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The design skeleton is the basis of the Top Down Skeletal Modeling concept. The principal is that you add enough sketching, parameters, and work geometry to guide the overall design. It can contain anything you like, and there is no hard and fast rule about how it must be established. You do what works for you.

Some things to remember about base skeletons are:

  • They establish the core instruction that influences the entire design structure. 1-stop shopping
  • They provide a consistent design schedule, all parameters are fed from same list
  • They guide the overall direction and limits of the design, like the skeleton of the human body
  • They should not contain any mass

The main things I’d like to focus your attention on today are:

  • How this concept works
  • What should be included in the base skeleton file

How this concept works

The principal is that you sketch out an overall plan for the design. This plan is then distributed to the project part components via the derived component process.  The individual part files then refer to features and parameter that are stored in the skeleton for consistent instructions.  When an overall design specification such as ‘overall width’ needs to be adjusted, the change is made in the skeleton, and all related components update automatically.

Recently I developed a VAO Nozzle project in inventor 2011 using these principals, and I’ll use that as an example through this discussion.


What should I include in the base skeleton file?

As stated earlier, you use what works for you. I’d like to point out some things that have made my skeletal modeling designs successful.


Recently I wrote an article about Passing Parameters that will explain more about this topic. In today’s discussion i would like to stress the importance of including all ‘global’, or  overall design parameters in the base skeleton. If you intend for a parameter such as ‘Minimum Material Thickness’ to be distributed through most of the components in the project, then it had better be established in the base skeleton file. This is the single most important thing you can do for your project to enable:

  • Consistent expectation of change
  • Design Automation
  • Fast turn around on revisions
  • Parametric Variations

Remember to enable the ‘Export’ checkbox on all parameters that you want handed off in the derived component process. They will automatically be available.


You will need to create enough sketches to guide and constrain the design process. This can be one sketch, or a series of sketches.

I used sketches and work geometry that delineate the opening and exit geometries, and the actuator and main moving functionality. First the length and centerline were established. Then the functional form was added to determine a range of motion. After that the housing and front and rear cover sketches completed the process. There was some intermediate sketching, but these represent the main sketches that make up the skeleton file.


Exit Geometry


Actuator Sketch


Housing Sketch


Cover Sketch

As you can see from the process, I layered the fundamental phases of design in separate sketches. This method keeps the skeleton easily organized, navigable, and stable. It also permits the derived component process to include only the skeleton features that are required, which will add stability to the overall design.

Note that many of the sketches in this example reference geometry from other sketches in the file via projected geometry and parameters. In my example above, I had to change the actuator shaft size and location numerous times before the design was complete. Each change to its location automatically reformed the housing. Planning for intended future changes, and careful constraint placement was key in this awesome design adaptability.

Knowing how much sketching to include will come with experience.  A skeleton that does not contain enough information is inefficient. If you make the skeleton too complex, it will become difficult to navigate. Remember, when a team shares the design task on large projects, the skeleton has to be straight forward and easy to understand. Overly complex sketches run the risk of failure that will affect the entire project.


Surfaces provide another layer of capability to the overall design. In the example above I used a sketch and revolved surface to delineate the interior surface of the nozzle. This provided a 3-dimensional representation of the design limits. This limitation could then be referenced by any part along the surface, even when they were not aligned to the sketch cross section. Sketching would have been far too difficult to do this with, but a surface was exactly what I needed to hand off the limitation to much of the design.

Work Geometry

Planes, Points, and Axis are the most stable components in any design. I like using Work Axis and Work Planes because they provide cleaner organization in my derived components. These are also invaluable when constraining sub-assembly components to the skeleton.

Autodesk Inventor 2011 derived components from skeleton

Derived components from skeleton

A word about Features and Mass

None, nyet, nein, nada, いいえ, NO NO NO NO NO!!!!

Do not include any thing that possesses MASS in the base skeleton file. Why am I being so freaky? Eventually you will need to insert this skeleton in an assembly, and when you do, it will add MASS to the assembly. Your assembly mass, weight, Center of Gravity, etc. will all be wrong. Just avoid it.

*Hint: if ya gotta create a feature, use a surface, not a solid.

Comments and a Challenge

I was running headlong into Skeletal Organization strategies, but decided that this article was long enough, so I’ll address that topic later.

Skeletal modeling is more than a sketch handed off to large component assemblies. It can manage the entire design, right down to the smallest shaft. I generally try to use a skeleton for every assembly project, no matter how big or small. Why?

Because you never know how a global variable will be useful later on. If the base skeleton framework is already in place, then it is SO EASY to add another variable. If the top down process was avoided, then there is no way to accomplish this without starting over.  You aren’t required to use features from the base skeleton during the derived component process.. However, but when you decide to do so, the framework is already in place.

A quote from one of my field supervisors when I was younger: “To have and not need is better than to need and not have”

If you are not using this method of design management because you are unsure of it’s effectiveness, then I challenge you to try it on a small assembly project. When the initial design is complete, go back to the skeleton and change a global parameter (such as ‘Minimum_hole_spacing’ or “OA_Length’). Watch the assembly update. If that does not bring a smile to your face, then you might need counseling.

Just keep trying. I guarantee that continuously applying these principals will eventually reveal the workflow and  balance that fits your company best and provide a streamlined consistent foundation for managing your projects.

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