What is Dimensional Control Testing?

Among the various tests on products and materials, one of the least known to the general public is the dimensional control test.

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The purpose of climatic and mechanical tests is easily understood even by non-experts: to verify the resistance of products and materials to certain conditions.

But what is the point of checking its size?

Today we will try to explain it in simple terms in our discussion of dimensional control testing.

What is the Dimensional Control Test for?

There are sectors in which product dimensions are of vital importance and their precision must be maximum.

One above all: electronics. Electronic components , in fact, have extremely low margins for error in the manufacturing of circuit boards, integrated circuits, and other components.

To a lesser but still significant extent, the automotive sector is also involved in dimensional control testing to verify the measurements of key engine components. 

The same need for precision is required in other sectors such as the production of medical instruments and of course construction.

In all these cases, the manufacturer selects some samples and subjects them to dimensional control testing, performed internally or through external laboratories, so as to be able to certify that the production cycle meets the required standards.

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How the Dimensional Control Test works

The dimensional control test uses some technical tools that come into play depending on the nature of the sample to be tested.

Among the most technologically sophisticated are coordinate measuring machines (CMMs): instruments that use a computer-controlled mechanical arm to measure the dimensions of an object. 

The object is placed on the CMM working platform and the mechanical arm moves along three axes to measure the x, y, and z coordinates of the object.

Another fundamental instrument is the profile projector , which can measure the dimensions of complex objects, such as curved surfaces, grooves, or holes. The sample is placed on a lighted table, and a lens magnifies the image, which is projected onto a graduated screen.

Other less complex instruments are used in dimensional control tests such as goniometers, micrometers and calipers.

All these tests must be carried out in accordance with the guidelines prescribed by a series of international standards among which we can mention: ISO 10012, ISO 14253 and ASTM E18.

Dimensional Control Testing and Climatic Chambers

play climatic chambers any role in the dimensional control testing trial?

Of course yes.

As with otherproduct and material tests we have already discussed, climatic tests play a complementary but fundamental role in the reliability of the results of a dimensional control test.

Especially for the most delicate components, such as electronic boards and integrated circuits, it is essential to know how they behave when exposed to extreme environments , with high relative humidity levels or significant temperature variations.

For this reason, in many cases it is essential to integrate the dimensional control test with targeted climatic tests that certify the integrity of the sample under all conditions of use.

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The FDM Environmental Chamber for Dimensional Control

Electronics is the sector most involved in climatic tests complementary to dimensional control testing.

For this reason, we recommend the FDM Environmental Chamber , which, unlike the standard climate chamber, specializes in so-called extreme climate tests. Here, for example, we discussed the environmental chamber test burn-in

For any further questions or concerns, please do not hesitate to contact us.

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Reference Standards

• ISO 1:2016 — Geometrical product specifications (GPS) – Standard reference temperature for the specification of geometrical and dimensional properties. Defines 20 °C as the metrological reference temperature.
• ISO 10012:2003 — Measurement management systems – Requirements for measurement processes and measuring equipment.
• ISO 14253-1:2017 — GPS – Inspection by measurement of workpieces and measuring equipment – ​​Decision rules for conformity/non-conformity.
• ISO/IEC 17025:2017 — General requirements for the competence of testing and calibration laboratories.
• JCGM 100:2008 (GUM) — Evaluation of measurement data – Guide to the expression of uncertainty in measurement.
• ISO 230-3:2020 — Test code for machine tools – Part 3: Determination of thermal effects.
• VDI/VDE 2627 — Measurement chambers – Classification and characteristics, classification of metrological chambers.

Authoritative Sources

• BIPM – International Bureau of Weights and Measures. International organization that maintains the International System of Units and publishes the GUM.
• INRIM – National Institute for Metrological Research. Italian metrological institute, a reference for primary traceability of physical quantities.
• ACCREDIA – Italian Accreditation Body (www.accredia.it). Italian accreditation body for calibration and testing laboratories according to ISO/IEC 17025.
• PTB – Physikalisch-Technische Bundesanstalt. German Metrological Institute, European reference for GPS and dimensional metrology.

Frequently Asked Questions about Dimensional Control in a Controlled Environment

Why is 20°C the metrological reference temperature?

