19th International Conference on Production Research

COMPUTER AIDED GEOMETRIC MODELING AND 5-AXIS MILLING OF A SCREW PROPELLER IN A SINGLE SETUP: A CASE STUDY

A.C. Munar1, E.L.J. Bohez2, M. Singh3, T. Lin4, S.S. Makhanov5

1,2,3Industrial Systems Engineering Department, Asian Institute of Technology (AIT), Pathumthani, Thailand 4Industrial Engineering Department, Konkuk University, Seoul, Korea

5Sirindhorn International Institute of Technology (SIIT), Thammasat University, Pathumthani, Thailand

Abstract This paper presents a geometric modeling method and 5-axis CNC machining algorithm for the manufacture of screw propellers in one single setup. A novel approach for 5-axis roughing is developed and implemented in addition to further streamline finishing sequences. The toolpath is generated by dividing the propeller model into milling regions viz. the front & rear blade faces, the leading & trailing blade edges and the lateral hub surfaces between adjacent blades. Cutting tools for each region are then selected along with the appropriate tool orientation for 5-axis flank milling. The CL-data is acquired using UnigraphicsTM and translated into NC code using a postprocessor for the Maho MH600E milling machine. The viability of the proposed method is verified by virtual machining on VericutTM and actual machining on Maho MH600E. Keywords: 5-Axis CNC machining, CAD/CAM, sculptured surface machining.

1 INTRODUCTION Sculptured surface machining has significantly developed ever since its inception in the 1950s under the historic project called Automatic Programmed Tool Language (APT). The term sculptured has earned popularity in machining as NC programmers have gained more control of the cutting tool thus resembling the movement of an artists chisel. Machining of free-form surfaces called for advanced CNC multi-axis machines which have a higher degree of flexibility and precision than conventional 3-axis types. Its implementation also demanded even more sophisticated CAD/CAM systems to ease designer work in modeling and programming. CAM technology has assisted designers in selecting cutting parameters in addition to preparing NC data based on the required design surface tolerance. The selection of cutting variables involves specifying cutting tools that are geometrically compatible with the design surface as well as choosing the appropriate milling technique. Countless research has been devoted to harness the full potential of multi-axis NC machining in both hardware and software aspects [1]. Manufacturing parts with complex geometry requires flexible methods of CNC programming and machining especially when the design part covers an area of several meters such as gas turbine blades and marine propellers. Among the numerous advantages of 5-axis machining, the three most significant are: reduced process time due to higher material removal rates, reduced setup time for intricate prismatic parts and improved surface quality thus minimizing the time required for subsequent finishing [2,3]. The inherent ability of 5-axis machines to position the tool and workpiece at any given relative point and angle allows them to produce the design part using several approaches [2,4]] that which is evidently a shortcoming of 3-axis machines. In contrast to their predecessors, 5-axis machines have considerable advantage in terms of accessibility and productivity. For example, the effect of employing 5-axis machines in the manufacture of die molds has resulted in 10-20 times more than the efficiency set by 3-axis machines [5,6]. Moreover, parts with irregular shapes such as turbo impellers can be machined using a single setup since areas previously inaccessible to 3-axis machines are made workable with added degress of freedom although under certain constraints [7,8].

