Buyer guide

CNC Machining Services London: What to Check Before You Order

['This guide is for engineers and sourcing staff in London who need machined parts and want to compare suppliers on facts, not sales pages.', 'It covers tolerance, lead time, order quantity, certifications, material range and how a quote is written.', "By the end you should be able to read a supplier's capability list and spot the gaps before you send a drawing."]

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±0.005 mm toleranceNo MOQ12-hour quoteISO 9001 / IATF 16949

On this page

1
Key takeaways
2
How CNC machining services London buyers compare suppliers
3
Tolerance: the number that sets the price
4
Lead time: when the spindle actually starts
5
Order quantity and the MOQ trap
6
Certifications: match them to your industry
7
Materials, finishes and where the cost hides
8
How to run a CNC machining RFQ in 6 steps

Quick answer

Key takeaways

Start with the tolerance you actually needA ±0.005 mm shop costs more per part than a ±0.05 mm shop. Only pay for the band your drawing calls out.

Ask how the quote was builtA single lump sum hides setup, stock and finishing. Itemized quotes tell you where the cost sits.

Certifications are a filter, not a scoreISO 9001 fits general work. IATF 16949 and ISO 13485 matter when your customer audits you.

Order quantity should not decide the shopA supplier that runs one prototype and 10,000 parts removes a second sourcing round.

Check the finish before the priceAs-machined at Ra 1.6–3.2 μm and fine finish at Ra 0.2–0.8 μm are different operations with different costs.

Selection criteria

How CNC machining services London buyers compare suppliers

Use this table as a first pass. A supplier that cannot answer three or more rows in writing is a risk on a tight program.

Check	What to ask	Good answer looks like	Red flag
Tolerance	What band do you hold on a 200 mm part?	±0.005 mm on critical features, stated per feature	One number quoted for the whole part
Lead time	When does the spindle start?	Quote in 12 hours, production in 24 hours	Lead time starts after drawing approval
Order size	Minimum order quantity?	No MOQ, one part to 10,000+	Setup fee that resets each run
Certifications	Which certificates are current?	ISO 9001, IATF 16949, ISO 13485, ISO 27001	Certificates shown without scope or date
Materials	Do you stock the alloy I need?	Aluminium, stainless, steel, titanium, plastics listed	Only "aluminium and steel" on the page
Finishing	Is finishing done in house?	Anodizing, plating, powder coat, bead blast, laser mark	Every finish subcontracted with no schedule
Inspection	What ships with the parts?	100% inspection, reports on request	Inspection only on request, no record
Confidentiality	How are files handled?	Secure upload, NDA available on request	Drawings sent by plain email only

Pick the shop that matches your tolerance, not the one with the longest list

For general parts, choose ISO 9001 and a clean inspection record. For regulated programs, choose the certificate your auditor asks for. For tight features, confirm ±0.005 mm and a CMM report before you commit.

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Criterion 1

Tolerance: the number that sets the price

Tolerance is the first thing to fix before you contact anyone. A shop holding ±0.005 mm runs slower feeds, checks temperature and inspects more often than a shop working to ±0.05 mm. That difference shows up in the unit price, so quoting a wide-tolerance bracket to a tight-tolerance shop wastes money.

Read your drawing feature by feature. Datum holes, bearing seats and sealing faces usually carry the tight callouts. Mounting holes, clearance slots and covers rarely do. When you send a drawing, list which features are critical. A supplier can then plan the setup so the tight features are cut in one fixturing instead of being chased across three operations.

For parts under 200 mm, ±0.005 mm is achievable on a well-maintained 5-axis center with a controlled shop floor. Above 500 mm, thermal drift and fixture deflection start to matter, and the same shop may reasonably quote a wider band on non-critical features.

Imperial drawings are fine. ±0.005 mm is about ±0.0002 in. If your drawing is in inches, say so on the RFQ. Mixed units are a common source of scrapped first articles.

1
Tight featuresBearing bores, seal faces, datum holes, mating pilots.
2
Loose featuresClearance holes, covers, non-mating slots, cosmetic edges.
3
Watch forA single general tolerance note covering a part with mixed requirements.

Criterion 2

Lead time: when the spindle actually starts

Lead time means different things to different suppliers. Ask when metal is first cut, not when the order is acknowledged. A quote that arrives in 12 hours and a start within 24 hours keeps a program moving. A quote that arrives in a week and starts after drawing approval costs you the same week twice.

Break the timeline into four parts: quote, DFM feedback, machining, and finishing plus inspection. Finishing is where schedules quietly slip. Anodizing, plating and powder coating add handling and queue time even when they are done in house. Ask for the finishing step to be named in the quote.

Batch size changes the shape of the schedule. A single prototype can often be slotted into an existing setup window. A 5,000-part run needs stock, tooling and a dedicated fixture, so the first part and the last part may be weeks apart.

If your program has a hard date, say it on the RFQ. A supplier can then tell you which parts fit and which do not, rather than accepting everything and running late.

1
Quote stageQuotation and free DFM analysis within 12 hours.
2
Start stageProduction can start within 24 hours of release.
3
Shipping stageParts ship in 3–5 days for standard work.

Criterion 3

Order quantity and the MOQ trap

Minimum order quantity is where prototypes get stuck. A shop with a 500-part minimum forces a startup to pay for 500 parts to test one idea. Look for a supplier that runs one prototype and 10,000+ part runs on the same floor. That removes a second sourcing round when the design freezes.

Ask how setup is charged. Some suppliers fold setup into the unit price at low volume, then the price drops sharply at a break point. Others charge setup once and hold the unit price flat. Both models are workable, but you need to know which one you are buying so your budget model is honest.

