How to plan assemblies for top-down design
Level = Intermediate

NEW! 16 customer video testimonials

When starting a new design, the first item that should be considered is the overall design intent and structure of the assembly. Just like part design, design intent should be captured when modeling the assembly.

Top-down design allows the designer to tie interrelated components together so if one changes, the related components also change. This is a powerful feature that, if managed intelligently, can help you take full advantage of the assembly model.

2D layout sketches, planes, and axes can be used to control large complex assemblies. These simple entities comprise an assembly skeleton.

Goals for Assembly Management
The goals of using the techniques described within this article are as follows:

• Collaboration (internal, external, data sources and types). Ability to break up the assembly into subassemblies so multiple designers and external suppliers or contractors can work on the same project
• Ability to easily define and document the design intent of the assembly
• Ability to analyze assembly fit, form, and function
• Ease of modification
• Ability to simulate assembly movement. This allows for the analysis of assembly motion and range of motion in a virtual prototype.
• Optimize the performance of large assemblies
• Control access to the design intent within a collaborative assembly

Top-down and Bottom-up Design
The ability to define geometry based on other related assembly components is called top-down design.

An example would be the front bucket assembly. If the range of motion for the bucket is not correct, the linkage would need to be adjusted. In a top-down design the change can be made easily if the skeleton is defined to accommodate the movement. The inter-related arm components will update automatically based on the layout change (skeleton).

Top-down design is a powerful feature that affects the entire design each time it is used. Therefore, it should be reserved for important design decisions only. Use this type of in-context reference only when it is important to the overall design.

Bottom-up design is the opposite. The assembly components are modeled as separate parts without referencing other assembly components. This means if the other assembly components change, the changes are not propagated to the assembly component. The changes need to be made in a number of places.

In most cases the best method used is a cross between top-down and bottom-up design. It depends on what data exists in which format, and how many parts come from suppliers versus designed, sourced, and manufactured within your company.

The main challenge for users is knowing how and when to use top-down design. It can also be a challenge understanding ways to simplify or modify the design process especially when features and components are related to other assembly components. Parts affected need to be opened and resolved to change the design intent.

Where to Start

1. Plan – Take a step back and resist the urge to just start modeling the assembly. While this is tempting, take the time to think about and plan your assembly. This is time well spent and can pay off later in the process.
2. Define what is known – While each project is different, there are certain requirements that are known. Capture these before you start. Having well-defined requirements can insure you are going in the right direction. It will also save time, indicating whether you have all the information needed to complete the project.
3. Create a hand sketch of the assembly skeleton – Before starting the assembly model, create a simple hand sketch. Outline during an initial design review ideas and functions that should be included within the design. Document how the design should move. A simple hand sketch allows you to try different ideas and document what you know quickly. Once this first sketch is completed, it can be created in the skeleton part. Simple skeletons are small, fast parts that serve as an aid to managing large assembly performance. (Assembly skeletons are defined more completely later in this article.)
4. Create the structure – Define the major subassemblies and skeleton parts. Create configurations with each major subassembly and the assembly skeleton and one of the skeletons. This allows a designer to open the assembly and not open and resolve the other subassemblies.
5. Review and capture the design intent – Complete and test the assembly skeleton. Insure the features are named and the assembly motion works as intended.
6. Create and mate the subassemblies and assembly componentst – Create the subassembly component using the skeleton to define and mate the components.

Assembly Skeletons
A skeleton is a simple structure – comprised of 2D sketches, planes, and axes – used to relate assembly components and define movement.

When defining the assembly skeleton, define the important features that are related to other assembly components.When defining the assembly skeleton, define the important features that are related to other assembly components.

In the example shown in Figure 3, the major mounting joints and the 2D sketch geometry to define the movement of the arm are good candidates for inclusion in the skeleton. This skeleton is one of a couple included in the overall tractor design.

Advantages of using skeletons:

• Assembly skeletons can help you define the design intent using simple objects
• Eases collaboration especially when the assembly is organized with subassemblies tied to the skeleton
• Stimulates design intent and provides basis for documenting the function
• Skeletons are more robust in nature and easier to understand and modify due to the use of simple sketches containing 2D geometry
• Assembly sketches can be used to document design intent, review fitment, and help analyze assembly tolerance stack-ups and motion
• These features can be named to document the design intent of the assembly
• A skeleton or layout can be created within the assembly or inserted as a component
• Allows for assembly components and part features to be attached to simple, robust, and named entities
• Assembly components and part features attached to the skeleton can be displayed with the other assembly components or part features suppressed
• Design tables can be used to define configurations that show different assembly positions, sizes, and more, or these explicit constraints can be suppressed to allow for dynamic movement

The next question that needs to be answered is how to create the skeleton. The 2D sketches, axes, and planes can be defined inside the assembly. But a better way to manage the skeleton is to create a part and insert that part into the assembly. The following are practices that make using skeletons more productive:

• Insert the skeleton as the first assembly component. Drag the origin from the skeleton part from the FeatureManager® design tree to the origin of the assembly. This will fix the skeleton to the same X-Y-Z origin. Otherwise, you will need to un-fix the first assembly component and add the mates.
• Insert the skeleton part as an envelope. This will insure the skeleton component does not appear on the BOM (bill of materials).
• If desired, make the skeleton part read-only for everyone except an administrator or project lead. This way you limit access to add or change this important design element.

The sketch in Figure 4 shows the 2D layout for the arm assembly. The position and movement of the arm are controlled using this sketch.

Assembly Structure
The assembly structure is an important component in assembly management for several reasons:

1. Enables effective control of geometry displayed
2. Allows others to collaborate on the design
3. Enables reuse of the structure of the BOM

It can be beneficial to break the assembly into logically grouped subassemblies. This way, subassemblies can be created separately based on the make-up of the design team, manufacturing and assembly considerations, or a combination of both.

Another benefit of breaking the assembly into subassemblies is performance. This allows just what is necessary to be viewed or fully resolved. The designer can suppress unrelated subassemblies. This is better than using lightweight assembly components. A configuration can be used to suppress the other subassemblies. Lightweight assembly components can also be used for performance reasons. A lightweight component or subassembly is not fully resolved and loads faster than a fully resolved assembly component.

The level and grouping of assembly components can be changed at any time. If you change your mind later as to the assembly structure, the change can be modified. Use the following commands to change the structure:

• Form New Subassembly Here
• Insert New Subassembly
• Dissolve Subassembly

Conclusion
Assembly skeletons help define the design intent using simple objects, top-down design is easier due to the simplification of the information, designs are more robust and easier to modify due to the skeleton, and the skeleton makes it easier to collaborate when the assembly is organized with sub-assemblies tied to the skeleton.

---

Rate this article     Not useful     Somewhat useful     Useful     Very useful     Extremely useful Comments: