Wireform prototype approval checklist

Why a good sample is not an approval

A prototype arrives, it fits the housing, and someone sends an email saying approved. Six weeks later the production parts do not fit. The sample was never an approval. It showed that one part, made by one person on one afternoon, could meet the drawing.

Approval is a different decision. It confirms that a defined process, with named material, tooling and inspection, will keep producing parts to the drawing. The sample is the evidence; the approval turns it into a production reference that a second shift, machine or supplier can work to. Each check below should leave a document. If it cannot, it has not been done.

The eight checks at a glance

The checks answer different questions. Drawing, wire and geometry describe the part. Load and finish describe what it does. Process, measurement and packing describe whether the result can be repeated and delivered.

Checks 1 and 2: the drawing and the wire

Start with the revision. Compare the drawing on the purchase order with the drawing used to make the sample and the CAD file the supplier programmed from. A prototype made to revision C and approved against revision D is an unapproved part with a nice report. Note the revision, issue date and signatory.

Balloon every characteristic before measuring, including those under the general tolerance. The numbers become the index for the report, so a reader can see whether a feature failed or was never measured. Mark bend centres, the angle of a load bearing leg, free length and hook position.

The wire check is often the weakest. Grade, temper, diameter and tolerance class all change the formed result. Two coils of one grade with different temper can spring back differently on the same program. Ask for the mill certificate and check the heat number against the wire label. A certificate that names the grade but not the temper has approved a family of wires, not a material.

Check 3: geometry in three dimensions

A formed wire part is not flat, so a list of lengths is a poor record. Measure bend centres, leg angles and hook positions in the orientation the part sees in the assembly. A fixture that holds the datum leg is worth more than a longer report built from loose parts.

Record nominal, actual and difference for each critical feature. A pass column hides how close the part sits to the limit. For illustration, a 90 degree leg with a tolerance of plus or minus 1 degree that measures 90.9 degrees has passed, but it has used most of the band.

Decide whether dimensions apply before or after plating. A layer of a few micrometres changes a wire diameter and a bend radius, and on a tight callout that change can consume the tolerance. If the sample came from prototype tooling, say so; geometry proved on a CNC former does not automatically transfer to a production tool.

Checks 4 and 5: load, finish and joints

A formed wire part that carries load is a spring, whether or not the drawing says so. Test at the installed and working lengths and report both. Speed, preload and settle time all change the reading, so agree the method before the sample is made.

For an extension spring, add the initial tension, the load that holds the coils closed. It is the number most often missing from a prototype report. Two springs with the same rate can behave differently if one has 1 N and the other 4 N, an illustrative comparison rather than a specification.

Finish needs the same care. Name the plating type, standard, thickness and coverage. On high strength spring wire, ask for the post plating bake and the delay between plating and baking. Hydrogen can enter the wire during cleaning, and a missed bake turns a ductile spring into one that cracks days later.

A weld, crimp or stake needs its own result. A peel or torsion test with the failure location is stronger evidence than a photograph. Record the filler, crimp height or weld schedule so the joint can be repeated.

Checks 6 and 7: process and measurement

The process record separates an approved part from a lucky one. Write down the machine, tool set, feed length, speed, bend sequence and stress relief batch. Add the adjustments made during the run, because the tweak that fixed the last leg is part of the process.

State which settings may not change without a new approval. Feed length and bend order usually belong on that list. A night shift that shortens the cycle by moving a tool has changed the part, even if the drawing has not moved.

Measurement needs its own check. For illustration, a 0.1 mm tolerance checked with a tool that repeats to 0.05 mm leaves little margin for the operator and fixture. List the instrument and fixture for each critical feature. If a CMM or vision system proved the bend planes, say so; if hand tools were used, say that too and explain why they are adequate.

Check 8: packing and the reference sample

An approved part that arrives bent has not been approved in any useful sense. Agree the packing, separators and box marking. Long legs and open loops distort in a bag, and plated parts that rub in transit fail a cosmetic check at goods inwards.

Keep a sealed reference sample with the pack, labelled with part number, revision, wire lot, date and approver. Store it where it will not rust, and keep a second sample with production if the part is plated. The sample is a physical cross check, not a golden part to copy by eye; if it disagrees with the report, the report wins.

Signing off and keeping the approval alive

Put the eight checks on one cover sheet with the part number, drawing revision, wire lot, machine, sample quantity and deviations. The approver signs the sheet, not an email thread. Anyone opening the pack later should see what was approved and what was excluded.

State the re-approval triggers: a drawing revision, a change of wire supplier or heat, a different machine, a tool repair that changes a bend, a new plating source or a factory move. Each should lead to a limited first article against the same eight checks.

A concession is a decision with a boundary. If a feature is accepted out of tolerance, record the value, the quantity and an expiry date. An unwritten concession becomes an undocumented change, and the next lot is judged against a drawing nobody meets. Finally, name the approval reference on the purchase order so the sample on the bench and the parts at the line are connected.