CNC Machine Wiki: Introduction

• 1
  What a CNC Machine Actually Does
• 2
  From CAD to G-code: The Information Chain
• 3
  3, 4, and 5 Axis: What the Numbers Mean
• 4
  Tolerance, Surface Finish, and Size Limits
• 5
  How Material Choice Changes the Process
• 6
  Workholding and Setup: Where Accuracy Is Won
• 7
  Choosing the Right Machine for the Part

CNC stands for computer numerical control. The machine removes material from a solid blank by driving a rotating cutter along a path defined by numbers. There is no mold and no pattern. The shape lives in a file, and the machine turns that file into motion.

The blank can be aluminium plate, stainless bar, titanium forgings, or engineering plastic. The cutter is harder than the work material, and it peels away thin layers until the remaining geometry matches the CAD model. This is subtractive manufacturing. A 4,000 mm gantry mill and a compact 500 mm mill follow the same logic, only the scale changes.

Accuracy is built from several links. The machine structure must be stiff, the spindle must run true, the tool must stay sharp, and the thermal state must be stable. If one link moves, the part moves. A common mistake is to blame the controller when the real cause is tool wear or a loose fixture.

The practical takeaway: CNC is best for parts with tight tolerances, complex geometry, or low-to-medium volumes. It becomes expensive when you need thousands of identical simple parts, because each one still consumes machine time. For those, casting or molding wins on unit cost.

The chain starts with a CAD model. A CAM programmer selects stock size, workholding, and cutting tools, then generates toolpaths. The output is G-code, a list of coordinates, feed rates, spindle speeds, coolant commands, and tool changes. The controller reads this list line by line.

Feeds and speeds follow the material. A 10 mm carbide end mill in 6061 aluminium may run at 8,000 rpm and 2,500 mm/min. The same cutter in 316 stainless might run at 1,200 rpm and 300 mm/min with heavy coolant. Wrong numbers cause chatter, poor finish, or broken tools. Speeds matter as much as the machine.

Tool access drives the plan. A deep pocket with a 90° internal corner needs a small cutter or a different setup. A part with features on five faces needs either multiple fixtures or a machine that can rotate the part. This decision is made before the first chip, not during the cut.

Verify before cutting. Simulation catches collisions, over-travel, and unmachined areas. A short simulation pass costs minutes. A scrapped titanium part costs days. For expensive materials, we also check stock allowance and tool reach manually.

• 1
  CAD modelDefines nominal geometry; include tolerances and datums.
• 2
  CAM setupSets stock, workholding, tools, and cutting strategy.
• 3
  G-codeCoordinates, feeds, speeds, and tool-change commands.
• 4
  VerificationSimulate, then probe or set the work offset on the machine.

A 3-axis machine moves in X, Y, and Z only. The cutter always points straight down. These machines are fast, rigid, and cheap to run. Most flat plates, brackets, and simple housings never need more. If all features are reachable from one or two directions, stay with 3 axes.

A 4-axis machine adds rotation around one axis, usually A. The part turns while the cutter works, so you can machine around a cylinder in one setup. Splines, cams, and shaft features fit here. The limitation is that the tool axis stays fixed relative to the machine, so undercuts remain a problem.

A 5-axis machine adds a second rotary axis. The cutter can tilt, which lets it reach undercuts and keep a short, stiff tool in the cut. Complex impellers, medical implants, and aerospace brackets are typical. Setup count drops, and accuracy improves because the part is not moved between operations.

More axes are not automatically better. A 5-axis machine is slower to program and usually has a smaller work envelope. Use it when geometry demands it or when setup reduction pays for the extra cost. Otherwise a well-fixtured 3-axis job is the better choice.

• 1
  3-axisFlat or prismatic parts with features from one or two sides.
• 2
  4-axisCylindrical parts with features around the circumference.
• 3
  5-axisFreeform surfaces, undercuts, and single-setup complex parts.

Tolerance is the allowed deviation from nominal. A general machining tolerance sits around ±0.05 mm. Tight features can reach ±0.005 mm on a stable setup with the right tool and temperature control. Not every dimension needs that. Over-tolerancing raises cost because it slows cutting and increases inspection.

Surface finish is measured as Ra, the average roughness. As-machined finishes run Ra 1.6–3.2 μm. A finer pass reaches Ra 0.8–1.6 μm. Polished or lapped surfaces go to Ra 0.2–0.8 μm. Finish affects sealing, wear, and coating adhesion. Specify it only where it matters.

Size limits depend on the machine. Our largest travel is 4,000 × 400 × 150 mm. Medium machines cover 750 × 1,150 × 550 mm and 600 × 600 × 600 mm. Compact machines handle 500 × 500 × 450 mm and 500 × 310 × 200 mm. A Ø400 mm rotary table supports round parts on 4-axis work.

Wall thickness is a quiet constraint. Thin walls deflect under cutting force, causing chatter and dimensional drift. Below about 0.8 mm in aluminium, the process needs light passes, sharp tools, and sometimes a support fixture. Tell the machinist early, because the fix is planned before cutting.

Aluminium is the default for prototypes and housings. Grades 6061 and 7075 cut fast and hold good finish. 7075 is stronger but less weldable and more prone to stress movement after heavy stock removal. 2024 and 6082 appear in aerospace and structural work.

Stainless 303 and 304 machine reasonably well with sharp tooling and steady coolant. 316L is tougher and gummier, so feeds drop and tool life shortens. 17-4PH adds strength and is often heat treated after machining. Plan the sequence so heat treatment does not distort critical bores.

Titanium Ti-6Al-4V and Inconel sit at the hard end. They generate heat at the cutting edge, so carbide grades, low surface speed, and high-pressure coolant matter. Cutting forces are high, and tool wear is fast. These parts reward a 5-axis machine because fewer setups mean fewer chances for error.

Plastics behave differently. POM and PEEK cut cleanly but may need stress relief. ABS and PC soften with heat, so sharp tools and air blast beat flooding. Carbon fibre composites demand diamond-coated tools and dust control. The material dictates the tool, not the other way around.

A machine tool only cuts as well as the part is held. A vise is fine for simple blocks. Complex parts need custom fixture plates, soft jaws, or vacuum chucks. The fixture must resist cutting force without deforming the part. Over-clamping thin walls is a common source of out-of-tolerance features.

Every setup adds error. Each time the part is unclamped and repositioned, the datum shifts slightly. Two setups can add 0.02 mm of variation. A single 5-axis setup removes that stack-up. This is why complex parts often cost less on a 5-axis machine despite the higher hourly rate.

Thermal growth matters on tight work. A 100 mm aluminium part grows about 0.0023 mm per °C. A shop that swings 5 °C between morning and afternoon will see dimension drift. Temperature-controlled rooms and in-process gauging keep this in check for ±0.005 mm work.

We inspect 100% of parts before shipment, with raw material checks, in-process monitoring, and final inspection. Reports are available on request. That is how the tolerance on the drawing becomes the tolerance in the box.