Throughout my time working as a design engineer the Bill of materials Structure always created issues for me and my colleagues. This is partly to do with the fact there are 5 types of structure and partly to do with every company operating by their own processes and procedures. I can’t help with the second issue but in this article, I hope to shed some light on the first to help you navigate to the right structures for you and your company.
The 5 BOM structures to choose from are:
Understanding which of these structures to use in which situation can mean the difference between a complete Bill of Material and the workshop getting held up due to missing parts.
Accessing the Bill of Materials
First let’s see how to find the bill of materials! Inventor has a very nice dialog box that allows users to see and manipulate BOM’s from either the assembly environment or the drawing environment. In the assembly environment you can use the Bill of Materials command (see Figure 1) located in the Manage panel of the Assemble tab. In the drafting environment select either the drawing view or the parts list, right click and select Bill of Materials from the context menu.
When you enter the Bill of Material command you will be presented with a table like dialog box (See figure 2). In this table there are three tabs, Model Data, Structured, and Parts Only. The Bill of Materials table can be customized, and you can add and remove columns to suit your company requirements.
Model Data shows all the components that are contained in the model and allows you to manipulate certain properties of the components from the top level assembly, a very useful tool when you need to manipulate the same property across many parts in an assembly.
Structured, as the name suggests, provides a structured view of the components in the assembly and displays it as it would be displayed in a drawing when placing a Parts List. This is an important distinction as the BOM structure of the components on the assembly comes into play here. More on this later.
Parts Only provides you a complete list of parts contained in the assembly. This can be especially handy for outputting a list of parts as it rolls up all common components irrespective of where they appear in the assembly structure. For example, you may have the same bolt that appears in both the top-level assembly and a lower-level sub assembly. The Parts Only would roll all these up to give you the total amount required of that bolt.
Bonus Tip: From the Bill of Materials dialog box you can export to Excel. With some filtering and the right BOM structure you can output a list of purchased components to send to you supply team!
Setting the Default BOM Structure
Setting the Default BOM Structure of a component can be done in two ways. Either through the BOM dialog box seen previously or using the Document Settings (see figure 3) when in the document you want to change. Once in the Document Setting dialog box navigate to the last tab (Bill of Materials). In this tab you can set the default BOM Structure (See Figure 4).
Understanding BOM Structures
Now to the meat of the subject and how BOM Structure effects the parts list. Below is a breakdown of what each BOM structure is. It is important to note that these breakdowns are a high-level view and each structure type may have nuances not described here.
This is the default BOM structure for part files and assembly files (excluding weldments). This means that a part/assembly will show up as a single line item in the BOM, the parts/assemblies will display normally in drawing views and the part/assembly will be considered in mass calculations.
This is a very handy BOM structure and has different effects depending on if the item is an assembly or a part. If you set a part to Phantom BOM Structure it simply will not show up in a BOM, however it remains a part of the assembly, displays in drawing views normally and will still be used in mass calculations. If you set an assembly to Phantom BOM Structure it has a very different effect. It promotes the components in the assembly up one level in the Structured BOM (this promotion is signified by a blue arrow in the BOM dialog box, see Figure 5).
N.B If you have multiple occurrences of a phantom assembly the quantity of parts being promoted is multiplied by the quantity of the phantom assembly. E.g. if you have 3 bolts in a phantom assembly and 3 phantom assemblies the parts will show in the Structured BOM with a quantity of 9.
Phantom Structures will also cascade. By this I mean components will continue to be promoted through the structured BOM until they encounter an assembly that does not have a Phantom Structure. An example of this is:
Assembly 0 is set to Normal and contains Assembly 1.
Assembly 1 is set to Phantom and contains Assembly 2.
Assembly 2 is also set to Phantom and contains Part 1.
The Structured BOM for Assembly 0 will show Part 1 as it will be promoted from Assembly 2 to Assembly 1 and then from Assembly 1 to Assembly 0.
Other than Normal Structure most CAD users will understand Reference Structure. What this means is the part or assembly will not show up in a Parts List, will be detailed with dashed lines in drawing views (this can be changed in the view dialog box if required) and will not be considered during mass calculations. This is an important distinction between a Reference and a Phantom Structure.
Now, there is another interesting nuance with the Reference Structure which is a component can be “set” as Reference by default or can be “overridden” as Reference in an assembly environment. To override the BOM Structure in an Assembly document you need to find it in the model browser, right click and select BOM Structure, and then select between Default and Reference. The difference between these two is often missed.
When a component is “overridden” as Reference it only appears as a reference in that particular assembly. Hence you can use manufactured components in drawing views (such as assembly documents) without it appearing in parts list or influencing the calculations but the manufactured part can be detailed normally in other assemblies or in its own context.
When a component is “set” as Reference by default it will always appear as a reference component in all the assemblies it is used in. If it is three levels down, it will still appear in the top-level drawing views as a reference file.
The difference between setting the default to reference and overriding the part in an assembly really is dependent on the component and the situation.
A purchased structure is just that. When you have a part or an assembly that is bought in you would set the default BOM Structure to Purchased. This essentially acts in the same way as a Normal BOM Structure where the item shows up in the Parts List, is displayed normally in drawing views and is considered in mass calculations with one major difference. When an assembly is set with a default BOM structure of Purchased none of the Children of that assembly will show up in the Parts Only list. The assembly will however show up in the Parts Only List as it is considered to be one part. See Figure 6.
It is also very useful when exporting BOM’s to Excel as it allows you to sort between Purchased Parts and Fabricated Parts.
N.B. All content center parts are by default set to a Purchased Structure.
Inseparable is the last BOM Structure available to you. This is the default setting for weldments but can be used for other situations as well. The Inseparable Structure is similar to the Normal Structure in that it appears as a single line item in a parts list, is displayed normally in drawing views and is considered during mass calculations. An Inseparable Structure will act as a Normal Structure when documented in its own context.
The Inseparable structure also shares a similarity to the Purchased Structure in that it appears as one line item in the Parts Only list. This is because it is considered as one part by the system despite it being fabricated with multiple components. An important difference between the Inseparable and Purchased Structure is that if you use a purchased part inside an Inseparable assembly it will show up in the Parts Only list as well. See Figure 7.
Figure 7 - Inseparable and Purchased
Understanding BOM Structure and how to set up your files can be complicated and will often require a lot of head scratching and multiple trial and error. However, once you have clarified where each structure should be used it can make BOMs much more complete and easier to manipulate, reduce errors in part ordering and improve company efficiency.