Top Down Design

TOP-DOWN DESIGN

The idea behind Top-Down design is to try to build in intelligence between the fit, form and function of parts that reside in an assembly.  You try to capture this fit, form and function into the assembly first, and then pass the appropriate information down to the part level so that a change made at the assembly level or to one component in the assembly can drive updates to the rest of the critical parts.

In this method, many of the components are created in the assembly, instead of being assembled into the assembly.

SKELETON MODELS

The best way to capture fit, form and function for the assembly is to create a special kind of component called a Skeleton Model.  A Skeleton model is similar to a regular part, but it is treated specially in the assembly.  For example, a skeleton model is automatically excluded from the Bill of Material, where if you just created a regular part and used it like a skeleton, it would still be reported.

There are also restrictions that can be placed on regular parts that skeleton models are exempt from, or get special rights to deal with.  For example, you can make it so you are not allowed to copy surfaces from a regular part to another regular part, but you can still pass surfaces from the skeleton model to a regular part.  In defining such restrictions, you avoid creating parent-child relationships between individual part files, making the model more robust.

To demonstrate this principal, we will first create a new assembly file called Stacker.  Be sure to use a Design sub-type for this assembly, just as we did for the last lesson.  Also, once you have the assembly started, be sure to turn on the features in the model tree.

Bottom Up Design

PLACEMENT CONSTRAINTS

If you recall, when we first got into the placement window for a new component, the current constraint was listed as Automatic.  It was awaiting entity selection to determine automatically which of the pre-defined constraints made the most sense.  I would recommend starting off with Automatic and then adjust the constraints accordingly.  In this section we will look at the pre-defined constraints that can be assumed or selected directly.

Mate

A mate takes two surfaces and points their normals towards each other and lines up both surfaces, as shown in the next figure.

Mate Offset

This is the same as a Mate except that the distance between the surfaces can be less than or greater than zero.

Align

An Align takes two surfaces and points their normals in the same direction and lines up both surfaces, as shown in the next figure.

Align Offset

This is the same as an Align except that the distance between the surfaces can be less than or greater than zero.

Orient (Parallel)

This is similar to an Align offset, except that you don’t specify the distance between the surfaces. 

Insert (Coaxial)

An insert takes two cylindrical surfaces and lines up their axes, as shown in the next figure.

Tangent

A Tangent constraint takes two cylindrical surfaces, or a planar and cylindrical surface and makes them tangent to each other, as shown in the next figure.

Point on Surface

Places a datum point or vertex on a surface, as shown in the following figure.

Edge on Surface

Places a straight edge on a cylindrical or planar surface, as shown in the next figure.