--- title: "CNC Machining for Steel — Tolerances, Fits, Surface Finish" description: "How contract CNC machining works for steel parts: ISO 2768 general tolerances, ISO 286 fit classes, achievable surface finish, and EN 10204 traceability." canonical: https://steelequivalents.com/services/cnc-machining language: en robots: index, follow og_type: website type: service-detail language_alternates: en: https://steelequivalents.com/services/cnc-machining.md cs: https://steelequivalents.com/cs/služby/cnc-obrábění-oceli.md de: https://steelequivalents.com/de/dienstleistungen/cnc-bearbeitung.md es: https://steelequivalents.com/es/servicios/mecanizado-cnc.md fr: https://steelequivalents.com/fr/services/usinage-cnc-acier.md hu: https://steelequivalents.com/hu/szolgáltatások/acél-cnc-megmunkálása.md it: https://steelequivalents.com/it/servizi/lavorazioni-cnc-acciaio.md nl: https://steelequivalents.com/nl/diensten/cnc-verspaning-staal.md pl: https://steelequivalents.com/pl/usługi/obróbka-cnc-stali.md pt: https://steelequivalents.com/pt/serviços/maquinagem-cnc-de-aço.md ru: https://steelequivalents.com/ru/услуги/чпу-обработка-металла.md tr: https://steelequivalents.com/tr/hizmetler/çelik-cnc-işleme.md zh: https://steelequivalents.com/zh/服务/钢材cnc加工.md ja: https://steelequivalents.com/ja/サービス/cnc加工.md ar: https://steelequivalents.com/ar/خدمات/التشغيل-الآلي-للحديد.md ko: https://steelequivalents.com/ko/서비스/cnc-가공.md pt-br: https://steelequivalents.com/pt-br/serviços/usinagem-cnc-de-aço.md hi: https://steelequivalents.com/hi/सेवाएं/सीएनसी-मशीनिंग-सेवा.md content_hash: sha256:f6d964d1cfd2c14dd0a947728bcae4d990231803ebe4fb9d709f604f7cc3af80 generated: 2026-09-24 --- > **Machine-readable Markdown rendering.** This is a Markdown representation of a web page, > served for AI agents and crawlers as a token-efficient alternative to the full HTML. > Full interactive HTML version: https://steelequivalents.com/services/cnc-machining > Canonical URL: https://steelequivalents.com/services/cnc-machining > Available languages: en, cs, de, es, fr, hu, it, nl, pl, pt, ru, tr, zh, ja, ar, ko, pt-br, hi > Change detection: `content_hash` is the SHA-256 of this document's Markdown body — everything > below this block, whitespace-trimmed, excluding the front-matter. Compare it to detect a real > content change; `generated` moves on every render and is not a change signal. ![CNC Machining](/images/uploads/b1a43aed-c581-409d-b625-ca2b236df55f/cnc-machining-service-turning-steel-bar-in-lathe-chuck-1920x1080.jpg) Category # CNC Machining CNC machining for steel: tolerances, fits, finishes and what a quote needs CNC machining is turning, milling and multi-axis work to bring a steel part to its finished drawing dimensions — as a service, in whatever combination of operations that takes. This page covers what actually decides the outcome: the tolerance class you can realistically hold, what an ISO 286 fit like H7 means on your drawing, the surface finish a shop delivers as standard versus what costs more, and what a provider needs from you before quoting. If your job is a single operation — only milling, or only drilling — those are narrower services in their own right; the last FAQ below draws that line. ## What drives the price - Material removal volume - Steel grade and its machinability - Tolerance class - Surface finish requirement - Setup and fixturing complexity - Total volume - Secondary operations ## What to send for a quote - A drawing or STEP file - Material grade - Tolerance class or specific tolerances that matter - Surface finish required, per surface - Quantity - Whether you supply the material or the shop sources it - Whether you need an inspection certificate ## Traceability, EN 10204 and dimensional reports - 2.1 - 2.2 - 3.1 - 3.2 Standard for steel purchases: 3.1 EN 10204 certifies the material, not the finished part. A tight tolerance may call for a separate dimensional inspection report proving the finished dimensions were held — a different document, answering a different question. Standard-defined Source: EN 10204 defines inspection-document types. ## Achievable tolerances (ISO 2768-1) ISO 2768-1 | 0.5 – 3 mm | Fine (f): 0.05 mm · Medium (m): 0.1 mm | | | |---|---|---|---| | 3 – 6 mm | Fine (f): 0.05 mm · Medium (m): 0.1 mm | | | | 6 – 30 mm | Fine (f): 0.1 mm · Medium (m): 0.2 mm | | | | 30 – 120 mm | Fine (f): 0.15 mm · Medium (m): 0.3 mm | | | | 120 – 400 mm | Fine (f): 0.2 mm · Medium (m): 0.5 mm | | | | 400 – 1000 mm | Fine (f): 0.3 mm · Medium (m): 0.8 mm | | | Medium (m) is the class most shops meet as standard; confirm which class a specific provider defaults to. Indicative figures. Confirm the achievable tolerance, fit and finish with the provider for your grade, geometry and quantity. Supplier-published ## Fit classes (ISO 286) ISO 286-2 | 6 – 10 mm | 0 – 0.015 mm · H7 | | | |---|---|---|---| | 10 – 18 mm | 0 – 0.018 mm · H7 | | | | 18 – 30 mm | 0 – 0.021 mm · H7 | | | H7/h6 is the standard locational-clearance fit for general engineering — a shaft that locates precisely in a bore without an interference press. Supplier-published ## Surface finish ISO 1302 Indicative figures. Confirm the achievable tolerance, fit and finish with the provider for your grade, geometry and quantity. Supplier-published ## Turning, milling and multi-axis ### Turning Best for: Round and rotationally symmetric parts Indicative ### Milling Best for: Flats, pockets and other non-rotational features Indicative ### Multi-axis