Aspects of Shape Decomposition for Thick Layered Object Manufacturing of Large Sized Prototypes Johan J. Broek, Imre Horváth, Bram de Smit, Alex F. Lennings, Joris S.M. Vergeest Faculty of Design Engineering and Production Sub-faculty of Industrial Design Engineering Delft University of Technology Jaffalaan 9, NL-2628 BX DELFT, The Netherlands E-mail :
[email protected], WWW : http://www.io.tudelft.nl/research/ica/
ABSTRACT Large sized freeform objects of different materials are widely used in various applications. Current layered rapid prototyping technologies are not suitable for the fabrication of this kind of objects, due to the necessity of a large number of layers and the limitations of size. This paper presents a novel approach of layered manufacturing that is most appropriate for the fabrication of these objects. A method of thick layered object manufacturing is described, which is based on higher approximation and the application of a flexible curved cutting tool. The method allows us to produce physical prototypes, which need less or no finishing. In order to meet the designer’s intend as good as possible, some provisions are introduced. A hierarchical decomposition of the CAD geometry into components, segments, layers and sectors, based on morphological analysis, is proposed which facilitates the manufacturing and the re-assembly of the parts to produce the physical prototype without affecting the requested functionality. Hollowing and degenerated layers are discussed. The proposed facilities will complicate the RP process very much and reasoning about the efficiency is needed. Finally the process is ordered in a sequence, which allows a highly automated process. Keywords: TLOM, Large sized prototypes, adaptive slicing, higher order approximation, hollowing, geometric decomposition, layer stacking 1. INTRODUCTION AND PROBLEM STATEMENT The application of physical models in design engineering has become commonly accepted among designers and stylists [51]. In conceptual product design a tendency can be observed towards the need of prototypes applied for testing and/or reasoning about one or a few aspects related to the functionality of the design. The prototype application is related to the appearance and the interface design of products taking into account aspects of mechanical engineering, electrical engineering, component arrangement and ergonomics. In conceptual design large-sized freeform physical models (prototypes) of various materials (e.g. Plastic foam, paper, plywood, clay, compound materials, etc.) are widely used in the domain of household appliances, mockups, automotive industry, aerodynamic and hydrodynamic simulation, decorations in advertisement industry, scenery in movie film making industry, stage settings in theaters and architecture. In the entertainment industry, extra-sized human and animal mannequins are special made of plastic foam. A complicating factor is however that the physical models are not only applied as shape models, but also for partly or fully functional prototypes, each application with it's own requirements such as finishing (accuracy, texture, painting, lettering, etc.), kinematic degree of freedom, stability, weight distribution, mechanical strength, etc. One specific example is the testing of kinematic behavior of a mechanism. It is obvious that the surface quality of that prototype for that function is of little or no importance for that testing aspect. However when the RP-system at great costs delivers a high quality surface that effort is wasted and makes materialization of the prototype too expensive related to the intended effective usage of the prototype. All are related to what the designer intends to reason about or to test the physical model for. The types of requested physical prototypes have not been yet unambiguously defined. A categorization seems to be helpful since the physical concept prototypes might serve different purposes, e.g. to help to decide on or reason about geometry, function, aesthetics, technological and economic aspects. Each of that purposes can have it’s own impact on the way the prototype is materialized. When we allow an approach of Rapid
Prototyping technologies that the designer has control over the way the materialization of the prototype is realized, then a fundamental issue is involved.
