BOM and Routing: The Recipe and the Method Behind Every Product

A cake recipe has two parts. The first lists the ingredients: 200 g flour, three eggs, 100 g sugar. The second lists the steps: mix, rest for ten minutes, bake at 350°F for 40 minutes. Lose the first half and you don't know what to buy. Lose the second and you don't know how long the oven is busy. Every manufactured product works the same way.

A bill of materials (BOM) is the ingredient list: every material and component, with the quantity needed for one unit of the product. A routing is the method: the sequence of operations, the work center that does each one, and how long it takes. Together they tell a factory what to buy, what to build and how long each machine is busy.

What a multi-level BOM looks like

Most products aren't made from raw materials in one step. They're made from intermediate products, which are made from other materials. That's a multi-level BOM. Here's a simple one for 5,000 m of a three-core power cable, 3 × 2.5 mm²:

• Level 0: Cable 3 × 2.5 mm², 5,000 m
• Level 1: Insulated core, 15,150 m (3 cores × 5,000 m, plus 1% allowance)
• Level 2: Copper conductor 2.5 mm², about 339.4 kg
• Level 2: PVC insulation compound, 227.3 kg
• Level 1: PVC sheathing compound, 450 kg
• Level 1: Wooden drum, 1 piece

Let's check the numbers. A 2.5 mm² conductor holds 2.5 cm³ of copper per meter, and copper weighs 8.96 g/cm³, so about 22.4 g per meter. For 15,150 m, that's 15,150 × 22.4 g = 339,360 g, or 339.4 kg. The insulation at 15 g/m gives 15,150 × 15 = 227,250 g, or 227.3 kg. Sheathing at 90 kg per km gives 5 × 90 = 450 kg.

The insulated core is a semi-finished product. It gets its own item number because it's made, stored and sometimes shared across several cables. Semi-finished goods covers when that's worth doing.

What a routing contains

The routing for the same cable:

1. Op 10 – Core insulation, extrusion line E1. Setup 60 min. Run 15,150 m at 250 m/min = 60.6 min.
2. Op 20 – Laying up (twisting three cores), cabling machine C2. Setup 45 min. Run 5,000 m at 50 m/min = 100 min.
3. Op 30 – Sheathing, extrusion line E2. Setup 60 min. Run 5,000 m at 40 m/min = 125 min.
4. Op 40 – Testing and drumming, test bay T1. Setup 30 min. Run 60 min.

Total setup: 60 + 45 + 60 + 30 = 195 min. Total run: 60.6 + 100 + 125 + 60 = 345.6 min. That's 540.6 minutes, about 9 hours of machine time. The order will still take days to get through the plant, because most of manufacturing lead time is queueing, not running. But those 9 hours are what the scheduler books on four different machines.

How the BOM and routing drive planning

The BOM feeds MRP. MRP multiplies the order quantity down every level: 5,000 m of cable means 15,150 m of core means 339.4 kg of copper. That's dependent demand: calculated from the BOM, never forecast.

The routing feeds capacity planning and scheduling. It tells the planner that line E2 is busy for 185 minutes (60 setup + 125 run) with this order, and that op 30 can't start before op 20 is done.

It also feeds costing: material from the BOM, machine and labor time from the routing.

Myth: "The parts list on the drawing is the BOM"

It's a BOM, but usually the wrong one for planning. The engineering BOM describes the finished design: copper, insulation, sheath. The manufacturing BOM describes how the plant actually builds it: the intermediate insulated core, the 1% allowance, the drum it ships on. MRP needs the manufacturing version. Plants that plan from the engineering BOM keep finding "missing" items that nobody ordered, like the drum.

The three most common BOM errors and what they break

1. Wrong unit of measure. Sheathing is entered as 90 kg per meter instead of per kilometer. MRP now asks for 450,000 kg of PVC instead of 450 kg. Buyers ignore the obvious nonsense, then stop trusting MRP for everything else. Breaks: MRP and costing.

2. Missing scrap or allowance. The core is entered as exactly 3 × 5,000 = 15,000 m. Startup and cut-off waste use about 150 m. Now the last cable length comes up short, and in cable you can't join two pieces to make one continuous length. Breaks: MRP (shortage found at the machine) and schedule (a rerun of op 10).

3. Stale routing times. Line E2 was upgraded, but the routing still says 30 m/min instead of 40. The run shows as 167 min instead of 125. The schedule looks overloaded, so orders get pushed out that could have shipped. Breaks: schedule and costing.

How this looks in factory.online

In factory.online, a planner opens the cable order and sees the multi-level BOM with the insulated core as its own item, plus the four operations on E1, C2, E2 and T1 on the Gantt. MRP explodes the 5,000 m through both levels and links any copper shortage back to this customer order. For families like 3 × 1.5, 3 × 2.5 and 4 × 2.5, matrix BOMs and routings generate the items from their characteristics, so one template replaces dozens of hand-kept BOMs. Item master data explains why that matters when one cable becomes thousands of SKUs.

If your BOMs and routings live in three places and agree in none of them, book a demo and bring one product family.