CNC Machining Newbury: How the Process Actually Works

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  Key takeaways
• 2
  What CNC machining removes, and why that matters
• 3
  How 3, 4 and 5 axis setups change the part
• 4
  Reading a tolerance band the way a machinist does
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  Material choice changes the cut, the finish and the price
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  How a shop proves the part is right
• 7
  Which setup and tolerance fits your part

CNC machining is subtractive. A cutter spins at a set surface speed and travels along a programmed path, and metal leaves the blank as chips. Nothing is molded or pressed, so the shape comes entirely from the toolpath and the fixture that holds the part. That is why the same drawing can cost very different amounts at two shops: the geometry is fixed, but the number of setups, the tool reach and the rigidity of the workholding are not.

The cutting edge does three things at once. It shears material, rubs against the chip, and pushes the workpiece away from the tool. On a light finishing pass the cutting force is small, but on a heavy roughing pass it can deflect a thin wall by more than the tolerance band. An engineer who understands this stops drawing 0.8 mm ribs next to open pockets and starts adding material where the part needs stiffness during the cut.

Heat is the second effect. Aluminium conducts heat away quickly, so the chip carries most of it. Titanium and stainless steels hold heat at the edge, which shortens tool life and pushes the shop toward lower surface speeds and more coolant. This is not a detail you can ignore when comparing quotes: a 17-4PH housing takes longer to machine than the same shape in 6061, and the price reflects that.

Chip evacuation is the third. Deep pockets and blind holes trap chips, and a recut chip breaks carbide edges fast. That is why shops add through-spindle coolant or peck cycles, and why a part with a 10:1 depth-to-diameter hole often costs more than its size suggests.

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  Subtractive means fixturing mattersThe part is only as accurate as the way it is held.
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  Cutting force deflects thin wallsRough, then semi-finish, then finish to control it.
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  Chip evacuation limits depthDeep pockets need coolant and shorter tools.

Every setup costs time and adds error. When a part is flipped or moved to a second machine, the datums shift slightly, and those shifts stack up in the final dimensions. A three-axis job on a complex housing might need four or five setups; each one adds a fixture, an operator touch-off and a chance for a small offset error. Five-axis machining, where the tool can approach from many directions in one clamping, removes most of that stacking.

Simultaneous five-axis means all five axes move together while cutting. That lets a ball-nose cutter stay normal to a curved surface, which gives a more even finish on impellers, turbine blades and contoured molds. It also lets a short, stiff tool reach into a cavity that a three-axis machine could only reach with a long, flexible tool that chatters.

The trade-off is programming and cost. Five-axis toolpaths take longer to prove out, and the machine itself costs more per hour. For a simple bracket with holes on two faces, a three-axis machine plus one flip is usually cheaper and just as accurate. The judgment call is about geometry, not about which machine sounds more advanced.

Mill-turn centers combine both ideas. A bar or blank is turned to a diameter, then milling tools cut flats, slots and cross-holes without a second setup. Parts like shafts with flats, valve bodies and small gear blanks fit this well because concentricity between the turned and milled features is held in one clamping.

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  Fewer setups, less stack-upEach re-clamping adds a small datum shift.
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  Short tools cut betterFive-axis reach avoids long, flexible end mills.
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  Simple parts stay on 3-axisMore axes only pay off when the geometry needs them.

A tolerance is a range, not a target. If a bore is called out at Ø20 mm ±0.005 mm, the machinist aims for the middle of that range, because being 0.004 mm high leaves no room for thermal drift or tool wear on the next part. On a long run, the operator watches the trend and offsets the cutter before the dimension walks out of band.

Not every feature needs the same band. A bearing seat, a dowel hole or a sealing face usually needs the tight number. An outside profile that bolts to a bracket often works fine at ±0.1 mm. When a drawing applies a single tight tolerance to the whole part, the shop has two choices: machine everything carefully and charge for it, or ask which features actually matter. The second answer is usually cheaper for everyone.

Surface finish is tied to tolerance. A Ra 0.8–1.6 μm finish on a sealing face needs a light finishing pass with a sharp tool and a stable setup. If the same face is left at Ra 1.6–3.2 μm, a tool mark can create a leak path. So the finish callout is a functional statement, not a cosmetic one.

Thermal effects are real at this scale. A 100 mm aluminium part grows about 0.0023 mm per degree Celsius. If the shop is warm when it cuts and the part is measured in a cooler inspection room, the reading changes. Good shops let parts stabilize before final inspection, especially on tight stainless and titanium work.

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  Aim for the middleCentre of the band leaves room for wear and drift.
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  Tight tolerance only where it functionsBlanket ±0.005 mm calls raise cost without benefit.
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  Let the part cool before measuringA 100 mm aluminium part moves 0.0023 mm per °C.

Aluminium 6061 and 7075 machine cleanly at high speed. They form a built-up edge if the tool is dull, so sharp carbide and good coolant matter. 7075 is stronger but less forgiving of sharp internal corners, where stress concentrates. For brackets, housings and heat sinks, 6061-T6 is usually the default because it balances strength, finish and cost.

Stainless 303 and 304 behave differently. 303 has sulfur added for chip breaking, so it machines faster but is slightly less corrosion-resistant. 304 galls and work-hardens if the tool rubs instead of cuts, which means a light feed can actually make the surface harder and harder to finish. 17-4PH (SUS630) is common in aerospace and medical parts because it can be heat-treated after machining to high strength, but it needs lower surface speeds and more tool changes.

Titanium TC4 (Ti-6Al-4V) and Inconel are the slow ones. They hold heat at the cutting edge, so tool life drops and the shop runs slower speeds with generous coolant. A part that takes 20 minutes in aluminium might take two hours in Inconel. That difference is not a markup; it is the physics of the material.

Plastics like POM, PEEK and ABS machine fast but move with temperature. POM is stable and holds tolerance well. PEEK is expensive and abrasive, so tool wear adds up. ABS is soft and can smear if the cutter rubs. For prototypes and light-duty parts, these materials are often the right answer, but the feeds and fixtures differ from metal work.

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  6061-T6 is the default aluminiumGood strength, finish and machinability.
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  304 work-hardens under light cutsKeep the tool cutting, not rubbing.
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  Titanium and Inconel run slowHeat at the edge limits speed and tool life.

Inspection starts before the first cut. The raw material certificate is checked against the drawing callout, so a 7075 part is not made from a 6061 bar. Then the first article is measured. On a tight job, the machinist checks the critical features with micrometers, bore gauges or a height gauge, and the setup is adjusted before the run continues.

In-process checks catch drift. If a tool wears 0.01 mm over 50 parts, the operator offsets the cutter at part 25 to keep the trend inside the band. Final inspection happens after the part has cooled and, on parts with tight sealing faces, after any stress relief or finishing step that could move dimensions.

A CMM report gives the customer a record. It maps measured points against the nominal geometry and shows where the part sits inside each tolerance. For aerospace, medical and automotive work, that report is often required, not optional. It is also the fastest way to settle a question about whether a feature is in or out.

GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring and final inspection. Reports are available on request. That covers the paperwork side, but the real value is earlier: a shop that checks the first article carefully usually has fewer surprises at final inspection.

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  Check material firstThe certificate must match the drawing alloy.
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  Offset before the trend leaves the bandTool wear is predictable and correctable.
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  CMM reports settle disputesMeasured points against nominal geometry.