Exploring the Best Modeling Approaches in Inventor: Top Down, Bottom Up or Middle Out?

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Modeling in Inventor [Source: ENGINEERING.com]

An Inventor tutorial.

Like most 3D mechanical modelers, Inventor supports different workflows for designing 3D models.

The traditional (and most common) is bottom-up. In this approach, you build parts and add them to assemblies. You continue working from the bottom level up to the top (hence the name). Because of the relationships between the components, changes to the parts can impact the levels above them.

Bottom-up is most useful when many details are known. This includes using purchased parts or existing components and basing a part off of established designs. You might know what is most compatible with your manufacturing processes. Alternatively, you might have a distinct idea of the end design’s capabilities and appearance. This approach also lends itself to teamwork, as different components can be tackled simultaneously.

Conversely, top-down is the antithesis of bottom-up. Here, the end result is initially defined. This result forms the primary structure or configuration of the design. This foundation then serves as the building block for the components and assemblies. Any alterations made to the base will extend to all associated parts’ components. This method is beneficial when the creation of a structure is vital and the finer details can be incorporated later.

Another approach, often referred to as middle-out, allows for a combination of methods. There is no one-size-fits-all technique for every scenario. However, Inventor can support a variety of modeling methods, which is encouraging news.

Model Layout

Autodesk refers to one of Inventor’s top-down methods as the Model Layout. In this approach, the process starts with a part where you establish primary parameters and develop the assembly layout. It’s possible to design this layout in either 2D, 3D, or a blend of the two.

The assembly structure is constructed using sketches based on the layout. After adding the layout to an assembly for reference, you can utilize it to arrange the components. The constraint pivots from individual components to the layout, meaning that any modifications to the layout would automatically reshape the components’ positions.

A secondary function of model layouts is to derive parts and assemblies from the layout. In turn, the layout becomes the foundation for new components while maintaining associativity. Consequently, any alterations made to the layout are mirrored in the associated models.

The layout does not necessitate a comprehensive assembly definition. It has utility in designing key components, constructing the assembly framework, or shaping its structure. Furthermore, it can be employed to position other components, inclusive of those created using the bottom-up methodology. Contemplation may be given to using multiple layouts for different parts of your design.

Getting Started

It starts with a part and at least one 2D sketch. Then you sketch the design footprint, as in the structure.

Pro tip: When working in an assembly, use Make Layout to automate the process. This creates a new part in the assembly (grounded at the origin) and edits it in place, so you are ready to jump in and start sketching.

Here, I have designed the shock in 2D to ensure that it will work. I have mixed design approaches and started by adding existing parts to the assembly. I then projected fixed locations of the frame to act as the anchoring geometry. The project (as per my current settings) creates a cross-part association.

After establishing the necessary layout structure, I created assembly relationships between it and the preexisting assembly components. You have the option to establish relationships between components and sketch objects. Should you manipulate the layout, the components will adapt accordingly. Furthermore, alterations to the frame prompt updates to the correlated projected geometry in the layout.

The creation process was swift as I worked in 2D, resulting in a shock that allowed me to evaluate the kinematics. My objective with this design was to ascertain whether the shock acted appropriately as I maneuvered the upper and lower control arms. I was also able to reverse the process and modify the 2D shock geometry to observe its impacts on the assembly.

Subsequently, I incorporated the components and constructed relationships with the layout. The Inventor is indifferent to whether the components are 2D or 3D. In my case, I opted for an existing strut. By constraining this strut model to the layout, modifications to the layout influenced the placement of the strut.

The detail level within your layout (and the amount of effort you put in) will entirely depend on your specific needs. Are you using the layout merely to verify the design, or do you have plans to utilize it for parts production?

Making Parts

Use Make Part to create a single part file from your layout. The difference with this feature compared to Derive Part is that it pushes. Make Part derives from the active file instead of into the existing file.

Start by selecting the objects you want to include in the new part. This can be sketch blocks, surface bodies or solids. Clicking OK will create the new part.

Pro tip: You can preselect objects before starting the command. This can be quicker (and easier) in many situations, especially when you know exactly what you want to derive.

Make Part operates on the structure of Derive, offering comparable capabilities. This entails establishing the Derive style, detaching from the main component, and defining a Scale factor. You may also incorporate additional components, such as work geometries and traits, in the fresh part.

Decide on the new part’s name, file positioning, and BOM blueprint, then choose the template to start from.

If desired, you can use Place part in the target assembly to incorporate the new component into a new target assembly.

Due to the fact that I picked a sketch block to form the foundation for my new piece, I utilized it to create the initial shape. Afterwards, I integrated features to finalize its shape.

Both the shape and position are derived and remain associated. As you change the layout’s block instances, the part file reflects the changes.

With Make Part, the position of a block instance in the target part is identical to its position in the source part relative to the part origin. This is a different behavior than Make Components, which we will look at next.

Although you can use Make Part multiple times to build components from a single layout, it is better to use Make Components to create multiple components from your layout.

Sketch Blocks

Sketch blocks can be described as a method for grouping sketch items together. As a result, these grouped objects act as one element. This technique, provided by Inventor, allows for the reuse of typical shapes and provides an approach for tackling designs.

Sketch blocks can be incorporated in your layout to symbolize components. As blocks can be reused, they can be used multiple times across the layout. Any changes made to the block definition will be applied to all instances. This allows for not only the design of the assembly layout but also the components.

Interested in learning more about sketch blocks? Do read my post on sketch blocks here.

Using sketched items can make design and simulation faster and simpler prior to transitioning into 3D modeling. Typically, sufficient details are included to represent a component’s basic shape. After the component has been derived into its independent file, the details can be further refined and expanded upon.

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