CAD BASED DRILLING USING CONVENTIONAL TWIST DRILLS

Transcription

1 CAD BASED DRILLING USING CONVENTIONAL TWIST DRILLS Panagiotis KYRATSIS 1, Nikolaos BILALIS 2, Vasilis DIMITRIOU 3 and Aristomenis ANTONIADIS 2 ABSTRACT: Twist drills are geometrically complex tools, which are used in industry for a variety of hole making processes. In the past, a number of researchers have adopted mathematical and experimental approaches for drilling simulation. The present paper is based on the ground that the increasing use of modern CAD systems provides an excellent platform for the development of CAD-based tools for machining simulations and optimizations. The drilling tools are modeled using the API (Application Programming Interface) of a general purpose CAD system. The model is parameterized in order to achieve different twist drill geometries, depending on design and manufacturing parameters, while at the same time, different cutting conditions are used. Both the drilling tool and the cut work piece are represented as solid models. The simulation is based on virtual kinematics of solid models, which leads to successive penetrations and material removal of the drilling tool into the cutting piece. The final outcome is the creation of solid models for the undeformed chip and the remaining cutting piece geometries. Finally, having such a CAD-based tool means that there is a solid foundation for further simulations (cutting forces and applied torque calculation, finite element analysis etc). KEY WORDS: Drilling, Simulation, 3D-modelling, CAD. 1 INTRODUCTION Drilling is one of the most commonly performed material removal processes, being increasingly popular over the years. The aforementioned process is used to create a cylindrical hole in a solid section of a work piece material. When using mechanical methods and tools, drilling involves rotational and linear feed motions. Invented almost two centuries ago, today s twist drill is a general purpose tool used to produce holes ranging in size from very small (0.1 mm) to quite large diameters (well over 100mm). Figure 1 identifies twist drill features and relevant dimensions. Although the conventional drilling tool is very common in industry, it is characterized by a complex geometry. This geometry involves two main cutting edges ( lips - designed to produce identical chips) and the chisel edge. 1 Technological Educational Institute of Western Macedonia, Kila, Kozani, Greece 2 Technological Educational Institute of Crete Romanou 3, Chalepa, GR73133 Chania, Crete, Greece pkyratsis@teikoz.gr; bilalis@dpem.tuc.gr; dimvasi@chania.teicrete.gr; antoniadis@dpem.tuc.gr The cutting action of the main cutting edges of the drill has been described closely with the classical oblique cutting theory but with variable cutting speeds, inclination angles and normal rake angles along the lip. Their rake angle is closely related to the helix angle and decreases progressively with the helix angle from the outer corner of the drill to the chisel edge. The nominal relief angle of the drills increases rapidly from the outer corner to the chisel edge. On the other hand, the cutting action on the chisel edge has been described to be closer to the orthogonal cutting theory at relatively high negative rake angles and very low cutting speeds. The chisel edge extrudes into the work piece material and contributes mainly to the thrust force and very little to the torque. The cutting lips cut out the material and produce the majority of the drilling torque. They also contribute significantly to the thrust force because of the point angle. During the drilling operation the chips are formed along the cutting lips and move up following the drilling helix. In the past, different researchers in order to simulate the drilling operation used either a mathematical approach in which the tool was described analytically by extremely complicated equations or an experimental one by which a large amount of experiments were carried out in order to gather all the different pieces of information required. Both methods required a large amount of 48 ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 7, ISSUE 1/2009

