The most important standards for CNC turned parts – without the standards jungle.

Standards are the common language between design engineer, buyer and supplier. If you reference them cleanly on your drawing, you avoid misunderstandings and reduce complaint risk. This article lists the most important standards for CNC turned parts – with a brief explanation of when each applies.

General tolerances: DIN ISO 2768 is the standard reference. Part 1 (linear, angular) has the classes fine (f), medium (m), coarse (c) and very coarse (v). Part 2 (form and position) has the classes H, K, L. If nothing is specified, we manufacture to 2768-m / 2768-K. If you want tighter tolerances, you must specify them explicitly.

Fits: DIN EN ISO 286-1 and -2 define the ISO fit system. Common functional fits for turned parts: H7/g6 (clearance fit), H7/k6 (transition fit), H7/p6 (interference fit). Working with the fit system avoids the old 'IT7 tolerance on shaft diameter' confusion.

Surface finishes: DIN EN ISO 1302 governs the notation on drawings. Common for turned parts: Ra 1.6 (standard), Ra 0.8 (elevated), Ra 0.4 (high). Lower values usually mean downstream grinding or polishing.

Materials: steels are classified per DIN EN 10027 (material no.) and DIN EN 10083 (quenched and tempered steels), 10084 (case-hardening steels), 10277 (bright steel). Stainless steels per DIN EN 10088. The old DIN material designations (e.g. C45, 42CrMo4) remain in common use and often appear on drawings.

Material certificates: DIN EN 10204 governs the types – 2.1 (declaration of compliance), 2.2 (test report), 3.1 (inspection certificate by the works inspector), 3.2 (inspection certificate by an external third party). Standard at Marquart: 3.1, 3.2 on request.

Threads: metric coarse threads per DIN 13, metric fine-pitch threads per DIN 13-2, Whitworth threads per DIN 11, trapezoidal threads per DIN 103. Special threads always with explicitly specified values.

First article inspection: VDA 2 (the automotive standard) is the most common reference. Three levels (1 for simple parts, 2 for functional parts, 3 for critical parts). For medical technology, often VDA 2 Level 3 or specific ISO 13485-oriented procedures. PPAP (Production Part Approval Process) is the US variant – we supply PPAP-equivalent documentation on request.

Quality management: DIN EN ISO 9001:2015 is the general standard. For automotive, additionally IATF 16949. For medical technology, DIN EN ISO 13485. Marquart is certified to 9001 – sufficient in most cases for non-implant medical parts.

An unambiguous title block is the foundation of complaint-free turned parts. Record the general tolerance, the material with material number, the surface specification and the edge condition consistently in the drawing. For edges, DIN ISO 13715 applies: it defines whether a transition is to be executed as a burr, burr-free or with a defined edge break, and specifies permissible size ranges. If this specification is missing, exactly those interpretation gaps arise that later lead to complaints. Clearly defined edge conditions save coordination effort and ensure reproducible results across all lots.

Above the individual standards stands the system of geometrical product specification (GPS). ISO 8015 forms its foundational standard and anchors the independency principle: every size, form and position specification applies on its own unless a relationship is explicitly required. Size tolerances therefore do not automatically limit form. This understanding significantly reduces misunderstandings between design and manufacturing, because both sides interpret drawing specifications by the same rules. Specifying GPS-compliantly avoids tacit assumptions and makes inspection requirements clearly traceable between the customer and the contract turning shop.

For threads, we recommend specifying the tolerance class concretely. Common are 6g for metric external threads and 6H for internal threads, which corresponds to a medium, well-mating fit. Inspection is done with gauges: the GO gauge must screw on, the NO-GO gauge may only engage a few turns. Use fine-pitch threads with smaller pitch where higher self-locking, finer adjustment or thin-walled parts are required. A clear tolerance class and inspection specification ensures the function of the threaded joint across all batches.

Industrial customers increasingly demand not only dimensional accuracy but also material compliance as a standards-based proof. The EU REACH regulation governs substances of very high concern; the RoHS directive limits heavy metals such as lead in parts used in electrical engineering. Hence the trend toward lead-free or low-lead materials, such as correspondingly designated free-cutting steels and brass alloys. In addition, conflict minerals declarations per the common industry frameworks are expected. Clarify these requirements early so the chosen material, including material certificate and conformity declarations, substantiates the compliance of your supply chain.

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