Machined parts for test, measurement and lab equipment are the structural and locating parts of an instrument: base plates, frames, mounts, adapters, shafts and enclosures that hold a sensor, a sample or a beam in a fixed position. Their tolerances are part of the measurement, because a mounting face that is 0.05 mm out of flat tilts everything fixed to it.
Which parts does a test or measurement instrument need?
Instruments are built from a small set of part families, and each carries its tolerance on a different feature. A base plate sets the datum for everything above it. A frame or gantry sets the distance between two axes. A mount or adapter sets the position of a sensor relative to a sample. Shafts and spacers set lengths in a drive or an adjustment. Enclosures and panels carry no measurement function, but they decide whether the instrument closes cleanly and shields what it should.
Table 1. Part families, the tolerance that decides fit, and the document to request
| Part | Tolerance that decides fit | Document |
|---|---|---|
| Base plate, breadboard | Flatness of the mounting face; position of the hole grid | Measurement report for flatness and hole positions |
| Frame, gantry | Squareness of the faces that carry axes; overall length | Dimensional report, EN 10204 3.1 material certificate |
| Mount, carrier, adapter | Bore fit to ISO 286; position to the datum | Measurement report with fit and position values |
| Shaft, spacer, pulley | Roundness, run-out, fit on the bearing seat | Measurement report, material certificate |
| Fixture and gauge plate | Position of locating holes; flatness | First article inspection report |
| Enclosure, panel | Cut-outs, flatness of the sealing face, burr-free edges | Dimensional report, certificate of conformity |
Source: drawing practice per ISO 1101:2017, ISO 286-1:2010 and ISO 2768-1:1989; document types per EN 10204:2004.
A part can belong to more than one row: a machined base plate welded into a frame must keep its flatness through the welding.
Which tolerances belong on an instrument drawing?
Start with the general tolerance and tighten only the features that carry the function. ISO 2768-1:1989 class m allows ±0.2 mm from 6 to 30 mm, ±0.3 mm over 30 to 120 mm and ±0.5 mm over 120 to 400 mm. On an instrument it covers clearance holes, cover edges and non-critical lengths. The ISO 2768 tables give the full range.
Functional features need individual tolerances written to ISO 1101:2017: flatness on a mounting face, position on a hole pattern, run-out on a shaft seat. Two rules decide whether those tolerances do what the designer expects.
The first is the independence principle of ISO 8015:2011. Size and form are separate requirements unless the drawing invokes the envelope requirement. A bore toleranced only as ⌀25 H7 is controlled at two points across its diameter, so it may be oval or barrel-shaped and still conform. The same bore allows 0 to +0.021 mm, which is IT7 for 18 to 30 mm and about ten times tighter than the ±0.2 mm of class m, and it still says nothing about roundness. Where a bearing or a dowel has to sit true, add cylindricity or a fit that controls the mating part.
The second is the shape of the tolerance zone. A ±0.1 mm coordinate tolerance around a hole centre describes a square of 0.2 mm by 0.2 mm, whose corner lies 0.141 mm from nominal. A position tolerance of ⌀0.2 mm describes a circle of 0.2 mm diameter and rejects that corner. The two are not interchangeable, and a drawing that mixes them across a hole pattern gives the supplier two different answers.
Surface texture belongs in the same pass. ISO 21920-1:2021 replaced ISO 1302:2002 in December 2021, so a drawing that still cites ISO 1302 asks for an indication the current standard no longer defines. The Ra and Rz guide covers the parameters and their conversion limits.
Why does a 20 °C reference temperature matter?
Every dimension on a drawing is the size at 20 °C. ISO 1:2022, fourth edition published 2022-06-14, fixes that reference temperature, and NIST records that industrial lengths have been defined as the size at 20 °C since 1931. A workshop at 25 °C measures parts that are larger than their drawings, and the difference is not noise.
