CNC Processing Services UK: What to Check Before You Order

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  What matters most
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  Matching the part to the process and the supplier
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  How CNC processing services UK buyers should read a tolerance callout
• 4
  Lead time, order quantity and the real cost of a cheap quote
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  Certification and inspection: what to ask for in writing
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  Materials and finishes that change the plan
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  Shipping machined parts to the UK: customs, packing and documents
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  Seven steps to qualify a supplier before you commit

Every drawing has a title-block tolerance, and then a handful of features that actually need something tighter. Those few features set the price. If a bearing bore is called at ±0.005 mm and the rest of the part sits at ±0.1 mm, the shop has to plan a separate finishing operation for one hole. That is fine, but it should be in the quote, not discovered at first article.

A general tolerance of ±0.1 mm is achievable on most three-axis work without special effort. Below ±0.02 mm, thermal drift starts to matter on aluminum parts longer than about 300 mm. Below ±0.005 mm, the shop needs a controlled environment and a metrology plan. Ask what instrument will verify the feature: a caliper is not the same as a coordinate measuring machine.

Surface finish is the second half of the same conversation. Ra 1.6–3.2 μm is a normal as-machined result. Ra 0.8–1.6 μm usually means a finishing pass with a smaller stepover. Ra 0.2–0.8 μm often needs polishing or a dedicated finishing operation after machining. Write the Ra number on the drawing next to the face it applies to, because a blanket note pushes cost onto every surface.

One more habit worth building: mark the datum. When a drawing has no clear datum, two shops can measure the same part and disagree by 0.03 mm while both being technically correct. Datum callouts remove that argument before it costs you a shipment.

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  Tight features onlyApply ±0.005 mm to the features that need it, not the whole part.
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  Name the gaugeAsk how the critical dimension will be verified before production.
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  Ra per faceA blanket finish note raises the price of surfaces nobody will touch.

Lead time has three separate clocks: quoting, production start, and shipping. A supplier can quote in 12 hours and still sit on the order for a week waiting for material. Ask directly when the machine will be booked. For a shop with 127 CNC machines, production can start within 24 hours once the drawing is frozen and the material is in stock.

Material availability is the hidden variable. Aluminum 6061 and 6082 plate, 304 stainless bar and POM sheet are usually on the shelf. Titanium Ti-6Al-4V, Inconel and 17-4PH often need to be ordered, which adds days before any chip is cut. If your schedule is tight, ask whether the quoted lead time includes material procurement.

Order quantity is the other lever. A shop with no minimum order quantity lets you run one prototype and then scale to a 10,000+ part run without changing supplier. That matters because the second supplier re-learns your part from scratch, and the fixture you paid for the first time is gone.

Be careful with a quote that is much lower than the others. It usually means one of three things: a looser tolerance band than you asked for, no inspection report, or a finish that will not pass your incoming inspection. Ask what is included. The answer tells you more than the number.

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  Ask when the machine is bookedQuoting speed and production speed are different promises.
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  Material adds daysTitanium and Inconel usually need to be ordered before machining.
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  No MOQ means no re-learningOne prototype to 10,000+ parts on the same process.

Certification is not a marketing badge. It defines the scope of work a supplier is allowed to take on and the records they must keep. For general industrial parts, ISO 9001:2015 is the baseline. Automotive and EV work usually requires IATF 16949:2016, which adds traceability and change-control requirements. Medical device components sit under ISO 13485:2016. If your data package is sensitive, ISO 27001:2022 covers information security.

Ask for the certificate and read the scope statement. A certificate that covers machining of aluminum parts may not cover a plating process that is subcontracted. If your part needs both, you want to know who holds the certificate for each step.

Inspection is the practical side of the same question. A supplier running 100% inspection before shipment checks raw material on arrival, monitors dimensions in process, and does a final inspection before packing. Reports are available on request. For a critical feature, ask for the actual measured values, not a pass stamp.

A qualification rate of 99.99% sounds like a slogan until you see how it is measured. Ask what counts as a defect and what happens to a rejected lot. A shop that reworks in-house and re-inspects will tell you the process. A shop that goes quiet will not.

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  Read the scopeThe certificate must cover the specific process you are buying.
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  Ask for valuesMeasured numbers beat a stamp on a packing list.
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  Check the sub-tierPlating, heat treat and anodizing may sit outside the main scope.

Material choice affects machinability, and machinability affects cost more than most buyers expect. Aluminum 6061 and 6082 cut fast and hold tight tolerances well, which makes them the default for prototypes and housings. 7075 is stronger but more prone to distortion on thin walls. Stainless 303 machines cleanly, while 316L and 17-4PH work-harden and need slower feeds.

Titanium Ti-6Al-4V, Inconel and magnesium AZ31B or AZ91D are all available, but each brings its own rules. Titanium needs sharp tooling and generous coolant to avoid overheating. Magnesium needs dedicated handling for chip safety. Inconel is slow, and the quote will reflect that. If a design can use 17-4PH instead of Inconel, the cost difference is usually large.

Finishes are where schedules slip. Anodizing in clear, color, hardcoat or conductive form is a separate process with its own queue. Electroless nickel, zinc, silver and gold plating, powder coating and black oxide all add handling steps. Bead blasting, tumbling, brushing and polishing change the surface but not the dimensions, so they are safe on most parts.

Laser marking and engraving need a minimum character height of 1.5 mm to stay legible after finishing. If your part number or logo is smaller than that, it may disappear into a coating. Send artwork with the drawing and confirm the marking step before anodizing, not after.

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  Thin walls distort7075 and stainless sections under 2 mm need stress-relieved stock.
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  Finishing has its own queueAnodizing and plating add days beyond machining.
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  Marking size limitLaser characters below 1.5 mm may not survive a coating.

Most parts for UK buyers ship by air express, with sea freight for larger runs. The paperwork matters as much as the box. A commercial invoice needs an accurate description and a declared value that matches the purchase order. A vague description like "metal parts" invites a customs hold and a phone call at the wrong moment.

Packing protects the tolerance you paid for. Precision bores and ground surfaces should not sit against each other, and anodized cosmetic faces should be separated by foam or paper. Ask for the packing method in the quote if the part has a visible surface. A part that arrives at spec but scratched is still a rejection.

Incoterms should be agreed before the first shipment. The usual split is FOB or EXW with the buyer's freight account, or DDP if you want the supplier to handle the last mile. Whichever you pick, make sure the quoted unit price matches the term, because a cheap unit price with DDP terms is not the same as a cheap unit price FOB.

Keep the documents with the parts. Material certificates, inspection reports and finish certificates should travel with the shipment or arrive by email the same week. If your quality team needs to trace a lot number six months later, finding it should not require an email chain.

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  Specific invoice wordingDescribe the part, not just the material.
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  Separate cosmetic facesFoam or paper between anodized surfaces.
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  Agree Incoterms firstA unit price means nothing without the trade term.