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Figure 1: Different slicing principles The strength of current Rapid Prototyping is that when the geometry of the CAD-model is transferred into the RP preparation section, the system can fully automatically and without any human interaction, generate the required prototype. The prototype, however, is for the same geometry data and the same RP-machine settings always the same. The intentions of the designer, such as considerations and decisions about the style or workmanship of the prototype are not under his control. He can provide some influence by adapting the CADmodel in such a way that some of the intentions are fulfilled already in the transferred data (at an extra amount of effort and cost). It becomes more and more reasonable that the designer wants full control over the manufactured prototype in terms of functionality, because the quality of a functional prototype will be influential for the quality of reasoning and testing about the design. In order to fulfill the demands of the designer transfer of his intentions is needed alongside with the geometric data. Nowadays this is mostly a discussion between the designer and the RP-expert. However, when that information can be provided by a formal language or a set of commands, it might be possible to embed it in a CAD transfer standard. This method offers also an opportunity to check the success of the designer intentions on the chosen RP-system during the data transfer. Another advantage can be that the designer does not need to adapt the CAD geometry too much to create a functional prototype. Although acquainted with this situation our research is not concentrating on this matter. The proposed process in this paper will take into account some intends of the designer, but leaving some parts of the process to be performed by human intervention. Most of the available conventional incremental Layered Manufacturing Technologies (LMT) for prototyping (e.g. laser stereolithography, selective laser sintering, fused filament deposition, etc.) are not qualified for the manufacturing of large-sized objects since these incremental layer forming methods are restricted to small and mid-sized objects [25], [29], [31]. The applied layer thickness in these processes is relatively small compared to the dimensions of the model. In LMT the number of deposited layers is proportional to the cost (effort and time) needed to manufacture prototypes. Rapid Prototyping is a computer-related process, which converts in a short turn-around time CAD geometry into a physical model and might be considered as an optimization process between productivity and accuracy [53], [54], [56]. The general principles of these layer producing methods are described in [5], [35]. For the efficient fabrication of large-sized models other rapid prototyping technologies are needed. Thick-layered object manufacturing (TLOM) is an appropriate technology for the large dimension domain.
The initial geometry, further on referred to as nominal shape, is based on a three-dimensional CAD-model. Main issues of producing physical prototypes are a selection of the Rapid Prototyping technology, intelligent geometry decomposition, approximation method of a nominal shape and slicing method. In the preparation phase of LMT processes the nominal shape is sliced to create layers and according to the TLOM process, the front surfaces of the thick layers are shaped and finally the layers are assembled and/or stacked to create a physical prototype. Slicing of the nominal shape depends on the geometric representation, the requested accuracy, the local geometry, the requested functionality, applied technology and on the final appearance of the prototype. These parameters jointly define the applicable layer thickness. Approximation methods, such as zero order, first order and higher order, are strongly related with the applied shaping technology, which relationship will be explained later. Slicing methods are based either on uniform slicing or adaptive slicing. An example of the different slicing and approximation techniques is presented in figure 1. When zero order approximation and uniform slicing of the nominal shape is applied the maximum layer thickness of the complete prototype is defined by the worst cusp-layer thickness ratio. This results into a situation that most of the layers are thinner than needed and that the appearance of the prototype is rather poor due to the staircase effect. The uniform slicing has been presented in [12], [57]. The staircase effect can be reduced when the uniform slicing method is substituted by an adaptive slicing [43], [32], [37]. But the appearance of the produced prototype based on adaptive slicing is still poor. In [40] a stepwise refinement method of adaptive slicing is presented. When zero order approximation is replaced by a first order approximation (slanted front faces) and has been combined with adaptive slicing the appearance of the prototype might be much better and smoother. This method results in a reduced number of layers (manufacturing time and costs) and an almost constant accuracy of each layer. The principles for layers of slanted front faces are discussed in [9], [18], [19], [57]. Some slicing efficiency comparisons about these methods are presented in [11], [33], [10]. Although the appearance of the resulting prototype is better, some finishing is still needed especially at the transition regions of the adjacent layers, in order to achieve a nicely presentable model. Shaping technologies for zero and first order approximations include water- or CO2-jet cutting, laser- or flame cutting, hot wire cutting, side face milling, etc. (the slanted front face machining process needs mostly more than 3 D.o.F. of the shaping machine). In recent years large object manufacturing or Thick Layered Object Manufacturing (TLOM) has been demonstrated by [48], [17], [18], [11]. Systems like Formus, Trusurf and Shapemaker offer specific technologies to fabricate large-sized objects. A survey of the progress in the field of TLOM is presented in [4]. 