An extended workbench (German: verlängerte Werkbank) is a build-to-print arrangement in which an outside machine shop makes parts to your drawings, material specification and inspection plan, so that its spindle hours work as an extension of your own shop floor. For a Tier 2 supplier it solves overflow, meaning orders above your own capacity, and it stays safe only if the drawing, the inspection plan and the release decision remain with you.
What is an extended workbench in machining?
The supplier brings machines, operators and tooling. You bring everything that defines the part: the released drawing, the material, the tolerances, the inspection requirements and the delivery schedule. The supplier does not design, does not change the process route without asking and does not decide whether a deviation is acceptable.
That separates it from two other models. A design-owning supplier sells you a part it has engineered itself. A fully managed supply contract hands the supplier the process validation as well. With an extended workbench your process knowledge stays in your drawing package and your control plan, which is why the model suits parts you already make and understand.
When does overflow capacity make sense for a Tier 2 supplier?
The test is whether the part can be fully described on paper. If a competent machinist can make it right from the drawing and a measurement report can prove it, outside capacity is low-risk. If the part depends on a setup nobody has written down, it is not.
Table 1. Decision table for overflow machining
| Situation | Extended workbench? | Condition or reason |
|---|---|---|
| Demand peak above own capacity, part released and repeating | Yes | Drawing at a fixed revision, inspection plan exists |
| Structural capacity shortage on a mature part family | Yes | Treat the shop as a qualified supplier, not as a one-off outlet |
| Part with features only your own gauges or CMM program can verify | Conditional | Agree measurement method and report format first, or buy inspection capacity as well |
| Prototype or part with frequent design changes | No | Every revision costs a round trip, and the supplier cannot learn the part |
| Part that depends on your own process know-how or a validated special process | No | Keep it in-house or hand over the process in writing |
| Parts needed within days | No | Road transport Bursa to central Europe typically takes 5–10 days |
Source: Ertaş Engineering assessment. Transit time: typical road transport, Bursa to central Europe.
Two points settle the question. First, drawing stability: an overflow supplier cannot absorb the engineering changes a prototype runs through. Second, inspection: if you cannot describe how the part is verified, you will find the problem at your own incoming inspection, after the delivery time has already been spent.
What must stay under your control?
You remain the supplier in the eyes of your own customer. ISO 9001:2015 clause 8.4 required the organisation to determine the controls for externally provided processes, products and services, and to ensure that they conform to requirements. ISO published ISO 9001:2026 on 2026-09-16 as the replacement for the 2015 edition. Certified organisations have until 2029-09-30 to transition, and from 2028-03-31 new accredited certifications may only be issued to the 2026 edition. As of 2026-10-30, check the external-provider clause in the new edition before you revise your supplier procedure.
Put the following in writing before the first order. Each line is something a supplier cannot decide for you.
Table 2. What to fix in writing before overflow parts are made
| Item | What the order must state | Reference |
|---|---|---|
| Drawing revision | One released revision per order; changes only through you | Your change process |
| General tolerances | A class on the drawing, for example ISO 2768-m, and individual tolerances for functional features | ISO 2768-1:1989 |
| Material and certificate | Grade by designation and certificate type, normally 3.1 | EN 10204:2004 |
| Inspection plan | Characteristics, sample size, gauge, report format | Your control plan |
| First article | Report approved by you before series release | Your release procedure |
| Delivery term | Named place and edition, for example FCA Bursa, Incoterms 2020 | Incoterms 2020 |
Source: ISO 2768-1:1989; EN 10204:2004; Incoterms 2020 (ICC).
EN 10204:2004 is the current edition, listed by DIN Media as DIN EN 10204:2005-01 with no replacement noted. A revision is in preparation in CEN/TC 459/SC 12, so date the reference on the drawing. ISO 2768-1:1989 is likewise current; the ISO 2768 article covers the tables and the withdrawn part 2.
The technical drawing checklist lists the title block entries a supplier needs, and the supplier qualification checklist covers the audit side.
