Rapid prototyping (3D printing) and parametric CAD systems

Article written with Marco Galloni, Umberto Cugini, and Giuseppe Bocchi and pubblicato in November 1994 for “27^th ISATA – International Symposium on Automotive Technology & Automation“, page 51-57, ISBN 0-947719-64-4

Today CAD systems are mainly conceived as tools for the production of detailed and fully defined geometric models. The technological improvement of such systems are dramatically reducing the time required to generate the final geometric model of a new product. The introduction of rapid prototyping techniques (nw called 3D printing) offers a new perspective in their usage and sets new requirements for the software tools. CAD systems must fully support the designer in implementing the feedback coming from the analysis of the prototype. In our experience, the feedback is often an indication for the need to further refine and optimize the part. While a physical prototype can catch minor design errors, in general does not affect the main logical and functional structure of the part. The designer should be able to change the shape of the part, add new minor features and modify geometrical dimensions without losing the model geometrical integrity and consistency; parametric and variational CAD systems support this kind of functionalities.

In order to verify the practical implications of coupling prototyping techniques with parametric design tools, we chose a real-life design problem: the design of a motorbike engine with six cylinders and five valves each cylinder. The design was already sketched in the main structure, with several characteristics and dimensions left undefined.

First, we defined a logical structure of the specific design process, the freedom degrees of the part and the full set of functional and geometrical relations. Then, on the basis of this analysis, we scrutinized the major CAD systems commercially available and selected Eureka, a new parametric and variational CAD system by Cad.Lab. Compared to other systems, Eureka provides a wider range of tools for representing the design relationships and offers great functionalities and performance combined with direct tech support from the software developers.

Given that Eureka, like other similar CAD systems, is intrinsically unable to represent all the constraints and relations identified in the analysis phase, we made a pondered selection and simplification of the original set. Then, we defined a solid model containing a subset of the selected relations and controlling parameters and imposed the remaining relations and constraints. The result was a geometric model enriched with non-geometric information and flexible enough to support deep dimensional and morphological modifications.

Starting from an initial configuration, we produced the first tentative real prototype by means of a specialized service supplier, employing photopolymerization techniques. Using the prototype and the drawings obtained from the CAD model, we checked the results with the designer and the technological and production experts. The meeting produced a lot of discussions based on the prototype with a minor interest in the traditional drawings. We collected and coordinated dimensional and morphological changes proposed by the experts and implemented them into the CAD model.

The use of a parametric and variational system was fundamental in this phase because the changes could be done interactively with immediate graphical feedback. The changes required by experts were fully supported by the CAD model we realized because, as expected, they do not have any impact on the logical structure of the design but only in non-functional detail and minor
aspects. This experience showed also the limits of these CAD systems; they are often unable to represent non-trivial design rules, as for gears and helical springs dimensioning, and to map values computed from constraints evaluation within availability catalogs, as for bearings.

Feedbacks from the experts showed that the immediate availability of a real prototype is effective in reducing the lead time and improving the part quality.

The test case

The impact of rapid prototyping in the design process and methodologies and the practical implications of coupling prototyping techniques with parametric design tools can be pointed out only experimenting with a complete design process on a significant test case. Therefore, we looked for a mechanical part or component satisfying the following criteria:

• it should come from the industrial world, in order to work with a real problem;
• it should exhibit a reasonable morphological complexity in order both to generalize the results and to ensure the feasibility of a complete parametric model with current technologies;
• the author of the original design should be involved in the project in order both to provide the know-how required for the design and to extend the study to different design solutions.

Components from three different industrial areas were analyzed: packaging machines, pumps, and motors. After the analysis of different alternatives, we decided to work on a mechanical component of a motorbike engine; in the engine, the head (Fig. 1) shows the mechanical and morphological characteristics we are looking for and satisfies all the criteria pointed out.

We selected a four strokes competition engine having the following characteristics: six cylinders of five valves each, “V” architecture of 90°, water cooled, total displacement: 750 cm3, bore: 65 mm, stroke: 37.5 mm, maximum rpm: 14000, compression ratio 12:1.

It was the result of a previous experimental work made by the designer G. Bocchi, stopped at the conceptual design phase, [1].

Our study started with documents and data produced in the conceptual design phase; the information was mainly available in the form of sketches with a lot of not fully defined details. We organized a series of meetings, involving the designer and CAD experts, for acquiring the basic information on the engine; according to the designer indications, we formalized the global requirements of the design, the main design rules and an operative procedure for completing the design.

The design requirements were at a high level and in implicit form (e.g., to maximize the specific power within the total displacement); we tried to explicit the requirements in order, both to get clearer targets for the parametric model definition and to define a set of verifications and analysis on the model and on data obtained from it. The main requirements of the head
design are:

• to optimize the combustion chamber size and shape in order both to avoid misfiring and to optimize the volumetric efficiency;
• to identify the threshold in the engine bore that made convenient the five valves solution over four valves solution and, if possible, to experiment with both solutions;
• to analyze the initial design for verifying the proposed solutions;
• to complete the design according to the defined performances and cost ranges.

The next activity was the acquisition and description of design rules, i.e., rules used by the designer for choosing among several alternatives and for positioning, connecting and dimensioning functional elements of the engine head. We tried to explicit these rules in terms of constraints, equations, geometrical relations, conditional statements, and so on. This phase was fundamental in order to obtain, in the following phases, a model satisfying the designer expectations. We experimented that only a small part of the designer know-how and engineering rules can be mapped in an explicit form useful for being inserted in a computational model, [2, 3]. For example, the optimization of inlet and exhausts ports shape involves so many and so complex rules,
methods and experiences that could be inserted only partially in the model. Furthermore, talking with the designer, we found out the design steps logical sequence and the priority associated with each target; this helped us to plan the work and to join results from each subproblem.

