Machined and Welded Parts for Special Machine Builders

Machined and welded parts for special machine builders are single pieces and small batches made to the machine builder's drawing: base plates, housings, shafts, fixtures and welded frames. What separates them from series parts is that every piece needs its own drawing check, material record and, for welded frames, weld documentation, because no later batch exists to correct a first-off error.

What do special machine builders buy from a contract machinist?

A special machine builder (German: Sondermaschinenbau) designs and assembles machines for one customer's process, in quantities from one to a few units. The purchased parts follow the same pattern: a base plate in quantity 1, a housing in quantity 4, a welded frame in quantity 2. Design changes during commissioning mean the same part number can return with a moved bore or an added thread, so the drawing revision has to be explicit and the order has to quote it.

That profile favours a supplier that can run several processes in one chain. A welded frame is cut, welded, possibly stress-relieved, machined and painted before it ships, and the machine builder wants one party answerable for the result. Our network in Bursa covers CNC milling, turning, sheet metal, welded assemblies, casting with machining and assembly.

Which documents does each part type need?

The documents follow the part type and its process, not the other way round. A plate cut from stock needs a material record; a welded frame needs weld records on top of that. The table lists what a purchase order can call for; the standards behind each entry are named in the last column.

Table 1. Part types for special machines, process route and documents to call for

Source: EN 10204:2004; ISO 3834-1:2021; ISO 5817:2023; ISO 13920:2023. The choice of documents per part type is our recommendation, not a requirement of these standards.

Type 3.1 is a statement of compliance with the order that gives the results of specific inspection, validated by the manufacturer's inspection representative, independent of the production department (Wermac summary of EN 10204:2004). It documents the material of the delivered batch, not the dimensions of the finished part. Quality documents for machined parts explains how it differs from a certificate of conformity, a measurement report and an FAI report.

How do you keep a welded frame to size?

A welded frame distorts. Heat input pulls the structure out of shape, and how far depends on weld sequence, fixturing and plate thickness. Mounting faces, bearing seats and guide surfaces that have to be accurate are therefore machined after welding, and a frame drawing should say so. Whether the frame is stress-relieved before that machining is also a drawing decision. If the drawing is silent, the supplier decides, and two suppliers may decide differently.

Three standards belong in the drawing or the order for a welded frame:

• ISO 5817:2023 sets quality levels B, C and D for imperfections in fusion-welded joints in steel, nickel, titanium and their alloys, from 0.5 mm material thickness. Level B is the highest requirement on the finished weld. The edition replaced ISO 5817:2014. The drawing states the level; a weld without one has no agreed acceptance limit.
• ISO 3834 defines the quality requirements for the fabricator in three levels: part 2 comprehensive, part 3 standard, part 4 elementary. Part 1 (2021) gives the criteria for choosing the level. It applies in workshops and at field installation sites and does not demand a full quality management system. ISO 3834-2:2021 was confirmed in September 2026 (status as of 2026-10-30).
• ISO 13920:2023 gives general tolerances for as-welded linear and angular dimensions, shape and position, in four tolerance classes chosen by the function of the structure. The 2023 edition replaced ISO 13920:1996, so write the year in the reference.

ISO 13920 and ISO 2768 do not overlap. Weld-affected dimensions follow the first; machined dimensions follow the second or an individual tolerance, as described in ISO 2768 general tolerances. A frame drawing that cites only one of them leaves the other half open.

How does a one-off or small batch run through a supplier?

The flow is the same for a quantity of one and a quantity of ten, and most of the risk sits before the first chip is cut.

• Data package: a STEP model and a PDF drawing with a complete title block go out together. The technical drawing checklist lists the entries a supplier needs.
• Drawing review: the supplier returns questions before quoting: missing tolerances, unclear weld callouts, conflicts between model and drawing.
• Order: the purchase order names the drawing revision, the material, the documents from Table 1 and the weld level for any welded part.
• First piece: on batches above one, the first piece is measured against the drawing before the rest is finished. In a quantity of one, the measurement report of that piece is the approval.
• Delivery with documents: the records ship with the parts and carry the part number and revision, so the machine builder can file them in the machine's technical file.

