CNC machining becomes powerful when the process fits the part

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  Where the metal actually separates
• 2
  What ±0.005 mm really costs you
• 3
  3-axis, 4-axis, or 5-axis: the trade
• 4
  How the alloy changes the cut
• 5
  From Ra 3.2 to Ra 0.2 μm: what it takes
• 6
  Setup and tolerance comparison

CNC machining removes material with a rotating tool that shears metal into chips. The machine follows a programmed path, and how closely that path matches the drawing depends on the tool, the fixturing, and how rigid the whole setup is.

The cut itself is a controlled fracture. A carbide edge presses into the workpiece until the material yields, then slides forward and breaks the chip off. Heat leaves mostly through the chip, not the part. That is why feed rate matters more than spindle speed for tool life on most steels.

Every pass leaves marks. A sharp tool at a moderate feed leaves a finish near Ra 0.8–1.6 μm on aluminum. Push the feed too high and the tool deflects, the wall bows, and your ±0.005 mm callout is gone before inspection even starts.

So CNC machining becomes powerful not because the machine is fast, but because the cut is predictable. Repeat the same path 500 times and you get the same part, which is the property that matters in a production run.

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  Rigidity firstTool overhang under 4× diameter keeps deflection low.
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  Heat goes into the chipCoolant helps, but chip load controls temperature.
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  Finish follows feedHalving feed rate improves Ra more than raising rpm.

A ±0.005 mm tolerance is achievable on a rigid machine with sharp tooling, but it is not free. It changes how the part is held, how it is measured, and how many passes it takes. Working to that band means finishing passes of 0.1–0.2 mm radial depth and a probe check before the part leaves the vise.

The harder problem is usually datum control, not the machine. If two faces are cut in separate setups, the stack-up between them is what eats your tolerance. Cutting both faces in one 5-axis setup removes one link in that chain.

Material behavior sets a floor. Aluminum 6061 and 7075 hold ±0.005 mm well. Stainless 316 and titanium TC4 move more after cutting, so a part that measures in spec off the machine can drift overnight as internal stress releases.

For those alloys, a stress-relief step before finishing is often cheaper than scrapping a batch. On long, thin parts the same rule applies to wall thickness: under 0.8 mm walls will deflect under clamping pressure alone.

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  One setup, one datumFewer setups means fewer stack-up errors.
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  Measure in the machineProbing catches drift before unclamping.
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  Thin walls moveKeep walls above 0.8 mm unless you accept spring passes.

Five-axis work earns its cost when the part has features on faces that would otherwise need re-fixturing. Angled ports, undercuts, and contoured pockets on five sides are the usual triggers. If your part is a plate with holes on one face, 3-axis is faster and cheaper.

A 4-axis mill adds a rotary table, so you can cut around a cylinder without stopping. Shafts, flanges, and parts with evenly spaced features along an axis fit this setup well.

Simultaneous 5-axis lets the tool tilt, which shortens the effective overhang and lets you reach deep pockets with a shorter, stiffer tool. That is a rigidity benefit as much as a reach benefit.

The trade is programming time and inspection complexity. On a one-off prototype, the extra engineering can outweigh the setup savings. On a 500-piece run, the same programming cost spreads thin and the setup reduction wins.

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  3-axisPrismatic parts, single-face features, lowest cost.
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  4-axisCylindrical parts with index positions.
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  5-axisMulti-face geometry, deep pockets, short tooling.

Aluminum cuts fast and holds tolerance. Grades 6061 and 6082 machine cleanly at high spindle speeds, while 7075 gives higher strength at a modest cost in tool wear. Both are common in Houston-area oil and gas fixtures and in EV housings.

Stainless 303 and 304 are the workhorses. 303 machines easily because of added sulfur, but it welds poorly. 304 holds up better in corrosive service and machines slower. 17-4PH (SUS630) is the pick when you need strength plus corrosion resistance, and it responds well to a post-machining heat treat.

Titanium TC4 (Ti-6Al-4V) and Inconel sit at the difficult end. Both hold heat at the cutting edge, so tool life drops fast if you push speed. Low surface speed, generous coolant, and a rigid setup are the only reliable route.

Plastics like POM and PEEK behave differently again. They cut easily but move with temperature, so keep the coolant off and take light finishing passes. PEEK holds dimension better than POM after machining.

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  Aluminum6061, 6082, 7075 — fast, stable, easy to hold tolerance.
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  Stainless303 for machinability, 304 for corrosion, 17-4PH for strength.
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  Titanium and InconelLow speed, high coolant, rigid setup required.

As-machined finish lands around Ra 1.6–3.2 μm. That is fine for brackets, housings, and most structural parts. It is not fine for sealing faces or bearing bores.

Getting to Ra 0.8–1.6 μm means a finishing pass with a fresh edge and a lighter feed. Going below that, to Ra 0.2–0.8 μm, usually means a dedicated finishing tool and sometimes a second operation. It costs time and it costs tool changes.

Surface finish and tolerance interact. A polished face is easier to measure, so tight-tolerance parts often get a finer finish for metrology reasons rather than functional ones.

If a drawing calls for Ra 0.4 μm on a deep bore, ask whether the function needs it or whether a seal is being over-specified. Loosening that callout can cut cost without changing performance.

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  Ra 1.6–3.2 μmStandard machining, no extra pass.
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  Ra 0.8–1.6 μmFinishing pass, fresh tool, lighter feed.
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  Ra 0.2–0.8 μmDedicated finishing operation, higher cost.