CNC UK processing: how offshore machining really works

• 1
  What CNC UK processing covers
• 2
  How tolerance and setup decide the route
• 3
  Material choice changes the cutting plan
• 4
  Inspection, finishes and paperwork
• 5
  Lead time, quantity and when to stay local
• 6
  Which machining route fits which part

CNC UK processing is a buying pattern, not a machine type. A UK company owns the design, the drawing and the end use. The cutting happens somewhere else, often in Dongguan or Singapore, and the finished parts come back to a UK address. The work itself is milling, turning, mill-turn and finishing on metal or plastic stock.

The split matters because two things travel across borders: the physical part and the quality data. A part can measure perfectly and still cause a problem if the inspection report does not match the drawing callouts. That is why the first question is never about price per hour. It is about which features are critical and how they will be proven.

GreatLight has run this route since 2011 from three wholly-owned plants, with 127 high-precision CNC machines and 150 technicians. We quote from a STEP file, run a free DFM check, and ship in 3–5 days once the drawing is released. Nothing here is UK-specific tooling. The same process serves buyers in Germany, the US and Singapore.

• 1
  Typical partsBrackets, housings, shafts, manifolds, fixture plates, prototype enclosures.
• 2
  Typical batchOne prototype to 10,000+ parts, with no minimum order quantity.
• 3
  Typical materialAluminium, stainless, carbon steel, copper alloys, titanium, engineering plastics.

Tolerance is the first filter. A general machining tolerance in the ±0.05 mm range is easy on a three-axis mill. Once a callout drops to ±0.005 mm, the machine, the fixture and the temperature of the shop all start to matter. The same feature on a thin wall is harder than on a solid block, because the metal moves when the cutter leaves.

Datums decide more than the machine. If a drawing locates a hole pattern from a face that is later polished, the inspector has no stable reference. Good practice is to name one primary datum, one secondary and one tertiary, then dimension critical features from those. We mark the fixture plan in the DFM note so the buyer can see how the part will be held.

Five-axis work earns its cost when a part has features on several faces. One setup replaces four or five fixtures, and each removal introduces a fresh error. On parts under 200 mm, a 5-axis cycle often beats a 3-axis cycle plus two fixtures, even though the hourly rate is higher.

The limit is part size and stiffness. Thin floor sections, deep pockets with a small cutter, and bores that need a mirror finish may need EDM, grinding or a second operation. We say so at quote stage rather than after the first article fails.

• 1
  ±0.005 mmAchievable on rigid parts with clear datums and stable stock.
• 2
  Ø400 mm rotary tableCovers most round and index-machined work in one setup.
• 3
  4,000 mm travelLarge plates and long frames fit on the big mills.

Aluminium 6061-T6 and 7075 are the common starting points. 6061 machines cleanly, takes anodizing well and holds ±0.005 mm on most features. 7075 is stronger but more prone to stress relief during heavy cuts, so we rough, let the part rest, then finish. For marine or pressure work, 5083 and 6082 are available.

Stainless 303 and 304 cut with a gummy chip and work-harden if the feed is too light. 316L behaves the same way and is usually chosen for corrosion or medical duty. 17-4PH (SUS630) can be machined in the annealed state, then aged to reach its final strength, which is why heat-treat sequencing belongs in the drawing notes.

Steel grades 1045, 4140 and 4340 need a different insert and a slower surface speed than aluminium. Tool steel and pre-hardened stock raise the cutting force, so a part that held tolerance in aluminium may need a grinding allowance in steel. Titanium TC4 (Ti-6Al-4V) and Inconel take low speeds, high coolant flow and sharp tooling.

Plastics are not a soft option. POM and PEEK move with heat, PMMA chips, and carbon fibre wears tools fast. We set feeds and clamping pressure per material rather than running one universal program. The material list on the quote tells you what stock we will actually cut.

• 1
  Aluminium6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075, ADC12.
• 2
  Stainless303, 304, 316, 316L, 420, 430, 431, 440C, 17-4PH.
• 3
  Steel1018, 1045, 4130, 4140, 4340, A36, tool steel.
• 4
  Titanium and specialTA1, TA2, TC4, Inconel, magnesium AZ31B / AZ91D.

Every part is inspected before shipment. That means a raw material check, in-process monitoring during the cycle, and a final inspection against the drawing. Reports are available on request, and we can list measured values for the features the buyer marks as critical. If a drawing calls for a first article inspection, we plan that step before the run starts.

Surface finish is a process choice, not a polish at the end. As-machined faces sit around Ra 1.6–3.2 μm. A finer pass reaches Ra 0.8–1.6 μm, and Ra 0.2–0.8 μm takes a dedicated finishing cycle. Bead blasting hides tool marks and gives a matte look. Anodizing, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, brushing and laser marking are all handled in-house.

Laser marking needs a minimum character height of 1.5 mm to stay legible after finishing. Serial numbers, logos and date codes are fine at that size. Very small text on a curved anodized face will lose contrast. Send the artwork with the drawing so we can check the mark area before cutting.

Confidentiality runs through the same process. Uploads are secure, and an NDA is available on request. Buyers who need a signed agreement before releasing a file can start there, then send the CAD once the paperwork is back.

• 1
  100% inspectionRaw material, in-process and final checks on every order.
• 2
  Finish rangeRa 0.2–0.8 μm fine, Ra 0.8–1.6 μm high, Ra 1.6–3.2 μm as machined.
• 3
  ReportsInspection data issued on request with the shipment.

Quotation and a free DFM analysis come back within 12 hours. Production can start within 24 hours after the drawing is released and the material is confirmed. Standard parts ship in 3–5 days. The historical late-delivery probability is below 2%, which matters when a UK assembly line is waiting on one bracket.

There is no minimum order quantity. One prototype and a 10,000-part run go through the same quoting route, though the unit cost curve is flat between one and a few dozen pieces because setup dominates. If a program needs 200 parts a month for a year, say so at quote stage. It changes the fixture plan and sometimes the process choice.

Some work should stay in the UK. Parts under 10 mm with a ±0.005 mm callout on every face, parts that need same-day iteration with the designer in the room, and assemblies where a courier delay stops a line are all better served by a local shop. The travel time and the phone call cost more than a local hourly rate would.

The useful split is by risk, not by patriotism. Send parts with a stable drawing, a clear datum scheme and no same-day change loop offshore. Keep the parts that are still moving on a local bench. That is the rule we use when buyers ask which of their parts we should quote.

• 1
  Quote in 12 hoursIncludes DFM feedback on the STEP file.
• 2
  Ship in 3–5 daysAfter drawing release and material confirmation.
• 3
  Keep localSame-day iteration, sub-10 mm tight-tolerance parts, line-stopper assemblies.