2. EXTENDING TLOM METHODS The utilization of the current TLOM methods might be extended by considering a more advanced slicing method based on second or higher order approximation and by decomposing the CAD-model in manufacturable parts. 2.1 Higher order approximation and the related shaping technology A brief outlook on the possibilities of higher order approximation is presented in [34], [18]. Higher order approximation create possibilities to apply thicker layers under the same or better accuracy and smoother outside surfaces of the prototype (a higher layer-thickness/accuracy ratio). The higher order approximation (circular and freeform) of the nominal shape however requires sophisticated shaping technologies. Shaping tools having a fixed curved shape or having a flexible curved shape are considered to manufacture the front faces of the layers. The shaping might be smearing soft material or cutting light materials like foam. Fixed shaped spherical or conicoid cutting tools [44] can be applied using the best fitting part of the cutting tool in order to approximate locally the nominal shape the best. The result will be a layer with a cusped front face, which might need some finishing action. When a flexible cutting tool is shaped dynamically according to an approximation of the local nominal shape curve then much thicker layer are applicable with the same accuracy and finally the prototype needs no or less finishing. For second or higher order approximation of the nominal shape a flexibly controllable cutting tool with infinite possible tool shapes is needed. To the best of our knowledge at this moment no cutting device or tool is readily available, which can meet these requirements. In the late seventies a flexible cutting system was under development at the Twente University in the Netherlands. A number of actuators, fixed to the blade, controlled the shape of the cutter. The cutting process was based on hot knife cutting (equivalent to hot wire cutting, [52]) and the application of polystyrene material. After some time the project was terminated. At the Utah University the usage of a flexible tool was considered for Shapemaker III, but the development was not completed.
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Our choice for the cutting technology based on higher order approximation is based on hot blade cutting. The blade is a heated, flexible, computer-controlled metal strip, which can cut polystyrene foam by melting the material, comparable with hot wire cutting. The blade is supported, but not completely fixed, by active controllable supports at both ends. By changing the orientation and the position of the supports the shape and length of the blade can be adjusted. The cutting blade has to be flexible enough to take up the requested tool profile, referred to as tool shape, in this case of a given shape curve (nominal shape) and rigid enough to sustain the tool shape during cutting (figure 2 and 3). From a mechanical point of view the blade behaves like a "physical spline", which takes up its shape by following the law of "minimum strain energy" [20]. The blade is in fact a very slender bar (small cross section / length ratio) [14]. Since the deformation is comparable to the nominal size of the blade, the linear theory is not applicable, therefor the higher order theory has been applied to describe the blade behavior. The higher order mechanical model results in a non-linear differential equation, for which no exact analytical solution has been found [21]. Geometry based modeling of the blade curve can solve the problem. The assumption is that irrespective of the blade cross section the curved blade can be substituted by its two-dimensional profile curve of "least strain energy" and prescribed length [23], [24], [39]. Kallay [27] presents a solution for this case, but the curve of least strain energy is generated as a three-dimensional one, which means that the curve will not bend in a planar way due to twist. Therefor another solution to the problem of representing the bent knife by a parametric curve has been worked out. Practical measurement is needed to calibrate the bending characteristics of the cutting blade in order to construct an accurate and reliable cutting tool. A supplementary issue is the cutting process itself. Further investigations are needed in order to have a clear understanding of the cutting parameters and material properties to control the cutting process. The challenge is the development of this cutting tool, together with the application methodology in a TLOM system. 2.2 Decomposing the CAD geometry First step in the RP preparation chain of activities is the transfer and the input of the geometry. The computer generated geometry (CAD model, nominal shape) is transferred into the RP slicing preparation section in a standard geometric transfer format. Obviously the slicing activity depends on the support of the applied format. Different standard formats are available [8]. At this moment no geometry transfer format supports such kind of functionality. The input of the RP preparation process is restricted to the transfer standard STEP and the geometric representation is the NonUniform Rational B-spline (NURBS). The STL format, which is a widely applied standard, is for a number of reasons not selected for the higher order approximation method. The parametric surface description [36] can be subdivided in a) surfaces positioned in 3D-space having no special relationship and b) surfaces with some relationship in so-called extended B-rep representations. NURBS representations are very elegant in terms of amount of data to be transferred and the exact representation of the nominal shape. So the STEP file/format combination with NURBS is preferred [7], [38]. Next step is the conversion of the geometry data into an internal representation, which facilitates the storage and elaboration of the data during the total process of calculation and manipulation of data. The RP preprocessing and preparation phase is based on the novel flexible cutting technology, on the analysis of the CAD model shape and when the functionality or the physical prototype realization requires it, the geometry will be decomposed in elements, which are easily manufactured, assembled, stacked and finished.
