Wireform cost drivers: material, bends, tooling, finish and volume

The five lines that make a wireform price

A formed wire part is priced from five lines: material, conversion, tooling, finish and packing. Freight, duty and inventory sit outside the piece price and belong in a landed cost model. The five lines are simple to write down and hard to compare, because suppliers group them differently.

A should-cost model does not need to predict the last penny. It needs to show which line dominates and how the price should move when the drawing, the volume or the wire price changes. That is enough to test a quote and to argue a design change with numbers rather than instinct.

The worked example in this guide is illustrative. It uses round numbers so that the arithmetic can be followed, and it is not a quotation for any part.

Material: small, but not nothing

Start with the mass of wire in the part. For round wire the mass per metre is the cross section multiplied by the density of the steel. A 2.0 mm wire has a cross section of about 3.14 square millimetres and a mass of about 0.0247 kg per metre. A part with a developed length of 0.42 m therefore carries about 0.0104 kg of wire before scrap.

Multiply by the wire price per kilogram, then add the scrap allowance. Scrap on a formed wire part comes from the cut allowance, the trial at setup and the parts rejected during the run. A 10 per cent allowance is common on a stable part and low on a part with a tight bend that cracks. At an illustrative £3.40 per kg, the wire in the example costs about £0.035, or £0.039 with scrap.

That number is small against the finished price, and it explains a common frustration. A buyer who negotiates 10 per cent off the wire price saves less than a penny on a part that sells for a quarter. Material matters most on large diameter wire, on expensive alloys such as beryllium copper, and on parts with a long developed length. On a small music wire clip it is a minor line.

Conversion: the line that dominates

Conversion is machine time plus the labour to set, run and check the part. On a CNC wire former, the cycle time is set by the number of bends, the feed length, the number of changes of plane and the speed at which the machine can index. A part with four bends in one plane may run in a few seconds. A part with fourteen bends, two loops and a change of plane can take several times longer.

Convert the cycle time into money with a machine rate. A rate of £54 per hour is £0.015 per second. A nine second cycle costs about £0.135. That single line is more than three times the material cost in the example, and it is the line that a design change can move most.

The practical rule is that every added bend costs both cycle time and a tool position, and the second bend in a new plane costs more than the second bend in the same plane. Removing one bend from a ten bend part can take 5 to 10 per cent off the piece price. Removing a bend and a tolerance call can take more, because it also simplifies the setup and the inspection.

Labour is not only the operator. Deburring, tumbling, gauge checks and packing all sit in conversion or in their own line.

Tooling and setup: a fixed cost looking for a volume

Tooling is a one-time charge that has to be recovered. On a CNC former it is mostly a program, a set of bend pins and a trial. On a four-slide it is a set of slides, cams and form tools made for the part. Either way, the charge is fixed and the volume decides what it adds to each piece.

Take an illustrative £900 of tooling. Over 2,000 parts it adds £0.45 each. Over 20,000 it adds £0.045. Over 100,000 it adds £0.009. The part, the machine and the wire are identical in all three cases, yet the price moves by more than 50 per cent. This is why a supplier asks for a volume before quoting tooling, and why a quote without a volume is a guess.

Setup is a smaller version of the same problem. Every production run has a setup, and every release size decides how many parts share it. A buyer who orders 1,000 pieces four times a year pays four setups. A buyer who orders 4,000 once pays one. The piece price difference is real and it has nothing to do with the supplier being difficult.

Finish: priced by the batch, not the part

Plating and coating are usually bought as a service, and the service has a batch minimum. A barrel of zinc plating costs roughly the same whether it holds 5 kg or 20 kg, so a light part carries a higher finish cost per piece than its surface area suggests. This is one reason small clips can have a finish line that looks out of proportion to their size.

The finish route also changes the process before and after it. Electroplating on high strength spring wire needs a bake to reduce hydrogen embrittlement risk, and the bake is a separate operation with its own handling. Powder coating needs masking, racking and a cure that can soften a temper if the temperature is not controlled.

Volume: the multiplier nobody prices

Volume enters the price three times. It spreads the tooling, it spreads the setup, and it decides which machine the supplier uses. At low volume a CNC former with a program is the sensible route. At high volume a four-slide with a dedicated tool can run several times faster and win on piece price, provided the geometry suits it.

Volume also changes the wire purchase. A supplier who buys a tonne of wire at a time has a different material cost from one who buys a single coil.

A worked example

The table below builds a piece price for the illustrative clip described above: 2.0 mm music wire, 0.42 m developed length, ten bends, zinc plated, 20,000 pieces a year. Every number is rounded and none of it is a quote.

Where to push and where not to

Push on bend count, tolerance and finish, because those are design calls that change the process. A general tolerance instead of a tight one on a cosmetic leg can remove a gauging step. A change from a plated finish to a passivated stainless grade can remove a whole subcontract operation, though it may raise the wire price.

Do not spend the negotiation on the wire price alone. On the example part, a 20 per cent fall in wire price saves about £0.008, or 3 per cent of the piece price. The same effort spent removing two bends would save more, and the saving repeats every year without depending on a metal index.

Ask for the one-time charge and the piece price separately, then rebuild the total at your own volume. A supplier who cannot split the two is either pricing a different scope or protecting a margin, and both are worth knowing before the purchase order is raised.

• Challenge any bend that does not locate, load or retain something
• Move tight tolerances to the features that touch the assembly
• Ask what the finish costs per kilogram at your batch size
• Check whether a different process route wins at your volume