Meaning of CNC Processing

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  What the meaning of CNC processing actually covers
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  How a CNC machine turns code into a finished surface
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  Axis count and what each one buys you
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  What tolerance and finish a CNC process can hold
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  Which materials change the economics of processing
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  When CNC processing is the wrong choice
• 7
  CNC processing compared with other forming processes

CNC stands for computer numerical control. A part is drawn in CAD, the CAM stage turns that geometry into toolpaths, and the post-processor writes them as G-code: coordinates, feed rates, spindle speeds, tool changes. The machine reads that code and drives the axes, so the shape of the part comes from numbers rather than from a template or a hand wheel.

The word processing covers the whole material-removal chain, not one machine. Milling uses a rotating cutter on a moving table. Turning spins the workpiece against a single-point tool. Drilling, boring, tapping and thread milling are usually folded into the same setup. A mill-turn center does turning and milling in one cycle, which removes a re-fixturing step on parts like valve bodies or hydraulic manifolds.

What separates CNC from manual machining is repeatability. Once the first article is inspected and the offsets are locked, part 200 should match part 1 within the machine's positioning error. That is why CNC processing fits production runs where every unit has to drop into the same assembly.

It also means the geometry has to be machinable in the first place. A pocket narrower than the smallest cutter, or a corner radius tighter than the tool nose, will stop the program cold. Design for the tool, not against it.

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  Programmed pathG-code sets position, feed and speed for every move
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  Setup repeatsWork offsets and tool offsets make run two match run one
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  One chainMilling, turning, drilling and tapping share the same digital model

The control loop is simple to describe. The controller reads a block of code, compares the commanded position with feedback from the servo or linear scale, and sends current to the motor until the error is near zero. On a modern machining center that loop closes thousands of times per second, which is what allows a 12 mm end mill to follow a curved wall without visible chatter marks.

Feed and speed come from the material, not from habit. Aluminium 6061 runs fast: 3,000 to 8,000 rpm on a 10 mm cutter, 1,500 to 4,000 mm/min feed, air blast or mist for chip clearing. Stainless 316 is the opposite. Surface speed drops to roughly 60 to 120 m/min, feed per tooth 0.05 to 0.12 mm, and you need flood coolant or the edge will work-harden.

Heat is the real constraint. Most of the energy of a cut leaves in the chip, and the rest goes into the tool and the part. Climb milling on a rigid setup keeps the chip thick at the start of the cut and thin at the end, which pulls heat out and pushes the cutter away from the finished wall. Conventional milling does the reverse and tends to rub.

When the tool deflects, the wall moves with it. A long 6 mm cutter at 4× diameter depth will bend under cutting force and leave a tapered wall, even if the code is perfect. Rough, then semi-finish, then finish with a short, stiff tool.

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  Closed loopController corrects position error continuously during the cut
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  Chip takes the heatThick-to-thin chip load keeps the edge cool and the wall clean
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  Deflection is geometricLong tools cut undersize and taper; short tools hold size

A three-axis mill moves X, Y and Z. The tool axis stays vertical, so every face you machine has to be reachable from the top. That suits plates, housings, brackets and any part where the work can be flipped once and re-datumed. GreatLight runs 27 three-axis machines for exactly that kind of work.

A four-axis machine adds rotation around one axis, usually A. The part can be indexed to four sides without a second setup, which is how you hold true position between a bore on the front face and a slot on the side. Twelve four-axis mills cover this range.

Five-axis means two rotary axes on top of the linear three, and the tool can approach the part from almost any direction. Simultaneous five-axis lets the cutter tilt so a ball nose stays normal to a curved surface, which is the only practical way to finish an impeller blade or a deep contoured pocket in one setup. GreatLight has 16 simultaneous five-axis centers, with a Ø400 mm rotary table on the smaller machines.

The remaining capacity is size. Our largest travel is 4,000 × 400 × 150 mm, and the medium class runs 750 × 1,150 × 550 mm and 600 × 600 × 600 mm. If a part is bigger than the table, it is not a five-axis problem. It is a different process.

