The Influence of Twist Drill Design on Rigidity and Chip Disposal

Abstract

The research work described in this report was carried out as part of the development of a CAD-CAM system for drills under a Teaching Company Scheme. A CAD drill modelling program with a force prediction module was already present. The main objective of this work was to add two sub-programs that could predict two other major performance criteria in drilling - chip disposal efficiency and drill rigidity and to develop the methodology for applying the programs to practical design environments. These are two conflicting requirements - an efficient chip disposal demands a large flute area, whereas sound drill rigidity needs a large drill cross-section. The fulfilment of these requirements must be a compromise. An extensive literature search was carried out to determine theoretical and experimental methods of assessing these two performance criteria. There is little published on either topic. Using the theories found and my own practical experience, methods for predicting chip disposal efficiency and drill rigidity were added to the existing CAD system. In the area of chip disposal, the flute area and the correct angle of chip transportation were calculated by computer. A best-fit circle placed within the flute area has previously been referred to as a relative indicator of the drill's chip disposal efficiency. Further work, reported here, has revealed other factors also have a critical influence on the chip disposal capability of the drill and these have been examined. Drill rigidity has been examined using Finite Element Analysis, by means of which any type of drill can be modelled and analyzed for stress, strain and displacement. Experimental work has also been carried out using a purpose-built torsion rig to demonstrate the accuracy of the FEA predictions. A CAE system now exists where the CAD program is linked to several performance prediction modules. A case study given in this thesis shows how the CAE system is used to create a new drill. This is the first documented piece of work known to discuss such a computerised approach to twist drill design.