Inventor In-Depth: Managing Bills of Materials with Autodesk Inventor 10

14 Jun, 2005 By: Kevin Schneider Cadalyst

Design changes don't have to risk mistakes in purchasing

For product designers, the BOM (bill of materials) is the single source for a list of assembly parts and component quantities. You manage assembly and subassembly structure of purchased and nonpurchased assemblies, including non-mechanical components such as grease and paint.

As your design changes, so does the BOM. Its accuracy is crucial to purchasing and other activities related to production. Autodesk Inventor 10 features a new BOM Editor to make adjusting assembly bills of materials easier. You can use the BOM Editor to change iProperties and BOM properties for all of a product's components -- and even change several components at once. New BOM structure parameters let you define the status of components in relation to one another, to capture more information about your designs and ensure it's reflected in accurate parts lists (figure 1).

This month, we'll focus on these new BOM structure parameters settings.

Figure 1. Capture and control the accuracy of BOM information using structure parameters.

BOM Structure Parameters
As defined in Autodesk Inventor 10, a component in an assembly now can have one of five basic characteristics that relate its parts to one another: Normal, Phantom, Reference, Purchased and Inseparable. The following definitions will help you choose the right settings for components in your BOM.

Normal is the default BOM structure for most components. Normal components have the following characteristics:

  • Their placement in the BOM is determined by their parent assembly.
  • They are numbered and included in quantity calculations.
  • They have no direct influence on their child components' inclusion in the BOM.

Reference components are used for construction geometry or to add context to a design. Examples of reference components 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, where the tote is added for the purposes of a technical drawing or publication, but is not a part of the actual design.

When a component has a BOM Structure of 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. In addition, they do not appear on any parts list.

Reference components also have special treatment in drawing views. You can choose special line styles and formatting for reference components in a drawing of an assembly.

Purchased components are elements that might not be fabricated or manufactured by you. Examples of purchased assemblies 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 lists, but they are hidden in parts-only parts lists.

Inseparable components generally are assemblies that you can't take apart without physical damage to one or more elements of the assembly. Many manufacturing processes treat inseparable assemblies as a single line item, similar to purchased components. However, inseparable assemblies are most often fabricated rather than purchased. Inseparable assemblies 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.

Inseparable components have the following characteristics:

  • 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.

Inseparable components and purchased components differ in one respect. In a parts-only parts list or BOM, all children of a purchased assembly are hidden, and the assembly 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 components are used to simplify the design process. They exist in the design, but are not included as specific line items in a BOM. Examples of phantom components include construction assemblies; sometimes you need an assembly as a container for other components, but do not want this assembly to add a level to the BOM (figures 2a, 2b).

Figure 2a. Fastener Kit and its parts appear in the assembly/file hierarchy for the convenience of grouping purchased fastener parts.

Figure 2b. Set Fastener Kit to Phantom to exclude the assembly 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 (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. Purchased components of a phantom subassembly are promoted to the final BOM; the blue arrow indicates the parts that were elevated from a phantom component.

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), then the parent is not displayed in a parts-only parts list.

Efficient product development depends on accurate product design that gets communicated all the way from R&D to the purchasing department. Autodesk Inventor 10 automates some of the painful detail work involved in making sure your BOM is an accurate and up-to-date foundation for cost-effective manufacture of your design.

About the Author: Kevin Schneider

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