Apart from their benefits in sculptured surface machining, 5-axis machines have also introduced both computational and functional difficulties. First, current CAM systems still do not provide adequate support for toolpath generation and verification such that designers still rely on iterative methods [8,9,10,11]. Apparently there are still huge numbers of research concerning the effective control of scallop heights based on tool geometry and positioning. Second, considering the rigorous task of developing complicated algorithms for interference and collision detection in addition to position correction [12], 5-axis is prone to machining errors of which many are classified as NC programming related [5]. 5-axis operations can be categorized as either point milling or flank milling [8]. In conventional point milling, material is removed using the tip of the tool. Although the process can be applied to machine any complex surface, the main drawback in using point milling is that is it time-consuming and the milled surface would require polishing in order to remove scallops [13]. The process of flank milling on the other hand removes material using the side of the tool, which then leads to higher machining efficiency and to a great extent eliminates the presence of surface scallops [14]. Yet it has disadvantages involving large overcuts and undercuts with increased chances of cutter interference and collision. Flank milling can be further classified as either ruled milling or skive cut [5]. Ruled milling refers to the machining of flat ruled surfaces or the more convoluted hyperbolic paraboloid surfaces both of which are bound by two guide strings. Common applications of ruled-milling include the manufacture of fan, compressor and impeller blade surfaces. The major drawback of ruled milling includes relatively large deflections when slender tools are employed as well as gouging for the case of concave or sharp cornered features. While the cut also mills with the side of the cutter, it is preferred for convex surfaces such as the leading and trailing edges of airfoils found in gas turbine blades. Screw propellers have been the primary products of 5-axis CNC machining since the beginning. With their visibly complex geometry, the manufacture of propellers presented NC programmers the difficulty of guiding the tool through narrow areas between adjacent blade surfaces without causing gouging or interference. Research on 5-axis machining of propellers however have

mostly focused on the semi-finishing and finishing sequences [15,16]. Roughing is still widely performed on 3-axis machines for two reasons mainly, cost-effectiveness as well as high material removal rate [6,8]. The motivation of this research is to deviate from such common practice where roughing would be performed straight-away in 5-axis mode thus further complimenting the procedure with a significant reduction in setup time and overall machining time. 2 GEOMETRIC MODEL OF SCREW PROPELLER The geometry of a propeller is generally derived from the following parameters: chord length, pitch, camber, skew, rake and the profile thickness [17]. From such dimensional and other non-dimensional parameters, the efficiency and aero-hydrodynamic performance of the propeller is estimated depending on its specific application. Since the main focus of the work on propeller modeling is not intended to support hydrodynamic testing such as in a cavitation tunnel therefore greater emphasis has been laid on geometric modeling and subsequent 5-axis machining. For this reason a more simplified approach is adopted to generate the 3D propeller model with UnigraphicsTM. The suggested method would first take into account the airfoil coordinates, overall diameter, mean pitch and pitch ratio. Thereafter the specific pitch angle and profile thickness distributions of the selected propeller class can be applied by forming the airfoils at each local propeller radii using a series of affine transforms [18,19]. The surface for the blade was then created employing the airfoils as section strings. To complete the procedure, the blend surface was generated about the root section to make a fillet. After the construction of a single blade, it was duplicated into 4 copies, which was then rotated about the hub centerline at 72 interval to make a total of 5 blades. The completed 3D model of the screw propeller along with the resulting mechanical drawing is depicted in Figure 1.The proposed modeling approach is summarized in Figure 2.

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(b)

Figure 1: Propeller mechanical drawing and CAD model

Figure 2: Modeling of screw propeller blade.

19th International Conference on Production Research

3 5-AXIS MACHINING OF SCREW PROPELLER

3.1 Process Planning and Setup A four pronged process plan was followed to mill the screw propeller [4]. The strategy started off with the geometric identification of the part surfaces which classified them into either convex, concave or saddle. Following is the grouping of the identified surfaces into milling regions depending on their curvature properties. Then the maximum allowable tool diameter was determined for each region after which the milling direction is selected. Consequently the drive surface to machine the collective milling regions are created. In this case the minimum distance between two adjacent blades determined the maximum tool diameter. The flat end mill was used due to its wider range of effective cutting radius in contrast to ball nose cutters. The setup of the blank part on the MH600E along with algorithm for collision avoidance is shown in Figure 3.

Figure 3: Gouging avoidance for 5-axis milling of propeller

3.2 Toolpath Planning and Generation Prior to the toolpath generation for the roughing phase, a preform geometry of the modeled propeller was first constructed. The geometry coined the bounding boxes is a crude approximation of the propeller which is composed of ruled surfaces that envelop each blade to form a polygon. The concept of using such preform geometry is similar to that of a cast propeller at the near net shape stage which would have to undergo finishing so that the assigned tolerance is achieved [20]. Since the preform geometry is relatively less complex than the final part geometry, a more straightforward machining strategy for the roughing phase can be employed as a result. The blade polygon as shown in Figure 4 was divided into three milling regions namely front face, leading side and back face. With these regions, the drive surfaces were assigned accordingly. The roughing toolpath for each milling region of the bounding boxes geometry is depicted in Figure 5.