Table 2. Length change of a 500 mm part measured at 25 °C instead of 20 °C
| Material | α, 10⁻⁶/K | Change over 500 mm |
|---|---|---|
| Carbon steel (EN 1993-1-1) | 12 | 0.030 mm |
| Aluminium (EN 1999-1-1) | 23 | 0.058 mm |
Source: coefficients from EN 1993-1-1 and EN 1999-1-1 as tabulated by BuildSoft; change calculated as α × 500 mm × 5 K.
Take a 6082 base plate with two locating holes 500 mm apart, machined and checked in a shop at 25 °C. At 20 °C the hole spacing is 0.058 mm shorter. If the drawing gives the second hole a position tolerance of ⌀0.1 mm, its zone has a radius of 0.05 mm, and the temperature difference alone moves the hole past that limit. It may fail at incoming inspection although it passed at the machine.
Two responses work. Measure at 20 °C, or measure at shop temperature and correct each result with the expansion coefficient and the recorded part temperature. The second needs the coefficient of the actual alloy and temper, not a generic value for the metal. Either way the report should state the temperature at which the values are valid.
Which materials suit frames, plates and mounts?
Aluminium EN AW-6082-T6 and 6061-T6 are the usual choice for machined plates and mounts. They machine cleanly, take an anodised finish and keep moving mass low. Austenitic stainless steel 1.4301 suits parts that see cleaning agents, humidity or vacuum, and welded frames. C45 works for shafts and adjustment spindles where a heat-treated steel is needed.
Thermal expansion is part of the choice. A steel mount on an aluminium plate moves relative to the plate by 11 × 10⁻⁶/K, the difference between the two coefficients in Table 2, which is 0.028 mm over 500 mm for a 5 K change. Where two parts have to stay aligned over a temperature cycle, use the same material for both or design the joint to slide.
A machined casting is common in instrument frames because it puts material where the stiffness is needed. The machining then establishes the datum on the cast surface, and the drawing must say which surfaces are machined and which stay as cast.
What should a measurement report contain?
A measurement report is only useful if another engineer can repeat the decision from it. It should identify the part, the drawing revision and the batch; list each characteristic with its nominal value, tolerance, measured value and the instrument used; state the measurement uncertainty; and state the decision rule.
Three standards sit behind those lines.
- ISO 10360-2:2009 is the acceptance and reverification test for Cartesian CMMs measuring linear dimensions. Its maximum permissible error describes the machine, not a specific measurement task. NPL Good Practice Guide 130 makes that distinction and gives methods for the task-specific uncertainty.
- ISO 14253-1:2017 sets the decision rules. Under the default rule, conformance is proven only when the measured value lies inside the tolerance zone reduced by the expanded uncertainty U on each side. A ±0.05 mm tolerance measured with U = 0.01 mm is therefore proven only within ±0.04 mm; a value between 0.04 mm and 0.05 mm is not proven either way.
- ISO/IEC 17025:2017 is the competence standard for testing and calibration laboratories. A calibration certificate issued under accreditation is a different document from a supplier's dimensional report, and a report from a machine shop should not be read as one.
Which documents come with the parts?
"With certificate" on a purchase order does not say which certificate. EN 10204:2004 defines the types used for metallic products. A 3.1 certificate reports specific inspection results for the delivered products and is validated by the manufacturer's inspection representative, independent of production. It covers the material, usually chemistry and mechanical properties by heat number, and says nothing about the dimensions of the finished part.
Dimensions are covered by the measurement report or a first article inspection report. A certificate of conformity is the supplier's declaration that the delivery matches the order. The EN 10204 guide sets out the four types, and quality documents for machined parts covers how they combine.
Name the document type, the characteristics it must cover and the standard the inspection follows. A 3.1 certificate for a batch of 6082 plate and a measurement report for the machined plate are two different documents, and an instrument builder usually needs both.
What belongs in the request for quotation?
A drawing that answers these questions gets a quote that holds:
- The general tolerance class, for example ISO 2768-m.
- The datum reference frame, and which faces are functional.
- Flatness, position and run-out values on the features that carry the measurement.
