CNC Machining Service London: How to Pick a Supplier

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  Key takeaways
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  CNC Machining Service London: Supplier Scorecard
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  What tolerance a cnc machining service london buyer should actually ask for
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  Lead time, MOQ and the cost of a slow quote
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  Certifications that matter for regulated London industries
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  Materials and finishing: where quotes quietly diverge
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  Step by step: qualifying a supplier in one week

Tolerance is the first number engineers compare, and the one most often quoted without context. A shop can hold ±0.005 mm on a 40 mm aluminium bracket on a good day. The same shop will struggle on a 900 mm steel shaft because thermal growth and fixturing dominate. So ask the question the right way: which machine, which material, which feature, and what inspection method proves it.

GreatLight runs 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That mix matters. Simple prismatic parts go on three-axis machines and stay cheap. Parts with angled holes, undercuts or five-sided features need 5-axis so you avoid a second setup, which is where stacked error creeps in.

Know when tighter tolerance is wasted money. If a bracket only locates a cover panel, ±0.05 mm is fine and costs less. If a part presses into a bearing or seals a fluid path, then ±0.005 mm and Ra 0.8–1.6 μm are worth paying for. Surface finish and tolerance travel together: a tight bore with a rough wall will not seal or slide.

For imperial drawings, ±0.005 mm equals ±0.0002 in. Send both if your team works in inches. Suppliers who quote in one unit and machine in the other introduce rounding errors that show up at first article, not in the quote.

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  Ask for the machineA tolerance claim without a machine model and setup description is a guess.
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  Match finish to functionRa 0.2–0.8 μm for sealing faces, Ra 1.6–3.2 μm for as-machined brackets.
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  Watch the size envelope4,000 × 400 × 150 mm is the long-part limit; compact work goes on 500 mm tables.

Most project delay does not happen at the spindle. It happens in the week of emails before anyone cuts metal. A supplier who returns a quote and a free DFM analysis within 12 hours removes that week. Production can then start within 24 hours, and parts ship in 3–5 days for standard work. Historical late-delivery probability sits below 2%, which is the number to ask about instead of a promised date.

MOQ is the second filter. A shop with no minimum order quantity will quote a single prototype and a 10,000+ part run through the same process. That sounds generous, but check the setup logic. Prototypes are often machined from billet; production volumes may move to die casting or vacuum casting. A supplier who offers all three can tell you when the crossover saves money.

Beware the low unit price with a long tooling lead. If the quote assumes a die cast mold that takes weeks to cut, your “cheap” part arrives after your deadline. Ask what process the quote assumes, and what the per-piece cost becomes at 50, 500 and 5,000 units.

Shipping into London adds a layer most quotes ignore. Confirm Incoterms, who clears customs, and whether the supplier has shipped to the UK before. A fast factory with slow paperwork is still a slow supplier.

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  Quote speed is a signal12-hour turnaround usually means an engineer read the drawing, not a sales desk.
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  Cross-check the processAsk whether the quoted route is machining, casting or printing at each volume.
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  Price the landingDuty, freight and customs brokerage belong in your comparison, not in a footnote.

London work clusters around medical devices, automotive and EV, aerospace, robotics and electronics. Each carries its own paperwork. ISO 9001:2015 is the baseline quality system and should be active, with a scope that names machining. If the certificate scope covers trading only, the audit did not look at the shop floor.

For automotive and EV parts, IATF 16949:2016 is the expected system. It forces traceability, change control and process monitoring that a generic quality manual does not. For medical hardware, ISO 13485:2016 covers the documentation and validation trail your notified body will ask about. For designs you would rather not share widely, ISO 27001:2022 covers information security, and an NDA is available on request.

Certificates do not replace inspection. GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring and a final inspection. Reports are available on request. Ask to see a sample report before you order, and check that the measured features match the drawing callouts, not just the easy ones.

Qualification rate is the number behind the paperwork: 99.99% of parts pass. Ask how a supplier defines a reject, and who pays for rework. That answer tells you more than the certificate wall.

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  Check the scopeA certificate that excludes machining does not cover your part.
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  Request a sample reportOne page of real dimensional data beats a folder of logos.

Two suppliers can quote the same drawing and differ by 40% because of material stock and finishing route. Aluminium 6061-T6, 7075 and 6082 machine cleanly and are usually stocked. Stainless 316L and 17-4PH cost more in tool wear and cycle time. Titanium TC4 (Ti-6Al-4V), Inconel and magnesium AZ31B need slower feeds and sharper tooling, which shows up as a higher unit price rather than a hidden fee.

Plastics behave differently again. POM and PA machine well but move after cutting. PEEK holds dimension but wears tools. ABS and PC are better suited to prototypes than to load-bearing parts. Tell the supplier the function, not only the grade, and you will get a more useful process note.

Finishing is where scope creep lives. Anodizing in clear, colour, hardcoat or conductive form changes the part. Electroless nickel and zinc plating add thickness that can close a tolerance. Powder coating and black oxide change fit. Bead blasting, tumbling, brushing and polishing set the cosmetic standard. Laser marking needs a minimum character height of 1.5 mm to stay legible.

Keep finishing under one roof if you can. Moving parts between three subcontractors adds handling damage and hides who is responsible when a batch arrives scratched.

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  Name the function“Bearing housing, wear surface” gets better advice than “aluminium”.
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  Allow for coating thicknessPlating and anodizing grow the part; check fits after finish, not before.
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  Marking needs roomLaser text below 1.5 mm character height may not read cleanly.