UK CNC processing plants: how the work actually gets placed

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  What a CNC processing plant really sells you
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  Which machine suits which UK part
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  Tolerance, finish and where cost jumps
• 4
  Material choice decides the machining plan
• 5
  How a shop proves the part is right
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  Lead time, lot size and what changes them
• 7
  Finishing and assembly before shipment
• 8
  Matching a part to the right process

A CNC processing plant sells machine time, setup skill and inspection records. The metal removal itself is the easy part. What decides whether your part comes back correct is how the shop plans the fixture, the tool path and the measurement loop before the first chip is cut.

In the UK market, a lot of work sits in high-value, low-volume categories: prototype pump housings, EV battery brackets, surgical instruments, radar chassis, test rigs. These parts rarely need a thousand pieces. They need the first one to be right, because a design freeze depends on it.

That changes what you should ask. Instead of a price per piece, ask which machine the job will run on, how the datum is set, and who signs off the first article. The answer tells you more than a quote sheet.

GreatLight runs three wholly-owned plants with 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers and 16 mill-turn centers. Capacity of that shape is built for mixed-part, mixed-material production rather than one long run.

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  Setup and fixturingOften a larger share of cost than cutting time on one-off parts.
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  In-process measurementCatches drift before a whole batch is scrapped.
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  DocumentationMaterial certs, inspection reports, traceability on request.

Three-axis machining is still the workhorse. If your part has features on one face, or can be reached in a few fixtures, a 3-axis mill with a good vise setup is faster and cheaper than anything else. Roughly 27 of our machines sit in this class.

Four-axis work adds a rotary table so you can index to a second or third face without re-clamping. That matters when hole patterns must stay concentric. We run 12 four-axis mills and Ø400 mm rotary tables for this kind of work.

Simultaneous 5-axis is where complex geometry gets solved. Impellers, turbine blades, angled ports and free-form surfaces can be cut in one setup, which removes the tolerance stack that builds up across three or four re-fixtures. Sixteen 5-axis centers cover this category.

Mill-turn centers cut rotating parts from bar in one cycle: turn the OD, then mill flats, slots and cross-holes without moving the part. For shaft-like components this removes a whole operation and the error that comes with it.

Size still sets the limit. Our largest travel is 4,000 × 400 × 150 mm, with medium envelopes of 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, and compact machines at 500 × 500 × 450 mm and 500 × 310 × 200 mm.

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  Pick 3-axisFlat plates, covers, one-face features, simple brackets.
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  Pick 4-axisParts needing indexed faces and tight hole-to-bore alignment.
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  Pick 5-axisAngled ports, contoured surfaces, parts that would need 3+ fixtures.
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  Pick mill-turnShafts, pins, bushings with cross features.

A tolerance is a cost curve, not a checkbox. Tightening a bore from ±0.05 mm to ±0.005 mm means finer tools, slower feeds, more temperature control and more measurement. The part may look identical on the drawing and cost twice as much.

Our working tolerance floor is ±0.005 mm, or ±0.0002 in. That is achievable on the right feature, in the right material, on the right machine. It is not a blanket promise across a 900 mm aluminium plate, because thermal expansion alone moves the number.

Surface finish follows the same logic. As-machined output sits around Ra 1.6–3.2 μm. A high-finish cut reaches Ra 0.8–1.6 μm. Fine finishing gets to Ra 0.2–0.8 μm, and usually needs a separate pass, a smaller stepover and sometimes a different tool.

The practical rule: put tight tolerances only on the features that touch something else. Datum faces, bearing bores, sealing surfaces. Let the rest float at general tolerance. That single change often removes 20–30 percent of the machining cost without touching function.

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  Tight where it mattersBearing seats, seal grooves, mating faces.
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  Loose elsewhereCosmetic edges, clearance holes, non-critical pockets.
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  Finish calloutsRa values drive tool choice and cycle time more than most buyers expect.

Aluminium is the default for prototypes and enclosures. We machine 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12. The 7075 grades cut cleanly and hold tolerance well, but they are less weldable and cost more per kilo than 6082.

