How Its Made CNC Machines: 7 Steps From Raw Stock to Finished Part

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  Key takeaways
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  What Happens Before a CNC Machine Cuts Metal
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  How Its Made CNC Machines: CAM Programming and Workholding
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  Cutting: Choosing 3-Axis, 4-Axis or 5-Axis
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  Finishing and Inspection: Where Good Parts Get Ruined
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  Step by Step: How Its Made CNC Machines Run a Job
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  Which Machine and Process Fits the Part

The first question on any job is not which machine, it is what the drawing actually requires. We read the tolerance block, the surface finish callouts, the datum scheme and the material spec together. A part drawn at ±0.005 mm on every dimension is a different job than the same geometry with three critical dimensions and general tolerances elsewhere. Marking the critical few saves setup time and money.

Stock choice follows geometry. For a 6061-T6 bracket we typically leave 0.5–1.0 mm per side on turned diameters and 1.0–2.0 mm on milled faces, more on thin webs that will spring. Cast or forged blanks may already be near net shape, but the skin is hard and abrasive, so the first pass has to cut under it. Inconel and Ti-6Al-4V get extra allowance and slower speeds because they work-harden at the cut.

This is also when DFM feedback lands. A 2 mm deep pocket with a 2 mm corner radius and a 6:1 depth-to-diameter ratio is machinable. Push the same pocket to 12:1 and the tool will chatter or snap. We flag it before programming, not after the parts are on the bench. Quotation plus that free DFM review usually comes back within 12 hours.

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  Read the tolerance block firstSeparate critical dimensions from general tolerances.
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  Match stock to geometry0.5–2.0 mm allowance per side on milled and turned surfaces.
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  Flag hard skin earlyCastings and forgings cut differently from bar stock.

In CAM we build the toolpath around the fixture, not the other way around. Roughing removes most of the stock with a large-diameter cutter at high feed, leaving 0.2–0.5 mm for semi-finishing. Finishing passes then use smaller stepovers and lower feed to hit the surface callout. For a Ra 0.8–1.6 μm finish on aluminum we run finish passes at 0.1–0.2 mm stepover with balanced radial and axial engagement.

Tool selection is a trade. A long tool reaches deep pockets but deflects. As a rule, keep the flute length under 4× the cutter diameter for finishing, and under 6× only if the toolpath is light. Corner radii should be at least one third of the pocket depth where the design allows it. These are the choices that decide whether the bore comes out round or the wall comes out tapered.

Workholding gets decided at the same time. A 5-axis job often needs a dovetail or a self-centering vise on a rotary table so the part can be reached from five faces without re-clamping. For thin plates we use vacuum chucks or sacrificial tabs, because clamping force alone can bend a 2 mm wall by more than the tolerance. If the part flexes under the clamp, no cutter compensation will fix it.

Simulation runs before the machine does. We verify stock removal, tool holder clearance and fixture collision in the CAM environment. A crash costs a spindle and a week. Ten minutes of simulation costs nothing, and it is standard on every 5-axis job.

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  Rough heavy, finish lightLeave 0.2–0.5 mm for semi-finish, 0.1–0.2 mm stepover for finish.
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  Keep tools shortFinishing flute length under 4× diameter where possible.
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  Fixture for rigidityDovetail, vacuum or tabs instead of heavy vise clamping on thin walls.

The machine choice is a geometry decision. A flat plate with holes and pockets on one face is a 3-axis job. Add features on the side and you need a fourth axis, or a second setup that introduces a new datum error. Add compound angles, undercuts or contoured surfaces on five faces and simultaneous 5-axis is the only way to hold position without stacking setups.

Setup count is the real cost driver. Every re-clamp adds a datum transfer and its own error. On a part with a true position callout of 0.05 mm between two features on different faces, three separate setups rarely hold. Cutting all five faces in one fixturing on a 5-axis center keeps one datum for the whole part.

Speeds and feeds follow the material, not a chart. Aluminum 6061 cuts at high surface speed with generous chip load. 17-4PH stainless and Inconel want lower surface speed, heavier feed per tooth and flood or high-pressure coolant to keep the edge from rubbing. Chatter on stainless usually means the feed is too light, not too heavy.

Titanium adds heat management. Ti-6Al-4V conducts heat poorly, so the cutter absorbs it. We keep speeds low, use high-pressure coolant and never let the tool dwell in the cut. As-machined titanium surfaces also work-harden, so a second pass over the same surface at the same depth often cuts worse than the first.

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  3-axisFlat parts, single-face features, simple holes and pockets.
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  4-axisCylindrical or prismatic parts with side features that must share one datum.
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  5-axisCompound angles, undercuts and five-face geometry in one setup.

Finishing is not cosmetic. Anodizing builds an oxide layer that grows into and out of the surface, so a 0.02 mm growth on a ±0.01 mm bore pushes the part out of tolerance. Plating adds thickness on all sides. Powder coating adds tens of microns. Threads, bores and mating faces should be masked, and the drawing needs to say so.

Surface prep also matters. Bead blasting before anodizing gives a matte look but removes a small amount of material and can round sharp edges. Tumbling softens burrs on aluminum but does nothing for stainless in a reasonable cycle. Brushing leaves directional grain that shows under raking light. Pick one finish and specify the Ra, not just the name.

Inspection closes the loop. On a job with a ±0.005 mm tolerance, we verify against the CAD model on a CMM rather than trusting calipers. Calipers read to 0.02 mm at best and depend on operator feel. For bores, use pin gauges or an air gauge. For surface finish, a portable roughness tester confirms the Ra callout.

Records matter for regulated work. Aerospace, medical and automotive programs need traceability from raw material lot to finished part. We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. Inspection reports and material certs ship with the parts when the order requires them. Uploads stay confidential, and an NDA is available on request.

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  Mask before coatingThreads, bores and mating faces should be protected from finish growth.
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  Specify Ra, not just the nameBead blast, tumble and brush all change the surface differently.
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  Verify on a CMMCalipers are not enough below ±0.02 mm.