Machining standards & quality control

HomeStandardsQuality & StandardsMachining standards & quality controlEvery Prolean order is machined and inspected against a documented standard. This page sets out the tolerances we apply by default, the tighter limits we can reach on a specified feature, what is checked on every batch, and which reports ship with the parts. Get a Quote Contact Us 🔒 All uploads are secure and confidential. ✓ ISO 9001:2015 Certified by SGS United Kingdom Ltd ISO 2768-fH / mKOur default class when a drawing has no tolerance call-out — stated on the inspection record± 0.0002 in≈ ± 0.005 mm — on a specified feature, subject to engineering review and confirmed at quoteISO 2859-1 · Level IISampling applied to production batches — code letter, sample size and Ac/Re stated on the inspection record7 optional documentsA packing list ships as standard — everything else is requested at quote stage Standards Tolerances Threads & knurling Surface finish Equipment Certification Handling Documents Sampling FAQ 01 StandardsFour published standards cover almost every orderIf your drawing calls out something different, we follow your drawing. Where it is silent, these are the defaults we apply. 01ISO 9001:2015 — quality management systemCertified by SGS United Kingdom Ltd and independently audited, so the process behind your parts is documented and externally checkable — not just claimed. See the certificate and how to verify it ↓ 02ISO 2768 — general tolerancesThe class letters carry two different things. f and m are the linear and angular classes of ISO 2768-1:1989 — f is fine, m is medium. H and K are the geometrical classes of ISO 2768-2:1989, which was withdrawn in February 2021 and replaced for new drawings by ISO 22081:2021. fH for metals and mK for plastics is Prolean's own agreed default, not something ISO fixes by material. Drawings still marked mK stay binding and we read them the same way — but we do not present the withdrawn part as a current general rule, and we do not silently re-map H and K to ISO 22081. If your drawing names its own class, the drawing wins. How the ISO 2768 classes work ↗ ISO 22081:2021 on iso.org ↗ 03ISO 965-1 — metric thread toleranceThread tolerance class for metric threads. Where your drawing does not name one we apply the medium class, and we state which class was used on the inspection record. Thread and knurling defaults ↓ 04ISO 2859-1 — acceptance samplingISO 2859-1:1999, General Inspection Level II, single sampling, normal inspection, applied to production batches. Severity is graded critical, major or minor against a documented defect book, and the plan's code letter, sample size and acceptance number are stated on the inspection record. AQL levels and defect classes ↓ 02 Tolerances± 0.0002 in± 0.005 mmOur tightest achievable limit — on a specified feature, after engineering review. A capability ceiling, not a default for your drawing. CoarseMediumFineGeneral tolerance — metal (drawing silent) ISO 2768-fH — Prolean defaultGeneral tolerance — plastic (drawing silent) ISO 2768-mK — Prolean defaultTightest achievable — specified feature ≈ ± 0.005 mm (± 0.0002 in), after reviewMinimum feature size 0.5 mm or Ø 0.50 mmMinimum wall thickness 0.8 mm metals / 1.5 mm plasticsThe achievable limit is not one number for every feature — it depends on the material, the feature size and geometry, the machining process, and the surface finish state. Send the drawing with the requirement called out and our engineers confirm feasibility before quoting: the tolerance we commit to is the one on your quote. See the full CNC machining capability list. ⚠ Two different numbers. The general tolerance applies when your drawing is silent; the achievable limit applies only to a feature that is called out and reviewed. Both are capability references — the tolerance, inspection method and report for a specific part are confirmed on the quote for that part. 03 Threads & knurlingThread call-outs are the most common thing missing from a drawingIf yours has one we follow it. If it does not, we apply the defaults below — and we tell you which default we used on the inspection report. UNC unified coarse · assembly fit UNF unified fine · thin walls Metric ISO 261 / 262 profile NPT ANSI/ASME B1.20.1 NPS / NPTF / STI / BSP on request For metric threads we default to ISO 965-1 medium class. For UNC and UNF without a call-out we apply ASME B1.1-2003 — 2A external and 2B internal. ACME and Thor-labs threads we do not accept. Default thread tolerance classThread sizeInternalExternalThreads up to M1.45H6hThreads M1.6 and larger6H6g Knurling — DIN 82 / ISO 13444:2012 by default. Mark the knurl position on the drawing. A different knurling standard can be raised at quote stage and we confirm feasibility before manufacture. 04 Surface finishFinish is guaranteed by examination under fixed conditions, not by assumptionThe same viewing setup is used on every batch — that is what keeps batch 1 and batch 10 looking the same when they reach your line. Viewing distance50 ± 10 cmwith corrected visionViewing angle45°across all directionsIllumination500–1000 luxneutral white Cosmetic — a consistent surface with no visible flaw on the primary face. Non-cosmetic — minor visual blemishes are permissible because appearance is not functional. Tell us which one your part needs. See all surface finish options. 05 EquipmentEvery dimensional check happens in our own quality roomDimensional inspection runs in our own quality room, on the same floor as the machines that cut your parts. Where a specification calls for external testing — material chemistry, for example — we use an accredited third-party lab and say so on the report. Primary Coordinate measuring machineProgrammed 3D measurement of critical dimensions, position and profile. Reports are generated from the measuring program rather than typed by hand, so the numbers on your dimensional report are repeatable run to run.