CNC Machining Experience Summaries: What Is CNC Machining?

The process means a cutting tool follows a program. The machine moves on axes driven by servomotors, and the controller reads G-code to decide where the tool goes, how fast it feeds, and how fast the spindle turns. Nothing about the shape depends on an operator's hand at the moment of the cut. That is the whole difference from manual work, and it is why a second run matches the first.

The practical consequence is repeatability. Once the program and the fixture are proven, part 500 sits in the same place as part 5. Tolerance follows from that geometry: we hold ±0.005 mm (±0.0002 in) on critical features when the setup supports it, and Ra 0.8–1.6 μm is a normal as-machined finish. Tighter surface values exist, but they come from a second operation, not from the same pass.

Where the numbers come from matters more than the numbers themselves. A tolerance is only meaningful together with the datum it refers to and the inspection method that verifies it. If a drawing calls out a 0.01 mm true position without a datum scheme, the shop has to guess, and the guess is not free.

The axis count is a fixturing decision before it is a cost decision. A part with features on five faces, or with compound angles, gets fewer setups on a simultaneous 5-axis center. Fewer setups means fewer datum transfers, and every datum transfer is a place where stack-up grows. We run 16 simultaneous 5-axis machining centers for exactly that class of work.

Four-axis work covers the middle ground. A shaft with cross holes, a housing with features around a bore, a part that needs indexing rather than full contouring. Twelve four-axis mills handle that, and the programming is simpler, so the quote is usually lower than the same part scheduled on a five-axis machine.

Three axes still cuts most parts. Twenty-seven three-axis machines handle prismatic work with features on one or two faces, and the setup is fast because the part sits flat and the tool comes down. If your part is a plate with pockets, holes, and a profile, three axes is not a compromise. It is the correct machine.

The wrong call is paying for simultaneous motion on a part that never leaves one orientation. We see this regularly in incoming RFQs. Ask which faces carry toleranced features before you ask which machine is best.

Aluminium 6061 and 7075 cut cleanly and hold tight tolerances without much drama. Stainless 304 and 17-4PH work-harden if the tool dwells, so the feed has to stay aggressive enough to cut under the hardened layer. Titanium TC4 (Ti-6Al-4V) moves under cutting forces and generates heat at the edge, which is why we slow the surface speed and accept a longer cycle. Inconel is slower still.

Thin walls are the most common geometry problem. A 0.5 mm wall on a 60 mm tall aluminium pocket will deflect and chatter. On stainless or titanium the same wall is worse. A practical floor is around 1 mm for aluminium and 1.5 mm for stainless at moderate height, and the ratio of wall height to thickness matters more than the absolute number.

Burrs are not cosmetic. A burr on a sealing face or a cross hole changes how the part seats. We deburr in process rather than at the end, because a burr left during roughing can push the semi-finish pass off the nominal surface. Bead blasting and tumbling remove the rest, and laser marking with a minimum character height of 1.5 mm stays legible after both.

A good fixture is often the real deliverable. When a part has no natural flat face, we build a soft jaw or a dedicated plate so the first operation establishes a datum that the rest of the part can trust. That fixture work is why a first article may take longer than the production pieces that follow it. It is also why changing one dimension after approval can reset the whole setup.

Inspection closes the loop. We check incoming material, monitor in process, and inspect 100% before shipment, with reports on request. A CMM report on a first article tells you whether the process is centered, not just whether one part passed. If the process is drifting, the report shows the trend before the parts go out of tolerance.

Some jobs we decline. A part with a 0.02 mm wall, an internal feature that no tool can reach, or a surface finish requirement that needs a process we do not run. Saying no early costs you a day. Saying no after the setup is built costs you a week.

Runs from one prototype to 10,000+ pieces go through the same shop. There is no minimum order quantity, and the same inspection applies whether we ship one part or a thousand.