Coordinate measuring machine (CMM) measurement
A probe records points on the part with great accuracy and compares dimensions with the drawing. It is the benchmark for machined parts with tight tolerances.
Expertise
Inspection and metrology subcontracting
Dimensional inspection, 3D measurement, non-destructive testing and reports: compare quotes from laboratories and metrologists.
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Inspecting means verifying that a part or assembly meets what was ordered. Dimensional metrology measures dimensions, shapes and surface finishes; non-destructive testing (NDT) looks for internal or surface defects without damaging the part; material tests check strength, hardness or composition. All these inspections result in a report, which serves as proof between the client and its supplier.
Needs are varied: a dimensional inspection report for machined parts, a first article inspection before a run, sorting on receipt, inspection of a part with tight tolerances that the workshop lacks the means to check, 3D digitisation of an object to compare it with the model, or weld inspections on a structure. Suppliers range from the accredited laboratory to the metrology workshop equipped with coordinate measuring machines, including mobile NDT technicians who work on site.
This page explains measuring equipment and its limits, the usual reports and certificates, price factors and how to define an inspection requirement so that it can be quoted and compared. It is aimed at quality managers, buyers and design offices who need to check, or have checked, parts.
Common services
Digitising and 3D scanning
A laser or structured-light scanner captures the complete shape and compares it with the CAD model: a colour map of deviations makes distortion visible in moulded, welded or sheet metal parts.
Optical inspection and profile measurement
Profile projectors and optical machines measure small parts, cut-outs or fine profiles without contact, particularly in watchmaking, bar turning and electronics.
Roughness, form and threads
Inspection of surface roughness, roundness, cylindricity, threads and bores using specialised equipment or dedicated gauges.
Non-destructive testing
Dye penetrant and magnetic particle testing for surface defects, ultrasonics and radiography for internal defects, eddy currents for certain cracks: standardised techniques applied by qualified personnel.
Material testing
Tensile, hardness, impact testing, composition analysis by spectrometry, material identification: to verify that a material matches what was ordered.
Calibration and reports
Calibration of measuring instruments, dimensional inspection reports, ballooned drawings, first article files, certificates: the documentary proof of conformity.
Materials worked
Machined parts in steel, aluminium, stainless steel and titaniumSheet metal, fabricated and welded partsCast, injection-moulded and 3D-printed partsMechanical assemblies and sub-assembliesPlastic and composite partsElectronic boards and components (visual and dimensional inspection)Tooling, fixtures and mouldsMeasuring instruments to be calibrated
Common applications
- First article inspection before launching a run
- Dimensional report required by a client
- Sorting and incoming inspection of supplied parts
- Weld inspection on structures and pressure vessels
- Comparison of a moulded or printed part with its 3D model
- Reverse engineering: measuring and surveying a part with no drawing
- Expert assessment after an incident or in a dispute
- Calibration of the workshop’s measuring tools
What affects the price
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1
Number of dimensions to measure
Measurement and programming time increases with the number of dimensions, how hard they are to access and the accuracy required. Checking ten key dimensions is not charged like checking every dimension on a drawing.
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2
Part size and complexity
A large part or a complex shape requires suitable equipment, holding fixtures and more set-up time.
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3
Quantity and sampling method
Inspecting one part, a statistical sample or every part in a batch does not cost the same. The first part requires the measurement programme to be created, which is then amortised.
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4
Uncertainty and environment
The tighter the tolerance, the lower the measurement uncertainty must be, which requires a more accurate instrument and a thermally stable environment.
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5
Report and certificates
A simple readout, a full report with ballooned drawing and analysis, or a first article file do not take the same time to write.
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6
Travel and accreditation
An inspection carried out at your premises involves travel; an accredited laboratory applies procedures that give reassurance but may be reflected in the price.
Tips for a successful request
Post a request- Attach the dimensioned drawing and, if possible, the 3D model; specify the dimensions to inspect (all of them or only the critical ones).
- Ask for a ballooned drawing, meaning a drawing with each dimension numbered, which serves as the reference for the report.
- State the number of parts to measure, the frequency and the sampling method (one part, a sample, all of them).
- Indicate the report format expected: table of measurements, conformity, deviation chart, certificate, first article file.
- Mention the standard or client specification to be met, for example aerospace, automotive or medical.
- Specify whether the inspection takes place at the supplier’s, at your premises or at your supplier’s, and the time slots available.
Which measuring equipment for which requirement?
There is no universal measuring method: the right choice depends on the tolerance to verify, the size of the part, its material and the quantity. A common-sense rule applies: measurement uncertainty must remain well below the tolerance of the dimension being checked. Measuring to the hundredth of a millimetre a dimension toleranced to the hundredth, with an instrument whose uncertainty is of the same order, proves nothing.
- Coordinate measuring machine (CMM): high accuracy, ideal for functional dimensions and geometric tolerancing (flatness, position, coaxiality). It works in a temperature-controlled room, because the reference temperature for dimensional measurement is 20 °C.
