8.3: Bill of Materials

8.3 Bill of Materials

A Bill of Materials, often abbreviated BOM, is a list of the parts and materials needed to build a project.

A good BOM connects the design to the actual lumber you need to purchase and prepare.

It can help you determine:

• what parts are required;
• how many of each part you need;
• what material each part uses;
• finished part dimensions;
• rough material dimensions;
• board-foot requirements; and
• estimated material cost.

Learning Outcomes

After completing this section, you should be able to:

• Explain the purpose of a Bill of Materials.
• Distinguish between finished size and rough size.
• Organize project parts by quantity, material, and dimensions.
• Estimate rough lumber requirements from finished parts.
• Use board-foot calculations as part of project planning.

Start With the Parts

Break the project into its individual components.

For a tool tote, for example, the major parts might include:

• long sides;
• short sides;
• bottom components;
• risers;
• handle; and
• other small parts.

Each distinct part should have its own line in the BOM.

Useful BOM Information

A practical woodworking BOM may include:

example information on a BOM

Part	Material	Quantity	Finished Thickness	Finished Width	Finished Length	Rough Thickness	Rough Width	Rough Length
Example Part	Hardwood	2	3/4 in.	3 in.	18 in.	4/4	3 1/2 in.	19 in.

Additional columns may include:

• board feet;
• extra material allowance;
• unit cost;
• total cost; and
• notes.

Not every project needs every column.

Use the information that helps you plan the work.

Finished Size

The finished size is the dimension the completed part should have after milling and machining.

Dimensions are normally listed:

Thickness × Width × Length

For example:

3/4 in. × 3 in. × 18 in.

Rough Size

The rough size is the amount of material you plan to begin with before final milling.

Rough stock should usually be larger than the finished part so you have material available to:

• flatten faces;
• straighten edges;
• square ends; and
• remove defects.

A common starting allowance for many WMT parts is approximately:

• 1/2 inch extra in width; and
• 1 inch extra in length.

The appropriate rough thickness is selected from available nominal hardwood sizes such as 4/4, 5/4, 6/4, or 8/4.

These are planning allowances, not absolute rules.

A badly warped board or a part requiring special grain selection may require more material.

Board-Foot Estimate

Once the rough dimensions are known, calculate the approximate board footage.

Remember:

Board Feet = Quantity × Rough Thickness × Rough Width × Rough Length ÷ 144

Use inches for the dimensions.

The BOM connects the board-foot calculation from Chapter 2 to an actual project.

Add Extra Material

The calculated volume of the rough parts is not necessarily the exact amount of lumber you should purchase.

Additional material may be required for:

• defects;
• milling loss;
• grain selection;
• color matching;
• layout within random-width boards;
• test pieces; and
• mistakes.

Your BOM may include an extra-material allowance to account for these realities.

Test Pieces Are Material Too

When a project uses a new joint or machine setup, include enough material for testing.

A test piece can be more valuable than trying to save every last inch of lumber.

It allows you to:

• verify a setup;
• check a joint fit;
• test a router depth;
• practice an operation; and
• make a mistake somewhere other than the finished project.

Plan for practice.

A BOM Is a Plan, Not a Command

A Bill of Materials is an estimate of the parts you expect to need.

It does not mean that every part should immediately be cut to its exact final dimension.

As a project develops, some dimensions may be more accurately determined from the project itself.

For example, two opposite sides being exactly the same length may be more important than both pieces matching a theoretical dimension perfectly.

The BOM gets you started.

Good woodworking judgment still guides the build.

Check Yourself

1. What is the purpose of a Bill of Materials?
2. What is the difference between finished size and rough size?
3. Why should rough dimensions normally be larger than finished dimensions?
4. Why might a BOM include an extra-material allowance?
5. Why should test material be considered when estimating lumber?
6. Does a BOM mean that every part should be cut to final size before construction begins?