Buyer guide

CNC London Services: How to Pick a Supplier You Can Audit

This guide is for engineers and buyers in London who need machined metal or plastic parts and do not want to chase a supplier after the PO is signed. It covers six checks you can run before you order: tolerance, materials, lead time, MOQ, certifications and quote format. By the end you should be able to tell whether a quote is workable or just optimistic.

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±0.005 mm toleranceZero minimum order quantityQuote in 12 hoursISO 9001 / IATF 16949

CNC London services processing a machined part

On this page

1
Six things to check before you send a PO
2
Which check decides your supplier choice
3
What CNC London services quotes should tell you about tolerance
4
Materials and the grades that change your price
5
Lead time, capacity and where the parts are made
6
MOQ, certifications and quote format
7
Six steps to place a first order

Key takeaways

Six things to check before you send a PO

Tolerance is a cost line, not a slogan±0.005 mm is achievable on 5-axis work, but it needs a clear datum and a part that can be held in one setup. Blanket tight tolerances raise the price.

Material grade decides the process7075 aluminium and 17-4PH stainless behave differently. Ask which grade the quote assumes before you compare prices.

Lead time has two halvesQuote and DFM feedback in 12 hours; production can start within 24 hours. Parts ship in 3–5 days once the first article is approved.

MOQ should not force you to buy stockRuns can start at one prototype and scale to 10,000+ parts, so you are not locked into a batch you cannot sell.

Certificates only matter if they match your industryISO 9001 fits general work. IATF 16949, ISO 13485 and ISO 27001 matter for automotive, medical and data-sensitive projects.

Decision table

Which check decides your supplier choice

Match the part to the check that carries the most risk.

Part or project type	Check that decides	Why it matters
Prototype, one-off bracket	MOQ and quote speed	A one-piece run must not carry a tooling charge.
Aerospace housing, thin walls	5-axis capability and datum plan	Wall deflection shows up as flatness error.
Automotive bracket, 5,000 pcs	IATF 16949 and process control	Traceability is audited at the tier-1 level.
Surgical instrument body	ISO 13485 and surface finish	Ra 0.2–0.8 μm may be needed for contact faces.
Sealed enclosure, IP-rated	Leak path and finish callout	As-machined Ra 1.6–3.2 μm can leak at a seal.
Part over 1 m long	Machine travel	4,000 × 400 × 150 mm covers most long frames.
Mixed metal and plastic batch	Material list depth	One supplier reduces split shipments and re-inspection.

The short version

If your part is simple and loose, any competent shop will do. If tolerance, material grade or certification carries the risk, pick the supplier that can show you how it will hold and measure the part, not the one with the lowest first number.

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Check 1

What CNC London services quotes should tell you about tolerance

A tolerance callout on a drawing is a promise. The question is whether the shop can hold it on your geometry, in your material, at the quantity you asked for. Blanket ±0.005 mm across every dimension on a 300 mm aluminium plate is possible, but it forces slow passes, temperature control and extra inspection. That cost lands in the unit price.

Ask for a datum scheme instead of a single tolerance number. On 5-axis work the part usually stays in one setup, so bore-to-bore position holds better than if the part is flipped between operations. If the shop plans three setups, expect the position tolerance to loosen.

The finish callout is part of the same conversation. As-machined surfaces sit around Ra 1.6–3.2 μm. Sealing faces, bearing bores and sliding surfaces often need Ra 0.8–1.6 μm, and optical or medical contact faces can need Ra 0.2–0.8 μm. Each step adds a finishing operation and a re-inspection.

One more thing to check: what the shop measures with. A CMM report on the critical dimensions is normal for tight work. If the quote does not name an inspection method, the tolerance is a claim, not a control.

Check 2

Materials and the grades that change your price

Most London enquiries land on aluminium 6061-T6, 7075, 304 or 316L stainless, and POM or PEEK for plastics. Those are not interchangeable. 7075 machines cleanly but is harder on tooling and less weldable. 316L galls if you tap deep holes without the right feed. PEEK needs sharp tools and controlled heat or the surface tears.

Titanium and nickel alloys are the slower group. TC4 (Ti-6Al-4V) and Inconel cut at low surface speed, so cycle time can triple against 6061 for the same shape. Magnesium AZ31B and AZ91D machine fast, but chips are a fire risk and the shop must handle them as such.

The grade also decides availability. A quote that assumes 6061 when your drawing says 6082 will not match the mechanical properties you designed for. Send the material spec with the RFQ, not after the PO.