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Figure 4: Bounding boxes perform geometry

Figure 5: Bounding box roughing toolpath generation

The number of tool pass for each segment of the bounding box is 30. Once generated for a single bounding box, each toolpath was duplicated for the remaining four consecutive boxes. Hence a total of 15 toolpaths were generated to form three independent CL files namely front, leading and back which would then lead to that illustrated in Figure 6.

(a) (b)

Figure 6: Bounding boxes roughing toolpaths For the semi-finishing and finishing phases, toolpaths were generated to machine the blade contour and the blade faces. The contour is divided into two parts: trailing edge and leading edge. Since the selected method of machining is ruled-milling, a ruled surface is constructed along the contour, which was used as the drive surface. However, ruled-milling is accessible starting from any segment of the trailing edge up to only the upper half of the leading edge. Due to the problem of overlapping areas between adjacent blades, ruled milling would result into gouging if used further down the leading edge. To address the problem, skive-cut combined with a long tapered tool is thus chosen to machine the difficult segment. The toolpaths used to mill

the contour of a single blade are summarized in Figure 7. The total number of tool passes for the trailing and leading edges are 5 and 20 respectively.

Figure 7: Blade contouring toolpath generation The duplication of the generated toolpaths for a single blade would lead to that illustrated in Figure 8 below.

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Figure 8: Combined contouring toolpaths. The procedure for semi-finishing and finishing is reserved for the front and back faces of the blades. Likewise to the roughing phase, flank milling with a tilted tool is used for the reason that it facilitates control of the tool axis and avoids probable instances of tool collision with adjacent surfaces. In fact the operation somewhat emulates point milling where the difference mainly lies in the formation of the cusps which are in effect triangular rather than elliptical since the tool used is a flat end mill. Different tilt angles were tested for each blade face depending on the level of gouging avoidance and the maximum travel limit of the swivel axis. The front face which has a concave surface was suggested to have a tilt angle sufficient to avoid tool interference with the blade tip and with the surface of the consecutive blade while at the same time ensuring no rear-cutter surface gouging. Conversely, the back face can have a smaller tilt angle given that it is a convex surface. Simulation have shown that the allowable tilt angle for the front face range within 5 to 50 relative to the surface normal vectors. It is found that a tilt angle less than or equal to 5 would eventually result into a collision between the tool and the blade tip as the tool approaches the root section. Furthermore the tool would interfere with the back face of the adjacent blade if the assigned tilt angle were greater than or equal to 50. These two cases are depicted in Figure 9.

(a) (b)

Figure 9: Blade and tool collisions at 5 and 50 tilt angles Similar trials to determine the permissible range of the tilt angle was conducted for the blade back face. Presumably the tilt angle would range from 0 to 50 for an interference free toolpath based on the trials done for the front face. However it was ascertained that the primary constraint in this case is the maximum travel limit of the swivel axis and not the level of gouging avoidance. At a tilt angle of 2 the postprocessed CL-data contained some lines where the B-axis rotation angle is greater than the machine maximum 105. As the tilt angle is increased so does the instances of lines with invalid values for the B-angle. If on the other hand ruled-milling is used that is with 0 swarf tilt angle, gouging would result when the cutter approaches the root section. Since the major constraint is given by the machine swivel axis, it was decided that a 2 tilt angle such that the G-code would be subsequently corrected by hand. The toolpaths employed to mill the front and the back faces of the blade with no interference is shown in Figure 10.

Figure 10: Blade faces finishing toolpath generation The generated toolpaths for the front and back blade faces were consequently instanced about the propeller centerline at 72 rotation angle resulting to that shown in Figure 11. Note that the intol and outol for finishing is 0.01 mm.