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  3-axisFlat faces reachable from one direction; lowest cost per part
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  4-axisIndexed sides, true position between features, one setup
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  5-axisTilted tool axis, contoured surfaces, fewer setups
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  Travel limits4,000 mm max length; check the part against the table first

General machining tolerance sits around ±0.05 mm on a well-fixtured aluminium part. Tightening to ±0.005 mm is possible, and we hold it on bores, spigots, bearing seats and mating faces, but it costs. The tighter you call, the more the process has to control: thermal growth, tool wear, clamping distortion, and the repeatability of the machine itself.

Surface finish follows the same logic. As-machined surfaces land at Ra 1.6–3.2 μm, which is fine for brackets and covers. A high-quality finish of Ra 0.8–1.6 μm needs a sharp tool, a light finishing pass and stable cutting. Fine finish at Ra 0.2–0.8 μm is a separate operation with small stepovers and often a polishing step after.

Not every dimension needs a tolerance. If a face is cosmetic, leave it general. If a hole locates a bearing, hold it tight. Over-tolerancing a whole drawing raises inspection time and scrap rate without improving function.

Inspection has to match the callout. We check 100% of parts before shipment: raw material check, in-process monitoring and final inspection, with reports on request. A ±0.005 mm bore checked with a caliper is not a measurement.

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  Standard±0.05 mm on most milled features
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  Tight±0.005 mm on bores and mating faces, with inspection cost
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  As-machinedRa 1.6–3.2 μm, fine for non-sealing surfaces
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  FineRa 0.2–0.8 μm needs a dedicated finishing pass

Aluminium is the default for prototypes and most enclosures. The 6061 grades machine cleanly, hold a good finish and take anodizing well. 7075 is stronger but gummier and tends to leave a poorer as-machined surface. 2024 machines well but corrodes without a coating.

Stainless 303 is the free-machining grade and the easiest of the 300 series. 304 and 316 are tougher, and 316L is common in medical and food-contact parts. Titanium TC4 (Ti-6Al-4V) cuts at roughly a quarter of the speed of aluminium and needs sharp tools and generous coolant, which shows up in the price.

Steel grades matter for strength. 1018 and 1045 are straightforward. 4130, 4140 and 4340 are used for shafts and stressed parts and may need pre-hardened or heat-treated stock, which is harder on the tool. Tool steel is usually machined soft and hardened afterward, with a finishing pass to correct the distortion.

Plastics behave differently. POM and PEEK hold tight tolerances well. ABS and PP cut easily but burr and move with temperature. Carbon fibre machines like a very abrasive plastic: carbide tooling wears quickly and dust extraction is mandatory.

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  Easy6061, 303 stainless, POM, brass C36000
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  Moderate7075, 304, 316L, 4140, PEEK
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  Hard on toolsTi-6Al-4V, Inconel, 4340, carbon fibre

CNC removes material, so it wastes it. For a thin-walled shell, a deep ribbed housing or a part that is mostly empty space, the cutting time climbs and so does the price. Die casting, injection moulding or sheet metal usually beat it once the annual volume passes a few thousand pieces.

Geometry sets another boundary. Tools are round and have a length. A blind internal corner with a sharp corner radius cannot be milled, because the cutter leaves its own radius behind. Deep narrow slots need long, thin tools that chatter. Undercuts need a special tool or a different process.

Cost behaves the same way. One prototype from a 20 mm plate is inexpensive because the setup is short and the material is standard. The same part in Inconel with a ±0.005 mm callout on every face is a different quote entirely. Setup amortizes over quantity, material does not.

That is why the honest answer to when to use CNC is: when the part is complex, the quantity is low to medium, the tolerance is tight, or the material is hard to form. Outside that window, look at casting, moulding or fabrication first.

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  High volumePast a few thousand parts, casting or moulding usually wins
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  Sharp internal cornersA round tool always leaves a radius; design a relief
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  Mostly airHollow parts waste cutting time and stock