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Figure 4 Decomposition and assembly process When geometric models are produced in the product development process it must be supposed that complete assemblies are to be prototyped. The term assembly is related to system of parts or units that are in a specific relational, functional and morphological connection to each other. An assembly is decomposed for reasons of structure, shape, size, materialization, fabrication, handling and/or functioning. In figure 4 the decomposition is subdivided in four hierarchical levels and is presented at the right side of the figure. In the same figure at the left side the hierarchical manufacturing levels of the physical prototype by stacking and assembling is presented. The decomposition must support an efficient and competitive RP-fabrication process and must produce a functional prototype. A functional decomposition of assemblies will mean defining parts, units and subassemblies that have a kinematic degree of freedom relative to each other. At the same level the parts, which are to be produced from a different material are detected and are considered as inserts (see paragraph 2.3). These inserts are not manufactured on the flexible cutting system but are imported from other manufacturing processes or RPtechnologies and are embedded in the foam structure. The parts, set of parts, units, etc. that might be fabricated as a single piece without loosing any functionality are labeled as components. The decision which combination of the assembly parts are functional or not-functional related to each other, can not be automated because the functional decomposition depends completely on the designer’s intend and his knowledge about the functioning of the prototype. However, when the components are decided on the generation of the enclosing boundary of the set of parts (defined as component) can be performed in an automatically way. A morphological decomposition of the components into technological advantageous segments is based on partly morphological, partly technological issues. The morphological issues are related to detect regions of large changing curvature (e.g. at sharp edges, the so-called singularities, description in [22]), regions of a moderated change of curvature and flat regions. The technological issues are related to choose an optimal segment orientation for slicing and stacking, the applied layer manufacturing technology and the manufacturing efficiency in time and in costs. An algorithm might automate the decomposition of components into segments. Different shape interrogation methods are available. An overview of shape interrogation methods and convexity analysis are described in [16], [3], [46], [2], [47], [30], [13], [42]. These methods are often related to geodesics and visualization techniques. A well-known method to gather and to catalogue shape details is a so-called curvature map. The curvature map or curvature index supports the detection of specific shape details, which will introduce problems in the fabrication of the prototype. The curvature map or equivalence will support recognition and distinction of shape singularities of any form and will support and provide information about the most appropriate decomposition of the components into segments ([41], [49]). The calculation of curvature (Gaussian- Mean curvature, etc.) however is a computational intensive method and another suitable method is needed. The current research is directed towards a less computational method, which represents the curvature well. The method of reasoning about the actual decomposition based on the morphology will be an issue of research. The same curvature information set informs us also about domains of almost equal curvature, which are easily manufactured by LMT.
