THE INFLUENCE OF DRILL CHARACTERISTICS AND ENTRY MECHANISMS ON DRILLING PERFORMANCE (WANDERING MOTION, POINT GEOMETRY)

The quality of drilled holes is critical in many manufacturing operations. The degree of instability of a drill during cutting is affected by its point geometry, cutting process parameters, chip formation mechanisms, etc. To improve and satisfactorily maintain the stability of the drill and hence the hole quality, an understanding of the cutting mechanics and dynamic characteristics of the drilling process is required. This thesis investigates the factors influencing hole quality in drilling processes such as asymmetry of the drill point geometries, dynamic characteristics of the drill bit and initial penetration mechanisms analytically and experimentally.;Second, various characteristics of the drill bit were investigated by analytical models. The drill bit was modeled as a twisted Euler-type beam under axial load. The cross-sectional shape of the drill was simplified to be symmetrical about its principal axis. From the model, the geometric, static and dynamic characteristics of the drill were obtained numerically by a developed computer program "CADRILL".;Third, a mathematical model for the initial penetration mechanism of the drilling operation is developed. It was shown why the resulting distorted polygon-shaped holes always have an odd number of sides during drill entry, and how the profile of the distorted hole gradually becomes round with increasing the depth of penetration.;Finally, the penetration process was observed experimentally by measuring the displacement of the drill point in two radial directions. The frequency corresponding to the maximal peak in the power spectrum in the radial displacement signal was correlated to the number of sides of the drilled hole profile. The simulation results obtained from the developed mathematical model showed a close coincidence with the experimental results. A suitable point geometry, multifacet point, which minimizes the skidding and wandering motion during drill entry was also suggested.;First, the hole oversize was studied by considering effects of the asymmetry of the drill point, such as relative lip heights and different half point angles. Drill point runout was also evaluated at certain depths by introducing moving boundary conditions and interaction effects between the hole and the drill.