubrication benefits of the cutting fluid must be achieved through other means (eg., drill coatings or allowing the drill to taper back).

If you have any comments or suggestions please e-mail jwsuther@mtu.edu.