A geometric decomposition of the segments into thick layers is called adaptive slicing. This process can be automated on the basis of a slicing algorithm. In the slicing process some special cases can come across, which require locally a special slicing treatment. These cases might be expected at the slicing of the first layer, the slicing of the last layer and the slicing of in general degenerated layers. In some situations degenerated layers might be avoided by choosing a optimal slicing orientation. The method of slicing based on higher order approximation and application of adaptive layers is discussed in [23]. The curve representing the nominal shape at a specific location, which is perpendicular to the slicing base of the object, is matched against the tool shape profile curve. In order to speed up the matching process and to minimize the computing effort a library of tool profile curves is created. All library curves satisfy the requirement of being a planar least energy curve. A section of the tool curve is fitted to the nominal shape curve with a tolerancing constraint [24]. When the matching fails another tool shape curve or another section of the tool curve is considered or when no match is found in the library of tool shape curves the section length of the tool shape curve is decreased (figure 2 and 3). All the tool shape curves of the library are applied for matching
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f b kj Figure 5 Cutting tool correction and when a successful match is found all the tool data is stored. The complete procedure is repeated for all positions and the related nominal shape curves of the periphery of a layer. The result is a set of tool shape curves at each selected position at the circumference of the base contour of the layer. When the matching is performed for all positions at the base contour the minimum calculated layer thickness is calculated from the matching data along the periphery. Next a standard layer thickness, which is the maximum slab thickness fitting inside the calculated layer thickness, is selected from the foam stock. The tool shape data at the periphery is analyzed and a family of tool shape curves is selected, which will ensure a minimal tool adaptation and movements during the cutting process. The appearance of a prototype will be acceptable when a first order continuity is guaranteed in the transient region of the layers. This means that the top contour of a layer is of the same shape as the base contour of the next layer. Finally error compensation is applied on the tool orientation to assure the required first order continuity. Rotating the tool curve slightly in order to compensate this error performs the correction (figure 5). The top of the sliced layer becomes subsequently the base of the next layer. The choice of slicing positions will be influenced by the achievable preciseness of the shape approximation. A more extended description of this issue is presented in [23]. A technological decomposition of the layers into sectors is a complete technological issue. The tool interference with the foam slab, the hollowing of the layer, the maximum working space of the flexible cutting machine, the transportability and handling of the layers (e.g. weight, dimensions), the stacking provisions, the rigidity of the stacked sectors and the whole model, etc., etc. are all considered in this decomposition. Each of the parameters can have its own decomposition in sectors, but some decompositions are very position related (e.g. due to curvature, undercutting, tool holder interference) and others are more general (e.g. due to hollowing and dimensions). In order to perform the decomposition efficiently the decomposition sequence must start with the position related sectoring. This sectoring might already satisfy the remaining sectoring requirements. When sectoring is completed the created sectors are closed in order to create completely closed objects and are marked according to the way of stacking.
2.3 Process facilities In the process of TLOM some facilities are provided to improve or to assure the functionality of the prototype. However, the more possibilities are created the more complex the manufacturing process will be. With an overspent usage of these facilities the efficiency and costs become in some cases troublesome. So the facilities must be applied in an intelligent way. The following facilities are considered: • Application of inserts. • Choice of stacking method • Hollowing of the prototype • Degenerated layers All these issues are strongly related to the decomposition of the geometry and have to be considered in the TLOM process preparation phase. The application of inserts allows the application of different materials to improve the functionality of the prototype. When e.g. the functionality requires that two components of the prototypes must slide in respect to each other, then the foam material is not so suitable due to the high friction, the introduced surface damage and the soft material. It is advisable to have at least a different material at the contact parts of the sliding components. Another example is the creation of a pivot point, the pen material must be of different material than foam. Local pressure on the surface of the prototype can be withstood by an insert. These examples are related to a local improvement of the surface properties. Other enhancements are related to the correct simulation of the weight and weight distribution of a prototyped product, the gravitational stability of the object (fixture to the ground, etc. The most advanced insert is a skeleton, which guarantees the rigidity and inner connection of the structure. This facility needs intensive geometric reasoning where the skeleton will be effective related to the decomposition of the nominal shape. The dimension and the embedding shape and tolerance is defined in the CAD geometry and transferred into a geometric representation. The inserts of other materials than foam are extracted from the parts lists and the embedding surfaces are set to a specific useful tolerance. Further research is needed in what way inserts