The CAD design phase

Within the previous phase, we chose a meaningful test case and collect unstructured knowledge on the engine head design problem. In order to select a parametric CAD system, we analyzed some commercial systems and a research prototype and finally decided to implement the model in the Eureka system, a new parametric and variational system produced by Cad.Lab SpA (Casalecchio di
Reno, Italy). The Eureka features relevant for our work are, [4]:

• it integrates solids modeling, surfaces modeling and simple features modeling;
• it offers a wide set of constraints typologies with good performances;
• it provides tools for the definition of non-geometrical relations and constraints;
• it supports a data format toward rapid prototyping systems;
• it is available for research use;
• direct support is available from the original software company.

Eureka system uses variational techniques to solve constrained 2D profiles and parametric techniques to represent 3D models. This mixture of parametric and variational techniques is usual in commercial CAD systems; it seems to be the best compromise between the flexibility of the numerically unstable and slow variational approach and the stiffness and performances of parametric approach [5, 6, 7]. The Eureka standard work procedure starts with the definition of a 2D profile, followed by the imposition of a set of constraints and the definition of simple relations among parameters. The behavior of the profile is then tested by changing the value of some driving parameter and analyzing the resulting geometry. When the behavior has been verified, the profile is ready to be extruded along a vector or a spine. In Eureka system, various profiles can be created in different work planes and connected by means of positional constraints; furthermore, the parameters of each profile can be related, via algebraic relations, to parameters of others profiles (e.g., the radius of this fillet is two times the radius of this hole).

We chose to work initially on a single module of the head, corresponding to a single cylinder and then to combine single modules in order to obtain a complete engine head, [8]. According to the Eureka standard procedure, we started defining a group of key profiles of the engine head. The first profile was the triangular combustion chamber cross-section, important for the
definition of valve angle. In order to generate the combustion chamber, the profile extrusion was intersected with a cylinder so providing the required geometrical references for positioning the valves (Fig. 2). Working on the combustion chamber volume, we studied both the four valves and the five valves solutions. Using advanced inquiring functionalities we were able to
verify that, under the design requirements, the five valves solution is the most convenient; in fact, the inertia of the valve reaches the optimal values even if the useful area decrease.

Then we attached the spines, a set of spatial curves, on the combustion chamber and used them to extrude a circular cross-section. The obtained solids, completed with the spark plug housing, refined with adequate fillets and connected to the combustion chamber, previously defined, represents the empty volume of the engine head. Offsetting the surfaces of resulting solid we generated the central solid structure of the head with complete inlet and outlet manifold, shown in Fig. 3.

The next activity was the definition of a new 2D profile: a cross-section of the engine head external box, shown in Fig. 4. In this activity we experimented the limits of variational techniques: the definition of a complete and not contradictory set of constraints requires a lot of work; the users spent a lot of resources facing the system idiosyncrasies instead than solving the problem. The external box was connected with the combustion chamber, inlet and outlet manifold thus obtaining the basic module of the head. The following activities were the insertion on the module of minor details such as holes to fix the cam shafts and the definition of relations among parameters of different sub-parts. In the end, we verify the behavior of the global model shown in Fig. 5.

We were not able to reach the expected flexibility on the final model: the main problems did not depend on dimensional and morphological variability but on the significance of the results. For example, we were unable to set a parameter that switches completely and correctly between four and five valves configurations. On other targets, not so ambitious, the model behavior was
satisfactory.

The prototyping phase

The use of rapid prototyping techniques introduces an interesting question: which is the best status when to stop the design process and to produce a prototype? In the engine head design, we stopped just before the insertions of minor details on the model that could limit the flexibility of the model and preclude some design alternative. At this point, the designer already made all
the important decisions on the engine head but he needed a validation mechanism.

The generation of a computer file containing the model description for the rapid prototyping services required a single command on the CAD system. The data format used by the CAD system was the SLA, the standard format used for data exchange in stereolithography systems; using SLA a solid model is described by means of triangulated surfaces represented in binary or ASCII
formats. The first tentative for obtaining a prototype failed because the Eureka triangulation algorithm left some hole along the boundary between NURBS patches of the model. The software company, involved in this problem, quickly fix the algorithm and we could produce a completely close triangulation of the part.

In a few days, we obtained the physical prototype (Fig. 6) by means of a local rapid prototyping service. When the prototype becomes available, we organized a meeting with designer and CAD experts providing also drawings plotted from the CAD model. The main interest was on the characteristics of the prototype and with minor use of drawings. The meeting confirmed the main design
choices shown by the prototype and suggested minor dimensional and morphological changes. We collected the indications and directly implemented them on the CAD model modifying the corresponding parameters. We observed that:

• the feedback from the physical prototype suggested only minor changes to the model and did not impact on the basic design choices;
• the parametric model, as it was, supported the immediate modifications suggested by the meeting;
• the combination of the parametric model and the physical prototype was effective in the verification and “tuning” of the mechanical part.

Peoples involved in the design, agree that in a more favorable situation, with not so expensive rapid prototyping tools, the designer should analyze more than one prototype corresponding to different design alternatives, e.g. four and five valves solutions.

Conclusions

The increasing diffusion of rapid prototyping techniques can really change and enhance the current design processes; the availability of a physical prototype, reproducing physically the model shape, cannot be effective without adequate software tools. Our experience demonstrated that parametric techniques can provide today the required modification mechanisms. We also pointed out the limits of these systems but the global valuation it is very positive.

Acknowledgment

We wish to tank the CEDI Laboratorio CAD Avanzato at the Università degli Studi di Parma for providing computing resources and technical support and Cad.Lab SpA for providing the software tools and founding the prototype.

Bibliography

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