2 time in order to develop accurate drilling models and in a number of cases, a combination of both approaches was necessary. Most of them encompass a variety of restrictions and limitations mainly because they were based on 2D strategies and the geometry of the tools was defined using the principles of projective geometry. Figure 1. Drill tool terminology The present paper is based on the concept that the increasing use of modern CAD systems provides an excellent platform for the development of CAD-based tools for machining simulations and optimizations. In order to proceed to simulation, the Application Programming Interface (API) of a general purpose CAD system was used. That strategy reveals the powerful modeling capabilities of up to date CAD software environments. The algorithm developed, is built in terms of a computer code, which is supported by a user friendly graphical interface. First, the user can choose the detailed geometry of the tool, by changing the available parameters and thus simulating a number of different conventional drilling tools geometries. One of the main advantages of that approach is the usage of solid modeling in order to describe both the drilling tool and the work piece. Second, the solid models are used in order to simulate the penetration of the tool inside the work piece and to produce 3D solid models of the cut work piece and of the undeformed chip. The undeformed chip geometry is an essential parameter in order to determine the cutting force components, as well as to predefine the tool wear development, both of them important cost related data of the drilling process. 2 TWIST DRILL TOOL PARAMETERS AND SIMULATION STRATEGY A common method of grinding the flank surfaces of the point of a twist drill is shown in Figure 2. The drill is rotated about the axis AA, which is fixed in space and the flank is ground by the plane face of the grinding wheel G. The axis AA and the plane of the grinding wheel face intersect in the point O. The motion of the wheel face in relation to the drill point may be deduced by considering the drill point as fixed and the wheel face rotating about the axis AA, the virtual motion of the wheel face being equal and opposite to the actual motion of the drill. A plane rotating about the fixed axis envelops a cone, so that the grinding wheel face envelops part of a conical surface in its motion relative to the drill point and the flank surface produced on the drill point is part of the surface of the cone. The vertex of the cone is the point O and the cone semi angle is the angle θ between the axis AA and the wheel face. The cone may be termed the grinding cone and this method of drill point grinding is referred to as conical grinding. A right hand coordinate system was used based on the following principles: a) the z-axis lies along the drilling tool axis (towards the drill shank), (b) the y-axis lies along the common perpendicular which intersects with the extensions of the two ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 7, ISSUE 1/

3 cutting edges and (c) the x-axis is perpendicular to the y and z-axes. According to the model described above, a number of parameters have to be chosen in order to produce a great deal of different drilling tools. Those are separated in two main categories. First, the following geometrical parameters are used to determine the main shape of the tool: - R: tool radius in mm; - W: tool web thickness in mm; - p: half point angle in deg (i.e. 2p=118 o ); - h: helix angle. Second, manufacturing parameters are used, for the detail shape of the tool based on the conventional grinding method: - g: distance of cone vertex along the x axis; - s: distance of cone vertex along the y axis; - θ: half cone angle. Figure 2. Conventional twist drill tool geometry Another main element for the correct twist drill W 2 2 geometrical representation is the shape of the flute ( r ( W ) ) 2 2 tan( h) v = arcsin( ) + (1) cross section. An appropriate drill flute surface for r D tan( p) straight cutting lips was considered. Galloway was 2 the first to derive the condition which the flute where ( r, v) are the necessary polar coordinates for surface must satisfy in order to have a straight the flute cross section creation, with r varying from cutting lip under conventional conical grinding W/2 to D/2. The profile for the secondary (non conditions. That condition is described by the cutting lips) flute can be defined in a manner to following equation: 50 ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 7, ISSUE 1/2009

4 perform easy chip removal, while at the same time ensures sufficient strength of the tool. The API of a CAD system was used in order to produce a piece of software that was utilizing the aforementioned geometrical description of the tool. The solid model of the twist drill fluted part was formed by sweeping helically the flute cross section described by equation 1. The twist drill point geometry is finalized by the boolean subtraction of the grinding cones mentioned earlier. The computer algorithm was based on Visual Basic for Application programming language and a number of conical twist drill tools can be modeled. It is very important to mention that Galloway s approach is purely geometrical, which means that it allows the analytical calculation of all the necessary characteristics of the tool (rake angle, nominal relief angle etc) based on 2D analysis of a variety of geometrical planes. The approach used at the present research is based on the API that creates a 3D solid model of the tool using the same principles, but the final model comprises all its geometrical properties. Based on the dimensions of the twist drill modeled previously, two more solid models were produced. One for the description of the drilling action on the main cutting edges and another for the application of drilling in the area of the chisel edge (Figure 3). Figure 3. The flow chart for the drill process simulation 3 MODELING THE 3D PENETRATION PROCESS Having built a 3D solid model of the tool, resulted in the creation of another software module, in order to virtually simulate the kinematics of the drilling process and achieve successive penetrations into the work piece. The virtual drilling process was separated in two actions. The first was based on the cutting action of the cutting lips and the second on the cutting action of the chisel edge. Both were treated in a similar way but analyzed individually. The final result was the creation of 3D solid models representing the undeformed chip and the shape of the remaining work piece geometries for each case. The tool was virtually moved transitionally towards the Z axis (feed) while at the same time, it was rotated around its Z axis of symmetry by a constant step. In every step, a boolean subtraction of the tool model from the remaining work piece model was carried out. The step value, for both cases, was selected comprising the motion accuracy, the calculation complexity and the CAD system capacity. First, a work piece specially shaped in order to achieve the cutting lips to be directly engaged ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 7, ISSUE 1/

