Bills of Materials

A bill of materials says what goes into one assembly

A bill of materials (BOM) lists the parts needed for a product. Give each part a stable number, a clear name, a quantity per assembly, a material and a make-or-buy designation. Identify the stock for made parts and the purchasing description for bought parts.

Made means the part needs a drawing and a route such as an operation method sheet. Bought means it arrives as a component; it still needs a size and specification so the buyer can obtain the correct item. Match the BOM revision to the assembly drawing. State whether quantities are per assembly or for the whole batch.

Material cost per part is its allocated stock share times stock price

Estimated material cost per part = stock share allocated to that part × price of that stock unit. Keep both amounts on the same basis: length for constant-section bar, area for sheet of the same thickness, or mass when purchased by mass.

Allow for the saw cut's width (kerf), trimming and unusable stock. Finished-part volume alone can understate what making the part consumes. State how you allocate waste and reusable leftovers so two estimates can be compared.

Consumption, purchase cost and batch yield answer different questions

You may need to buy a whole sheet even when one part uses a small fraction. That purchase is a cash requirement; the allocated fraction is the material estimate for the part. For a batch that consumes the entire stock unit, dividing its cost by the number of acceptable parts includes the loss from rejected pieces. Avoid charging the same waste once in the allocated share and again as an added allowance.

Worked BOM: one display label assembly costs $1.20 in allocated materials

This original example matches DL-01 in Operation Method Sheets and Critical Dimensions for the Shop. All prices are invented teaching values, not quotations. Assembly DL-A, revision A, contains one made plywood plate and two purchased closed cord loops.

The 110 × 50 × 6 mm starting blank is allocated a 120 × 60 mm area of a 1200 × 120 × 6 mm birch plywood strip costing $18. The larger allocation includes room for cutting and trimming. Allocation share = (120 × 60) / (1200 × 120) = 0.05. Plate material estimate = 0.05 × $18 = $0.90.

Extended cost is quantity × cost each. The $1.20 excludes labor, machine time, tooling, consumables, taxes and delivery. It is not a complete manufacturing cost or a selling price. Broader cost roll-up and design-for-manufacturing decisions belong with manufacturing economics.

Buying one strip and one pack costs $21, even for one assembly. Unused material remains inventory; do not confuse that purchase total with $1.20 of allocated consumption.