Stainless is where the cycle time changes. 303 machines freely because of its sulfur content. 304 and 316L work-harden, so light cuts and constant feed matter. 17-4PH (SUS630) machines in the annealed state, then gains strength after aging, which is why it appears in aerospace and medical parts.

Steel grades 1018, 1045, 4130, 4140 and 4340 all see regular work, plus A36 and tool steel. Pre-hardened 4140 at 28–32 HRC is common for mould inserts, but it eats tool life and needs a more conservative path.

Titanium and nickel alloys are a different order of difficulty. TA1, TA2, TC4 (Ti-6Al-4V), Inconel and magnesium AZ31B or AZ91D all generate heat at the cutting edge. Tool paths need high-pressure coolant and lower surface speed. If a design can use aluminium instead, the saving is real.

Plastics run from ABS, PC, PMMA, POM, PA, PEEK, PP and HDPE to carbon fibre. PEEK holds tolerance at temperature but is expensive; POM is stable and cheap for fixtures and test parts.

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  Aluminium 6082 vs 70756082 for weldability and cost; 7075 for strength and finish.
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  Stainless 303 vs 316L303 for free machining; 316L for corrosion and medical use.
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  Titanium and InconelExpect slower cutting and shorter tool life.

Inspection is where a low quote usually hides its cost. A shop that checks only the first article can ship a batch with drift in it. A shop that measures in-process catches the drift while the part is still in the machine.

Our sequence is raw material check, in-process monitoring, then final inspection. Inspection is 100 percent before shipment and reports are available on request. The qualification rate across that flow is 99.99 percent.

For regulated work the paperwork matters as much as the metal. ISO 9001:2015 covers general quality systems, IATF 16949:2016 covers automotive, ISO 13485:2016 covers medical devices, and ISO 27001:2022 covers information security for customer data and drawings.

If your part goes into a car, a patient or an aircraft, ask which certificate applies and whether the supplier can show the current scope. A certificate number without a scope tells you very little.

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  First articleConfirms the setup, the datum and the program.
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  In-process checksCatches tool wear and thermal drift during the run.
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  Final inspection100 percent before shipment, reports on request.
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  Scope checkMatch the certificate scope to your industry.

Quotation and a free DFM analysis come back within 12 hours. Production can start within 24 hours after that, and parts ship in 3–5 days. Those numbers hold for straightforward work with released drawings. A part that needs a new fixture, a custom tool or a special material will take longer, and a shop that says otherwise is guessing.

Lot size changes the economics more than anything else. There is no minimum order quantity here, so one prototype and a 10,000+ part run both go through the same floor. One-off parts carry the full setup cost. Past a few hundred pieces, the setup is amortised and the per-part price falls fast.

Historically, the late-delivery probability on our orders is below 2 percent. That figure comes from tracking actual shipments, not from a promise. It is the number to ask any supplier for, because it exposes how they plan capacity.

Confidentiality is part of the same conversation. Uploads are secure and confidential, and an NDA is available on request. For defence, medical or unreleased consumer products, get that in place before the first STEP file moves.

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  12 hoursQuote and free DFM analysis.
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  24 hoursProduction start after release.
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  3–5 daysTypical shipment window.
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  No MOQFrom one prototype to 10,000+ parts.

Machining leaves marks. Whether those marks matter depends on the function. Bead blasting and tumbling even out tool paths and remove sharp edges. Brushing gives a directional grain that hides handling scratches. Polishing goes further for optical or cosmetic parts.

Coatings change dimensions slightly, so they belong in the tolerance plan from the start. We offer anodizing in clear, colour, hardcoat and conductive versions, plus electroless nickel, zinc, silver and gold plating, powder coating and black oxide.

Laser marking and engraving handle part numbers, logos and traceability codes. Minimum character height is 1.5 mm. Below that, the mark becomes hard to read and hard to inspect, so plan the marking area accordingly.

Keeping finishing in the same supply chain removes a common failure mode: the part leaves tolerance at the coater and nobody owns it. One supplier, one inspection report, one point of contact.

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  Bead blastingEven matte surface, deburred edges.
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  Hardcoat anodizingWear resistance on aluminium, adds a few micrometres.
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  Laser markingTraceability codes at 1.5 mm minimum character height.