CNC machining removes material from a blank until the drawing is met. The choice of machine — 3-axis mill, lathe, 4-axis or 5-axis — decides how many setups the part needs, how tight the tolerances can be held and, ultimately, what it costs. A buyer who understands the trade-offs writes drawings that manufacture themselves.
SEC. 01
3-axis, 4-axis, 5-axis — when to use each
- 3-axis mill — flat and simple parts, low cost, needs multiple setups for complex geometry.
- 4-axis mill — adds a rotary A-axis, ideal for around-the-part features on shafts and manifolds.
- 5-axis mill — one setup for parts with compound-angle features, best for aerospace and turbine components.
- CNC lathe with live tooling — all turning plus limited milling in one setup, unbeatable for shafts and bushings.
SEC. 02
Realistic tolerance envelope
General machining reaches IT8 (±0.05 mm on a 100 mm dimension) without special effort. IT7 (±0.03 mm) is achievable with hard tooling and controlled coolant. IT6 (±0.02 mm) requires precision grinding or lapping — flag it clearly on the drawing.
Surface finish follows the same rule: Ra 3.2 μm is standard, Ra 1.6 μm needs a semi-finish pass, Ra 0.4 μm needs grinding.
SEC. 03
How to write a drawing that quotes low
- Only tolerance features that need to be tight — free-tolerance the rest.
- Call out reference datums; avoid nested tolerance chains.
- State surface finish only on functional surfaces, not the whole part.
- Use standard fasteners and thread pitches; avoid custom tooling.
Key Takeaways
- Match machine class to part complexity — don't overbuy 5-axis for a flat plate.
- IT8 is standard, IT7 with care, IT6 needs grinding.
- Free-tolerance non-critical features to keep price down.
- Consolidate setups by moving to 4- or 5-axis when the part warrants it.