What changes when the parts come from Türkiye?
Three things: duty status, proof of status and transit time.
The EU–Türkiye customs union has been in force since 1995 and covers industrial products. The Commission lists zero customs duties and zero quotas for goods covered by it. Goods benefit when they are accompanied by proof of their customs status, which is the A.TR movement certificate. Agricultural products and coal and steel products as covered by the ECSC Treaty are outside the customs union, so confirm the tariff classification of each part number once, not per shipment. The A.TR article describes the certificate in detail.
The carbon border adjustment mechanism is the second check. CBAM has applied in its definitive regime since 2026-01-01 to iron and steel, aluminium and four other sectors. On 2025-12-17 the Commission proposed adding 180 downstream products (COM(2025) 989). Check each steel or aluminium part number against the goods listed in Annex I of Regulation (EU) 2023/956 and the current state of that proposal, because the list can change after this article is published.
Transit time is the third. Road transport from Bursa to central Europe typically takes 5–10 days. That is acceptable for overflow planned a few weeks ahead and unsuitable for fire-fighting. The practical consequence is to release overflow orders earlier and to hold a small buffer at your plant for repeat parts.
How do you start without risking a live order?
Start with one repeating part that is not on your customer's critical path. Give the supplier the full package from Table 2, ask for a first-article report and compare it with your own measurement of the same parts.
Judge the result on four points:
- The measured values match your own measurement within the agreed method.
- The certificate type and heat or batch number match the order.
- The supplier asked questions about unclear drawing points before cutting, rather than guessing.
- The parts arrived with the documents the order named.
If all four hold, add the second part family. If one fails, the failure shows you which line of Table 2 was too vague, and that is cheaper to learn on one lot than on a series.
Our network in Bursa works to ISO 2768-m as standard on CNC milling and other machined parts, and tighter where the drawing specifies it.
Frequently asked questions
- What does verlängerte Werkbank mean?
- It is the German term for outsourced machining capacity that works to the buyer's drawings, material specification and inspection plan, as if it were part of the buyer's own shop floor. In English it is called an extended workbench, and the contract form is build-to-print.
- When is overflow machining worth outsourcing?
- When the part is released, the drawing is stable and the demand peak is larger than your own spindle hours. It is a poor fit for prototypes with frequent design changes and for parts that carry your own process know-how.
- Who is responsible for quality when a supplier machines overflow parts?
- You are, towards your own customer. ISO 9001:2015 required the buyer to determine the controls for externally provided processes, products and services, and to ensure they conform to requirements. Supplier audit, first-article approval and incoming inspection are how you meet that.
- Do machined parts from Türkiye attract customs duty in the EU?
- Industrial goods covered by the EU–Türkiye customs union move at zero customs duties when they are accompanied by an A.TR movement certificate as proof of free circulation status. Agricultural products and coal and steel products are outside the customs union, so the tariff classification of each part has to be checked.
- Which material certificate should the drawing call for?
- Name the certificate type from EN 10204:2004, most commonly 3.1 for machined metal parts. A drawing that only says "with certificate" leaves the type open.
Sources
- EU–Türkiye Customs Union (European Commission, Access2Markets, page version 2026-10-14)
- Türkiye: customs unions and preferential arrangements, A.TR movement certificate (European Commission, DG TAXUD)
- ISO 9001:2026 has been released, transition deadlines (ISO/TC 176 news page)
- DIN EN 10204:2005-01, Metallic products, types of inspection documents, German version of EN 10204:2004 (DIN Media)
- DIN ISO 2768-1:1991-06, General tolerances, identical with ISO 2768-1:1989 (DIN Media)
- Incoterms 2020 rules (International Chamber of Commerce)
- Carbon Border Adjustment Mechanism, definitive regime from 2026-01-01 (European Commission, DG TAXUD)
- CBAM: extension of scope to downstream goods, COM(2025) 989 of 2025-12-17 (EPRS briefing, 2026-07-06)
Questions on a drawing? sales@ertas.engineering