machining Best for: Complex geometry that needs several faces machined in one setup Machining several faces in one setup on a 4- or 5-axis centre avoids the alignment error a part accumulates when it is unclamped and re-fixtured between separate operations. Indicative ## Services What tolerance can CNC machining achieve? It depends on the feature size and the tolerance class you specify. Under ISO 2768-1, most shops meet class m (medium) as standard: on a 6 to 30 mm feature that's ±0.2 mm, tightening to ±0.1 mm at class f (fine) if you ask for it. For a toleranced bore or shaft fit rather than a general dimension, ISO 286 defines fit classes such as H7 — on an 18 to 30 mm hole, H7 runs from 0 to +0.021 mm. These tables give the range; confirm the exact figure for your size and class with the shop. What does an H7 fit mean and do you machine to it? H7 is the hole side of the most common general-engineering fit, H7/h6 — a locational clearance fit for a shaft that needs to locate precisely in a bore without an interference press. Under ISO 286-2, an H7 hole's lower deviation is always 0 and the upper deviation grows with diameter: +0.015 mm at 6 to 10 mm, +0.018 mm at 10 to 18 mm, +0.021 mm at 18 to 30 mm. Yes, we cut to H7 and other ISO 286 fit classes — specify the class and the mating part on the drawing. What surface finish can you achieve? Ra 3.2 µm is the default as-machined finish most shops deliver without extra steps, commonly specified under BS EN ISO 1302. Tighter finishes are available in steps — 1.6, 0.8 and 0.4 µm — but each step usually means an extra finishing pass, which adds cost. State the Ra you need per surface on the drawing rather than 'smooth': different surfaces on the same part often need different finishes. Do you do turning, milling, or both? Turning holds the bar or blank in a rotating chuck and takes material off with a fixed tool — the natural choice for round, cylindrical or threaded features. Milling holds the part still and moves a rotating cutter across it, which is how flats, pockets, slots and off-axis holes get made. Many parts need both, done in one setup on a multi-axis machining centre, which avoids the alignment error of moving the part between separate turning and milling operations. Tell us which features the part needs and we plan the operations. What do you need to quote a machined part? Send a drawing or a STEP file, the material grade, the tolerance class or the specific tolerances that matter, the surface finish you need per surface, the quantity, and whether you are supplying the material or we are sourcing it. Those six things are what turn an enquiry into a firm price instead of a range. Can I send my own material to be machined? Machining customer-supplied material is common, but whether a given shop accepts it — and on what terms for liability and any remnant — varies by provider. If your material carries an EN 10204 certificate, say so up front and ask how they will preserve traceability through the machining operation. Do I get an EN 10204 certificate for a machined part? EN 10204 certifies the material — types 2.1, 2.2, 3.1 and 3.2 exist, and 3.1 is the usual buyer default in steel procurement. It does not certify the finished machined part. If your tolerance is tight enough that you need proof the finished dimensions were actually held, that's a separate document — typically a dimensional inspection report from a CMM or similar measurement — and it is not standardized the same way EN 10204 is. Ask for both separately if you need both. Is there a minimum order quantity? There is no industry-wide minimum order quantity for CNC machining; it varies by provider and by how much setup the part needs relative to the run length. Ask as part of your enquiry rather than assuming a single piece is either impossible or free of a setup charge. What is the lead time? No general lead time can be stated honestly — it depends on the provider, the part's complexity, how many operations and setups it needs, and their current load. Ask for a committed date with your quote rather than planning around an average. Does the steel grade affect machinability? Steel grade does affect machinability — harder and more alloyed grades generally cut at lower speeds, wear tooling faster, and sometimes need different tooling — but how much varies by grade, condition and the specific operation, and we have not found a sourced, general figure worth quoting here. Ask the shop about your specific grade rather than assuming a free-machining steel and a hardened tool steel cut the same way. What's the difference between CNC machining and milling or drilling as separate services? CNC machining, as we use the term here, is the broader service: turning, milling and multi-axis work to bring a part to its finished drawing dimensions, in whatever combination of operations that takes. Milling and drilling on their own are narrower, single-operation services — useful when that is genuinely all a job needs. If your part needs more than one operation to reach its finished geometry, that's a CNC machining job even if milling or drilling is most of the work. What does CNC machining cost? CNC machining is not priced from a single rate — it depends on material removed, the steel grade's machinability, the tolerance class, the surface finish required, how complex the setup and fixturing is, the quantity, and any secondary operations. 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