Wire forming design guide: 12 rules for parts that quote well

Why twelve rules, and what they protect

A formed wire part has few dimensions but many ways to be difficult. The wire can crack on a tight bend. A leg can vibrate during forming and finish out of position. A bend can sit so close to another that the tool cannot reach it. A finish can bridge a radius or introduce hydrogen embrittlement. None of these show up as a line on the drawing, and all of them show up in the price.

The twelve rules below are the ones that come up most often in a drawing review. They are not a standard and they do not replace a supplier's DFM note. They are a way to get the drawing close enough that the first quote is a real number.

Rules 1 to 3: radius and temper

The bend radius is the most consequential call on a wire drawing. It sets the strain on the outside of the bend, the springback and the tool access. It is also the call most often copied from a catalogue without checking the temper.

• Rule 1: keep the inside bend radius at or above one wire diameter for spring temper material. Below that, ask for a DFM note before release. The strain on the outside fibre rises quickly as the radius falls.
• Rule 2: match the radius to the temper, not just the grade. Annealed 316 will bend at half a diameter. Spring temper 302 at the same radius may crack. State the temper, then set the radius.
• Rule 3: do not mix radii without a reason. A part with four different radii needs four tool settings and gives the inspector four features to check. A common radius, where the function allows it, is cheaper and more consistent.

Rules 4 to 6: spacing, sequence and legs

A bend needs room. The tool has to reach the bend line, apply force and withdraw without marking the adjacent leg. When two bends are too close, the tool fouls the first leg or the part has to be re-chucked, and both outcomes add cost.

Long unsupported legs deserve their own note. A leg that is long relative to its diameter will vibrate during forming and can finish out of angle. If the design needs a long leg, expect the supplier to form it in a controlled order or to support it with a temporary bridge that is cut later. Ask which approach is planned.

• Rule 4: leave at least 1.5 wire diameters of straight between adjacent bends. Closer than that, the tool radius and the wire radius compete for the same space.
• Rule 5: keep bends in one plane where the function allows, and group the bends that share a plane. Every out-of-plane bend adds a rotation and a new setup risk.
• Rule 6: avoid a bend closer than one wire diameter to a cut end. The end has no support, so it can slip in the gripper or deform as the tool withdraws.

Rules 7 to 9: ends, joins and threads

The ends of a wire part do real work. They locate, retain, contact or connect. A drawing that leaves the end form vague invites the supplier to choose a default, and the default may not fit the mating part.

Loops are a common source of argument. A loop that must pass a pin needs an inside diameter and a position. A loop that only needs to guide a cable needs less. Decide which one the design needs before the drawing is released, because the tolerance and the tooling follow from it.

• Rule 7: give every end a defined form. A loop, a hook, a chamfer, a flattened tab or a plain cut is fine, but say which one and give the dimensions that matter.
• Rule 8: do not call a thread on a formed wire end without a thread standard and enough straight stock. A rolled thread needs a shank diameter and a runout. A cut thread on spring wire is a specialist operation.
• Rule 9: put the joint specification on the drawing when a second element is welded, crimped or staked. State the process, the joint location and the inspection method. A weld symbol without a process is a guess.

Rules 10 and 11: tolerance the function

A wire part is a chain of bends. Every bend has a position and an angle, and the error accumulates along the chain. A drawing that dimensions every point from a single origin looks tidy but does not match how the part is made or measured.

Rule 10 is to dimension the features that locate the part and give the rest a general tolerance. For most parts that means the distance between bend centres, the angle of the load-bearing leg, the free length and the load at a stated deflection. Rule 11 is to put a general tolerance in the title block so that the undimensioned features have a defined default.

Rule 12: make the finish a process call

Finish is not a cosmetic line at the bottom of the drawing. It decides handling, masking, baking and sometimes the material itself. Zinc plating on high strength spring wire needs a bake to reduce the risk of hydrogen embrittlement, and the bake has to happen within a defined window after plating. If the drawing does not ask for it, the process may not include it.

Powder coating adds thickness. On a tight bend, the coating can bridge the inside radius, crack at the outside or change the fit. If the part is coated, say whether the coating is decorative or functional, and whether the bend radii are coated or masked.

For stainless parts, passivation is the usual call. For contacts and terminals, the finish may be tin or silver, and the thickness and the contact resistance matter more than the appearance. Name the standard and the thickness, and say which areas must be free of finish.

A worked review, illustrative

Take a retaining clip in 1.2 mm music wire with six bends, one of them at R0.6 and one leg 45 mm long. The first quote comes back high and the supplier notes that the tight bend is outside the working range for the temper. The design does not need R0.6; it needs the clip to fit inside a 3 mm gap, and a larger radius with a shorter leg still fits.

Change the tight radius to R1.2, shorten the long leg by 8 mm and move one bend 2 mm to give tool clearance. The bend count stays at six and the function is unchanged. In this illustrative case the supplier can now run the part on a standard setup, and the quoted price falls because the reject risk and the special tooling have gone.

That is the pattern. The expensive features are usually not the ones the designer thought were expensive. They are the ones that push the part outside a standard setup: a tight radius, a crowded bend, an unsupported leg or a finish that needs a special handling route.

How to run the review

Run the twelve rules as a checklist before the drawing leaves the department. Mark each rule pass, fail or not applicable, and write a one-line reason for every fail. That record is more useful to a supplier than a clean drawing with no notes, because it shows which compromises were deliberate.

Then send the drawing for a DFM note. Ask the supplier to comment on the bend sequence, the tool access, the material and the finish, not just the price. A supplier who returns the drawing with three specific comments is telling you that the quote is real. A supplier who returns a price with no comments is telling you less.

• Radius at or above one diameter for spring temper, or a documented exception
• At least 1.5 diameters of straight between bends
• No bend closer than one diameter to a cut end
• One dominant plane, with out-of-plane bends grouped
• Defined end forms and joint specifications
• General tolerance in the title block
• Finish standard, thickness and any bake or mask