Key takeaways
- A wireform FAI proves one part on one machine from one wire lot, not the capability of the process over a year.
- Dimensions, load at a stated length and the material certificate are three separate claims and each needs its own evidence.
- Report measured values with the method and the instrument, not a column of tick marks.
- Any deviation should be written into the pack with a disposition, not discovered later at goods inwards.
What a wireform FAI is for
A first article inspection is a one-time proof. It shows that a particular part, made on a particular machine from a particular lot of wire, meets the drawing. It is not a capability study, and it does not prove that part 5,000 will match part one. It is the record that the drawing was read correctly and that the process was set up before production started.
On a formed wire part the evidence usually arrives in three uneven piles. One supplier sends forty dimensions and no load test. Another sends a load curve and no material certificate. A third sends a certificate for the wire but nothing that connects it to the parts in the box. A useful pack covers geometry, load and material, and it names the drawing revision that all three were measured against.
Start with a ballooned drawing
The pack should begin with the drawing at the revision that was quoted, with every dimension, note and tolerance ballooned and numbered. The balloon numbers are the index for the rest of the report. Without them, a reader cannot tell whether a missing line was measured and passed or never measured at all.
Ballooning a wireform drawing takes longer than ballooning a flat part because bends are described in three dimensions. A leg angle may be called out from a datum leg, from a plane or from a second bend. Record which convention the drawing uses before the part is measured, because the same number can mean two different things.
Mark the characteristics that control assembly: bend centres, the angle of a load bearing leg, free length, the position of a hook and any feature that locates the part in its housing.
- Drawing revision, sheet count and the date the revision was issued
- Every dimension ballooned, including those covered by a general tolerance note
- The datum scheme, and the leg or face used as datum A
- Critical characteristics agreed at the RFQ stage
- Any note on finish, weld, stress relief or marking that needs its own evidence
The dimensional run
The report should list nominal, tolerance, actual and the difference for each ballooned feature. A pass or fail column on its own is weak evidence, because it hides how close the part sits to the limit. Two parts can both pass while one is centred in the band and the other is one micron from the edge, and only the second one is a warning.
Method matters as much as the number. Wire diameter should be measured with a micrometer at three positions along a straight run, not with a calliper across a bend. Bend centres are best taken on a fixture or a vision system that holds the part in the same orientation as the assembly. Angles need a stated reference, because a leg that reads 89.6 degrees from one datum may read 90.2 degrees from another.
If the part is plated after forming, decide whether the drawing dimensions apply before or after plating. A zinc or tin layer of a few micrometres changes a wire diameter and a bend radius, and the difference can consume a tight tolerance. The pack should say which state was measured.
Three dimensional wireforms need a coordinate report rather than a list of lengths. A CMM or a vision system can report the bend planes, the leg vectors and the position of a hook in space. A height gauge and a rule cannot, and a report built only from hand tools should say so.
Load, rate and function
A formed wire part that carries load is a spring, whether or not the drawing calls it one. The functional evidence is the load at a stated length, not the free length. For a compression or extension spring, test at the installed length and at the working length, and report both. For a clip, test the retention force in the direction the assembly pulls.
Test speed, preload and the number of cycles before the reading all change the number. A spring that is compressed quickly reads higher than one that is allowed to settle. Agree the test method in the RFQ so that the supplier and the buyer measure the same event, and put the method on the report.
For an extension spring, add the initial tension. It is the load that holds the coils closed before any extension, and it is the number that most often goes unmeasured. Two springs with the same rate can feel very different in an assembly if one has 1 N of initial tension and the other has 4 N.
Test at the length the assembly uses
A load quoted at a convenient test length is not proof of function. Put the installed length and the working length on the drawing, and ask for both loads in the first article.
Material, finish and joint evidence
The material certificate should identify the wire standard, the grade, the temper, the heat number and the wire diameter with its tolerance class. A mill certificate is a declaration about the lot that was drawn, not a test of the finished part. It still matters, because it is the only document that ties the chemistry and the mechanical properties to the wire that went into the machine.
