Wire forming vs stamping: how to choose the right process

The decision in one paragraph

Wire forming takes round wire from a coil, straightens it and bends it into a shape. Stamping takes flat strip, cuts a profile from it and forms that profile in a press. The choice is not a matter of which process is better. It is a matter of which one can produce the geometry the assembly needs at the volume the programme can support.

In practice, four inputs decide most cases: the cross-section, the annual volume, the tooling budget and the tolerance the part has to hold. Work through them in that order. Price comes last, because a piece price without the tooling amortisation and the scrap rate is not a comparison.

Start with the cross-section, not the price

A wireform has a constant cross-section by definition. The wire may be round, or it may be a drawn profile such as a flattened oval or a D-shape, but it does not change along the part. That single fact explains most of what wire forming can and cannot do.

If the part needs a flat mounting face, a hole pattern, a lanced tab, a locating slot or a contact pad, wire cannot produce it in the primary operation. A secondary stamping, a machining cut or a welded flat plate is needed, and each of those adds a handling step and a tolerance stack. If the part is a clip, hook, loop, cage, spring, retaining ring or a wire harness form, the constant section is an advantage: there is no strip layout to waste and no die to build around the profile.

Stamped parts earn their place when the design uses the flat. A terminal with a folded barrel, a bracket with two mounting holes and a stiffening rib, or a contact with a coined pad is a stamping problem. Trying to reproduce those features from wire produces a part with more operations than the stamping would have had in the first place.

Volume and tooling economics

The two processes carry different fixed costs. A stamping programme carries a die, a strip layout, a press setting and usually a coil feed and a scrap arrangement. A wire forming programme carries a machine program, a set of bend pins or slides, and a first article. At low volume the wire route is cheaper to start. At high volume the stamping route usually has the shorter cycle time and the lower unit cost.

The crossover is not a fixed number. It moves with the number of bends, the wire diameter, the press tonnage the stamping needs and the amount of scrap in the strip. A part with four bends and a simple profile may cross over later than a part with fourteen bends. A part that needs a 200 tonne press crosses over earlier than one that runs in a 40 tonne press, because the hourly rate is higher.

The table below gives the shape of the decision. The quantities are illustrative and should be replaced with quotes for the actual part.

The geometry each process handles well

Wire forming is comfortable with three-dimensional shapes. A bend can be made in one plane, then the part can be rotated and bent in another, which is how cages, hooks and retaining forms are made. A stamping can also form in more than one plane, but each direction needs a cam, a slide or a second operation, and the die cost rises with each one.

Stamping is comfortable with features in the flat: holes, slots, tabs, windows, knuckles and coining. It is also better at a sharp internal corner, because the die can shear one. Wire has no internal corner in that sense. A tight wire bend is a small radius, and the material on the outside of that radius is drawn thin.

• Choose wire for loops, hooks, torsion arms, cages, retaining rings and springs.
• Choose stamping for flat bases, hole patterns, folded boxes, lanced contacts and coined pads.
• Choose a hybrid when a wire shape needs a mounting plate or a stamped contact at one end.
• Do not choose wire for a part whose stiffness comes from a wide flat face.

Material and temper change the answer

Wire is supplied in a temper, and that temper is part of the process. Music wire and oil tempered spring wire arrive with high tensile strength and limited ductility. They bend well around a generous radius and crack around a tight one. Annealed stainless 316 bends almost anywhere but will not carry a spring load without work hardening. Phosphor bronze and beryllium copper sit between those extremes and are chosen when conductivity and fatigue life both matter.

Strip has a rolling direction. A bend made across the rolling direction behaves differently from one made along it, and the difference shows up as cracking or as a change in springback. A stamping drawing that does not state the bend direction relative to the grain leaves the toolmaker to guess. Wire has no equivalent grain direction, although drawn wire does carry a direction of drawing and a cast that affects straightness.

If the part is a spring, the material is usually a wire material and the decision is already made. If the part is a conductor, a copper or brass strip may be the only practical route, because the current path needs a section that wire cannot provide without a secondary operation.

Tolerance, burr and flatness

Stamping holds position tolerances in the flat very well. A hole pattern can be held to a few hundredths of a millimetre in a good die, and the die maintains that relationship for the life of the tool. The weak points are the formed features: a bend angle depends on springback, and a coining or embossing operation depends on tonnage and material thickness.

Wire forming holds bend positions less tightly. Each bend has a position and an angle, and the errors accumulate along the part. A general tolerance on a wireform is typically wider than on a stamping, and a drawing that carries stamping tolerances onto a wire part will attract either a high price or a polite refusal. The compensating advantage is that a wire part is often compliant: it can flex into place, which absorbs some of the variation.

Burr is a stamping issue. A sheared edge has a burr side and a rollover side, and the drawing should say which side matters. Wire has a cut end with a small burr, but there is no long sheared edge to control. Flatness is a stamping issue too, because strip can carry coil set and a formed part can distort after the press.

A five-step decision sequence

Use the same order every time so that a review does not drift into a price argument before the geometry is settled.

1. 01

   Describe the part as a section and a path

   Write down the cross-section and the centreline. If the path plus a constant section is enough, wire is viable.

2. 02

   List the features that need a flat

   Holes, pads, slots and local thickness changes are the features that force a stamped or hybrid route.

3. 03

   Estimate the annual volume and the programme life

   Use the real life, not the first order. A die amortised over three years looks different from one amortised over one.

4. 04

   Check the tolerance and inspection route

   Decide how the part will be measured. If a functional gauge is enough, wire is easier. If a CMM report on a hole pattern is needed, stamping is easier.

5. 05

   Quote both routes on the same basis

   Include tooling, scrap, finishing, inspection and freight. Compare cost per good part, not piece price.

Hybrid parts and what to put in the RFQ

Many production parts use both processes. A stamped bracket with a wire bail welded to it, or a wire spring with a stamped contact riveted to one leg, is common. The design question then moves to the joint: who makes which element, who owns the weld specification, and who inspects the assembly. A joint between two processes has two sets of tolerances feeding into it, and the stack needs to be checked before the drawing is released.

For a useful quote, send the drawing at its current revision, the material grade and temper, the wire diameter or strip thickness, the finish, the annual volume, the release size and the inspection level. Say which features are functional and which are cosmetic. If the part is a hybrid, say which element you expect the supplier to make and which will be free-issued. That single note prevents a quote built on the wrong scope.

• Drawing with a title block, general tolerance and revision
• Material standard, grade and temper for wire or strip
• Wire diameter and tolerance, or strip thickness and width
• Finish, plating thickness and any masked areas
• Annual volume, release size and programme life
• Inspection level and any functional test
• For hybrids, the joint specification and the free-issue scope