Autodesk User Group International

For product designers, the bill of materials (BOM) is the single source for a list of assembly parts and component quantities. It includes purchased and nonpurchased assemblies�even �virtual� or non-mechanical components, such as grease and paint.

Since the BOM impacts purchasing and production, it has to be accurate�and it has to change as your design does. Accuracy gets complicated when some components are purchased, others are shown in drawings in order to better represent design intent, and still others cannot be physically separated. The BOM Editor feature in Autodesk Inventor� makes it easier to adjust bills of materials for your assemblies by setting structure parameters that automatically promote items to the BOM, or ensure they are not represented, as appropriate.

It all starts with setting BOM structure parameters. Here�s how.

BOM Structure Parameters
BOM structure parameters define the status of components in relation to one another. Setting them helps you capture more information about your designs, for accurate parts lists (see Figure 1).

Figure 1. Structure parameters help control the accuracy of BOM information.

In Autodesk Inventor 10, a component in an assembly can designating as having one of five types of relationships with other parts. These conditions are normal, phantom, reference, purchased, and inseparable. The following definitions will help you choose the right settings for components in your BOM.

Normal
This is the default BOM structure for most components, characterized by:

Placement in the BOM that is determined by their parent assembly.
Numbering and inclusion in quantity calculations.
No direct influence on their child components� inclusion in the BOM.

Reference
Reference components are used for construction geometry or to add context to a design, and include:

Construction elements such as a skeleton part or assembly for skeleton modeling.
Visual enhancements such as a tote filled with parts sitting on a desk, added for the purposes of illustration, but not a part of the actual design.

When a component is designated �reference,� the BOM treats the component and all the component's children as if they do not exist. All of the elements that are a part of a reference component are excluded from quantity, mass or volume calculations, regardless of their own BOM Structure value. Parts lists do not show reference components, either.

In drawing views, reference components have special treatment. You can choose different line styles and formatting to call out these components in a drawing of an assembly.

Purchased
Components designated �purchased� are those that may not be produced by you. Examples include:

Cylinders, pistons, or shock absorbers.
Fasteners such as bolts, nuts and washers.

Purchased components have the following characteristics:

Each is considered a single BOM line item, whether it is a part or an assembly. For example, an assembly marked as �purchased� will be listed in a parts-only parts list.
If the purchased component is an assembly, its children are not usually included in the BOM. Children are also excluded from quantity calculations in the BOM.
Normal child components of a purchased assembly are numbered and included in structured parts list, but they are hidden in parts-only parts lists.

Inseparable
Components that are deemed �inseparable� are typically assemblies that cannot be taken apart without physical damage to one or more of their elements. Manufacturing processes often treat inseparable assemblies as a single line item, much like purchased components. However, inseparable assemblies are more likely to be fabricated than purchased, and can include:

Weldments, such as assemblies that are glued or bonded.
Riveted components attached with semi-permanent fasteners that must be destroyed to separate the elements.
Assemblies in which components are press-fit together, such as dowel pins pressed into a part.

A component is considered inseparable if:

Some component or multiple components must be physically damaged in order to separate the assembly.
One or more child components are considered part of the parent and are never tracked or revised separately.
In a parts-only parts list, the inseparable assembly is treated as a part, just like a purchased assembly.

When documented in its own context, an inseparable assembly is treated as a standard one.

So what�s the difference between inseparable components and purchased components? In a parts-only parts list or BOM, all children of a purchased assembly are hidden, and the assembly itself shows up as a line item in the BOM. For inseparable assemblies, child components with a normal or inseparable BOM structure are hidden. Purchased child components that are part of an inseparable assembly are still displayed in the parts-only parts list.

Phantom
Phantom components exist in the design, but they are not meant to appear as specific line items in a bill of materials. Construction assemblies are a good example, as when an assembly serves as a container for other components, but does not add a level to the BOM (see Figures 2a and 2b).

Figure 2a. Purchased fastener parts are grouped within the �Fastener Kit� assembly file hierarchy.

Figure 2b. When �Fastener Kit� is designated as phantom, it�s excluded from BOM calculations.

Phantom components have the following characteristics:

They are �ignored� by the BOM.
They are not numbered, and are not included in quantity calculations.
They influence their children�s participation in the BOM by promoting them as visible elements in BOM Structure views (see Figure 3). The children components of a phantom element are treated as siblings to the phantom component�s siblings, although this is not true for the structure model.
The quantity of phantom-dependent children is multiplied by the quantity of the phantom component.

Figure 3. The final BOM reflects purchased components of a phantom sub-assembly, and a blue arrow indicates which parts have been promoted from phantom components.

When a parent component is phantom and it has children that are normal, purchased, or inseparable, then:

The BOM visibly promotes the children in structured views to a higher level than their model structure dictates.
The quantity of promoted child components is multiplied by the quantity of phantom parent components.
The promoted child components are combined with any other matching components at that same assembly level.
The order, sorting, and numbering of promoted child components are determined as if they were at the promoted level.
In the case of multiple phantom parent components, children are promoted until they reach a level where the parent is not phantom.

If a parent component has a BOM Structure set to �normal� and all its children are phantom (or reference) components, then the parent is not displayed in a parts-only parts list.

The relationships between design elements can be complex. With tools such as the BOM Editor in Autodesk Inventor, some of the painstaking work to keep track of these relationships can be automated for peace-of-mind that accurate product design information will be communicated all the way from R&D to the purchasing department.

(Discuss this Article! in AUGI's new Discussion Forums.)

Kevin Schneider is technical marketing manager for Autodesk Inventor.