will be applied effectively. The assembly of the prototype is a logistic process since the layers are manufactured not necessarily in a stacking sequence. All fabricated sectors and layers should be marked separately and are assembled according to a specific scheme. Arrangements must be provided in the layer to perform the stacking of each layer in an accurate and unique way. This arrangement (e.g. a pattern of holes) is considered to be used during manufacturing to fix and to clamp the slab accurately into the layer cutting machine providing a better controlled tolerance of the final result. The stacking procedure must result in a rigid prototype with well interconnected layers During the free stacking of the prototype assembly of the complete prototype some stability problems will be expected. These unwanted situations can be avoided by simulating the actual stacking process and check the assembly in progress for stability [50]. When stability problems are encountered extra supports can be generated, or the sequence of layer building or the stacking orientation can be changed, or the stacking (fixed stacking) is performed with long stacking pins avoiding the layers to fall over. The support structure must be created and manufactured, which can be realized by the same TLOM manufacturing system. The method of applying a support structure tends to be not so very efficient, due to the extra machining time needed to shape the structure and the wasted material. In current depositing RP technologies the layers are solidified or deposited layer by layer and when an overhanging structure has to be produced, a supporting structure is created in an automated way [1], [28]. However, in TLOM the support structure is not needed during manufacturing but only considered during stacking and assembling of the prototype. Two different stacking methods are proposed: • free stacking • fixed stacking The stacking is performed on a construction table. The decomposed and manufactured parts are used to reconstruct the prototype. Each part, marked according to the position and the stacking sequence, will be provided and known during assembly. Free stacking starts with a first layer, which is orientated stable on the construction table. It is advisable to avoid support structures when stability is an issue. In some cases it might be realized by changing the stacked sequence and stacking orientation. During stacking the subsequent layers are accurately positioned with help of dowels, which are put in some accurately positioned holes assuring a proper and accurately positioning of the stacking part respectively to the adjacent part(s). This allows an accurate way of stacking and will assure the final tolerance of the complete physical prototype. The stacking provisions (pattern of holes) are freely chosen inside the contour geometry of the adjacent layers. The fixed stacking is based on stacking of the sectors and layers also on a construction table, though the positioning of the parts is realized by long stacking poles, piercing from bottom to top the stacked layers. This means that all
layers and sectors must have a common domain in which the stacking pole can pass through. This might be a very dominant constraint. This method will produce prototypes with better overall tolerances. The creation of stacking provisions can be supported by an automatic generation of solutions. An automatic stacking method is described in [57]. Another stacking issue is the assembly of inserts during the stacking process, which probably needs some fixture and the proper moment of inclusion of the part into the stacking process. Hollowing of a prototype will improve functionality in the following way: • Reduction of weight. • Minimization of the material volume • Functional hollowing. The stacking procedure involves the manipulation of the manufactured parts by human force. Also the manipulation of the complete prototype is important. It will be obvious that manipulation is only possible when the weight is limited reasonably. For reasons of costs, environmental issues and weight the minimum amount of material is chosen for TLOM without affecting the functionality of the prototype. In most of the cases the designer is not aware of the hollowing issues, because it depends also on the constraints of other facilities, such as stacking, efficiency of hollowing, etc. The decision of hollowing is only efficient when the decomposition into layers is performed. At that moment it is attractive to consider a kind of free hollowing. The outside shape of the layer is defined the room for stacking provisions is a selected according to existing experience and the remaining domain is available for effective hollowing. When hollowing is possible then the hollow inside must be defined. Methods for creating hollow layers are scaling down the outer contour [15], applying a dexel model [6], generating inner contours based on different standard geometric elements and decomposing layers into sectors, which contain
Figure 6 Layer hollowing the approximated nominal surface of the layer and keeping the inside of the layer hollow. Free hollowing will support cutting with a linear cutting edge, because no tolerance is set for this cutting. When CAD geometry includes a full definition of inside hollow shapes and these voids are functional from the point of view of the designer, the TLOM process must check the successfulness of realizing the prototype. For example, when thin wall structures are involved, the method of cutting and stacking may not be applied successfully. The required functionality is not met and a partial functionality of shaping the outside and the inside of the shape separately might be considered. However, when no hampering constraints are detected the inside hollow structure will be reproduced. This kind of hollowing influences the decomposition of CAD geometry in a very decisive way. Considering the following that during segmenting of the CAD geometry, a constraint not to segment the void of the prototype exists. The segmenting action will result in a completely different decomposition in contrast with the situation of having no constraints. When the segmenting is performed in a proper way, the slicing of the segment might also influenced by the hollowing. E.g., when the hollowing starts inside the sliced layer (figure 6). For the manufacturing of this type of hollowing the inside of the layer is shaped according to the same procedure of the shaping of the front faces of the layer (nominal shape curve matched with the tool shape curve). In this situation the occurrence of tool interference is due to the shape of the void more likely and must be checked intensively [45]. Finally some remarks about degenerated layers, branching and correspondence problems. With the introduction of segmenting the component the possibility is created to adapt and to avoid such kind of difficulties. The
