What CNC processing professionals actually control

• 1
  Where CNC processing professionals really decide tolerance
• 2
  Axis count, fixturing and how the part is held
• 3
  Material behavior that changes the process
• 4
  Inspection, finish and the last 5 percent
• 5
  Which setup fits the part

A tolerance on a drawing is a request. What the machine delivers is the result of the whole setup: spindle condition, tool holder runout, thermal drift, material batch, and how the part is held. Shops quoting ±0.005 mm without asking about the part are quoting a number, not a process.

The practical limit is not one value. A 4,000 mm long aluminum extrusion and a 40 mm stainless fitting behave differently on the same machine. Long parts move with temperature and with their own weight. Short, stiff parts hold size more easily. CNC processing professionals ask about geometry before they quote a tolerance.

Temperature is the quiet variable. Aluminum grows roughly 23 µm per meter per °C. A shop that machines a 500 mm part in a warm afternoon and inspects it in a cold morning can lose 10–15 µm without touching a single offset. Controlled shops keep the floor stable and let parts rest before final inspection.

So the ±0.005 mm figure is real, but it comes with conditions. It applies to features the shop can reach, measure and hold. Anyone promising it on every dimension of a thin-wall part is selling confidence, not capability.

More axes do not automatically mean better parts. A 3-axis machine with a good vise and a clean fixture will beat a 5-axis machine on the wrong setup. The question is how many times the part has to move. Every re-fixture adds a new datum error and a new chance to scratch a finished face.

That is where simultaneous 5-axis earns its place. Complex angles, undercuts and deep pockets can be cut in one setup. GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, so the setup can match the part instead of the other way around.

Fixturing is the part of the job most quotes hide. Soft jaws machined to the part profile, vacuum plates for thin panels, custom tombstone plates for repeat runs. A shop that plans fixtures in the DFM stage removes a whole class of scrap before the first chip.

Watch for the part that needs a fixture nobody wants to build. Very thin walls, parts with no flat face, or geometry that only exists on one side. These are the jobs where an experienced shop will tell you the honest option: change the design, or accept a longer setup.

Aluminum 6061 and 7075 cut fast and hold size well. They also burr, and thin sections can distort after machining as residual stress releases. A stress-relieved blank or a light roughing pass before finishing usually solves it.

Stainless 303 and 304 work-harden. A dull tool rubbing instead of cutting will harden the surface and kill the next pass. Feed and speed matter more here than on aluminum. 17-4PH in the H900 condition is harder again, and shops often prefer to machine it before aging when the drawing allows.

Titanium Ti-6Al-4V and Inconel sit at the other end. Low thermal conductivity pushes heat into the tool, so tool life drops and cycle time climbs. These are the jobs where a shop should quote more cautiously, use high-pressure coolant, and plan for more inspection.

Plastics behave like a separate discipline. POM and PEEK move with temperature, ABS and PC can chip, and carbon fibre wears tools quickly. The right answer for a plastic part is often a different cutter geometry and a slower finishing pass, not a different machine.

A part is not finished when the last pass ends. It is finished when someone has measured the features the customer cares about. GreatLight inspects 100 percent of parts before shipment, with a raw material check, in-process monitoring and a final inspection. Reports are available on request.

Surface finish is tied to tolerance. Ra 0.2–0.8 μm takes a fine finishing pass and often a polished tool. Ra 1.6–3.2 μm is a normal as-machined result. Asking for a mirror finish on a large face costs time, and the drawing should say which faces actually need it.

Post-processing changes size. Anodizing adds a few micrometers per surface and can shift a tight bore. Hardcoat adds more. Plating and powder coating change dimensions too. The shop has to know whether the tolerance applies before or after the coating, and mask the surfaces that must stay bare.

This is the step where CNC processing professionals separate themselves. Not the spindle speed. The willingness to hold a part back, re-measure it and tell the customer why.