CyberCut: A Coordinated Pipeline of Design, Process Planning and Manufacture
4.2 Conventional Approach
In CAD/CAM environments typical of the last three decades, there exists a loose interaction between the designer and the manufacturer (Figure 4.1(a)). During the design phase, the manufacturer is seldom consulted and the design is driven by requirements such as function, aesthetics and ergonomics. Occasionally, the design of the component is superficially checked for assembly problems by detecting interference with other components. Sometimes, crude tool availability checks such as tool diameter and depth are incorporated. However, the designer seldom has knowledge about fixturing, tool access problems and other manufacturing issues.
The design is completed and sent to the process planner. Manufacturing problems typically manifest themselves at this stage and the design has to be modified iteratively, thus leading to a longer time to manufacture. The disadvantages of this approach forces one to consider other alternatives. In particular, a closer coupling
between design and the manufacturing throughout the design phase is desired. Two such architectures to enforce the coupling are suggested below.
4.2.1 Manufacturing-dependent CAD Systems
The CAD system in this approach is geared towards the requirements of a manufacturing process. Only parts that can be produced by the downstream process can be designed [4.6]. The rules and restrictions for the design come from the manufacturing process and the designer is constrained by these rules during the creation of the part [4.7] (Figure 4.1(b)). This mode of operation is common in VLSI circuit design or MEMS (micro-electromechanical systems). Systems that have been constructed for these domains based in this principle are MOSIS (metal oxide semiconductor implementation service), MUMPS (multi-user MEMS process), and LIGA. A machining service that employs the manufacturing dependent CAD philosophy is described in [4.8][4.9]. The guarantee of manufacture of the parts or circuits obeying the rules makes these systems attractive for designers who want simple parts or circuits. Similarly, the manufacturers can easily vouch for the manufacturability of the design without much strain, since they can adhere to the common practices that they follow on the shop floor.
The principal disadvantages of these systems are the limitations that they impose on the designer. Manufacturers have a tendency to use rather conservative rules in order to assure manufacturability. Thus, it is common for parts to get rejected by the rules although they can be made with a little additional effort on the part of the manufacturer and would increase the value of the design at a small extra cost.
Additional problems arise in the mechanical domain. Abstraction of manufacturing information is often a very difficult problem, since the range of components that may be produced by conventional means is huge. When one reflects on the components of a car, airplane or any machine, one is confronted with an immense diversity of size, form and intricacy, all of which have been manufactured using some conventional process. Even if the constraint of rapid manufacturing is imposed, a bewildering choice of manufacturing techniques presents itself.
Assuming that some abstraction has been obtained, the presentation of this information to the designer creates a difficulty and the manufacturers are frequently unable to express their full capability. The designer, in turn, cannot exploit the full potential of the available manufacturing processes. Manufacturing constraints and rules are usually enforced using features that are a collection of geometrical or other entities that have significance for manufacturing. However, the design process often uses features that are different from the manufacturing features, making the design phase artificial and difficult [4.4]. The problem becomes acute if the component is to be used in an assembly. In this case, the chief characteristics of the interacting component are decided simultaneously, and one part is used to create the other for ease of design and to ensure the critical characteristics are consistent. Manufacturing dependent CAD breaks up this natural design process by demanding that the design be done in terms of manufacturing features. Thus, these systems may successfully be used when the desired part is within or at the boundary of the manufacturing rules and the interaction of the part with other components of the assembly is minimal.
When these systems reject the part due to rule violations, a third alternative, described below, may be found to be more attractive.
4.2.2 Bidirectionally Coupled CAD Systems
From the above discussion, it can be seen that both the traditional over-the-wall approach and the constrained CAD approach have disadvantages that impede the design process. A happy compromise may be attained by allowing the designer to use conventional techniques for the design phase, but enforcing checking with a process planner for manufacturability from time to time. An architecture is shown in Figure 4.1(c).
It is clear that too frequent a check for manufacturability would slow down the design phase; hence the process planning phase has to be automated. Recall that the conventional approach where the process planning is done after the design is completed does not mandate automation and can also be done by a human process planner (perhaps even better). However, automation in the bidirectional architecture of Figure 4.1(c) is imperative.
Also, since the design phase uses information in a representation that is different from the format of manufacturing information, a translation between the two representations is required. The translator is popularly known as “feature recognition”. The recognition presents the process planner with features that are used for planning. Feedback from the process planner can be communicated to the designer by mapping the manufacturing features back to the design. This approach allows considerable flexibility for both the designer and the manufacturer. It achieves a coupling of CAD and CAM through a cooperative mode of operation.
The design phase is kept decoupled from the process planning phase in terms of representation and manipulation of component data.
This approach, though highly attractive, is fraught with difficulties arising from the feasibility of the automation of feature recognition and process planning. Both are considered to be difficult problems and have been areas of intense research for the last two decades, as can be seen from [4.4][4.10]–[4.12]. Even most systems based on this approach use large number of heuristics and computationally expensive algorithms that limit their use.
Early systems using feature recognition borrowed much of the terminology from the manual process planning domain and algorithms were sought to recognise these features. The process planning system was supposed to assist and not replace humans. This approach, however, tended to force the feature recognition algorithm into making fine distinctions between different types of features, which then needed special routines for each type that needed to be identified. Recent systems have moved towards greater automation of the entire process, eliminating many arbitrary distinctions between features. For instance, the older systems maintained distinctions between slots, shoulders, rectangular profile pockets and pockets with arbitrary contour. By contrast, newer systems such as the one in Ref. [4.13] and also the one described here unify these concepts into a single feature called a pocket.
Distinction is maintained by flagging “open edges” for shoulder edges and leaving the tool-path planner to deal with these special edges, as will be explained later.
In summary, the conventional approach of over-the-wall manufacturing is clearly an undesirable scenario. The second approach of incorporating manufacturing into the design has the advantage of guaranteeing manufacturability, but suffers from the drawback of over constraining the designer. The third approach of having the design
checked from time-to-time has the advantage of flexibility and ease of use for the designer, but requires sophisticated algorithms.