UK CNC machining parts: how the process works and where it fails

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  How UK CNC machining parts are cut
• 2
  Why 5-axis changes the tolerance chain
• 3
  Material choice and what it does to the cut
• 4
  Tolerance, finish and the cost curve
• 5
  Inspection, documentation and the UK paperwork
• 6
  Quoting, lead time and the questions that save a batch
• 7
  Machine and process selection by part feature

CNC machining is subtractive. A CAM program converts a CAD model into toolpaths, and the machine moves a rotating cutter through a solid billet until the remaining material is the part. Nothing is added back, so the starting stock and the fixturing matter as much as the drawing.

The cut itself is a controlled failure of the material. The tool edge shears metal ahead of it, and the chip carries away most of the heat. When feeds and speeds are right, the chip breaks cleanly and the surface comes off at Ra 1.6–3.2 μm as machined. When they are wrong, the tool rubs instead of cutting, and the same pass leaves a work-hardened skin.

Three variables set the outcome: cutting speed, feed per tooth, and depth of cut. Aluminium 6082 runs fast and shallow. Titanium TC4 runs slow with heavy coolant. Stainless 316L sits between them and work-hardens if the tool dwells in the cut.

Rigidity closes the loop. A 4,000 mm bed part needs more support than a 500 mm bracket, and thin walls deflect under the same load that leaves a thick boss untouched. On long parts we check the travel envelope before quoting: 4,000 × 400 × 150 mm on the large machines, 750 × 1,150 × 550 mm on the medium ones.

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  Chip loadLight feeds rub the tool and burnish the surface instead of cutting.
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  WorkholdingDeflection under clamping shows up as taper, not as a size error.
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  Stock sizeBillet choice sets how many setups the part needs.

On a 3-axis machine, every new face needs a new setup. Each setup adds a datum transfer, and each datum transfer adds error. A part with six machined faces on a 3-axis mill may carry four or five re-clampings. On a simultaneous 5-axis center the same part often comes off in one or two.

The rotary table is the reason. With a Ø400 mm table and a trunnion, the tool reaches the part from angles that would otherwise need the part moved. Holes on different faces keep their position relative to each other because the datum never changes. That is where a ±0.005 mm callout becomes achievable rather than optimistic.

5-axis is not automatically tighter. It is faster to set up and better on angled features, but a tall thin rib still deflects, and a deep pocket still needs a long tool that chatters. If the feature is a simple flat plate with a few holes, a 3-axis machine with a good fixture often holds the same tolerance for less money.

We run 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. The choice of machine follows the feature, not the marketing. Complex angled ports, impellers and housings go to 5-axis. Shafts and fittings with turned and milled features go to mill-turn so the part stays on one spindle.

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  One setup, one datumFewer re-clampings means less stack-up across faces.
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  Angled featuresUndercuts and compound angles cut without a second fixture.
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  Not a cure-allThin walls and deep pockets still limit what any machine can hold.

Aluminium is the default for prototypes and enclosures. 6061 and 6082 machine cleanly, take anodizing well, and hold Ra 0.8–1.6 μm without extra work. 7075 is stronger and cuts almost as easily, but it anodizes to a darker, less uniform colour. 2024 has better fatigue behaviour and poor corrosion resistance, so it usually needs a coating.

Stainless is where drawings get corrected. 303 is free-machining and the easiest of the family. 304 and 316L are tougher, they work-harden, and a tool that dwells will glaze the surface. 17-4PH (SUS630) machines in the annealed state and gains strength after heat treatment, which means the finishing cut happens before the final hardness.

Titanium TC4 (Ti-6Al-4V) and Inconel are slow. Tool life is short, coolant flow matters more than speed, and thin sections move after the cut because of residual stress. For these we plan a rough, a stress-relief pause for critical parts, then a finish pass.

Plastics behave differently again. POM and PEEK hold tight tolerances but move with temperature. ABS and PC are soft enough to burr, so we use sharp tooling and air blast rather than flood coolant. Carbon fibre eats tool edges and needs dust extraction, not just coolant.

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  6061 / 6082General purpose, anodizes cleanly, best finish per hour.
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  316LCorrosion resistance for medical and marine parts, watch work hardening.
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  TC4 / InconelHigh strength at temperature, plan for short tool life.

Tolerance is not a single number. A ±0.005 mm callout on one bore is routine. The same callout on ten bores spread across a 600 mm part is a different job, because each feature shares the machine's positioning error and the part's thermal state.

Finish follows the same logic. Ra 1.6–3.2 μm comes straight off the cutter on most aluminium. Ra 0.8–1.6 μm needs a controlled finishing pass and sharp tooling. Ra 0.2–0.8 μm usually means a secondary operation such as lapping or polishing, and it should be specified only where a seal, a bearing or an optical surface needs it.

The cost curve is not linear. Going from ±0.05 mm to ±0.02 mm adds inspection time. Going from ±0.02 mm to ±0.005 mm adds machine time, temperature control and sometimes a rejected first batch. Specify the loosest tolerance that still lets the assembly work.

Surface texture and tolerance interact. A rough surface has peaks that sit outside the measured diameter, so a shaft that gauges to size can still bind. If the fit matters, call out both the diameter and the Ra, and say which one is functional.

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  Functional onlyTighten the fits that move or seal, leave the rest open.
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  Datum firstA clear datum scheme removes most tolerance arguments.
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  Both numbersSize and finish together describe a working fit.

Inspection is where the drawing and the part meet. We check raw material certificates on arrival, monitor dimensions in process, and inspect 100% of parts before shipment. Reports are available on request, and for regulated work the report set is agreed before the first cut.

For UK buyers the paperwork usually means a material certificate, a dimensional report for the critical features, and a certificate of conformity. Aerospace and medical work adds traceability by lot and, in some cases, first article inspection. We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022.

Inspection equipment sets the floor for what can be proved. A caliper reads to 0.02 mm. A micrometer reads to 0.001 mm. A coordinate measuring machine can confirm position and form, but only against a defined datum. If the drawing has no datum scheme, the inspector invents one, and that is where disputes start.

Keep the inspection plan proportional. A bracket for a test rig does not need a full dimensional report. A part that goes into a surgical device does. Saying which one you are buying saves both sides time.

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  Datum schemeDefine A, B and C on the drawing so inspection is repeatable.
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  Report scopeList the features that need numbers, not the whole drawing.
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  TraceabilityLot control for regulated industries, agreed before production.

A quote is only as good as the information behind it. We return a quotation and a free DFM analysis within 12 hours, and production can start within 24 hours of approval. Parts ship in 3–5 days for standard work. Our historical late-delivery probability is below 2%.

The DFM pass catches the usual problems: a corner radius smaller than any cutter can reach, a thread too close to a wall, a tolerance that cannot be measured, a finish spec that belongs on a different surface. Fixing these before cutting is cheaper than fixing them after.

There is no minimum order quantity. One prototype and a 10,000-part run go through the same first-article process. That matters for UK buyers running a pilot build before committing to tooling.

Confidentiality is part of the quote. Uploads are secure, and an NDA is available on request. Drawings for defence, medical and automotive programmes often arrive under NDA, and we would rather sign before the files move than after.

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  12-hour quoteQuotation and free DFM analysis within 12 hours.
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  No MOQOne part or 10,000+, same process.
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  NDA on requestSigned before files are shared when needed.