What Is CNC Machining?
CNC (Computer Numerical Control) machining is a subtractive manufacturing process where pre-programmed software controls the movement of cutting tools to remove material from a solid block — called a workpiece or billet. The result is a finished part machined to precise dimensions.
Unlike 3D printing (which builds up), CNC cuts away. Unlike manual machining (where a human turns handwheels), CNC follows a digital program. The machine does what the code says — no more, no less.
Types of CNC Machines
CNC Mill
The workhorse. A rotating cutting tool moves across a stationary workpiece (or the table moves under the tool). 3-axis mills handle X, Y, Z movement. 4-axis and 5-axis mills add rotation, allowing complex geometry without re-fixturing.
- 3-axis: Flat surfaces, pockets, holes, simple 3D contours
- 4-axis: Adds rotation around one axis — good for engraving cylinders, cutting features on multiple faces
- 5-axis: Full freedom — undercuts, compound angles, complex aerospace parts
CNC Lathe (Turning Center)
The workpiece spins while the cutting tool moves. Best for round parts: shafts, bushings, fittings, threaded components. Modern CNC lathes with live tooling can also mill flats, drill cross-holes, and cut keyways — reducing operations.
CNC Router
Similar to a mill but designed for softer materials and larger workpieces. Common in woodworking, plastics, and aluminum sheet. Lower rigidity, higher speed.
Wire EDM
Uses electrical discharge to cut conductive materials with a thin wire. No cutting forces, so it handles hardened steel and delicate features that would break a conventional tool. Slow but extremely precise (±0.0001″).
Common CNC Materials
| Material | Machinability | Typical Use |
|---|---|---|
| 6061-T6 Aluminum | Excellent | Brackets, housings, prototypes |
| 7075 Aluminum | Good | Aerospace, high-stress components |
| 304 Stainless | Fair — work hardens | Food/medical, corrosion resistance |
| 4140 Steel | Good | Shafts, gears, structural parts |
| 1018 Steel | Excellent | Low-cost structural, pins, fixtures |
| Brass 360 | Excellent | Fittings, valves, decorative |
| Delrin (POM) | Excellent | Bearings, gears, insulators |
| PEEK | Good — expensive | Medical, aerospace, high-temp |
Tolerances
Standard CNC tolerances are ±0.005″ (±0.127mm). Tighter tolerances cost more because they require slower feeds, more passes, and sometimes grinding or lapping as a secondary operation.
| Tolerance | Difficulty | Typical Cost Impact |
|---|---|---|
| ±0.010″ | Easy — standard roughing | Baseline |
| ±0.005″ | Standard — most shops default here | 1x |
| ±0.002″ | Precision — requires good machines | 1.5–2x |
| ±0.001″ | High precision — grinding may be needed | 2–4x |
| ±0.0005″ | Ultra — specialized equipment | 5–10x |
When to Use CNC Machining
- Tight tolerances — nothing beats machining for dimensional accuracy
- Metal parts — steel, aluminum, stainless, brass, titanium
- Functional prototypes — real material, real strength
- Low-to-mid volume — 1 to 10,000 pieces (beyond that, consider casting or stamping)
- Complex geometry — 5-axis can handle almost anything
Design Tips for CNC
- Avoid internal corners sharper than 1/8″ radius — end mills are round
- Keep wall thickness above 0.030″ for metals, 0.060″ for plastics
- Standard hole sizes (drill chart dimensions) are cheaper than odd sizes
- Deep pockets (>4x width) require special tooling — adds cost
- Add chamfers or fillets to edges — sharp edges burr and cost time to deburr
Need CNC machined parts? Get a free quote — we’ll match your project to the right shop in our DFW network.