The classic question in purchasing departments: "Should we buy these 12 parts separately and assemble them ourselves, or ask the supplier for a finished sub-group?" Turkish contract shops are traditionally part suppliers; in recent years, however, sub-group delivery including simple assembly, coating coordination and testing has become widespread. We touched on kit delivery in our robotics parts article; this article deepens the decision.
Definition of the Two Models
- Single-part sourcing: The supplier delivers machined (and, if required, coated) parts; assembly, procurement of purchased components and functional testing remain with the buyer.
- Assembled sub-group: The supplier combines the parts it produces with purchased components (bearings, seals, springs, bolts, inserts); performs simple tests if required; delivers a sub-group ready for the buyer's assembly line.
Advantages of Sub-Group Sourcing
1. Total cost
The buyer's assembly labour (EUR 45–70 per hour in Western Europe) drops to EUR 10–15 in Turkey. Procurement and stock management of purchased components (ordering, receiving, warehousing for 10–15 items) is taken off the buyer. Packaging and freight volume decrease: an assembled group ships more compactly than separate parts.
2. Time
The buyer's assembly line is one station shorter; the sub-group goes straight into final assembly. On the supplier side, because parts and assembly are under one roof, intermediate stock and waiting are reduced.
3. Quality responsibility
When a non-conformity occurs, the "is the part faulty or the assembly" argument ends; the supplier is responsible for the functional sub-group. It can optimise fit tolerances to the working of the sub-group; unnecessarily tight tolerances are relaxed.
4. Design feedback
The supplier that assembles the sub-group offers DFA suggestions that reduce assembly difficulty and part count; improvements emerge that a part supplier could not see.
Risks of Sub-Group Sourcing
1. Supplier competence
A machining shop may not have assembly discipline, a clean area, torque control, bearing-pressing equipment and a test rig. Simple mechanical assembly is possible in most shops; hydraulic leak testing, electrical-electronic integration or classified cleanliness is a separate competence.
2. Purchased components
The brand and quality of bearings, seals and fasteners are critical for the buyer. Risk of the supplier using equivalents or counterfeit products; the solution is an approved brand list and a certificate expectation. Some buyers supply these components themselves (free-issue).
3. Dependency and IP
If the supplier makes the entire sub-group, a significant part of the know-how accumulates at the supplier; switching to a second source becomes harder. Precautions are in our intellectual property article.
4. Customs and classification
An assembled sub-group may fall under a different HS code from the separate parts; this makes no difference in the EU with the A.TR, but the rate may change in duty-bearing markets such as the USA.
Which Sub-Groups Are Suitable for Contract Assembly?
| Suitable | With care | Not suitable (generally) |
|---|---|---|
| Mechanical groups with bearings and pins fitted | Hydraulic/pneumatic blocks (testing required) | Groups containing electronic boards |
| Thread inserts, helicoils, press-fit parts | Springs and preloaded mechanisms | Classified clean-room assembly |
| Coated + assembled flange/adapter groups | Measuring groups requiring calibration | Safety-critical certified groups |
| Labelled part sets as kits | Bonding/sealing applications | Groups requiring software loading |
Cost Comparison: Example
A linear-axis bearing block: 4 machined parts (aluminium body, steel shaft, 2 covers), 2 bearings, 4 bolts, 1 seal. Single-part model: parts EUR 38, buyer's component procurement EUR 9, buyer's assembly 12 min × EUR 60/hour = EUR 12, stock and order management ~EUR 3; total EUR 62. Sub-group model: parts EUR 38, supplier's component procurement (approved brand) EUR 9.5, assembly 12 min × EUR 14/hour = EUR 2.8, assembly fixture amortisation EUR 0.5, function check EUR 0.7; total EUR 51.5. The difference is about 17%; at an annual volume of a thousand pieces it exceeds EUR 10,000. If the buyer's assembly capacity is already in place and the labour is considered "sunk", the difference shrinks.
Decision Matrix
- Assembly complexity: Simple mechanical → sub-group; testing/cleanliness/electronics → parts.
- Quantity: 100+ per year → sub-group economics are clear; 10–50 → parts or kit.
- Criticality: Safety or certification scope → assembly at the buyer.
- Buyer's assembly capacity: If there is a bottleneck, sub-group; if there is spare capacity, parts.
- Component control: If an approved brand list can be given, sub-group; if not, free-issue or parts.
- IP criticality: If high, buy the critical part from a separate supplier and do the assembly yourself.
- Supplier maturity: See the assembly area, procedures and test equipment on site.
Phased Transition Proposal
Rather than starting directly with a sub-group, three stages: (1) part sourcing and quality proof; (2) kit delivery (parts + free-issue components as labelled sets); (3) simple assembly and finally a function check. At each stage the assembly instruction, torque values and checklist are given to the supplier in writing; the first 3 batches are verified with 100% incoming inspection. Responsibility clauses are exemplified in our quality agreement article.
How to Prepare an Assembly Instruction
The precondition for handing a sub-group to the supplier is that a party you do not know can perform the assembly repeatably. A good assembly instruction contains: the parts list and revisions, assembly sequence and photographed steps, torque values and tools to be used, bearing-pressing method (press force, heating), adhesive or grease definition and quantity, check points (rotational play, axial play, leak test) with acceptance criteria and the packaging method. The instruction is applied together with the supplier on the first batch, gaps are filled and it is frozen with a revision number. This document is also an insurance policy that eases the transition to a second source.
Frequently Asked Questions
Do Turkish CNC shops do assembly?
Simple mechanical assembly (bearings, pins, inserts, coating coordination) is common; for hydraulic testing or electronic integration a specialist supplier is needed. Do not assume the scope without asking.
Who should procure the purchased components?
Critical branded components (bearings, seals) can be bought by the supplier against an approved list or supplied free-issue by the buyer; standard fasteners should be bought by the supplier.
How does warranty work on a sub-group?
The supplier is responsible for the function of the sub-group; the chain of responsibility in the event of a purchased-component failure must be defined in the contract.
What does kit delivery mean?
Delivering all parts of a sub-group in a single labelled package in assembly order; assembly stays with the buyer but material picking time disappears.
Conclusion
For simple mechanical sub-groups, contract assembly provides a 10–20% total cost advantage and a simpler quality responsibility; for groups involving testing, cleanliness or electronics, part sourcing is safer. To assess which model suits your sub-group, send the BOM and assembly instruction via our quote form; we price the part, kit and sub-group options separately. The process is summarised on our How we work page.
