What Is CNC Machining? A Working Explanation

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  What is CNC machining on the shop floor
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  3-axis, 4-axis and 5-axis: what the extra axes buy you
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  What tolerance and surface finish CNC can hold
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  What CNC does well and what it does badly
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  Choosing the right CNC setup for the part

CNC stands for computer numerical control. A CAM programmer converts a 3D model into toolpaths, and those toolpaths become G-code: a list of coordinates, feed rates, spindle speeds and tool changes. The machine controller reads that list and moves the axes to match. Nothing about the cut depends on an operator turning a handwheel, which is why the same program run twice produces the same geometry twice.

The cutting itself is ordinary metal removal. A spinning cutter, usually carbide, is pushed into the workpiece at a set feed per tooth. Each tooth bites a chip of a known thickness. If the chip is too thin, the tool rubs and work-hardens the surface. If it is too thick, the tool deflects and the wall tapers. Feed and speed are chosen together, not separately.

Three numbers describe the machine envelope: X, Y and Z travel. A compact mill might offer 500 × 500 × 450 mm. Our large gantry machines reach 4,000 × 400 × 150 mm, which covers long extrusion profiles and frame rails. A part that fits the envelope can still fail if the tool cannot reach a feature, so reach is checked before the quote, not after.

Rigidity sets the real limit. A heavy cast frame and preloaded linear guides hold ±0.005 mm on a stable cut. A thin floor, a deep pocket, or a tall thin rib will move under cutting force no matter how good the machine is. We look at wall thickness and depth-to-width ratio first when we review a drawing.

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  SubtractiveMaterial is removed, not added or formed.
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  Program drivenGeometry comes from the file, not the operator.
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  RepeatableThe same program repeats the same cut.

A 3-axis mill moves the table in X and Y and the spindle in Z. Every feature is cut from the direction the tool points. That is fine for plates, brackets, housings with open faces and parts you can reach from six sides in separate setups. Most machined parts in the world are 3-axis parts, and they are the cheapest to run.

A 4-axis machine adds a rotary table, usually Ø400 mm or smaller, that indexes or turns the part. This removes one or two setups and lets you cut around a cylindrical part in one program. Shafts with cross holes, cams, and parts with features on four sides fit here. The tool still points the same way, so undercut features stay out of reach.

A 5-axis machine adds a tilting head or a trunnion, so the tool can approach the part from an angle. That solves three problems: undercuts and compound angles, short tools for deep cavities, and one-setup finishing of complex surfaces. Short tools deflect less, so a 5-axis cut often holds tighter tolerance on a deep feature than a 3-axis cut with a long tool.

The trade-off is cost and programming time. Five-axis toolpaths take longer to prove out, and not every part needs one. We run 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Matching the part to the right machine is a bigger cost lever than negotiating the hourly rate.

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  Choose 3-axisFlat plates, open pockets, six-sided access.
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  Choose 4-axisCylindrical parts, cross holes, fewer setups.
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  Choose 5-axisUndercuts, compound angles, deep cavities.

Tolerance is a band, not a number. A drawing marked ±0.005 mm on every dimension will cost far more than one that reserves the tight band for the features that need it. Datum callouts matter as much as the value. If two tight features are dimensioned from different datums, the machine has to split the error between them, and scrap risk rises.

Surface finish follows the tool. As-machined surfaces land around Ra 1.6–3.2 μm. A finishing pass with a smaller stepover reaches Ra 0.8–1.6 μm, which is a common target for sealing faces and bearing bores. Fine finishing at Ra 0.2–0.8 μm needs a dedicated pass, sharp tooling and a stable setup, so we quote it as its own operation.

Material changes the answer. Aluminium 6061 and 7075 cut clean and hold tight bands well. Stainless 316 and 17-4PH work-harden, so light rubbing passes are avoided. Titanium TC4 and Inconel generate heat at the cutting edge and spring back after the pass, which pushes practical limits wider than aluminium.

Inspection closes the loop. We check incoming material, monitor in process and inspect 100% before shipment, with reports on request. A tolerance that cannot be measured repeatably is not a tolerance. If your drawing calls for a band tighter than the gauge can resolve, the number will not mean anything on the receiving dock.

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  Reserve tight bandsTighten only the features that function.
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  Name one datumStacked datums split the error.
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  Match materialTitanium and Inconel cut wider than aluminium.

CNC wins when the part needs accurate geometry from a solid block: prototype housings, fixture plates, manifolds, impellers, gear blanks, medical instrument bodies. Setup cost is low relative to tooling, so one piece is economic. We hold no minimum order quantity, from a single prototype to 10,000+ part runs, and parts ship in 3–5 days on typical jobs.

CNC loses on hollow thin-wall shapes that a mould or a die can form in one shot. A die-cast or injection-moulded shell at 200,000 pieces per year will beat machining on unit cost every time. Machining also struggles with very soft, gummy plastics that smear instead of chip, and with parts whose internal cavities cannot be reached by any tool geometry.

Material choice is wide. We cut aluminium 6061, 2024, 5052, 6082 and 7075, stainless 303, 304, 316L and 17-4PH, steels 1018, 1045, 4140 and 4340, copper and brass grades, titanium TA2 and TC4, Inconel, magnesium, plus ABS, POM, PEEK, PC and carbon fibre. Each family has its own feeds, and mixing them in one quote hides that.

Finishing decides how the part reads. Anodizing in clear, colour, hardcoat or conductive, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing are all available. Laser marking holds a minimum character height of 1.5 mm, so plan part numbers accordingly.

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  Good fitAccurate geometry from solid stock, low volume to mid volume.
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  Poor fitHigh-volume hollow shells, unreachable internal cavities.
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  Plan marking1.5 mm minimum character height for laser marks.