Designing top-down and bottom-up are the two approaches for assembly design. In actual fact, the design and development of new products combines both of them.
Bottom-up design begins the design of the individual parts and then inserts them into an assembly. In contrast, top-down design is the design methodology used most often when creating new parts in an assembly context. In this case, constraints are made between the existing geometric elements within the assembly. The most important thing to understand is how to manage this kind of association between related components.
I’d like to describe a tool I found most useful in managing associations: reference geometry. ZW3D 2012 SP2 provides three ways of handling reference geometry, which I will describe to you. (See figure 1.) The three types are as follows:
Associative reference (without a timestamp)
Associative reference with a timestamp.
(Note that for each of these, five types of 3D reference geometry are supported, curve, plane, point, face, and shape.)
Figure 1. The dialog box for controlling reference geometry
1) Non-Associative Reference
When the Associative Copy option is left unchecked, ZW3D creates a one-time static copy of the selected geometry from the external part. This is the simplest form of reference, with no association.
Now, you may wonder, “What happensto the reference geometry whenthe feature on which it depends is edited or deleted?”
It turns out that the reference geometry is unchanged, even when the selected external feature is deleted. Because this is a one-time static copy, it is un affected by any change from external parts, as shown in figure 2 below.
Figure 2. Example of using non-associative reference
2) Associative Reference (without a timestamp)
When only the Associative Copy option is checked in the dialog box, an associative reference is created without a timestamp. This is the option used most often for top-down assembly design. The most important point to note here is that the reference geometry changes with the selected external feature.
Again you may be wondering, “But whathappen to the reference geometry if the feature on which it depends is edited or deleted?” When the selected external feature is edited or modified, the reference geometry is updated automatically, as shown in figure 3.
There is one exception: when the selected external feature is deleted, then it can no longer regenerate the reference geometry.
Figure 3. Example of using associative reference without a timestamp
3) Associative Reference with a Timestamp
When both options are checked in the dialog box, then ZW3D create associative reference with a timestamp. See figure 4. This provides greater flexibility for designers to modify the status of selected external features, because the reference geometry depends on two factors: (a) the selected external feature, and (b) assigned state of the feature.
What happen to reference geometry if the feature on which it depends is edited or deleted?Well, it turns out that the reference geometry updates to the recorded state of the selected external feature. I’ve illustrated this effect in figure 5 to show this effect more clearly. So when modifications occur after the recorded state of the selected external feature, then the reference geometry will remain unchanged. On the other hand, the designer can change the recorded state to redefine the reference geometry.
Figure 4. Example of using associative reference with a timestamp
Figure 5. Specific application of associative references with timestamps
For the three different reference types, let’s take a look at a summary, as shown in the table below.
What happen to reference geometry when the feature on which it depends is edited or deleted
The reference geometry is unchanged; it is unaffected by any changes
Associative Reference without a Timestamp
The reference geometry changes with the selected external feature
Associated Reference with a Timestamp
The reference geometry is updated to the recorded state of selected external feature
During the design and development of products, these three approaches allow designers to apply reference geometry in assemblies flexibly. The combination of parametric and associative design provides a much more efficient product design process.
The primary difference between top-down design and bottom-up designis associative design. This determines the references between different components within assemblies. Updated reference geometry provides three type references: non-associative reference, associative reference without a timestamp, and associative references with a timestamp.Designers can apply reference geometry during assembly design in a flexible manner.
ZW3D, Assembly design, Inter-part modeling, Reference, Associative, Non-associative, Timestamp, Reference geometry, Top-down, Bottom-up
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