Design Patterns Explained

5O Part II • The Limitations of Traditional Object-Oriented Design
... because it must
determine the order
of the features
The problem is, I cannot just generate the NC set commands for the
three features independently of one another and hope the part
comes out properly— there is often a particular order that must be
used. In the example, if I do the slots first and then the cutout, as
shown in Figure 3-3, when the cutout is made (remember a cutout
is created by using a high-impact punch), the sheet metal will bend
because the slots will have weakened the metal.
Figure 3-3 A bad approach to cutting out the openings. This sequence
results in weakened, bent sheet metal.
I must create the shape shown in Figure 3-2 by punching out the
cutout first, then doing the slots. This works because the slots are
created using a router, which applies sideways pressure. Making the
cutout first actually makes the job easier, not harder. This is shown in
Figure 3-4.
Fortunately, someone had already worked out the rules for the
expert system. I did not have to worry about that. I took the time to
explain these challenges so that you could understand the type of
information needed by the expert system.