ISO 1 has set 20 °C as the global standard temperature for all dimensional measurements since 1931. The choice is a compromise between industrial and laboratory environments: close enough to typical shop-floor conditions to reduce thermal gradients, yet stable enough for precision metrology. Every mechanical part expands or contracts from its nominal size as a function of temperature and the linear expansion coefficient α: ΔL = L₀ × α × ΔT. For carbon steel, α = 11.5 × 10^-6 /K; a 1000 mm shaft at 25 °C measures 1000.058 mm (+58 µm compared to 20 °C). For aluminum (α = 23 × 10^-6) the change doubles to +115 µm. For IT3-IT5 tolerance checks (high precision classes, ± 5-10 µm on 100 mm) it is essential to operate at 20 ± 1 °C or apply thermal correction (temperature compensation) with measurement of the actual temperature of the piece and tool.

What temperature tolerance is required for a metrology chamber?

The VDI/VDE 2627 classifies measuring chambers into five classes: Class 1 (high precision) 20 ± 0.2 °C with gradient ≤ 0.2 K/m, stability ≤ 0.2 K/h, RH 45 ± 5%; Class 2 20 ± 0.5 °C, gradient ≤ 0.5 K/m, stability ≤ 0.5 K/h; Class 3 20 ± 1 °C; Class 4 20 ± 2 °C; Class 5 20 ± 3 °C. The choice depends on the IT tolerances of the products being inspected. For high-precision CMMs (Coordinate Measuring Machines) (MPE ≤ 2 µm + L/300) Class 1-2 is required; For quality control in workshops IT8-IT11, Class 4-5 is sufficient. The chamber must have double doors with airlocks, insulated walls shielded from direct sunlight, high Ra LED lighting without heat dissipation, and an antistatic floor on a raised, vibration-damping platform. Parts and instruments must thermally stabilize for 4-24 hours before measurement.

How does uncertainty propagate in a dimensional measurement?

According to GUM (JCGM 100), the combined uncertainty u _c of a dimensional measurement is calculated as the square root of the sum of the squares of the components (Type A statistical and Type B contributions from specification): u _c ² = u _repet ² + u cal _² + u _temp ² + u _op ² + u _geom ² + … Typical contributions are: repeatability (standard deviation of n measurements, Type A), instrument calibration (from ACCREDIA certificate, typically 0.5-2 µm for CMMs), thermal effect (u _temp = L × α × u _T ), operator effect , uncertainty in the shape of the piece. The expanded uncertainty U = k × u _c with k=2 for a 95% confidence level. The result is declared as L = (value ± U) mm, k=2. For the ISO 14253-1 decision rule, the accepted conformity zone is reduced by the uncertainty band: a piece is declared compliant only if the measured value + U falls within the specified tolerance; in the indecision zone (tolerance ± U), customer-supplier agreement is required. A tolerance/uncertainty ratio (TUR ≥ 4:1) minimizes the risk of an incorrect decision.

Which climatic chamber for precision dimensional control?

For dimensional control in a dedicated climatic chamber, a reduced range but very high stability: typically 20 ± 0.2 °C for class 1 metrology, 20 ± 0.5 °C for class 2. Critical specifications: spatial uniformity ≤ 0.2 °C over the useful volume (mapped to 9 IEC 60068-3-6 points at full load), temporal stability ≤ 0.1 °C/hour, recovery time after door opening ≤ 5 min. Adaptive PID with anti-overshoot algorithm, Pt100 class 1/10 DIN sensors (uncertainty ± 0.03 °C) calibrated by ACCREDIA. Humidity 45 ± 5% RH to neutralize electrostatic charges on CMMs. Vertical laminar air flow 0.1-0.3 m/s with low turbulence. Granite (α = 4–8 × 10–6^,low thermal sensitivity) with vibration damping < 1 Hz. Glass wall for observation. Part passage through a pre-chamber at 20 ± 1 °C for stabilization. Metrological-class FDM chambers offer 1–20 m³ volumes with ISO 14644 class 7 (clean chamber) certification for aerospace and semiconductor applications where particulate contamination is as critical as thermal stability.