How to Qualify a Second Source for a Running Part
Adding a second supplier to a part that already runs well is one of the few sourcing projects that can make quality worse while everything looks fine on paper. The new shop passes its first article, ships two lots, and then a dimension starts drifting because their fixture clamps on a different surface. Qualification is not a repeat of the original exercise. It is a comparison, and it has to be explicit enough that a 0.02 mm shift in process mean shows up before your customer finds it.
Why second sources fail at the interface
Three interfaces cause most of the trouble. Datum selection comes first: a loosely defined datum B may be fixtured off a cast surface by one shop and a machined face by another, shifting every downstream tolerance by 0.05 mm. Deburr standard is second: a 0.2 mm edge break versus a 0.5 mm chamfer changes fit in a pocket and looks identical on a CMM report that does not measure edges. Cosmetic acceptance is third, and nobody writes it down.
What goes into the transfer package
Send more than the drawing. A working package contains the current model and drawing revision, the incumbent's approved first article report with actual measured values, the last three lot inspection records, a marked-up photo set showing the datums as the incumbent interprets them, three good sample parts, and the packaging specification.
The sampled parts are the most valuable item and the one buyers skip. A physical part settles arguments about edge break, surface finish and what "blend smoothly" means in a way no note can. Mark them clearly as reference samples and state that they do not constitute acceptance of the new supplier's process.
See also: Air vs Sea Freight Cost for Machined Parts: When Air Actually Wins and CBAM and Machined Metal Parts: The Supplier View.
How many samples before you trust a lot
Three parts for the first article is standard and sufficient to prove the process can produce a conforming part. It proves nothing about consistency. For that you need a short capability run.
Ask for 30 consecutive parts from a single setup, measured on the critical characteristics, with raw values rather than pass/fail. Thirty is the minimum that gives a usable estimate of standard deviation; 50 is better if the part is cheap and the feature tight. For a part with five or more critical features, measure all of them on all 30. The extra inspection costs $80 to $200 and it is the cheapest insurance in the project.
If 30 pieces is commercially unreasonable, ask for 10 consecutive parts plus a documented control method — tool life counters, in-process gauges, first-off and last-off checks — so consistency is controlled by procedure rather than proven by sample.
Comparing first articles and capability
Do not read the new FAI alone. Lay it beside the incumbent's and compare actual values, not pass/fail marks.
A delta under 20 percent of the total tolerance band is comfortable. Between 20 and 40 percent is workable but worth a discussion, because the two processes will drift in different directions. Above 40 percent means the parts are functionally different even if both conform, and your assembly will notice.
Conformance asks whether the parts are in tolerance; capability asks whether the process will stay there. A Cpk of 1.33 is the usual floor for a production feature and 1.67 where the dimension is safety-related or not inspected on receipt. If the incumbent runs at 1.8 and the new source at 1.1, the new source will produce occasional bad parts at a rate a 30-piece sample will not reveal.
| Step | What you do | Typical duration | Exit criteria |
|---|---|---|---|
| 1. Transfer package | Drawing, model, incumbent FAI, 3 samples | 3 – 5 days | Supplier confirms receipt and raises questions |
| 2. DFM review | Supplier returns notes on tolerance and fixtures | 2 – 3 days | Open issues closed in writing |
| 3. Tooling | Fixture, program and gauges built | 5 – 10 days | Fixture proven on a first part |
| 4. First article | 3 parts, full dimensional report | 2 – 4 days | All in tolerance, deltas under 40% of band |
| 5. Capability run | 30 consecutive parts measured | 3 – 7 days | Cpk ≥ 1.33 on every critical feature |
| 6. Pilot lot | 10 – 25% of volume, 100% inspected | 1 – 2 weeks | Zero rejects, packaging intact |
| 7. Volume split | 70/30 for two lots, then review | 4 – 8 weeks | No trend in incoming reject rate |
| 8. Steady state | Split per commercial terms, audit annually | Ongoing | Normal sampling at receiving |
Total elapsed time is four to seven weeks for a simple part and eight to twelve where several critical features or a qualified finish are involved. The table is the schedule; the pilot lot is where it usually slips.
Tooling duplication and who pays
The new supplier needs a fixture, a program and possibly gauges. Expect $600 to $2,500 for a plate fixture, $150 to $400 of programming and $150 to $450 for a gauge set on a part with several checked features.
Someone has to pay, and the answer shapes commitment. If you pay, you own tooling at two shops and can move volume freely, but you have doubled your tooling spend for the same output. If the supplier pays, they amortize it into the unit price and expect volume in return.
The middle path that works: the supplier builds the fixture at their cost, and you commit to a minimum annual quantity, typically 60 to 70 percent of planned volume, with a shortfall clause. That gives them a defensible payback and gives you a real second source without capital outlay.
Splitting volume without drift
Do not split 50/50 on day one. Start at 10 to 25 percent with 100 percent inspection on the pilot lot, move to 30/70 for two or three lots, then settle into whatever split your commercial terms require.
Keep the split stable once set. Suppliers improve when they run a part regularly; a split swinging between 20 and 80 percent month to month keeps both shops on a learning curve and neither reaches steady state.
Watch for the drift signature: a slow change in one dimension over three or four lots, always within tolerance. Plot the incoming values rather than the pass rate. A mean moving 0.005 mm per lot exits the band in two months, and pass rates will not warn you until it does.
Finally, keep one owner for the part. Two suppliers, two quality contacts and two sets of records is how a change made at one shop fails to reach the other. A single part owner holding the master drawing, the approved FAI from each site and the change log is worth more than any clause in the supply agreement.
If you are adding a source for a part already in production elsewhere, send the drawing and the incumbent's FAI through the contact page and ask for a comparison note. Our how we work page covers the pilot-lot and capability steps.
Frequently asked questions
How many parts should a second source make before approval?
Three parts for the first article, then 30 consecutive parts from one setup for capability. The 30-piece run is what tells you the process is stable, and it costs $80 to $200 in inspection on a typical part.
What Cpk should I require from a new supplier?
1.33 is the standard floor for production features, and 1.67 where the dimension is safety-related or not inspected on receipt. Compare against the incumbent rather than demanding an identical number.
Who should pay for the second set of tooling?
Either can work, but the common structure is supplier-funded tooling against a written minimum annual quantity, usually 60 to 70 percent of planned volume, with a shortfall clause. Paying yourself buys freedom to move volume but doubles tooling spend.
How long does it take to qualify a second source?
Four to seven weeks for a simple part, and eight to twelve weeks where several tight dimensions or a qualified finish are involved. Most of that is tooling and the pilot lot.
Related reading
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