CNC Processing Services London Buyers Can Qualify in One Call

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  What matters most when you shortlist a supplier
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  Five checks and what a good answer looks like
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  Tolerance and surface finish: get the number before you send a drawing
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  Machine setup and part size: why 5-axis changes the quote
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  Lead time, order quantity, and what the numbers really mean
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  Certifications: read the scope, not the logo
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  How CNC processing services London teams should read a quote
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  Five steps to qualify a supplier before you commit

Every machined feature has a cost curve, and tolerance sits at the bottom of it. A general tolerance block on your drawing might say ±0.1 mm, but if three bores need to fit a bearing, those features need their own callout. Before you compare quotes, write down the tightest tolerance on the part and the finish you need. Those two numbers drive the setup count, the tooling, and the inspection time.

GreatLight works to ±0.005 mm (approximately ±0.0002 in) on qualified features. That is not a blanket promise on every surface. It applies to features the shop has planned for, with the right machine, workholding, and temperature control. A 4,000 mm long part will not hold ±0.005 mm across its full length, and no honest shop will tell you otherwise.

Surface finish follows the same logic. Ra 0.8–1.6 μm covers most functional mating surfaces and seals. Ra 0.2–0.8 μm is for sliding seals, optical bores, and bearing journals where the finish is part of the function. If your drawing asks for Ra 0.2 μm on a part that gets painted, you are paying for a step nobody will measure.

The practical test: ask the supplier which features they cannot hold on your drawing, and why. A shop that names the hard features has already read the part. A shop that says everything is fine has not.

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  As-machinedRa 1.6–3.2 μm, fine for brackets, housings, and painted parts.
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  High finishRa 0.8–1.6 μm, standard for sealing faces and bearing seats.
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  Fine finishRa 0.2–0.8 μm, use only where the finish does a job.
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  Tolerance±0.005 mm applies to planned features, not the whole part.

A part that needs four sides machined can be done on a 3-axis mill with four setups, or on a 5-axis center with one. The difference shows up in three places: setup error, lead time, and the number of fixtures you pay for. Each extra setup adds a datum shift, and datum shifts are where tolerance stacks go wrong.

Simultaneous 5-axis machining keeps the tool normal to the surface while the part rotates, so undercuts, angled ports, and compound curves get cut in one pass. That is the reason 5-axis matters for impellers, medical instruments, and complex manifolds. It is not automatically better for a flat plate with six holes.

Size limits matter more than most buyers expect. GreatLight runs travel ranges from 500 × 310 × 200 mm up to 4,000 × 400 × 150 mm, plus a Ø400 mm rotary table. If your part is 900 mm long, it fits the 750 × 1,150 × 550 mm envelope but not the compact machines. Send the bounding box in the RFQ, not just the STEP file.

The trap: a supplier quotes a 5-axis part and then machines it on a 3-axis mill with extra setups. The price looks competitive. The first article shows the error. Ask which machine will run the job, and ask to see it named on the setup sheet.

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  3-axisSimple prismatic parts, one face at a time, lowest hourly rate.
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  4-axisCylindrical parts with features around the axis, mill-turn work.
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  5-axisCompound angles, undercuts, deep cavities cut in one setup.
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  Rotary tableØ400 mm, good for round parts that need side drilling.

Lead time is the easiest number to promise and the hardest to hold. Break it into three parts: quote turnaround, production start, and shipping. A shop that quotes in 12 hours and starts production within 24 hours is staffed for small-batch work. A shop that quotes in a week is usually optimized for large runs.

GreatLight quotes with a free DFM analysis within 12 hours, can start production within 24 hours, and ships parts in 3–5 days. That is the working range for standard parts in common materials. It is not a guarantee on every geometry. A part that needs a custom fixture or a difficult material adds setup time, and the DFM note in the quote should say so.

Order quantity is the other filter. No minimum order quantity means the shop can run one prototype and still take a 10,000+ part order later. That flexibility matters in London, where a design team may need three prototypes in week one and a pilot run in week six. If a supplier asks for a 500-piece minimum, they are not set up for that pattern.

Late delivery risk is worth asking about directly. A historical late-delivery probability below 2% is a real number a shop can report. Vague answers about reliability are not.

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  Quote12 hours, including free DFM analysis.
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  Production startWithin 24 hours after approval.
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  Shipping3–5 days for standard parts and materials.
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  QuantityOne prototype up to 10,000+ parts, no MOQ.

A certificate is a scope statement. ISO 9001:2015 covers quality management: documented processes, corrective action, traceability. It does not mean a shop can make medical implants. IATF 16949:2016 adds automotive rules on change control and traceability. ISO 13485:2016 adds medical device rules on risk and documentation. ISO 27001:2022 covers information security, which matters when you send CAD files.

For a London buyer, the practical question is which certificate applies to your part. A robotics bracket needs ISO 9001:2015. An EV busbar needs IATF 16949:2016. A surgical instrument needs ISO 13485:2016. If your supplier holds all four, they can take work across those sectors without subcontracting the quality system.

Certificates also tell you something about how the shop handles your data. ISO 27001:2022 is not common in small machine shops. It means uploads, drawings, and revision history are handled under a defined security process, and an NDA can reference it.

Check the certificate number and expiry date. Ask which site it covers. A group certificate that names a different plant than the one making your parts is not the same thing.

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  ISO 9001:2015Baseline quality system for industrial and electronics parts.
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  IATF 16949:2016Automotive and EV work, adds change control and traceability.
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  ISO 13485:2016Medical devices, adds risk management and documentation rules.
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  ISO 27001:2022Information security for drawings and uploads.

A quote is a technical document, not a price tag. A useful quote lists the material grade, the machine, the finish, the tolerance callouts, the inspection method, and the lead time. If any of those are missing, the price is not comparable to another supplier's price, because the scope is not the same.

Material substitution is the most common hidden difference. Aluminum 6061-T6 and 7075 are both aluminum, but they machine and behave differently. 316L stainless costs more than 303 and machines slower. If a quote does not name the grade, assume the cheapest one that meets the drawing.

Finishing scope is the second trap. Anodizing, electroless nickel, powder coating, and bead blasting all change the part dimensions slightly and add days to the schedule. A quote that says machined only is not the same as one that includes hardcoat anodizing and laser marking at a 1.5 mm minimum character height.

Finally, ask what happens when a dimension is out. A shop that inspects 100% before shipment and shares reports on request has a process. A shop that only checks the first article is hoping the rest are fine.

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  Material gradeNamed alloy, not just aluminum or stainless.
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  FinishNamed process with masking and marking notes.
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  InspectionMethod, sample rate, and report format.
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  Data handlingNDA available, uploads kept confidential.