If the part is plated, the pack should give the plating type, the standard, the thickness range and the coverage. For high strength spring wire, add the post plating bake and the time to bake. Hydrogen can enter the wire during acid cleaning and plating, and a missed bake turns a ductile spring into one that cracks under load days later. A certificate that lists the plating thickness but not the bake has left out the part that matters most.
Welded assemblies need a joint result as well as a dimensional result. A peel or torsion test on a sample from the same setup, with the failure location described, is more useful than a photograph of a weld. If the joint is brazed or crimped, name the filler or the crimp height and report the pull value.
What the pack should contain
| Element | What it contains | What it proves |
|---|---|---|
| Ballooned drawing | Current revision with every characteristic numbered | The report was built against the right drawing |
| Dimensional report | Nominal, tolerance, actual, difference and instrument | The geometry is inside the drawing |
| Load or function result | Load at stated lengths, rate, initial tension or pull force | The part performs its job, not just its shape |
| Material certificate | Standard, grade, temper, heat number and wire diameter class | The wire is the grade that was specified |
| Finish record | Plating type, thickness, coverage and bake where required | Corrosion and embrittlement controls were applied |
| Joint result | Peel, torsion or pull value with the failure location | A welded or crimped joint carries the design load |
| Deviation list | Every out of tolerance feature with a disposition | Nothing was quietly left out of the pack |
The exact content should be agreed in the RFQ. A pack that reports more than the drawing requires is useful only if the extra data is read.
How to review the pack
Read the revision line first. A perfect report against the previous drawing is a failed submission. Then check the arithmetic on three or four rows at random: nominal plus tolerance, actual minus nominal, and the pass decision.
Look for features that are marked as not measured, not applicable or covered by the general tolerance. Some of those answers are legitimate. A general tolerance can cover a cosmetic leg length. It cannot cover a bend that locates the part. Judge each omission against the function, not against the length of the report.
- Revision on the drawing matches the revision on the purchase order
- Instrument and method named for each critical characteristic
- Actual values reported, not only pass or fail
- Load test method, speed and test length stated
- Material certificate tied to the wire lot used for the samples
- Samples marked and retained with the report
Deviations, concessions and resubmission
A first article with one feature out of tolerance is not automatically a rejection. It is a decision. The supplier should state the deviation, the measured value, the cause and the proposed disposition. The buyer then decides whether to accept by concession, ask for a change to the drawing or require a process correction and a new submission.
A concession should be time limited and tied to a quantity. Accepting a leg angle that is 0.4 degrees out on the first article is not the same as accepting it for three years of production. If the deviation is acceptable, change the drawing so that the next supplier quotes to the same requirement.
Frequently asked questions
How many parts should a wireform FAI cover?
One part per process stream is the usual minimum, and many buyers ask for three to five consecutive parts so that they can see the spread. AS9102 uses one part per part number per process stream unless the contract says otherwise. For a spring or a clip, consecutive parts from the same setup are more informative than parts picked from different hours.
Does a first article need a CMM?
No. Hand tools, height gauges and functional fixtures can be valid when the method suits the tolerance and the geometry. A CMM or vision system is needed when bend planes and hook positions have to be proved in three dimensions. What matters is that the method is named and that its uncertainty is small compared with the tolerance.
Is a mill certificate the same as a material test?
No. A mill certificate declares the chemistry and mechanical properties of the wire lot. It does not test the finished part, and it does not prove that the certificate belongs to the wire in the machine. Ask for the heat number and check it against the wire label and the production record.
Can I accept an FAI with one dimension out of tolerance?
Yes, by concession, if the deviation does not affect fit, function or safety. Record the measured value, the cause and the quantity covered. A concession that is not written down becomes an undocumented change, and the next shipment will be judged against a drawing that nobody meets.
Sources and further reading
- SAE AS9102C, Aerospace first article inspection requirement
- ISO 2768-1:1989, General tolerances, linear and angular dimensions
- EN 10270-1, Steel wire for mechanical springs, patented cold drawn unalloyed spring steel wire
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