For machined parts, the economic picture is simple. Setup and programming are fixed. Material and cycle time scale with quantity. If a quote shows a large price drop between 10 and 100 parts, that is normal. If it shows the same drop between 1,000 and 10,000, ask why.

CNC machining services London buyers often run mixed orders: machined housings, cast joints, finished covers. Consolidating those at one supplier cuts freight, incoming inspection and supplier admin.

1
PrototypeOne part is a valid order. No minimum.
2
Bridge volume10 to 500 parts, usually the most price-sensitive band.
3
Production1,000 to 10,000+ parts, where fixture design pays off.

Criterion 4

Certifications: match them to your industry

Certificates matter when your customer audits your supply chain. For general industrial parts, ISO 9001:2015 covers the quality system. For automotive and EV work, IATF 16949:2016 is the one buyers ask for. For medical devices, ISO 13485:2016. For data handling on customer drawings, ISO 27001:2022 is the relevant standard.

Ask for the certificate scope and issue date. A certificate for a different legal entity, or one that expired two years ago, is not a filter. A supplier that holds all four standards can take a program from prototype through regulated production without a supplier change.

Certification is not a substitute for inspection. A certified shop still needs to inspect your parts. What certification tells you is that the process is documented, calibrated and auditable. That is what saves you a second audit.

If your industry does not require a specific standard, do not pay for one. A general machine shop with ISO 9001 and a clean inspection record is often the better value for brackets, fixtures and enclosures.

1
Industrial machineryISO 9001:2015 is usually enough.
2
Automotive and EVIATF 16949:2016 is the expected baseline.
3
Medical devicesISO 13485:2016 plus traceable inspection records.
4
IP-sensitive workISO 27001:2022 and an NDA on request.

Criterion 5

Materials, finishes and where the cost hides

Material availability drives lead time more than machining does. A shop that stocks 6061, 7075, 304, 316L, 17-4PH, Ti-6Al-4V and common plastics can start without a mill order. A shop that orders every bar per job adds days before the first cut.

Finishing is the second cost center. As-machined at Ra 1.6–3.2 μm is fine for brackets and internal parts. Ra 0.8–1.6 μm suits visible covers and sliding surfaces. Ra 0.2–0.8 μm is for sealing faces, optical mounts and bearing fits. Each step up adds a separate operation.

Watch for finishes that are subcontracted. Anodizing, electroless nickel, zinc, silver and gold plating, powder coating and black oxide all require handling between shops. If the finishing house is not named in the quote, the schedule is a guess.

Laser marking has a practical limit. Minimum character height is 1.5 mm. If your part number or logo needs to be smaller than that, plan for a different marking method.

1
As-machinedRa 1.6–3.2 μm. Brackets, internal parts.
2
High finishRa 0.8–1.6 μm. Visible covers, sliding faces.
3
Fine finishRa 0.2–0.8 μm. Seals, optics, bearing fits.

RFQ process

How to run a CNC machining RFQ in 6 steps

This sequence works for a single prototype or a repeat production order. Each step removes a reason for the quote to come back vague.

1
Send the 3D model and 2D drawing togetherSTEP or IGES plus a PDF drawing with tolerances, finish callouts and material. If the two disagree, the drawing wins, so keep them consistent.
2
Mark critical featuresFlag datum holes, seal faces and bearing seats. State which dimensions must hold ±0.005 mm and which can run to general tolerance.
3
State quantity and target dateGive the prototype count and the expected production volume separately. A shop can then quote both bands instead of guessing.
4
Name the finishing and markingSay anodize type, color, plating, or blast. For laser marking, confirm character height is at least 1.5 mm.
5
Ask for a DFM note with the quoteA useful quote includes a short note on wall thickness, tool reach and any feature that will need a second setup. Free DFM analysis within 12 hours is a reasonable target.
6
Confirm inspection and paperworkAsk what ships with the parts: material certs, dimensional report, first article. 100% inspection before shipment should be standard, with reports on request.

FAQs

Questions London engineers ask before ordering

Can a supplier outside London hold ±0.005 mm on my parts?

Yes, if the shop has climate control and calibrated machines. Tolerance depends on the machine, the fixture and the inspection process, not the postcode.

Ask for the inspection method on the tightest feature. A CMM report on the first article is a fair request.

What is a realistic lead time for a machined prototype?

For standard materials and simple geometry, a quote within 12 hours and a start within 24 hours is achievable. Parts typically ship in 3–5 days.

Add time for finishing. Anodizing or plating adds a separate operation, so budget extra days even when the machining is fast.

Do I need IATF 16949 if I am not an automotive OEM?

No. ISO 9001:2015 covers general industrial and electronics work. IATF 16949 matters when your customer's quality system requires it, or when you supply automotive and EV programs.

Medical devices follow ISO 13485:2016. Ask for the certificate scope, not just the logo.

Is there a minimum order quantity for CNC machining?

It depends on the shop. Some set a minimum to cover setup. Others accept a single prototype and scale to 10,000+ parts without changing the process.

If you plan to scale, ask whether the prototype fixture carries over to production. A yes answer saves a second qualification round.

How should I handle confidential drawings?

Use a supplier with a secure upload channel and a signed NDA on request. ISO 27001:2022 is the standard that covers information security management.

Avoid sending controlled drawings as plain email attachments without an agreement in place.

Which finishes can be done in house?

Look for anodizing, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing, polishing, and laser marking under one roof.

In-house finishing shortens the schedule because the parts do not travel between suppliers.