Careful examination about which representation scheme suits machining feature modelling systems best, suggests that the volumetric scheme has more advantages over the surface scheme (Xu, 2001, Xu & Hinduja, 1997). With the volumetric feature representation, it is easier to extend feature concepts to general machining volumes that are associated with particular machining operations. As feature volume can define precisely the stock to be machined, the machining cost and time can be easily determined. In Figure 4.8(c), volumetric features F1, F2 and F3 are in essence the machining volumes for the part shown in Figure 4.8(a), if the blank is assumed to be the minimum enclosing box (MEB) of the part. The presence of a feature volume also makes it possible to represent other machining informa-tion such as tool approach and withdrawal direcinforma-tions. The normal to face f1 of feature F1 in Figure 4.8 represents the tool approach/withdrawal direction for machining feature F1. Face f2 can also serve the same purpose if feature F2 is removed first. With a volumetric
approach, it becomes easier to manipulate, decompose and merge a feature into volumes of material to be removed. One such example is a compound slot (Figure 4.9(a)). The surface representation of the slot (Figure 4.9(b)) presents difficulties in decomposing the feature into basic volumes, whereas the volumetric representation scheme (Figure 4.9(c)) does not.
The volumetric representation enables a more complete description of feature-feature interactions. Figure 4.10 shows an example with two blind slots interacting with a step. The interacting elements in the volumetric representation scheme are given by face patches (p1, p2) which are common to features F1, F2 and F3 (Figure 4.10(b)). In the case of surface representation scheme, the interacting elements will be two open wires, (w1, w2 as in Figure 10(c)). These open wires only designate part of the interacting boundaries, whereas the face patches can also tell the area of interaction as well as define the complete boundary of the interacting area. In fact, the incapability of handling arbitrary feature interactions by a system using surface representation scheme has been due to the inherent limitations of the pattern-matching nature of the surface representation scheme. Volumetric feature representation also makes it possible to detect certain feature interactions which would be otherwise difficult, if not impossible, for a surface representation scheme. This usually happens to those interacting features with boundary faces of un-equal heights. One of such examples is a closed pocket with an island whose height is different from that of the pocket boundary face(s) (Figure 4.11(a)). When these kinds of features are represented in volumes as seen in Figures 4.11(b) and (c), it leads to two features interacting to each other. A detailed account on feature interactions is given in Chapter VI. Furthermore, the two possible interpretations depend on the ways feature volumes are constructed. This is somehow to say that constructing feature volumes is also an indispensable part of feature recognition. This issue is essentially an “integration” issue which will also be discussed in detail in Chapter VII.
The intermediate shape of a workpiece can be obtained by using Boolean operations between volumetric features and the workpiece. The intermediate specification of a work-piece may include information relating to the current workwork-piece geometry, dimensions, and tolerances, relationships with the blank, stock material, component, previous and next intermediate workpieces, and features. This information is important not only for related manufacturing activities such as process selection, cutting condition determination and
Figure 4.9. A compound slot represented as surface and volume features
(a)
(b) (c)
NC code generation, but also for manual/computerised fixture selection/design, and qual-ity inspection. Using volumetric scheme, feature modelling operations for a FBD system are much simpler to implement. In fact, most FBD systems adopt the volumetric feature representation scheme. Detailed discussions on FBD are given in the next section. The obvious disadvantage of using a volumetric representation scheme is the additional com-plexity but this cannot be regarded as a serious objection since current solid modellers are powerful enough to handle a large number of complex components.
When the volumetric representation scheme is adopted by defining machining features as volumes to be machined, three propositions may have to be observed.
• Features should not be distinctively classified as depressions and protrusions because they do not all represent the volumes to be removed. Two such examples are an island Figure 4.10. Surface and volumetric feature interactions
w2
w1
F1 F2
F3 p2
p1
(b)
(a) (c)
Figure 4.11. Feature interactions and protrusion features
(f) (c)
F3
F4
F5
(d)
F1
F2
(a) (b)
(e)
(F1) in a pocket (F2) which is in turn on top of a face as shown in Figure 4.11(a). In this case, it is the surrounding material that constitutes machining features, F3, F4 and F5 (Figure 4.11(d)).
• To say that a machining feature is the actual material to be removed, a feature has to be an independent volume with no portion of it overlapping with another (machining) feature, i.e. there should be nil intersection between any pair of machining features.
Machining features illustrated in Figure 4.12(b) are not valid but those in Figures 4.12(c) and (d) are.
• To guarantee valid machining volumes from a design part, the initial state of the workpiece, i.e. the blank, has to be taken into consideration. The importance of considering the blank is illustrated in Figure 4.13 (Xu, 2001). By assuming the blank to be a billet, most systems would recognise two volumetric features as shown in Figure 4.13(a). However, if this part were to be machined from a casting/forging as shown in Figure 4.13(b), the system would recognise two brick-shaped volumes and a hole. Furthermore, if the part were to be machined from a blank with a pre-cast hole as shown in Figure 4.13(c), the system would recognise another different set of features. Take the component shown in Figure 4.11(a) and its blank shown in Figure 4.11(e) as another example. A valid set of feature volumes should be as shown in Figure 4.11(f). Note that feature F5 does not count in this instance.