CNC machining GA: how metal parts move from model to measured part

• 1
  What CNC machining GA changes about the part, not the drawing
• 2
  Tolerance, finish and the cost curve
• 3
  Why 5-axis setups reduce error stacking
• 4
  Material choice drives tool wear and stability
• 5
  Inspection is part of the process, not a final gate
• 6
  Which machining route fits your part

CNC machining GA is subtractive manufacturing driven by a toolpath file. The geometry comes from your CAD model, but the finished part comes from a chain of decisions the machine cannot make on its own: tool selection, cutting order, workholding, coolant, and stock allowance. Change any one of those and the same program produces a different part.

That is why a print alone does not define a machined part. Two shops can both hit the nominal dimensions on a drawing and still ship parts that assemble differently. The difference sits in surface texture, burr condition, residual stress, and how tightly the datum scheme was controlled during each setup.

The practical consequence: when you specify a part, you are specifying a process window, not just a shape. Useful questions are how many setups the part needs, which faces can be reached in one orientation, and whether any feature forces the part to be re-fixtured mid-cut.

A part that needs five setups costs more than a part that needs two, even when the geometry is similar. Setup count, not part size, is usually the first cost driver.

• 1
  Design intent firstWhich dimensions carry function, and which are only reference?
• 2
  Datums drive repeatabilityEvery re-fixturing step adds a stacking error.
• 3
  Stock is not freeExtra allowance adds cycle time on every pass.

Tolerance and surface finish move together. A face cut with a sharp tool and light finishing pass lands around Ra 0.8–1.6 μm. Push for Ra 0.2–0.8 μm and you add a separate finishing operation, sometimes a different machine, and often a polishing step after that.

Our floor holds ±0.005 mm on critical features when the part geometry allows it. That number is a capability, not a default. Applying it to every dimension on a print multiplies inspection time and slows the cycle, because the operator has to prove each one.

Here is the honest trade: a part with three tight bores and open tolerances everywhere else is cheap. A part where every dimension is tight is expensive, even if the shape is simple. Concentrate tolerances on mating surfaces, bearing seats, and sealing faces.

Surface texture also controls function. A rough face on a hydraulic manifold leaks. A mirror finish on a bracket wastes money. Match finish to the job, not to a habit.

• 1
  Ra 1.6–3.2 μmAs-machined, fine for brackets and covers.
• 2
  Ra 0.8–1.6 μmStandard for mating faces and bores.
• 3
  Ra 0.2–0.8 μmSealing and sliding surfaces; adds a finishing step.

Every time a part leaves its fixture, a small error is baked in. Re-clamping on a new datum repeats that error. On a part with six faces to machine, three-axis work might need four or five setups. Five-axis work can reach most of those faces in one or two.

Simultaneous 5-axis machines tilt both the tool and the table, so a single tool can approach a face from an angle that would otherwise require a re-fixture. For parts with compound angles, undercuts, or features on multiple planes, this is not a speed trick. It removes a whole class of positional error.

The catch: 5-axis programming takes longer and the machine is more expensive to run. It pays off on complex geometry and on parts where position between faces matters. On a flat plate with holes, three-axis is faster and cheaper.

We run 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. The right machine is the one that finishes the part in the fewest setups, not the most capable one.

• 1
  One setup, one datumFewer re-clamps means less accumulated error.
• 2
  Compound anglesTilted tool axis reaches faces a 3-axis setup cannot.
• 3
  Watch the overheadSimple prismatic parts do not need 5 axes.

Aluminium 6061 cuts fast and holds tolerance well. It is the default for prototype housings, brackets, and fixtures. 7075 is stronger but gummier; it needs sharper tools and more attention to chip evacuation. 2024 behaves differently again and tends to move after machining if stock is removed unevenly.

Stainless 303 machines cleanly because of its sulfur content, but it is not ideal for every corrosive environment. 304 and 316L are tougher on tools and generate more heat. 17-4PH adds a heat-treat step, which means the part may need a second machining pass after hardening.

Titanium Ti-6Al-4V and Inconel are in a different category. Low thermal conductivity keeps heat in the cutting zone, so tool life drops and cycle times rise. These materials are machinable, but they change the cost structure of the part.

Plastics are their own problem. POM and PEEK hold dimension well; ABS and PP flex under clamping pressure and can spring back after the vise releases. Climb milling and light passes help. Deep pockets in soft plastic need support or the wall will deflect.

• 1
  Aluminium 6061-T6Best balance of speed, finish and stability.
• 2
  Stainless 316LCorrosion resistance, more heat, slower cycles.
• 3
  Ti-6Al-4VStrong and light, but hard on tooling.

A part that is measured only at the end is a part you cannot correct. In-process checks catch drift while the part is still in the machine. That matters on long runs, where a tool wear offset that is fine on part 5 is out of tolerance by part 40.

We inspect raw material before cutting, monitor during the run, and do a final check before shipment. Reports are available on request. On a first article, that documentation is usually worth more than the part itself, because it tells you whether the process is stable.

The measurement method has to match the tolerance. A caliper is fine for ±0.1 mm. A tight bore needs a bore gauge or a coordinate measuring machine. If the inspection method is less capable than the tolerance, the number on the report is not meaningful.

For regulated work, process control matters as much as the measurement. IATF 16949, ISO 13485, ISO 9001 and ISO 27001 shape how records are kept and how changes are approved. They do not change the cutting, but they change what you can prove afterward.

• 1
  Raw material checkGrade and condition verified before the first cut.
• 2
  In-process monitoringCatches tool wear drift before parts go out of spec.
• 3
  Final inspection100% before shipment; reports on request.