Dimensional inspection: inspection report, critical dimensions, first article

Inspecting a part means checking that its dimensions and forms comply with the drawing, within tolerance. The means range from the simple to the sophisticated:

• hand instruments (calliper, micrometer, dial indicator) and “go / no-go” gauges for common dimensions;
• the profile projector and optical measurement for small parts and contours;
• the coordinate measuring machine (CMM), which probes the part and measures dimensions, positions and forms in space;
• the 3D scanner, which compares a freeform surface with the digital model.

Then there is what you receive on delivery. Three documents are often confused:

• the certificate of conformity: a declaration that the parts comply, with no measured value;
• the inspection report: the measured values, dimension by dimension;
• the material certificate: the composition and characteristics of the material, for example type 3.1 under standard EN 10204.

State which one you need: it is the first source of misunderstandings between a client and their workshop. For workshops that specialise in this, see the page on inspection and metrology subcontracting.

Not all dimensions on a drawing are equal. A dimension is critical when a deviation could prevent assembly or degrade function, sealing or safety. The others, known as free dimensions, fall under general tolerances (see the guide on machining tolerances). To spot them:

1. Start from the function. Interfaces with other parts (fits, centre distances), sealing, guiding or bearing surfaces.
2. Take your sector’s requirements into account. Some customers or standards designate “key” or “special” characteristics that must be identified and monitored.
3. Rely on a risk analysis. An FMEA (failure mode and effects analysis) shows which deviations would have the heaviest consequences.

Then mark them on the drawing (symbol, balloon or table) and communicate them to the workshop. Resist the temptation to declare everything critical: if every dimension is, inspection becomes long, expensive and hard to read. Ask the workshop to flag those that seem difficult to hold or to measure: it is often an opportunity to relax an unnecessary tolerance.

A usable report allows a third party to link each measurement to the dimension on the drawing and to the part measured. It includes:

• the identification: drawing reference and revision, part or batch number, quantity, date, operator;
• the ballooned drawing: the drawing on which each inspected dimension carries a number in a balloon, repeated in the table;
• the results table: dimension number, nominal value, tolerances, measured value, deviation and conclusion (compliant or not);
• the measuring equipment used and its calibration status;
• the non-conformities found and the decision taken: rework, scrap or a concession requested from the client.

Two technical points deserve agreement. First, measurement uncertainty: the measuring equipment must be clearly more accurate than the tolerance being checked (a ratio of one to ten is often cited as a guideline). Second, the decision rule for a value close to the limit: is the part declared compliant or not? This question is covered by ISO 14253-1; agree on it in advance. Finally, the reference temperature for measurement is 20 °C: a part straight off the machine must stabilise before a fine measurement.

The first article is a full inspection of the first parts made with the production tooling and process. It proves, before the run is launched, that the supplier can make the part in accordance with all the requirements of the drawing and the order. It covers all the characteristics on the drawing, not only the critical dimensions, as well as the materials and special processes.

In aerospace, it generally follows EN 9102 (the same requirement is published as AS 9102). It relies on three forms: the first identifies the part and the reason for the inspection; the second lists the materials, special processes (heat or surface treatments, for example) and functional tests, with supporting evidence; the third lists each characteristic of the ballooned drawing with its measured value. The standard applies when the contract or the buyer requires it. It has to be redone, in whole or in part, when manufacturing changes significantly: a change of design, process, manufacturing location or tooling, or restart after a long interruption.

Outside aerospace, practice varies. The automotive industry has its production part approval process, PPAP, with a file of measured initial samples. In medical, traceability and validation requirements depend on the device (see the guide on ISO 9001, EN 9100 and ISO 13485 certifications). In other cases, a dimensional report on the first part, on a ballooned drawing and accompanied by the material certificate, makes an excellent first article, provided you have defined it in the order.

A control plan (or monitoring plan) answers five questions for each characteristic: which dimension (marked on the drawing), with what equipment, how often, against what acceptance criterion and with what record. Frequency is decided dimension by dimension, according to risk:

100% inspection of all dimensions lengthens lead times and makes each part more expensive: reserve it for what justifies it. Write this plan into the order or into the specification: the workshop then prices the inspection time, and you compare quotes on an equal scope.

Inspection is first of all for detecting deviations before delivery. Agree in advance what happens next:

• Rework is only possible if the deviation is recoverable (there is material left to remove, for example) and if the reworked part meets the whole drawing.
• Scrap means making a new part: specify who bears the cost of the material and the additional lead time.
• A concession (or waiver) means the client knowingly accepts an out-of-tolerance but usable part. It is requested in writing, with the part and the measured deviation: the workshop does not deliver a non-conformity without the client’s agreement.

Also provide for a claims period on receipt and keep the reports: in the event of a dispute, they serve as evidence for both sides.

The CMM is the reference tool for complex forms, positions (centre distances, coaxiality) and the geometric tolerances of GPS dimensioning (GD&T). Its cost is made up of a measurement programme, written once, and a measurement time per part. It also requires respecting the datums on the drawing (A, B, C): they must reflect how the part is actually assembled, otherwise the measurement says nothing about function.

For a common diameter or length, a hand instrument or a gauge is enough and costs less. Flexible or deformable parts (plastics, thin sheet) do not lend themselves to probing and call for optical measurement instead. Finally, ask the workshop for the planned equipment and its calibration status. If you have the parts inspected by a third party, an accredited laboratory (in France, accredited by Cofrac, the French accreditation body, to ISO/IEC 17025) provides an additional guarantee.

Inspection that is too light exposes you to scrap; inspection that is too thorough increases price and lead times. Nothing obliges you to apply the same level to all parts:

In your request, state the current drawing and the critical dimensions, the level and format of the report (PDF, spreadsheet), the frequency, how non-conformities are handled (rework, concession) and any material certificate. Specify whether you supply the ballooned drawing or the workshop must produce it. Your quotes will then compare equivalent services.

Need inspection or a production run to be made? Post your request on Meaflow: it is free and commission-free, and you compare quotes from several workshops.