slicing and especially adaptive slicing causes degenerated layers. One of the conditions for slicing is the maximum volume per layer and that results in an unpredictable slicing geometry. Near the base, near the top or near a local maximum in a surface of the component the requested tool orientations are sometimes much larger than the freedom of rotation of the actual tool. Those regions are defined as degenerated. However, when segmenting is applied the slicing direction might be changed and the degenerated layer is manufactured easily, at cost of more layers and sectors, by the same system. It must be clear that the efficiency of the complete process must kept in mind in order to decide to apply these facilities. The branching and correspondence problems are also solved by the facility of segmenting. Designer’s Intend
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Figure 7 Decomposition process sequence
3. TLOM PROCESS DESCRIPTION The TLOM process is described as a straight forward sequence of activities without taking into account the complete range of possibilities and measures to assure a continuity of the process (figure 7). The process description starts at the situation that CAD geometry is present, and the designer’s intend to create a prototype with a certain amount of functional properties. The elaboration of the CAD model into manufacturable pieces is stepwise described. The process description ends with the simulation of the manufacturing and the data transfer into the manufacturing part of a TLOM system. The designer prepared a CAD model for RP and has an intention to have a prototype which needs to have some characteristics, which simulated by the prototype would provide a better insight of the involved matters. The CAD geometry is transferred into the RP preparation section and parallel to that transfer the designer’s intend must be transformed in functionality of the RP object and subsequently the required functionality is realized by selecting the proper facilities of the TLOM process. For example the tolerance, approximation method, kinematic properties, final finishing, usage of inserts, hollowing, stacking method and other facilities are to be
defined. It should be obvious that in each step of the process, some reasoning must take place about the efficiency of that step. When the CAD geometry represents a complete assembly, then the assembly is taken apart in basic elements. Elements from a different material, which are considered as inserts, are taken out of the set of elements. When needed the common faces of the insert and the embedded environment are set to a different tolerance. Taking into account the kinematics properties and functionality of the prototype a recombination of the basic elements is performed. The enclosing boundary of the newly created subassemblies is calculated and these functional units are defined as components. Each component is geometrically analyzed for symmetry and other specific shape characteristics and subsequently a morphological analysis is performed which results in a set of surface points and the related surface properties in that point. The result of that analysis is applied for decomposition of the components into segments, based on change of curvature. Depending on the required functionality, such as hollowing, stacking method, etc., this might be a complex action. The decomposition of the morphological structure into segments must take all these constraints into consideration. In the next phase the segments are prepared for the slicing into layers. The principle issues are the orientation of the slicing, the stacking method and the avoidance of degenerated layers. The convex hull of the segment is created and analyzed for flat areas, which might be used to have a stable setup for the stacking of the object. A stable setup of the stacking is not important when fixed stacking is provided, but at this point it is necessary to take into account only free stacking because fixed stacking might not be guaranteed at the end by geometric constraints. The position of the center of gravity is calculated and for each flat face of the convex hull the stability is defined and ranked. When no stable orientation is detected the segment is analyzed at a later stage for stability (application of special stacking sequences or support structures) and a best slicing direction is selected. Starting with the most stable segment orientation, the orientation of the surface normal vectors in respect to the slicing base are analyzed, giving information about the possibility of degenerated layers. Degenerated layers require a different slicing approach and complicate the slicing of the segment. Commonly the first and the last layer of a segment or CAD defined hollowing will cause slicing problems, because the nominal shape of those regions are, as to be expected, also freeform shaped, which might result in too large tool orientations and not applicable layer thicknesses. These situations are treated with dynamic segmenting during slicing, which allows inside the created segment to choose for a different slicing direction, e.g. perpendicular to the base. The slicing itself is based on the usage of a flexible cutter, represented by a tool shape curve, which is matched with the local nominal shape curve of the segment (paragraph 2.1). When the match is within the