- Measuring arm and 3D scanning: portable, suited to bulky parts, free-form shapes and on-site inspections. They give an overall view with somewhat lower accuracy than a CMM.
- Optical methods: non-contact, for fragile, flexible or tiny parts.
- Gauges: a go/no-go plug gauge quickly checks a bore or a thread in series.
CNC machining workshops often have a measuring room; for high requirements or large volumes, independent inspection provides a second opinion. The guide on machining tolerances helps you identify the dimensions that really deserve to be inspected.
Non-destructive testing, certificates and reports: useful vocabulary
NDT (non-destructive testing) makes it possible to verify the integrity of a part without damaging it. Dye penetrant testing reveals surface-breaking cracks through the penetration of a coloured liquid; magnetic particle testing detects surface defects in ferromagnetic materials; ultrasonics and radiography probe the inside of the material, notably welds; eddy currents are mainly used on conductive metals. The personnel who carry them out are qualified and certified by level, according to standard EN ISO 9712. These techniques often accompany a welding and fabrication job.
As for documents, a distinction is made between the material certificate, issued by the material manufacturer (type 3.1 of standard EN 10204 is the most common, while type 3.2 involves a third-party body), the dimensional inspection report, which compares measured dimensions with nominal dimensions together with their deviations, and the first article file, which gathers the evidence of conformity of a first part before a run is launched, according to specifications specific to each sector such as aerospace or automotive.
A laboratory can be accredited (in France, by COFRAC, the French accreditation body, under standard ISO/IEC 17025) for certain tests or calibrations. Accreditation is useful if your client or your specification requires it; it is not always necessary for an internal inspection. State this in your request, because it reduces the number of suppliers able to respond and may affect the price.
Defining an inspection plan: what, how many, how
A useful inspection is a targeted one. Inspecting every dimension of every part is rarely necessary and very costly. It is better to identify the critical dimensions (fits, interfaces, safety), decide on a sampling method and state the frequency. The sampling plans of the ISO 2859 series give benchmarks for setting a statistical inspection level; in production, capability indices (Cp, Cpk) measure process stability, a threshold of around 1.33 often being required in the automotive industry.
Plan for document needs: simple report, ballooned drawing, raw measurement file, calibration certificates. Specify the return of the parts, their packaging and transport, since precise parts can deteriorate during handling. For parts intended for a surface treatment, decide whether the inspection should take place before, after, or both, since the coating adds thickness.
A Meaflow request that specifies the critical dimensions, the number of parts and the type of report expected lets laboratories and metrologists respond on an identical scope; you then compare method, any accreditation, lead times and price. Submit your inspection brief with the drawing and, if you have it, the 3D model.
Guides to go further
Machining tolerances: ISO 2768, fits and dimensioning
ISO 2768 general tolerances, ISO 286 fits, ISO 1101 geometric tolerancing, Ra roughness: read a drawing and dimension sensibly to control the cost of a machined part.
Writing a manufacturing specification: method and template
What a workshop needs to know to quote and make your part: a method in a few points, a ready-to-copy template and common mistakes to avoid.
How to choose an industrial subcontractor: 10 criteria
Skills, capacity, quality, proximity, responsiveness, references, financial strength, communication, price and contract: 10 criteria and a comparison grid.
Related expertise
CNC machining
Milling, turning, 3- to 5-axis machining centres
Welding & boilermaking
TIG, MIG, structures, tanks, metalwork
Surface treatment
Painting, powder coating, anodising, galvanising
Mechanics & assembly
Assembly, integration, machine maintenance
Frequently asked questions
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How much does a CMM dimensional inspection cost?
The cost depends on the number of dimensions, the size and complexity of the part, the quantity, the accuracy required and the report requested. Creating the measurement programme is a fixed cost, then amortised over subsequent parts. Send the drawing with the dimensions to inspect to receive precise, comparable quotes.
What documents should I provide for an inspection quote?
The dimensioned drawing, if possible the 3D model, the list of dimensions to inspect or a ballooned drawing, the number of parts, the material and the type of report expected. Add any standard or client specification and the place of inspection. A photo of the part also helps the laboratory choose the right measuring method.
What is the lead time for an inspection report?
A few days is often enough for a dimensional inspection of one or a few parts, once they have been received. A complete first article file or a large batch takes longer, depending on the laboratory’s workload. An urgent inspection is sometimes possible at a surcharge. State the date you need and allow for transporting the parts there and back.
What is the difference between a 3.1 and a 3.2 material certificate?
Under standard EN 10204, the 3.1 certificate is issued by the material manufacturer, validated by its inspection department independent of production, with the test results for the batch. The 3.2 certificate adds validation by a third-party body or the client’s representative. The 3.2 is more demanding, takes longer to obtain and costs more.
Can a laboratory inspect welds?
Yes, specialist suppliers carry out visual, dye penetrant, magnetic particle, ultrasonic or radiographic inspections, with certified personnel. State the quality level aimed for, the applicable standard and the extent of the inspection. Some work on site, which avoids moving the parts or structures.
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