- Fits by ISO 286 code, and surface texture by ISO 21920.
- The reference temperature for critical dimensions, if it is not 20 °C.
- Material grade with the product form, and the certificate type.
- The scope of the measurement report, the uncertainty and the decision rule.
- Cleaning, marking and packaging.
Our network in Bursa mills and turns, cuts and bends sheet metal, welds, machines castings and assembles. The standard general tolerance is ISO 2768-m, tighter where the drawing specifies it. Parts travel by road from Bursa to central Europe in 5 to 10 days typical, inside the EU–Turkey customs union.
Frequently asked questions
- What tolerance do machined parts for test equipment carry?
- Untoleranced dimensions usually follow ISO 2768-1 class m: ±0.2 mm from 6 to 30 mm, ±0.3 mm over 30 to 120 mm, ±0.5 mm over 120 to 400 mm. Mounting faces, hole patterns and bearing seats carry individual tolerances to ISO 1101 or ISO 286, because the general class is too wide for them.
- Why is 20 °C the reference temperature for dimensional measurements?
- ISO 1:2022 fixes the standard reference temperature at 20 °C, and NIST records that industrial lengths have been defined as the size at 20 °C since 1931. A part measured at 25 °C is larger than its drawing, and the difference grows with length and with the expansion coefficient.
- How much does a 500 mm aluminium part change between 20 °C and 25 °C?
- 0.058 mm, using the coefficient of 23 × 10⁻⁶/K that EN 1999-1-1 gives for aluminium. A carbon steel part of the same length changes by 0.030 mm, using 12 × 10⁻⁶/K from EN 1993-1-1.
- Does an EN 10204 3.1 certificate prove the dimensions of a machined part?
- No. A 3.1 certificate reports specific inspection results for the material, usually chemistry and mechanical properties by heat number, and is validated by the manufacturer's inspection representative. The dimensions of the finished part are covered by a measurement report or a first article inspection report.
- Is a supplier measurement report a calibration certificate?
- No. A calibration certificate is issued under ISO/IEC 17025:2017 accreditation and covers a measuring instrument or a reference. A supplier's measurement report states measured values for the delivered parts, and the two documents answer different questions.
Sources
- ISO 1:2022, Standard reference temperature for geometrical and dimensional properties, 4th edition, published 2022-06-14 (ISO catalogue)
- NIST, Selected publications on dimensional metrology: the 20 °C paper, Doiron, J. Res. NIST 112 (2007)
- ISO 2768-1:1989, General tolerances, Part 1 (ISO catalogue, to be revised)
- ISO 286-1:2010, ISO code system for tolerances on linear sizes, Part 1 (ISO catalogue, confirmed 2026)
- ISO 8015:2011, GPS fundamentals, concepts, principles and rules (ISO catalogue, confirmed 2021)
- ISO 1101:2017, Geometrical tolerancing, tolerances of form, orientation, location and run-out (Genorma catalogue, confirmed 2022)
- ISO 21920-1:2021, Surface texture: Profile, Part 1, replaced ISO 1302:2002 (ISO catalogue)
- ISO 14253-1:2017, Decision rules for verifying conformity or nonconformity with specifications (ISO catalogue, confirmed 2023)
- ISO 10360-2:2009, Acceptance and reverification tests for CMMs measuring linear dimensions (ISO catalogue, confirmed 2026)
- ISO/IEC 17025:2017, General requirements for the competence of testing and calibration laboratories (ISO catalogue, confirmed 2023)
- EN 10204:2004, Metallic products, types of inspection documents (Standards Ireland catalogue, current)
- Carbon steel (EN 1993-1-1) versus aluminium (EN 1999-1-1), material parameter table, α 12 and 23 × 10⁻⁶ m/mK (BuildSoft)
- Flack, Co-ordinate measuring machine task-specific measurement uncertainties, NPL Good Practice Guide No. 130, September 2013
- ISO 286-1 IT grades, IT7 = 21 µm for 18 to 30 mm (table reproduced on Wikipedia)
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