Busbar cost model: metal, conversion, plating and scrap

Why a should-cost model beats a comparison of quotes

Three quotes for the same busbar can differ by a wide margin without any of the suppliers being dishonest. One may have bought copper at a different point in the price cycle. Another may be plating in a barrel rather than on a rack. A third may have included a bend that the drawing implies but does not state. Comparing the totals tells you which number is lower. A cost model tells you why.

For a busbar, the model is unusually transparent because the part is a simple solid. Mass is length times width times thickness times density. From there, the only real judgement calls are the yield, the operation count and the plating method.

Start with the mass of metal in the part

Calculate the finished mass first, then the input mass. Copper density is about 8.96 g/cm3 at room temperature. A bar of 40 mm by 5 mm by 600 mm has a volume of 120,000 mm3, which is 120 cm3, and a mass of about 1.08 kg. Doubling the length doubles the mass; doubling the width does the same. Thickness has the largest effect per millimetre because it is usually the smallest dimension.

The input mass is the finished mass divided by the yield. If the blank layout uses 78 per cent of the strip or bar, the input is about 1.28 times the finished mass. That difference is the scrap, and it has a recovery value. A fabricator that buys cut-to-length bar and punches holes has a different yield from one that nests parts on a coil.

Apply the copper price to the input mass, not the finished mass. That is the single most common error in a buyer's spreadsheet. The supplier pays for the metal it buys, and the scrap credit comes back later, often at a discount to the market price.

Conversion: operations, handling and tolerance

Conversion is the cost of turning bar into a finished part: cutting, punching, bending, deburring, inspection and moving the part between those steps. It scales with the number of operations and the handling time, not with the mass. A small bracket with six operations can cost more to convert than a long straight bar with two.

Build the operation list from the drawing. A typical busbar needs cut to length, punch or drill the holes, bend one or more times, deburr the edges and inspect. Add a separate operation for a twist, an offset bend, a countersink or a tapped hole. Every extra setup adds handling and a chance of scrap.

Tolerance affects conversion in a way that is easy to miss. A hole position held to a tight tolerance may need a dedicated fixture rather than a stop on the press. A bend angle that must hold across a long bar may need a check after springback. A flatness call on a mounting face may need a straightening pass. If the drawing asks for these, the model should carry them.

Plating is priced by area, not by weight

Plating cost follows the surface area to be covered, the thickness, the method and the amount of masking. A tin coating at a few micrometres on a bar of 2 kg and a bar of 1 kg can cost the same if the surface area is the same. That is why a wide, thin bar can be more expensive to finish than a narrow, thick one of the same mass.

Estimate the area from the developed shape, including the two faces, the edges, the hole walls and both sides of any bend. Add a handling allowance for racking or wiring if the parts are plated on a rack. Barrel plating is cheaper per part for small parts but can damage long or thin bars, so it is rarely the right route for a busbar.

Silver is more expensive than tin, and the cost is driven by both the metal and the process control. Thicker silver on a joint face, selective plating and masking all add cost. If the drawing only needs silver on the contact areas, selective plating can be cheaper than coating the whole bar, but it adds a masking operation and a handling risk.

Scrap, yield and the credit that comes back

Scrap has two values. The first is the metal that never becomes a part: the skeleton, the punched slugs, the ends cut off a bar and the parts rejected at inspection. The second is the price a scrap dealer pays for that material, which is usually below the price of new metal and depends on cleanliness, alloy and volume.

Model the scrap as a credit, not as free money. If the input is 1.28 kg and the finished part is 1.00 kg, the 0.28 kg of scrap might recover most of the metal price, depending on the market, the alloy and the dealer. Using the full metal price overstates the credit and makes the part look cheaper than it is.

Rejected parts are a separate line. A part that fails after plating has lost the metal, the conversion and the plating, and the scrap credit only recovers part of the metal. That is why the reject rate belongs in the model as a yield loss on the full cost, not only on the metal.

An illustrative build for one busbar

The example below is illustrative. It uses a 40 mm by 5 mm copper bar, 600 mm long, tin plated, with two holes and two bends, at a volume of 1,000 pieces. Copper is taken at £8.50 per kg, which is a placeholder rather than a current price.

The metal input is 1.28 kg per part after yield, giving £10.88 of copper. The scrap credit recovers 0.28 kg at £6.00 per kg, or £1.68, so the net metal line is £9.20. Conversion is £5.20, plating £1.95, packing £0.60 and freight £0.80, for a total of about £17.75 per part. The percentages in the infographic show how quickly metal dominates.

Using the model in a negotiation

Take the model into the conversation as a set of questions, not as an accusation. Ask which copper reference the supplier used and when it was fixed. Ask what yield the nesting assumes. Ask whether the plating is rack or barrel and what thickness is priced. Ask how the scrap credit is calculated and who keeps it. The answers will explain most of the gap between two quotes.

Then agree the parts of the price that should move and the parts that should not. Metal should track an index. Conversion should move with volume and with drawing changes. Plating should move with thickness and surface area. Packing and freight should be reviewed when the route changes. A price that moves as one number hides all of this.