(a) (b)

Figure 11: Combined front and back faces toolpaths

19th International Conference on Production Research

The next and final toolpath was generated to mill residual material between blades about the lateral hub surface as depicted in Figure 12 below. An auxiliary drive geometry representing the area located between consecutive blades was developed at the onset. The tapered ball-nosed end mill was again used given its long reach and small milling diameter hence allowing for material in the narrow area to be removed without causing gouging or interference.

Figure 12: Hub lateral face finishing toolpath

3.3 Toolpath Verification and Actual Machining After postprocessing the CL data from UnigraphicsTM, the acquired G-codes were imported into VericutTM for initial verification prior to actual machining. The parameters used in the simulation are given as follows: XYZ traverse speed = 6m/min; A-axis angular speed = 235/sec; B-axis angular speed = 162/sec; and Feedrate = 300 mm/min. The initial part stock volume is 14.777x105 mm3. The outcome of both virtual and physical verification of each toolpath is shown in Figure 13 to Figure 16. The total time incurred and net volume of material removed for each machining phase is summarized in Table 1 to Table 4.

Figure 13: Bounding boxes roughing verification

Table 1: Bounding boxes roughing performance

Milling Region Material Removed Cutting Time Front Faces 531909 mm3 87.81 min

Leading Sides 38357 mm3 11.29 min Back Faces 502813 mm3 84.86 min

Total 1073079 mm3 184.48 min

Figure 14: Contouring verification

Table 2: Contouring performance

Milling Region Material Removed Cutting Time Trailing Edges 65126 mm3 22.56 min Leading Edges 68005 mm3 145.62 min

Total 133131 mm3 168.18 min

Figure 15: Propeller blades semi-finishing verification

Table 3: Propeller blades semi-finishing performance

Milling Region Material Removed Cutting Time Concave Faces 204055 mm3 706.55 min Convex Faces 79188 mm3 212.65 min

Total 283243 mm3 919.20 min

Figure 16: Lateral hub finishing verification

Table 4: Propeller blades and hub finishing performance

Milling Region Material Removed Cutting Time Concave Faces 14250 mm3 1213.4 min Convex Faces 37060 mm3 212.65 min

Hub Faces 22766 mm3 199.17 min Total 74076 mm3 1625.2 min

Results from the simulation show that the entire roughing phase took a minimum total of 3 hours where an estimated 10.7x105mm3 of material was removed or 70% of the initial part stock volume. Altogether it took 48 hours to transform a cylindrical blank part into a propeller with 5 blades. A total of 15.64x105 mm3 of material was milled during the roughing and finishing sequences. 4 CONCLUSION The proposed modeling and 5-axis machining approach for marine propellers has highlighted the advantage of having the roughing phase performed in 5-axis rather than in the conventional 3-axis mode. A huge reduction in setup and machining lead time was achieved by having a more unified approach such that roughing and finishing were conducted in a single setup. Moreover a higher material removal rate was achieved for the roughing phase by using a preform bounding boxes geometry combined with 5-axis flank milling. Further research in the presented method would include the optimization of toolpath parameters in order to address surface quality and tool wear rate. 5 ACKNOWLEDGMENTS This research work was funded by AIT and SIIT. The authors are greatly appreciative to AIT CIM Laboratory supervisor Somchai Taopanich for his valuable expertise in machining and also to Suradash Chungpaiboonpatana for his assistance in editing this paper. 6 REFERENCES [1] Dragomatz, D., Mann, S., A Classified Bibliography of

Literature on NC Toolpath Generation, Computer Aided Design, 29(3), 1997, 239-247.

[2] Bohez, E.L.J., Five-axis Milling Machine Tool Kinematic Chain Design and Analysis, International Journal of Machine Tools & Manufacture, 42, 2002, 505-520.

[3] Lai, X.D., Zhou, Y.F., Zhou, J., Peng, F.Y., Yan, S.J., Geometrical Error Analysis and Control for 5-Axis Machining of Large Sculpture Surfaces, International Journal of Advanced Manufacturing Technology, 21, 2003, 110-118.

