Guide
Design for wire forming
A wire form is a centerline plus a diameter plus ends. These rules are written for 4–14 mm CNC — the radii and legs get large. Light-wire clip rules do not scale.
What the print must say
- Wire diameter and material spec (alloy, tensile or temper, coating)
- Inside bend radii — not “sharp” and not a broken edge from CAD
- Critical-to-fit dimensions only, with datums from the mating part
- End condition: square, chamfer, coin, flatten, thread, loop
- Finish: none, zinc, nickel, powder, passivate — and whether it is pre-coated wire
- Open vs closed, and whether a weld is allowed
“Spring steel, form to match sample” is not a print. It is a request for a reverse-engineering job, which is valid, but quote it as one.
Bend radius
CAD likes zero-radius polylines. Wire does not. In the 4–14 mm band:
- Mild carbon: inside radius ≥ 1× diameter (14 mm wire → 14 mm IR)
- Stainless and high-tensile: often ≥ 1.5–2× diameter
- Soft copper / aluminum: can go tighter, but watch marking
The outside fiber is in tension. Too tight a pin and you crack stainless or take the coating off carbon. If the assembly needs a sharper corner than the wire will take, that is a flattened section, a weldment, or a different process — not a smaller CAD radius.
Details by process: 3D CNC and 2D CNC.
Minimum legs and reversing bends
Tools need room. A straight between bends under about 2–3× diameter is where 3D programs get slow or impossible without special tooling. Reversing zigzags in 2D have the same problem in plane.
If the function is “pack as many bends as possible into a short clip,” talk to the former before you freeze the plastic housing around it.
Springback is a material property
The machine commands an angle. The wire returns some of it. Higher yield, larger radius, and certain stainless grades return more. Compensation lives in the program, not on the print — but the print must name the alloy, or the compensation is a guess.
Changing from A228 to 302 “equivalent” without a new first article is how a clip stops snapping onto the mating rib.
Tolerancing like a formed part
Do not chain ±0.005" down a 3D centerline. Pick the interfaces:
- Hole-to-hook span that locates the part
- Clip gap that sets retention force
- Overall envelope that has to clear a cover
Put those on a fixture or a CMM program. Let non-mating bends float at process capability. Tightening everything raises scrap without raising function.
| Default (starting) | Value |
|---|---|
| Linear, non-critical | ±0.015" |
| Linear, critical | ±0.005" with a fixture |
| Angle, non-critical | ±2° |
| Angle, critical | ±0.5° to ±1° |
Ends, loops, and closed forms
Square cut is the default. Chamfer for insertion. Flatten/coin for weld or rivet. Thread for a fastener. Loops need an inside diameter and a gap (or a weld) called out.
Fully closed rectangles and rings can trap on CNC tooling. Either leave a gap, add a weld as a secondary, or accept a two-piece assembly. Do not discover this after the housing is tooled.
CAD that a former can use
- A wire centerline (curve) plus diameter is better than a solid sweep with no path
- STEP or DXF plus a PDF with the critical dims
- Bend radii modeled, not sharp polylines
- One material spec, not “or equivalent”
If CAD does not exist, a sample can be reverse-measured. A photo cannot.
Related processes
- Wire form shapes — L, U, J, S, C, serpentine, frames
- Coil materials — 1010, 1018, 300-series, 330, brass, copper
- End forming
- Inspection
- 2D CNC · 3D CNC
- Process index