For mixed assemblies, one supplier covering aluminium, stainless, copper and plastics avoids split shipments and separate incoming inspection. That matters more than a small unit price gap on a single line item.

Check 3

Lead time, capacity and where the parts are made

Lead time is two clocks, not one. The first clock is quoting and DFM feedback, which should come back within 12 hours. The second clock is production, which can start within 24 hours of approval, with parts shipping in 3–5 days. Ask which clock the supplier is quoting.

Capacity decides whether that promise holds. A shop running 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, can absorb a spike without pushing your job out. A small shop cannot.

Size limits matter too. Maximum processing size is 4,000 mm, with travels of 4,000 × 400 × 150 mm for long parts, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm for mid-size work, and 500 × 500 × 450 mm or 500 × 310 × 200 mm for compact parts. A Ø400 mm rotary table covers most round work.

GreatLight runs three wholly-owned plants covering 7,600 m² with 150 technicians, in Dongguan and at a Singapore factory. Shipments to London go by air or sea from either site, so ask which plant the quote assumes before you compare freight.

Check 4

MOQ, certifications and quote format

A minimum order quantity that forces you to buy parts you cannot sell is a hidden cost. GreatLight quotes from one prototype to 10,000+ part runs with no minimum order quantity, so a design change does not leave you with dead stock.

Certifications should match your industry, not the supplier's marketing. ISO 9001:2015 covers general machining quality systems. IATF 16949:2016 is what automotive tier work expects. ISO 13485:2016 applies to medical device parts. ISO 27001:2022 matters when your drawings and CAD files are sensitive.

Quote format is the last check. A usable quote lists material grade, tolerance class, finish, inspection method, quantity breaks and freight terms. If those are missing, two quotes cannot be compared. You are comparing two different scopes.

Confidentiality belongs in the same conversation. Uploads are handled as secure and confidential, and an NDA is available on request. Get that in place before you send controlled drawings, not after.

Workflow

Six steps to place a first order

1
Send drawings with the material specInclude 2D PDF, 3D STEP and the grade, for example 6061-T6 or 316L. Missing grade is the most common cause of a slow quote.
2
Read the DFM feedback, not just the priceExpect feedback within 12 hours. Look for notes on thin walls, deep pockets and tolerances that force multiple setups.
3
Confirm the datum and inspection planAsk which dimensions go on the CMM report. If the plan does not cover your critical features, fix it before the first cut.
4
Fix the finish callout in writingState Ra values per face, for example Ra 0.8–1.6 μm on seal faces and Ra 1.6–3.2 μm elsewhere. Vague 'smooth finish' notes cause rework.
5
Approve the first article before the batchFor runs above 100 pcs, hold production until you sign off dimensions and finish. Changes after that point are expensive.
6
Check the packing and markingLaser marking needs at least 1.5 mm character height. Below that, readability drops and you may not pass incoming inspection.

FAQs

Questions London buyers ask before ordering

Can a supplier hold ±0.005 mm on a large part?

Yes, but only on selected features, not every dimension. On a 4,000 × 400 × 150 mm part the geometry, material and fixturing decide what is realistic.

Send the drawing and we will mark the dimensions we can hold at ±0.005 mm and the ones that need a looser class. That saves rework later.

What is the smallest order you accept?

One piece. There is no minimum order quantity, so prototyping and 10,000+ part runs go through the same quoting process.

Unit price drops as quantity rises, but a single part is a normal order, not a special case.

Which certifications apply to my project?

ISO 9001:2015 for general machining. IATF 16949:2016 for automotive. ISO 13485:2016 for medical devices. ISO 27001:2022 where file security is part of the contract.

Tell us the end market and we will confirm which system the order runs under before you place it.

How fast can parts reach London?

Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours, and parts ship in 3–5 days.

Freight time from Dongguan or Singapore to London is separate. We quote the machining clock, then you choose air or sea.

Which finishes are available after machining?

Anodizing in clear, colour, hardcoat and conductive grades; electroless nickel, zinc, silver and gold plating; powder coating and black oxide; bead blasting, tumbling, brushing and polishing.

Laser marking and engraving are also available, with a minimum character height of 1.5 mm.

How do you protect our drawings?

Uploads are handled as secure and confidential, and we hold ISO 27001:2022 for information security.

An NDA is available on request. Sign it before sending controlled CAD files.