Basic Guide to CNC Processing

CNC processing is subtractive. A cutting tool removes material from a solid workpiece along a path defined by a program. The program comes from CAM software, which converts a 3D model into toolpaths and then into G-code: coordinates, feed rates, spindle speeds, tool changes, coolant commands. The machine executes that code without an operator turning handwheels.

The sequence is short and repeatable. A blank is cut and faced. The operator loads it into a vise, chuck, or fixture and sets the work offset. Tools are loaded into the carousel and measured. The program runs, and the part is checked against the drawing. For a second operation, the part is flipped or moved to another machine, and the offset is set again.

Two things decide whether the part comes out right. The first is rigidity: how firmly the workpiece, fixture, and tool are held. Chatter, taper, and poor finish usually trace back to a weak setup, not to a weak program. The second is the datum strategy. If the drawing datums and the machining datums do not match, dimensions drift even when the machine is accurate to ±0.005 mm.

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  Design intent firstRead the drawing for function, not just dimensions. Which faces mate? Which bore carries the bearing?
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  Datums drive everythingSet machining datums from drawing datums, or dimensions stack up in the wrong direction.
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  Setup count is costEvery extra flip or fixture adds labor and error. Design for fewer setups when you can.

Milling cuts with a rotating multi-point tool while the workpiece moves or stays fixed. It suits pockets, slots, flats, bosses, and contoured surfaces. A 3-axis mill moves in X, Y, and Z only. It handles prismatic parts well when every feature is reachable from one direction, or from a few directions with refixturing.

Turning rotates the workpiece against a single-point tool. It produces cylindrical parts: shafts, bushings, fittings, pins, and threaded components. A mill-turn center combines both in one machine, so a turned part with cross-holes or milled flats does not need to move to a second setup. That reduces handling and keeps concentricity between the bore and the outside diameter.

Add a fourth axis, and the part can rotate around X or Y. Add a fifth, and the tool can tilt relative to the workpiece. Simultaneous 5-axis machining lets the cutter stay normal to a curved surface, which improves finish on impellers, turbine blades, and complex housings. It also reaches undercuts and deep pockets that a 3-axis setup cannot clean without a special tool.

Not every part needs five axes. A plate with drilled holes and a milled pocket is faster and cheaper on a 3-axis machine. Five-axis work pays off when the geometry is genuinely curved in three dimensions, when one setup must hold tight true position between features on different faces, or when a long tool would otherwise deflect.

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  3-axisPrismatic parts, plates, brackets, simple pockets. Fewest setups needed, lowest cost.
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  4-axisCylindrical parts with features around the circumference, or multiple faces in one setup.
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  5-axis simultaneousCurved surfaces, undercuts, deep cavities, and tight true position across faces.

Tolerance is the allowed variation on a dimension. Basic CNC processing holds ±0.005 mm on critical features when the setup is rigid and the material behaves. That is roughly ±0.0002 in. Not every dimension needs that. A clearance hole at ±0.1 mm is fine; a bearing bore at ±0.005 mm is not optional. The tighter the tolerance, the more likely the part needs a finishing pass, slower feeds, and more inspection time.

Surface finish is measured as Ra, the arithmetic average roughness. As-machined finish lands around Ra 1.6–3.2 μm. A high-quality machined finish is Ra 0.8–1.6 μm. Fine finishes reach Ra 0.2–0.8 μm, usually with a dedicated finishing toolpath or a secondary operation such as polishing. Call out finish only where it matters: sealing faces, sliding surfaces, optical mounts. A cosmetic finish on every face adds cost without adding function.

Material choice changes the whole calculation. Aluminum 6061 cuts fast and holds tolerances well. Stainless 316 work-hardens if the tool rubs, so the program must keep a steady chip load. Titanium Ti-6Al-4V generates heat at the cutting edge and needs lower speeds and generous coolant. Plastics such as POM and PEEK cut easily but move with temperature, so fixturing and measuring temperature matter as much as the toolpath.

Cost drivers are predictable. Tight tolerances, hard materials, thin walls, deep pockets, and features that need a second or third setup all raise the price. So does a surface finish callout on a face that never touches anything. If you want to reduce cost, start there: relax what does not function, and keep the tight callouts on the datums and mating features.

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  Tight where it functionsApply ±0.005 mm to mating features, not to every dimension on the sheet.
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  Finish with a purposeSealing and sliding faces need Ra 0.8–1.6 μm. Cosmetic faces rarely do.
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  Fewer setupsDesign so most features are reachable from one direction, or use mill-turn.

Material selection starts with the environment, not the machinist. Anodized 6061 works for enclosures and brackets. 7075 gives higher strength for aerospace fittings, though it machines differently and is less weldable. 17-4PH stainless gives corrosion resistance with high strength for medical and valve parts. Beryllium copper and C36000 brass suit electrical contacts and bushings. Magnesium AZ31B and AZ91D are used where weight matters most.

Finishing follows function. Anodizing builds a hard, corrosion-resistant oxide layer and can be clear, colored, hardcoat, or conductive. Electroless nickel gives uniform coverage on complex geometry. Zinc plating protects steel; silver and gold plating serve electrical contact resistance. Powder coating and black oxide handle appearance and mild corrosion. Bead blasting, tumbling, brushing, and polishing change texture and remove tool marks.

Laser marking and engraving add part numbers, logos, and traceability. Minimum character height is 1.5 mm, so plan the marking area before the design is frozen. A marking that is too small or placed on a curved surface will not read cleanly.

Some parts need more than machining. Heat treatment, passivation, and stress relief are common; they change dimensions slightly, so the machining allowance must account for them. When a part needs heat treatment after roughing, leave stock and finish after treatment. That sequence keeps final dimensions in tolerance.

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  Match material to environmentCorrosion, temperature, weight, and electrical properties decide the alloy.
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  Finish after heat treatmentHeat treatment moves dimensions. Rough first, treat, then finish machine.
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  Plan marking earlyMinimum character height is 1.5 mm. Marking area must be reachable.

Inspection starts before cutting. Raw material is checked against the certificate for grade and condition. During machining, in-process checks catch drift before a whole batch goes wrong. After machining, the part is measured against the drawing. GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring, and final inspection. Reports are available on request.

For a first article, agree on the inspection method before production. Calipers and micrometers cover most dimensions. A coordinate measuring machine handles true position, profile, and complex geometry. Surface roughness testers confirm Ra callouts. If a dimension is not measurable with the tools available, it cannot be verified, so flag it during DFM review.

The DFM step is where most problems get caught cheaply. A drawing with an unreachable feature, an impossible tolerance stack, or a finish callout that cannot be produced will surface in review, not on the shop floor. GreatLight returns a quotation and a free DFM analysis within 12 hours, so this feedback arrives before you commit to a design.

Traceability matters in regulated industries. ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022 cover quality management, automotive, medical devices, and information security. Uploads are kept secure and confidential, and an NDA is available on request.

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  First article agreementDecide the measurement method before the first chip is cut.
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  100% inspectionEvery part is checked before shipment, not just a sample.
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  DFM before productionFree DFM analysis with the quote catches design issues early.