Part III: Working with Assemblies
For example, you may create a box and put it into an assembly. You must then create a lid that is
parametrically linked to the size and shape of the box. You can create a lid part in the context of
the assembly such that the lid always matches the box. Sketch relationships, dimensions, and fea-
ture end conditions from the lid can reference the box. When the box changes, the lid also changes
if the assembly is open.The assembly maintains a record of each in-context reference. If the box is changed with both the
assembly and the top open, then the top updates, but if the box is changed without the assembly
being open, then the lid will not update until the assembly is opened. The record of the reference
that the assembly maintains is held in what is called an update holder, and in recent versions, it is
all but forgotten.Cross-Reference
Chapter 12 discusses in-context reference update holders. These are the pointers in the assembly that hold the
reference information. These holders are hidden by default and do not enable any real functionality, but they
do serve as a reminder that the assembly has in-context references and can be queried to tell you what parts
the in-context relations go between. n
Advantages of in-context modeling
The advantages of in-context modeling are obvious. I spent the first couple of chapters of this book
discussing the strengths of parametric design, and in-context modeling is just an extension of para-
metric techniques to include parts in the context of an assembly. Making a change to one part and
having all related parts update offers indisputable advantages.Some users approach modeling haphazardly, where if it works, it’s good enough. For some types
of work, this is acceptable and “good enough.” For example, it usually works when you create
something that will never be changed or if you are working on concept models.On the other hand, users who need to build models that will be reused frequently and changed
often must approach the decisions they make during modeling as if they were playing chess: Each
decision has consequences; you rarely know exactly how things are going to turn out but need to
prepare for the most likely contingencies.In-context modeling presents the same types of decisions. You can do it fast and dirty or methodi-
cally and with much consideration beforehand.Potential problems with in-context modeling
Some issues may arise from the technique of driving changes from a different part model within an
assembly. There are no problems with the overall concept of in-context modeling; the problems
occur with the practical application of the technique. In particular, the biggest problems seem to arise
when in-context techniques are combined with other techniques. You must be very careful about
file management issues when in-context references exist in your assembly.