CNC machining fundamentals explain how a CAD model becomes a finished part

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  CNC machining fundamentals explain the chain from CAD file to first cut
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  Axes decide which shapes are easy and which are expensive
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  Speed, feed, and depth set the surface and the tool life
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  Workholding is often the real limit on accuracy
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  What makes a part machinable, and what raises the cost
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  Which machining route fits which part

CNC machining means a computer moves a cutting tool along a path a programmer defined. No mold or die fixes the shape. The tool removes material until what is left matches the model. That is the whole idea, and it is also where the difficulty starts.

The chain has five links. A solid model defines the geometry. CAM software turns that geometry into toolpaths. A post-processor converts toolpaths into G-code for one specific machine. The machine executes the code, and inspection compares the result back to the model. Break any link and the part is scrap.

Every link carries tolerance. The model may be clean but the toolpath leaves too much stock. The post-processor may be right but the fixture lets the part move. Machining is not one operation. It is a stack of small errors that either stay inside the print or do not.

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  ModelGeometry with datums and tolerances that a machinist can actually measure.
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  ToolpathStock removal strategy, stepover, and tool engagement angles.
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  G-codeMachine-specific output from a verified post-processor.
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  InspectionMeasurement that closes the loop back to the model.

A three-axis mill moves X, Y, and Z. The tool always points down. That is fine for prismatic parts with features reachable from a few setup directions. Pockets, slots, holes, and flat faces cut quickly on three axes, and the setup is cheap.

A four-axis machine adds rotation around one axis, usually A. The part turns while the tool cuts. This suits cylindrical work with cross features: shafts, bushings, connectors, cam profiles. One rotary setup replaces several manual re-clamps, which removes accumulated position error.

Five-axis machines tilt the tool or the table on two rotary axes at once. The cutter can approach a surface from an angle instead of straight down. Undercuts, deep cavities, and contoured faces become reachable in one setup. The trade is programming time and machine cost.

More axes is not automatically better. A flat bracket with six drilled holes does not need five axes. Adding them raises the hourly rate and can slow the cycle. The honest question is whether the geometry is reachable without re-clamping. If it is, three axes wins.

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  3-axisPrismatic parts, simple pockets, fast setup, lowest rate.
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  4-axisCylindrical parts with cross holes or flats.
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  5-axisUndercuts, contoured surfaces, one-setup complex parts.
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  Mill-turnParts that combine turning and milling in one cycle.

Cutting speed is how fast the tool edge moves through the material, measured in surface meters per minute. Feed is how far the tool advances per tooth or per revolution. Depth of cut is how much material one pass removes. These three numbers, plus tool material and coolant, decide whether the cut is stable.

Aluminium 6061 runs fast, often 300 to 600 m/min with carbide and good chip evacuation. Stainless 316 is the opposite: it work-hardens, so a light rubbing pass dulls the tool fast. Feed it hard enough to cut under the hardened layer, keep speed moderate, and flood the cut with coolant.

Titanium Ti-6Al-4V burns hot and conducts heat poorly. Most of the heat goes into the cutting edge. That means lower surface speed, generous feed, and rigid setup. Chatter in titanium is expensive because the tool fails before the part does.

Surface finish follows the toolpath. A Ra 0.8–1.6 μm finish is normal for a well-run finishing pass. Ra 0.2–0.8 μm needs a finer stepover, a sharp tool, and often a separate finishing operation. Asking for a mirror finish on a roughing cycle is not a tolerance issue. It is a process mismatch.

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  AluminiumHigh speed, high feed, watch chip packing in deep pockets.
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  StainlessAvoid rubbing, control work hardening, flood coolant.
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  TitaniumLower speed, rigid setup, heat stays in the edge.
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  PlasticsSharp tools, air blast, avoid melting and burrs.

A machine can position to ±0.005 mm and still produce a bad part if the blank moves. Fixtures, vises, chucks, and vacuum plates hold the workpiece against cutting forces. Thin walls, long parts, and unsupported overhangs deflect under load even when the toolpath is perfect.

The usual fix is to plan the setup before the toolpath. Leave support material that gets removed in a later operation. Use soft jaws shaped to the part instead of a standard vise. For thin plates, vacuum workholding spreads the clamping force instead of concentrating it at three points.

Part size matters here. GreatLight runs three-axis machines with travels up to 4,000 × 400 × 150 mm, plus five-axis centers with a Ø400 mm rotary table. A part that fits the envelope but flexes in the middle is still a problem. Rigidity, not envelope, is the binding constraint.

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  Thin wallsSupport with sacrificial material or vacuum fixturing.
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  Deep pocketsUse long-reach tools with reduced radial engagement.
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  Long partsAdd intermediate supports to limit deflection.

Machinability is mostly about tool access. A cutter is a cylinder with a radius. Any internal corner smaller than that radius cannot be cut square. The fix is a corner radius slightly larger than the tool, or a relieved corner. Sharp internal corners force a tiny tool, slow passes, and a higher risk of tool breakage.

Deep holes are a related problem. A drill needs length-to-diameter ratio under about 10:1 for reliable chips. Beyond that, the drill wanders, chips pack, and the hole drifts. Deep pockets have the same issue with end mills, which deflect as the reach grows.

Tolerances should go where they matter. Marking a whole drawing ±0.005 mm when only two bores need it raises cost with no benefit. Datum choice matters too. If a tolerance references a surface that cannot be fixtured, the machinist has to guess, and guessing shows up in inspection.

Some features are better made a different way. Fine lattice, internal channels, and thin curved shells are usually additive or casting work. Machining them is possible in a few cases, but the cost curve turns steep.

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  Corner radiiMatch the radius to a standard cutter size.
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  Hole depthKeep length-to-diameter under 10:1 where possible.
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  ToleranceApply tight limits only to functional features.
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  DatumsReference surfaces a machinist can actually hold.