5 (steady state case) and with a small hole in the middle (chisel edge area), was used. A rotation step of one degree was successfully incorporated into the model. The resulted undeformed chip and the remaining work piece solid models are presented in Figure 4. Second, another work piece of pure cylindrical shape having a diameter equal to each tool s chisel edge was used. The step selected in that case, was in the range of three to five degrees in order to achieve the appropriate motion accuracy without encountering software limitations due to extremely small size of the chip involved. Once more, the produced undeformed chip and the remaining work piece involved were the result of the virtual cutting process in the area that comes in contact with the chisel edge (Figure 5). Both cases were very computing intensive and the simulations had to run for a significant amount of cycles in order to achieve constant chip geometry and simulate the steady state condition. Figure 4. Steady state simulation of the cutting lips penetration into the remaining work piece Figure 5. Steady state simulation of the chisel edge penetration into the remaining work piece 52 ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 7, ISSUE 1/2009

6 Figure 6. 3D solid models of undeformed chip sets Figure 6 illustrates the solid geometry of a number of chips produced in both the areas of cut (lips and chisel edge). Every layer of the chips is cut by a different cutting edge and an angle of 210 degrees is selected in order to emphasize their 3D motion. The selected approach, in contrast to former modeling efforts, is primitively realistic, since the produced remaining cut work piece and the chips 3D geometry are the results of the material removal of the cutting tool into a solid model of the work piece. The model is able to output formats provided from the host CAD system (.ipt,.sat,.iges,.stl,.dxf etc) and offer realistic solid parts, chips and cut work pieces, which can be easily managed for further investigations individually, or as an input to other commercial CAD, CAM or FEA environments. 4 DRILL SIMULATION RESULTS A representative number of simulation results of the drilling process are presented. The simulations were performed with three different values of tool diameters and three different values of feed. The resulted 3D solid geometries of the drilling simulation are presented at Figures 7 and 8. In both figures, the nine cited chip solid geometries represent the undeformed chips (produced from the cutting lips and chisel edge respectively) for three different values of tool diameters: 20 mm, 10 mm and 8 mm and three different values of feed: 0.18 mm/rev, 0.36 mm/rev and 0.72 mm/rev, as it is shown at their left and bottom sections. The geometrical characteristics of each produced solid may be directly analyzed so as to provide easily the values of the geometrical parameters needed from the end user. As presented at Figures 7 and 8, under each solid geometry, the length and the appropriate width of each chip has been evaluated. Obviously, from each of the three rows of the two Figures tables, the length of the produced chips is not influenced from the augmentation of the feed, while the appropriate width values follow the increment of feed as expected. The observation of the columns of Figure 7 table shows that the value of the average width depends on two main characteristics, the continuous change of the rake and relief angles from the outer corner to the chisel edge (which depends on the tool diameter) and the value of the feed used. In the case ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 7, ISSUE 1/