[4] Lauwers, B., Kruth, J.P., Dejonghe, P., An Operation Planning System for Multi-Axis Milling of Sculptured Surfaces, International Journal of Advanced Manufacturing Technology, 17, 2001, 799-804.

[5] Choi, B.K., Jerard, R.B., Sculptured Surface Machining Theory and Applications, Kluwer Academic Publishers, Dordrecht, 1998.

[6] Gray, P., Bedi, S., Ismaili, F., Rao, N., Morphy, G., Comparison of 5-Axis and 3-Axis Finish Machining of Hydro Forming Die Inserts, International Journal of Advanced Manufacturing Technology, 17, 2001, 562-569.

[7] Chen, S.L., Wang, W.T., Computer Aided Manufacturing Technologies for Centrifugal Compressor Impellers, Journal of Materials Processing Technology, 115, 2004, 284-293.

[8] Yilmaz, O., Noble, D., Gindy, N.N.Z., Gao, J., A Study of Turbomachinery Components Machining and Repairing Methodologies, International Journal of Aircraft Engineering and Aerospace Technology, 77(6), 2005, 455-466.

[9] Bliko, I., Kowerich, S. and Paulik, P., Experiences from a Quantum Leap Improvement in Turbine Manufacturing. In Machining Impossible Shapes, ed. Gustav J. Olling, Byoung K. Choi and Robert B. Jerard, Kluwer Academic Publishers, Boston, 1999, 8-23.

[10] Bohez, E.L.J., Senadhera, S.D.R, Pole, K., Duflou, J.R., Tar, T., A Geometric Modeling and Five-Axis Machining Algorithm for Centrifugal Impellers, Journal of Manufacturing Systems 16(6), 1997, 422-436.

[11] Lai, X.D., Zhou, Y.F., Zhou, J., Peng, F.Y., Yan, S.J., Geometrical Error Analysis and Control for 5-Axis Machining of Large Sculpture Surfaces, International Journal of Advanced Manufacturing Technology, 21, 2003, 110-118.

[12] Bohez, E.L.,J., Minh, N.T.H., Kiatsrithanakorn, B., Natasukon, P., Ruei-Yun, H., Son, L.T., The Stencil Buffer Sweep Plane Algorithm for 5-Axis CNC Tool Path Verification, Computer Aided Design, 35(12), 2003, 1129-1142.

[13] Gong, H., Cao, L.X., and Liu, J. 2005. Improved Positioning of Cylindrical Cutter for Flank Milling Ruled Surfaces. Computer-Aided Design 37:1205-1213.

[14] Bedi, S., Mann, S., Menzel, C., Flank Milling with Flat End Cutters, Computer Aided Design, 35, 2003, 293-300.

[15] Elber, G., Fish, R., 1994, 5-Axis Freeform Surface Milling using Piecewise Ruled Surface Approximation, Technical Paper, Israel Institute of Technology, Haifa.

[16] Youn, J.W., Jun, Y., Park, S., Interference-free Toolpath Generation in Five-Axis Machining of a Marine Propeller, International Journal of Production Research, 41(18), 2003, 4383-4402.

[17] Van Beek, T., Technology Guidelines for Efficient Design and Operation of Ship Propulsors, Marine News, Wrtsill Propulsion, Netherlands BV, 2004.

[18] Cortes, J.L., 2004, Do-It-Yourself Project, Designing and drawing a classical wood propeller (in Solidworks), http://www.nmine.com/propeller.htm.

[19] Kouh, J.S., Han, C.H., Chen, Y.J., 2004, A New Geometric Modeling Method for Marine Propellers, 9th Symposium on Practical Design of Ships and Other Floating Structures.

[20] Gao, J., Chen, X., Zhang, D., Yilmaz, O., Gindy, N., Adaptive Restoration of Complex Geometry Parts Through Reverse Engineering Application, Advances in Engineering Software, 2006.