What decides the lead time?

We do not publish a standard production lead time, because the sequence of the part decides it. A plate takes one machining route. A welded frame takes cutting, fit-up, welding, optional stress relief, machining, surface treatment and inspection, and each step waits for the one before it. Four things move the total most:

• Availability of the material, especially plate in an unusual grade or thickness.
• Whether the frame is machined after welding, which adds a second setup on a large machine.
• External steps such as heat treatment, plating or painting, which depend on the capacity of the subcontractor.
• Inspection scope: a full dimensional report on a large frame takes measuring time of its own.

Transport adds a calculable block. Road freight from Bursa to central Europe typically takes 5 to 10 days. Industrial goods under the EU–Türkiye customs union move at zero customs duty when accompanied by an A.TR movement certificate (European Commission, Access2Markets, page version 2026-10-14); import VAT and customs formalities still apply. The article on the customs union and machined parts covers the scope.

What stays with the machine builder?

The finished machine, not the part, is what the machine builder places on the market. Until 20 January 2027, machinery placed on the EU market must meet Directive 2006/42/EC; from that date Regulation (EU) 2023/1230 applies on a mandatory basis (European Commission, status as of 2026-10-30). Conformity of the finished machine therefore remains with the machine builder, and a parts supplier contributes traceable records for the parts: material certificates, weld documentation and measurement reports that match the drawing revision.

The practical consequence is a rule for the order: ask for the records you will need to file at the time of ordering, because reconstructing a material certificate for a frame that has already been welded and painted is rarely possible.

Frequently asked questions

What are machined and welded parts for special machine builders?

Single pieces and small batches made to the machine builder's drawing: base plates, housings, shafts, fixtures and welded frames. Each piece needs its own drawing check, material record and, for welded frames, weld documentation.

What must a drawing for a welded machine frame state?

At least the weld quality level under ISO 5817 (B, C or D), the ISO 3834 part the fabricator works to, the general tolerance standard for as-welded dimensions (ISO 13920), individual tolerances for machined faces and bores, and whether machining happens after welding.

Which part of ISO 3834 should a machine builder ask for?

ISO 3834 has three levels of quality requirements: part 2 comprehensive, part 3 standard and part 4 elementary. Part 1 gives the criteria for choosing between them. The choice is the buyer's and depends on the use of the frame, so it is agreed in the order.

Does ISO 2768 apply to welded frames?

ISO 2768-1 covers machined and sheet metal parts. As-welded dimensions fall under ISO 13920, which has its own tolerance classes. A frame with machined features carries both, each named for the dimensions it governs.

Is a 3.1 certificate needed for every part?

Only if the purchase order asks for it. A 3.1 certificate documents the material of the delivered batch and is validated by the manufacturer's inspection representative, independent of production. It does not prove dimensions.

Are machined parts from Türkiye duty-free in the EU?

Industrial goods covered by the EU–Türkiye customs union move at zero customs duty when accompanied by an A.TR movement certificate, according to the European Commission. Import VAT and customs formalities still apply.

Sources

1. ISO 3834-1:2021, Quality requirements for fusion welding of metallic materials, Part 1: criteria for selecting the level (ISO catalogue)
2. ISO 3834-2:2021, Part 2: comprehensive quality requirements, confirmed 2026-09-22 (ISO catalogue)
3. ISO 5817:2023, Fusion-welded joints in steel, nickel, titanium and their alloys, quality levels for imperfections (ISO catalogue)
4. ISO 13920:2023, General tolerances for welded constructions, dimensions for lengths and angles, shape and position (ISO catalogue)
5. EN 10204:2004, Metallic products, Types of inspection documents (Irish standards shop, status current)
6. EN 10204 certificate types and validation (Wermac, summary of the standard)
7. Machinery Regulation (EU) 2023/1230, application from 20 January 2027 (European Commission, DG GROW)
8. EU–Türkiye Customs Union (European Commission, Access2Markets, page version 2026-10-14)

Questions on a drawing? sales@ertas.engineering

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