7 presented at Figure 8, as the values of the tool diameters raise from 8 to 10 and finally 20mm the maximum width increases respectively. The determination of the undeformed chips, together with the number of constant coefficients stored in material databases depending on the couple of material used (tool and work piece), can led to the calculation of the applied cutting forces during the drilling process. Figure 7. 3D solid geometry of chips produced by the main cutting edges for different tools and feeds Figure 8. 3D solid geometry of chips produced by the chisel edge for different tools and feeds 5 CONCLUSIONS The novelty of the current research is that the algorithm has been developed and embedded in a commercial CAD environment, by exploiting its modeling and graphics capabilities. Full 3D solid models of tools and work pieces were used while in previous studies their strategies were mainly based on the principles of projective geometry (2D) and either the classical oblique cutting theory or the orthogonal cutting theory. Such an approach provides comprehensive 3D definitions of the drill tool, as well as the modeling of the initial work piece and the cut work pieces together with the undeformed chips during each step of the cutting process. 54 ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 7, ISSUE 1/2009

8 These 3D geometric definitions provide the appropriate data required for a number of downstream applications such as finite element analysis, geometric assessment of tool stresses and wear, prediction of the cutting forces, 3D scanning of tool geometry, optimization of the drilling process etc. 6 REFERENCES Armarego, E.J.A. (2000). The unifiedgeneralized mechanics of cutting approach - a step towards a house of predictive performance models for machining operations, Machining Science and Technology, Vol. 4, No. 3, pp Armarego, E.J.A.; Ostafiev, D.; Wongand, S. & Verezub, S. (2000). An appraisal of empirical modelling and proprietary software databases for performance prediction of machining conditions, Machining Science and Technology, Vol. 4, No. 3, pp Chapman, W. (2002). Handbook for the metalworking industry, Hansen Gardner Publications, X, Cincinati, Ohio. Ehmann, K.F.; Kapoor, S.G.; DeVr, R.E. & Lazoglu, I. (1997). Machining Process Modeling: A Review, Journal of Manufacturing Science and Engineering, Vol. 119, pp Fujii, S.; DeVries, M.F. & Wu, S.M. (1970). An analysis of drill geometry for optimum drill design by computer. Part I-Drill geometry analysis, Journal of Engineering for Industry, Vol. 92, No. 3, pp Fujii, S.; DeVries, M.F. & Wu, S.M. (1970). An analysis of drill geometry for optimum drill design by computer. Part II-Computer aided design, Journal of Engineering for Industry, Vol. 92, No. 3, pp Galloway, D. (1957). Some experiments on the influence of various factors on drill performance, Trans. A.S.M.E., Vol. 79, pp Hsieh, J.F. (2005). Mathematical model for helical drill point, International Journal of Machine Tools & Manufacture, 45, pp Hsieh, J.F. & Lin, P.D. (2005). Drill point geometry of multi flute drills, International Journal of Advance Manufacturing Technology, 26, pp Kang, D. & Armarego, E.J.A. (2003). Computeraided geometrical analysis of the fluting operation for the twist drill design and production. I. Forward analysis and generated flute profile, Machine Science and Technology, Vol. 7, No. 2, pp Kang, D. & Armarego, E.J.A. (2003). Computeraided geometrical analysis of the fluting operation for the twist drill design and production. II. Backward analysis, wheel profile and simulation studies, Machine Science and Technology, Vol. 7, No. 2, pp Paul, A.; Kappor, S.G. & Devor R.E. (2005). Chisel edge and cutting lip shape optimization for improved twist drill point design, International Journal of Machine Tools & Manufacture, 45, pp Tsai, W. & Wu, S.M. (1979). A mathematical model for drill point design and grinding, Journal of Engineering for Industry, Vol. 101, pp The Department for Permanent Education offers: post-graduate courses in the fields of: informatics electrotehnics-electronics... with the professionalism of the mechanics trainers from UPT and the experience constructions of specialists from the economic environment. trainings consultancy and counseling networking training - Cisco Networking Academy our European partner s experience Coming soon: Authorized Training Center B-dul. Republicii nr Timişoara, Timiş ROMANIA Tel: Fax: dep@mail.utt.ro ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 7, ISSUE 1/