A Close Look at CNC Machining Technology

A 3-axis mill moves the tool in X, Y, and Z while the part stays still. One setup, one face. If your part has features on four or five sides, you either flip it and re-zero, or you move up in axis count. Flipping adds a second setup, and every setup adds error to the stack.

A 4-axis machine adds a rotary table, usually Ø400 mm on our floor, so the part can index around one axis. Shafts with cross-drilled holes, cams, and parts with features at 90° to each other are a natural fit. The catch: the rotary axis is one more thing to indicate before the first cut.

A 5-axis center tilts the tool or the table in two rotary axes at once. Cutting stays in one setup, which is why impellers, turbine housings, and deep pockets with undercuts land here. We run 16 simultaneous 5-axis centers out of 127 machines, so the choice is never forced by availability.

Twenty-seven 3-axis machines and 12 four-axis mills handle the simpler work. Putting a flat bracket on a 5-axis machine does not make it more accurate. It makes it slower to quote and slower to run.

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  One face, flat features3-axis. Brackets, plates, housings with a single machined face.
• 2
  Features at 90°4-axis. Shafts with cross holes, cams, index-style work.
• 3
  Undercuts, compound angles5-axis. Impellers, turbine parts, contoured pockets.
• 4
  Very long partsMill-turn or large-travel 3-axis, up to 4,000 mm.

We hold ±0.005 mm, or ±0.0002 in, on features that need it. That number is not a blanket callout. It applies to a specific dimension on a specific feature, with the right material and the right setup. Put it on every dimension on a 300 mm aluminum plate and the quote will tell you what you asked for.

Temperature matters at this level. Aluminum expands roughly 23 μm per meter per degree Celsius. A part measured at 25 °C and used at 40 °C moves before it ever touches a load. If your tolerance is ±0.005 mm over 500 mm, you need to tell us the measuring temperature, not just the number.

Tool deflection sets the real floor. A long, thin end mill pushed hard will bend more than the tolerance band. We shorten the gauge length, take lighter radial cuts, and sometimes change the order of operations so the finishing pass runs on a part that is already stiff. That is where the tolerance actually comes from.

Surface finish and tolerance travel together. A Ra 0.2–0.8 μm finish usually means a fine finishing pass, which means time. As-machined at Ra 1.6–3.2 μm is fine for most brackets and covers. Only call the finer finish where a seal, bearing, or sliding surface sees it.

• 1
  ±0.005 mm / ±0.0002 inHeld on selected features, not the whole drawing.
• 2
  Ra 0.2–0.8 μmSealing faces, bearing bores, sliding surfaces.
• 3
  Ra 0.8–1.6 μmGeneral mating surfaces, cosmetic faces.
• 4
  Ra 1.6–3.2 μmCovers, brackets, non-contact surfaces.

6061-T6 aluminum cuts fast and holds tolerance well. 7075 is stronger but gummier, so it wants sharper tools and lighter cuts. Both are common here, along with 2024, 5052, 5083, 6063, 6082, and ADC12 for die-cast blanks.

Stainless is a different conversation. 303 machines cleanly. 304 and 316 work-harden if the tool rubs instead of cuts, so feed and speed need to stay in the cut. 17-4PH in the H900 condition is hard on tooling, but it is the right answer for a high-strength shaft. Titanium TC4 (Ti-6Al-4V) is worse still: low thermal conductivity, so heat goes into the tool, and climb milling with coolant is not optional.

Plastics are not a soft fallback. POM and PEEK machine well but move with temperature. ABS and PC are fine for prototypes but will not hold ±0.005 mm over a long dimension. Carbon fibre eats tool edges and needs dust control. If you need a plastic part at tight tolerance, say so early and we will pick the grade.

Setup is where most of the variance lives. We check raw material on arrival, monitor in-process, and inspect 100% before shipment, with reports on request. None of that replaces a clean setup. A fixture that lets the part move by 0.01 mm under cutting load will show up in the final numbers no matter how good the inspection is.

• 1
  Aluminum6061-T6, 7075, 2024, 6082. Fast, stable, holds tolerance.
• 2
  Stainless303, 304, 316L, 17-4PH. Watch work-hardening and tool wear.
• 3
  Titanium / InconelTC4, Inconel. Slow speeds, heavy coolant, rigid setup.
• 4
  PlasticsPOM, PEEK hold well. ABS, PC move with heat.

CNC is subtractive, so it starts from a solid block and removes what you do not want. That is wasteful for a thin-walled enclosure with no tight features. Sheet metal fabrication bends and cuts that part faster and cheaper. Die casting wins on a housing you need in the thousands with moderate tolerance.

3D printing is the right call for a geometry check before tooling, or for a part with internal channels a cutter cannot reach. Vacuum casting covers small runs of a urethane part that looks and feels like production. None of these replace CNC when the drawing calls for ±0.005 mm on a metal face.

The mixed case is common. A machined aluminum prototype validates the design, then the production part moves to die casting with only the critical faces machined. We do both under one roof, so the DFM notes from the prototype carry into the casting tool.

Tool reach is the hard limit people forget. A deep pocket narrower than the cutter is long will not clean out. A hole on a face that the spindle cannot point at needs a different axis count or a different process. Send the model and we will tell you which.

• 1
  Thin-wall enclosuresSheet metal is faster and cheaper.
• 2
  Thousands of identical housingsDie casting, with critical faces machined after.
• 3
  Geometry check only3D printing before any tooling is cut.
• 4
  Unreachable internal channelsAdditive or casting, not a cutter.