required tolerance that tool shape curve is a candidate to be applied. Each successful matching is stored and ranked in an order, which is most functional. The base contour is analyzed and related to the curvature and tolerance a distribution of points along the base contour is defined. The matching activity is performed circumferentially at each point of the slicing base contour. From all those matching calculations the minimal matching height hm is chosen. The matching ranking is analyzed and for each base contour point a tool shape curve is chosen which is related to the next tool shape curve in creating an efficient motion and tool path. Next it is possible to analyze the nominal shape contour at height hm and extra contour points are defined to represent correctly the shape. With this extra set of contour points the matching calculations are performed again and with the extended set of calculations the height hm is once more defined. Next to consider is the effect of hollowing. We have a sliced layer and a set of tool shape curves, positions and orientations and when solid layer or free hollowing is requested it is suitable to define the final slab thickness of the layer. However when CAD defined hollowing is required the matching sequence with its own tolerance value must be applied for the inside hollow shape too. This results in a different set of tool shape curves, positions and orientations and may also have an effect on the height hm. Finally a decision about the slab thickness can be provided, because the maximum slab thickness fitting in the height hm is selected from the stock of foam. The slab thickness hs defines the top of the layer. For a good appearance of the prototype at least first order continuity must be guaranteed minimally at the layer transitions. The tool shape curves need an orientational correction with regard to a fixed rotation point on the base contour. A check on the maximal rotation limitations is performed and might result in choosing a different tool shape curve form the calculated tool shape ranking. When free hollowing is selected, the inside hollow domain must be shaped and depending on the chosen shaping of the hollow inside. This void is supposed to be present in the overall height of the layer, the straight shape of the cutting edge and the required wall thickness for stacking and rigidity the tool motions are defined. The tool paths are generated using the correct set of tool shape curves, positions and orientations. The tool shape curves are interpolated and at each interpolation point a check is performed on the tolerance of the tool shape curve and the nominal shape curve and on tool interference of the tool holder with the layer. Each case of
detected tool interference is stored in terms of position and which side of the tool is involved. This information is used later in the process for the layer sectoring. Next phase of the process is the sectoring of the layers. First of all, when hollowing of the layer is selected, sectoring of a layer is needed. The overall dimension of each layer is known and is to be checked for restrictions of the working space of the cutting machine, the stacking method and when needed for transportation.. This sectoring is arbitrary. However, layer sectoring due to tool holder interference with the slab to manufacture, is dominant and must be applied with priority over other sectoring requirements, because the sectoring is completely related to the specific tool position and leaves no opportunities to generate alternatives. The decomposed sectors are, when required, tested for the limitations of dimension and further sectored when needed. All sectoring activity is taking into account the minimal stacking requirements of having fixed points for an accurate positioning of the parts during assembly. After sectoring the layers, the sector side faces are constructed and the related tool paths are generated. The related front face outside and inside sequence of tool paths are selected from the complete set of tool paths. Some tool movements to and from the home position, some tool movements to inter-link the tool paths and the process related data is inserted in order to create a useful total manufacturing process for each sector. Generally, the front face of a layer will be manufactured in one circumferential cutting movement without any tool interference and the layer is defined solid, then no technological decomposition is needed. For logistic and stacking reasons the layers and sectors are marked and provided with accurate positioned holes which will correspond with holes at adjacent layers. Into the corresponding holes dowels are inserted creating an accurate method of positioning and stacking. The layers and sectors are assembled according to a stacking scheme onto each other at a construction table. The interconnection of the layers is applied by spraying glue on the contact surfaces and finally the layers are bonded together. In that way the segments are produced. Putting the segments together we create the components and finally the components will complete the prototype as manufactured. According to the requirements the prototype is finished in the required color and texture. An important part of the process is the visualization of correct matching, tool path generation, decomposition, cutting process, interference checking, correctly generated tool paths and tool shapes, etc. and for situation analysis [55].
4. CONCLUSIONS The proposed process is efficient for the manufacturing for large sized freeform objects. The process supports the designer’s need to control the functional requirements in the materialization of the prototype. This will offer an optimal mean, in terms of effort and costs, for reasoning and testing different product aspects with a tailored prototype. The facilities of decomposing the CAD geometry, hollowing and stacking to optimize the functionality of the prototype tend towards complexity and not competitive usage of the process. Reasoning about efficiency of each process step is needed to avoid this. 5. REFERENCES 1
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