CNC Machining Design Guide
Engineering rules for features, tolerances, threads, deep cavities and wall thickness
CNC (computer numerical control) machining is a subtractive process family — milling, turning, drilling, tapping and wire EDM — in which a program-controlled spindle removes stock from a solid billet. Design quality is governed less by the machine than by geometry rules: minimum feature size, wall thickness, tool reach, internal corner radii and tolerance stacking all decide whether a part is machinable at a quoted cost.
1. Process fundamentals and capability envelope
Milling removes material with rotating multi-edge tools on 3-, 4- or 5-axis machine tools; turning generates axi-symmetric parts by rotating the workpiece against a stationary tool. Practical milling envelopes for one setup are typically up to 2032 × 1219 × 610 mm, and turning length up to about 1575 mm — beyond these, multiple setups or special machines raise cost sharply.
Standard machining tolerance is ±0.127 mm (≈ ±0.005 in) with ISO 2768-m as the usual default; precision work can hold ±0.025 mm (±0.001 in) on critical features at an added cost. Surface finish on standard milling is Ra 1.6–3.2 µm; grinding or fine boring reaches Ra 0.4–0.8 µm.
2. Minimum features and wall thickness
The minimum machinable feature depends on tool diameter and material: a practical floor is ~0.5 mm (0.020 in) for slots, ribs and text, while fine features below 0.25 mm are unreliable in production. Deep slots require longer tools, and depth-to-width ratios beyond 4:1 demand specialty tooling or EDM.
Wall thickness should stay above 0.8 mm in metal and 1.0 mm in plastic to avoid vibration, deflection and tolerance loss during machining. Abrupt thickness changes create stress concentration and chatter; blend section changes where function allows.
3. Internal corners, radii and threads
Milling tools are round, so internal corners cannot be sharp: specify an internal fillet at least 0.5 mm (0.020 in), larger for deep pockets. External edges can be sharp or chamfered; by default machined parts are deburred to remove stress risers and burrs.
Prefer standard metric (ISO 965-1) or unified (ASME B1.1) thread sizes; the smallest reliable internal thread is about M1.6. Thread engagement should be 1.5–2× the screw diameter in aluminum and steel; blind tapped holes need drill-point clearance beyond the thread length.
4. Tolerances, datums and cost
Every tolerance carries cost. A general tolerance of ±0.127 mm is free; tightening to ±0.025 mm roughly doubles machining time on that feature; sub-micron claims require grinding, EDM or lapping and are rarely needed. Apply tight tolerances only to features that mate or index.
Use GD&T datums (ASME Y14.5 / ISO 1101) to communicate what matters: flatness on sealing faces, perpendicularity on mounting bosses, true position on hole patterns. A clean datum scheme avoids ambiguous inspection and repeated rejects.
Quick reference
| Design rule | Standard / typical value |
|---|---|
| Minimum feature size | ≥ 0.5 mm; fine features 0.25 mm+ with added cost |
| Wall thickness | Metal ≥ 0.8 mm; plastic ≥ 1.0 mm |
| Standard tolerance | ±0.127 mm (ISO 2768-m); precision ±0.025 mm |
| Internal corner radius | ≥ 0.5 mm (tool radius) |
| Max envelope | Milling 2032×1219×610 mm; turning length 1575 mm |
| Deep-cavity depth:width | ≤ 4:1 standard |
| Smallest thread | Internal M1.6; engagement 1.5–2× diameter |
| Typical finish | As-machined Ra 1.6–3.2 µm |
Sources & references
- ISO 2768-1:1989 — General tolerances for linear and angular dimensions
- ASME Y14.5-2018 — Dimensioning and Tolerancing
- ISO 965-1 — ISO metric screw threads
- ASME B1.1 — Unified Inch Screw Threads
- Xometry Learning Center — CNC Machining Design Guide (public engineering reference)