Design

CAD basics: Top down modeling

Guest blog by Patrick Gannon

There are various methods for top down modeling in CAD design. From a high level, this is the use of a central file to define an overall design. This central file can come in the form of an assembly, part, or a design table (excel file). Top down methods allow an engineer or designer to control the design in one location. Instead of needing to open all files within the design to perform updates, all dimensions can be changed in the one file, updating the entire assembly with ease. In contrast, when using the bottom up method, you model each component of the design independently. Matching features can be verified in the assembly model once the parts are inserted, but changes need to be made to each part individually.

Assembly
The use of an assembly file as your central file involves the creation of adaptive parts within the assembly file. A blank part can be created within the assembly—and references from the assembly used to create the sketches that define the features in the part files. The workflow is relatively simple, but this method can create complex relationships. This is particularly troublesome if you need to change components used as references in other components. The result can be unwanted crashes of the features or the software itself.

Parts
Part files can be used in a couple of different ways. Two of the main methods using part files are skeletal modeling and multi-body modeling.

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Figure 1: Skeletal model.

Skeletal model
The part file in the skeletal modeling method contains all the base geometry (sketches, work features, and parameters) needed to define the design. The file is used as a reference within the individual components, keeping the relationship between the parts of the overall design. The skeleton can be derived into each part file contained within the design. In this way, any the sketches, work features, and parameters contained in the skeleton (see Figure 1) can be used in the part file.

Figure 2: Multi-body part.
Figure 2: Multi-body part.

Multi-body part
The multi-body method goes a little bit further. Instead of just the base geometry (as in the skeletal model), the multi-body model can contain all the ‘physical’ geometry of the design components. This allows matching features within an assembly to reference the same sketches (see Figure 2). The geometry is no longer created in separate files. It is created using separate bodies within a single part file, eliminating the need to create multiple files then referencing specific features within that file. Instead, the process is more automated within the multi-body part to create the assembly from the separate bodies.

Design table
A design table is used to define parameters within files. The benefit of the external spreadsheet file is the ability to reference it in multiple files as well as the abilities within the spreadsheet itself. You can manage complex functions to define relationships between dimensions within the parts. If you know that the length of a gusset (Dimension A) within the design will always be 1/8” shorter than the form it fits within (Dimension B) to allow for welding tolerances, you can define Dimension A as [Dimension B-1/8). This is clearly a simple example, but succeeds in demonstrating the point.

 

Written by

Paul Heney

Paul J. Heney is Vice President and Editorial Director of Design World magazine. A graduate of the Georgia Institute of Technology (Georgia Tech), Paul has a Bachelor of Science degree in Engineering Science & Mechanics with a minor in Technical Communications and Biomedical Engineering. He has written about fluid power, aerospace, robotics, medical, green engineering and general manufacturing topics for more than 25 years.