Basic Knowledge of CNC Processing: How Metal Actually Comes Off

• 1
  What happens where the tool meets the material
• 2
  3-axis, 4-axis, or 5-axis: what each one can and cannot reach
• 3
  What ±0.005 mm really costs
• 4
  How material choice changes the process
• 5
  Where accuracy is won or lost before the spindle turns
• 6
  Choosing a process by geometry and tolerance

CNC processing is subtractive. A rotating cutter or a rotating workpiece meets a stationary edge, and the material in front of that edge shears away as a chip. Everything else on the machine exists to hold that meeting point steady: the bed, the ballscrews, the spindle bearings, the tool holder, the fixture.

Three variables define the cut. Surface speed is how fast the edge travels through the material. Feed per tooth is how much material each flute takes. Depth of cut is how deep the tool engages. Change one and the other two must follow, or the tool rubs instead of cutting.

Heat is the visible result. In aluminium, most heat leaves with the chip, so the part stays cool and you can run fast. In titanium and stainless steel, heat stays in the edge and the part. Slow the surface speed, keep the feed per tooth high enough to stay under the work-hardened layer, and use plenty of coolant.

Chip shape tells you whether the cut is healthy. Long curled chips in 6061 mean the feed is adequate. Fine powder in 304 means the tool is rubbing. Blue chips in steel mean the speed is too high for the coolant supply. Read the chips before you touch the offset.

A 3-axis mill moves the table in X and Y while the spindle moves in Z. Every feature you machine must be reachable from the top, or you flip the part and set a second origin. That second setup is where most positional error enters a job.

A 4-axis mill adds a rotary table, usually Ø400 mm class. The part turns about one axis, so you can machine four faces plus the ends without re-fixturing. Useful for shafts, manifolds, and parts with pockets on several sides.

A simultaneous 5-axis center tilts the tool as it moves. This is what makes undercuts, deep cavities with short cutters, and impeller blades practical. The trade-off is programming time and the need for a post-processor that matches the machine kinematics.

Turning is a different family. The workpiece spins and a single-point tool feeds along it. Mill-turn centers combine both, so a part that needs a turned OD and milled flats can finish in one setup. For parts under Ø60 mm with tight concentricity, that single setup is often the deciding factor.

Tolerance is not a single number for the whole part. It applies to the features you dimension. A drawing with ±0.005 mm on one bore and ±0.1 mm on everything else tells the shop where to spend time. Marking the whole print ±0.005 mm raises cost without improving function.

The achievable number depends on the feature. A bored hole in aluminium can hold ±0.005 mm with a good boring head and a warm machine. The same callout on a 300 mm long slot in 316 stainless is a different problem, because thermal drift and tool deflection grow with length.

Surface finish and tolerance travel together. Turning to Ra 0.8–1.6 μm is routine. Pushing to Ra 0.2–0.8 μm usually needs a finishing pass with a small nose radius, a sharp insert, and a stable setup. If the print does not need that finish, ask for it anyway and pay for it.

Stack-up matters more than any single tolerance. If three features locate off one datum and each carries ±0.02 mm, the assembly sees up to ±0.06 mm. Calling out a tighter datum scheme is often cheaper than tightening every feature.

Aluminium 6061-T6 is the default for prototypes and enclosures. It cuts fast, holds a good finish, and anodizes cleanly. 7075 gives higher strength for aerospace brackets but is less weldable and more prone to stress movement after roughing.

Stainless 304 and 316 work-harden. If the tool dwells, the surface hardens and the next pass cuts through hardened metal. The fix is constant feed, no rubbing, and a cutter that stays sharp. 17-4PH can be machined in the solution-treated state and then aged to reach strength.

Titanium Ti-6Al-4V and Inconel sit at the hard end. Cutting speeds drop, tool life drops, and the machine time rises. These are the parts where a 5-axis setup pays for itself, because one accurate setup beats three slow ones.

Plastics behave differently again. POM and PEEK machine to tight tolerance but move with temperature. ABS and PC are soft and prone to burrs, so a sharp cutter and a light finishing pass matter more than spindle speed. Carbon fibre needs diamond-coated tooling or the edge wears in minutes.

A rigid setup beats a fast spindle every time. Workholding that lets the part vibrate will chatter no matter how good the tool is. For thin walls, support the back side or leave a sacrificial web and remove it in a later pass.

Datum choice drives repeatability. Pick a face and two holes that exist in every operation, and reference everything to them. If the first operation has no good datum, machine one in the first setup and use it for the rest.

Thermal drift is real. A machine that has been sitting cold will move as the spindle warms. For tight work, run a warm-up cycle and check the first part before releasing the run. In-process probing catches drift that a cold first-article check misses.

Inspection closes the loop. We check raw material on arrival, monitor dimensions during the run, and inspect 100% of parts before shipment. Reports are available on request. A first article that passes and a process that drifts will still ship bad parts, so the in-process check is the one that matters.