CNC machining in Southampton

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  What CNC machining removes, and why the machine choice decides the part
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  Three, four or five axes: choosing by geometry, not by habit
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  What tolerance and surface finish actually mean on the shop floor
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  Material choice changes the cutting strategy
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  When CNC machining is the wrong answer
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  How to get a usable quote in 12 hours
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  Machine configuration compared by part type

CNC machining is subtractive. A cutter follows a toolpath generated from CAM, and material is removed until the part matches the model. Nothing about the process is additive, so the geometry you can produce is limited by what a spinning tool can reach without colliding with the part or the fixture.

That single constraint explains most of the decisions. A three-axis vertical mill cuts from one direction, so holes on the side faces need a second setup. A four-axis machine adds a rotary table that indexes the part between faces. A five-axis machine tilts the tool or the table continuously, so undercuts, deep pockets and compound angles can be reached in one pass.

CNC machining in Southampton usually means parts that feed marine, aerospace, automotive and medical supply chains along the south coast. Those buyers tend to care about three things: can the geometry be reached, can the tolerance hold across the batch, and how fast can a revised design come back. The answers come from the machine list, not from a brochure.

GreatLight runs 127 high-precision CNC machines across three wholly-owned plants, including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. The largest travel is 4,000 × 400 × 150 mm. That mix matters because not every part belongs on a 5-axis machine.

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  ReachabilityIf a tool cannot enter the feature without hitting the part, the feature needs another setup or another machine.
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  StiffnessLong tools in deep cavities deflect. That shows up as taper or chatter, not as a clean wall.
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  Setup countEvery additional setup adds a re-datum operation and a chance for stack-up error.

A three-axis part is flat work with features on one or two faces. Brackets, plates, manifolds with parallel holes, and most housings fit here. Cycle times are short and the fixturing is simple, which keeps unit cost low at volume.

Four-axis work adds rotation around one axis. Shafts with cross-holes, cylindrical parts with milled flats, and cam profiles belong here. Because the rotary table indexes rather than tilts, the cutting conditions stay predictable and the programmer can use shorter tools.

Five-axis work is for compound angles, deep cavities, and parts where one datum must be held across many faces. Impellers, turbine housings, medical instrument bodies and complex automotive brackets are typical. Continuous five-axis motion keeps the tool normal to the surface, which improves both finish consistency and tool life on curved geometry.

Mill-turn centers deserve a mention because they collapse two operations into one. A part that would otherwise be turned, then re-fixtured on a mill, can be finished in a single program. That removes a datum transfer and usually removes the largest source of concentricity error.

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  Pick 3-axis whenFeatures are prismatic and reachable from two or three orthogonal directions.
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  Pick 4-axis whenThe part is rotational but needs cross-features or flats indexed around its axis.
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  Pick 5-axis whenCompound angles or deep curved surfaces make manual setup impractical.
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  Pick mill-turn whenConcentricity between turned and milled features is the critical requirement.

A tolerance of ±0.005 mm is achievable, but it is not a default. It has to be earned by controlling thermal expansion, tool wear and clamping force. Aluminium expands roughly 23 μm per metre per degree Celsius, so a part that is measured hot can pass inspection in the morning and fail it in the afternoon.

Surface finish behaves the same way. As-machined surfaces sit around Ra 1.6–3.2 μm. A high-quality finish of Ra 0.8–1.6 μm is normal for mating faces and bearing seats. Fine finishes of Ra 0.2–0.8 μm need either a finishing pass with a small stepover, a lapping operation, or a change of process such as grinding.

The engineering meaning is simple. Tolerance controls how well the part fits and whether an assembly closes. Finish controls friction, fatigue life and how a seal behaves. A hydraulic manifold can have a generous general tolerance and still need a very fine finish on one sealing face.

GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring and a final inspection. Inspection reports are available on request. The qualification rate is 99.99%. None of that replaces a clear drawing, though. Put the critical dimensions on the print with a defined datum, and let the non-critical ones carry a general tolerance.

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  As-machinedRa 1.6–3.2 μm. Fine for brackets, covers and non-contact surfaces.
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  High qualityRa 0.8–1.6 μm. Mating faces, bearing bores, seal grooves.
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  FineRa 0.2–0.8 μm. Requires extra passes or a secondary process.

Aluminium is the default for prototypes and most housings. Grades 6061 and 6061-T6 machine cleanly, hold a good finish and resist corrosion after anodizing. 7075 is stronger and used where weight matters, but it is less forgiving and tends to show tool marks. 2024, 5052, 5083, 6063, 6082 and ADC12 are all stocked.

Stainless grades 303, 304, 316 and 316L are common in marine and medical work. They work-harden, so the cutter must stay engaged and the feed must not be allowed to rub. 17-4PH (SUS630) machines well in the annealed state and is often chosen where both strength and corrosion resistance are needed.

Steels such as 1018, 1045, 4130, 4140, 4340 and A36 cover most structural and shaft work. Tool steel appears where wear resistance matters. Titanium grades TA1, TA2 and TC4 (Ti-6Al-4V) are machinable but generate heat at the cutting edge, so speeds drop and cycle times rise. Inconel is slower still.

Plastics and composites behave differently again. ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre are all available. PEEK and carbon fibre are abrasive and need carbide or diamond tooling. Copper and brass grades C101, C103, C110, C27400, C28000 and C36000 cut freely but are soft, so they mark easily during handling.

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  Easy to machine6061 aluminium, 303 stainless, C36000 brass, POM.
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  Moderate316L, 4140, 17-4PH, titanium TC4.
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  DifficultInconel, PEEK, carbon fibre, magnesium AZ31B and AZ91D.

CNC machining is the wrong process when the part is hollow, thin-walled and needed in thousands. A die-cast or injection-moulded part will beat it on unit cost once the tooling is amortized. The crossover usually sits somewhere in the low thousands of units, depending on geometry, but a machined prototype is still the right first step before committing to tooling.

It is also wrong when the material cannot be cut economically. Very hard tool steels, ceramics and some composites cost more in tool wear than the part is worth. In those cases, consider a different route such as die casting for non-ferrous parts or 3D printing for low-stress prototypes.

There is a size boundary too. GreatLight's largest travel is 4,000 × 400 × 150 mm. Parts beyond that need to be split, welded or sourced elsewhere. The Ø400 mm rotary table sets a limit on four-axis work as well.

Finally, CNC machining is a poor fit when the drawing is still moving. If the design changes weekly, the fixture and program have to change with it. That is not a problem at prototype quantity, but it is expensive at volume.

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  Too many unitsAbove a few thousand, casting or moulding usually wins.
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  Too hardCeramics and hardened tool steel wear out tooling fast.
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  Too bigBeyond 4,000 × 400 × 150 mm the part must be split.