Preliminary BOM Creation

1. The Anatomy of a Bill of Materials

At its core, a Bill of Materials is the "recipe" for an assembly. It is a comprehensive list of all the raw materials, sub-assemblies, intermediate assemblies, sub-components, parts, and the quantities of each needed to manufacture a product.

For a design engineer, the BOM serves several critical functions:

• Procurement: It tells the purchasing department exactly what to buy.
• Cost Estimation: It's the foundation for calculating the cost of a product.
• Manufacturing: It provides the part list for the assembly team.
• Communication: It ensures everyone, from engineering to supply chain, is working from the same information.

To get a practical sense of how a BOM is organized, let's look at an industry-focused guide.

As you can see from the reading, a BOM isn't just a simple list. It's a structured document. Here are the essential columns you'll find in a preliminary engineering BOM:

• Part Number: A unique identifier for each item. This is the single source of truth for a part.
• Part Name / Title: A human-readable name for the part (e.g., "M5x20 Socket Head Cap Screw").
• Description: More detailed information, such as material, finish, or standard (e.g., "Alloy Steel, Black Oxide, DIN 912").
• Quantity: The number of this specific part required for one top-level assembly.
• Unit of Measure: How the quantity is measured (e.g., "each," "mm," "liters"). This is crucial for bulk items like adhesives or lubricants.
• Procurement Type / Source: Specifies whether the part is "make" (custom manufactured) or "buy" (purchased off-the-shelf).

Here is a visual example of a simple, clean BOM for an engineering project.

2. Best Practices for Professional BOM Management

Creating a BOM is straightforward, but maintaining a professional BOM that prevents costly errors requires discipline. As your designs evolve, so will your BOM. Adopting good habits early is essential.

Let's summarize the key takeaways that are most relevant to your work:

• Establish a Part Numbering Scheme:

  • Sequential: Simple and easy to implement (e.g., 100-001, 100-002...). The number itself contains no information.
  • Intelligent: The number contains information about the part (e.g., SCR-05-020 for a screw, BRK-AL-001 for an aluminum bracket). This can be helpful but becomes complex to manage.
  • For your portfolio projects, a simple sequential system is a great starting point. The most important thing is that every unique part has a unique number.

• Control Your Revisions:

  • A BOM is a "living document." When you change a part, the BOM must be updated.
  • Each time you release a BOM for a purpose (e.g., for a design review, for a prototype quote), it should have a revision number (e.g., Rev A, Rev B, or 1, 2, 3...).
  • Keep a revision history log to track what changed, when, and why. This is crucial for traceability.

• Differentiate "Make" vs. "Buy":

  • Make: Parts you design from scratch and will be custom-manufactured (e.g., a mounting bracket, a custom housing). These parts will require a detailed engineering drawing.
  • Buy: Off-the-shelf components you purchase from a supplier (e.g., screws, bearings, motors). These are typically identified by a manufacturer part number.

3. Developing a BOM in Onshape

Modern CAD platforms like Onshape have powerful, integrated BOM tools that automate much of this process. Because the BOM is linked directly to the assembly, it updates in real-time as you add, remove, or change parts. This eliminates the manual effort and potential for error that comes with managing a separate spreadsheet.

Let's explore how Onshape handles BOMs.

In an Onshape assembly, you can open the BOM table at any time. It automatically populates with all the parts in your model. The key is to customize it to show the properties you need.

Here’s a typical workflow for creating a preliminary BOM in Onshape for a design review:

1. Assign Properties: For each part in your Part Studio (both custom and standard components), you must fill out its properties. Right-click the part in the Parts list and select "Properties." At a minimum, you should set the:

   • Name: (e.g., "Motor Mount Bracket")
   • Description: (e.g., "6061-T6 Aluminum, 5mm thick")
   • Part Number: (e.g., "PM1-100-001")
   • Vendor/Source: (e.g., "Make" or "McMaster-Carr")

2. Open the BOM Table: In your assembly tab, click the BOM icon on the right side of the screen.

3. Customize Columns: By default, the BOM might only show Item, Quantity, and Part Name. Use the dialog to add columns for Part Number, Description, Material, etc.

4. Manage Structure: You can choose different views:

   • Flattened: Shows a total count of every single part, regardless of sub-assemblies. Good for purchasing.
   • Structured: Shows the assembly hierarchy. Good for understanding the assembly structure.

5. Export: Once configured, you can export the BOM as a CSV file, which can be opened in Excel or Google Sheets for further formatting or cost analysis.

Conclusion

You have now learned how to develop a preliminary Bill of Materials, a cornerstone of professional engineering documentation. This document is the critical link between your digital design and the physical product.

Key Takeaways:

• A BOM is the complete "recipe" for your product, listing all parts, quantities, and key specifications.
• Essential BOM information includes a unique Part Number, Name, Quantity, and Source (Make vs. Buy).
• Maintaining revision control is crucial for managing design changes and avoiding errors.
• CAD tools like Onshape automate BOM creation, ensuring the BOM is always synchronized with the 3D model.

This lesson concludes the "Engineering Workflow and System Design" module. You've now covered the essential non-CAD skills of the design process: defining requirements, managing files, analyzing mechanisms, selecting materials and fasteners, structuring design reviews, and now, creating a BOM.

In our next lesson, we will officially kick off Project 1: Linear Motion System Design. Your first task will be to apply what you've learned by defining the requirements for a linear actuator system, which will serve as the foundation for the entire project.