Inside corner radius in cnc machining: what is the rule?

Use an inside corner radius of at least one third of the pocket depth; leave sharp inside corners out. The cutter is round, so inside corners always have a radius; large radii allow bigger, faster, cheaper tools.

Pocket depth in cnc machining: what is the rule?

Keep pocket depth to about 4× the pocket width or tool diameter (up to about 10× with special tooling). Deep narrow pockets need long, flexible tools that chatter.

Wall thickness in cnc machining: what is the rule?

At least 0.8 mm for metal and 1.5 mm for plastic. Thin walls vibrate and deflect while being cut.

Hole depth in cnc machining: what is the rule?

Standard drilled holes up to about 4× the diameter; up to about 10× with special drilling. Deep holes cause tool wander and chip-clearing problems.

Threads in cnc machining: what is the rule?

Use standard thread sizes (M3 and larger are easiest) and a thread depth of about 1–1.5× the diameter. Deeper engagement adds little strength but adds machining time.

CNC machining: design rules

Milling and turning parts from solid metal or plastic with computer-controlled machines.

Typical tolerance

Typical standard tolerance ±0.125 mm (±0.005"). Tighter tolerances (±0.025 mm) add cost: specify them only on critical features.

Files to send

STEP for the geometry plus a PDF drawing that marks tolerances, threads, finish and critical surfaces.

Common materials

Aluminium (6061, 7075), steel, stainless (303, 304, 316), brass, titanium, plastics (POM, nylon, PEEK).

The rules, with reasons

Inside corner radius. Use an inside corner radius of at least one third of the pocket depth; leave sharp inside corners out. Why: The cutter is round, so inside corners always have a radius; large radii allow bigger, faster, cheaper tools.
Pocket depth. Keep pocket depth to about 4× the pocket width or tool diameter (up to about 10× with special tooling). Why: Deep narrow pockets need long, flexible tools that chatter.
Wall thickness. At least 0.8 mm for metal and 1.5 mm for plastic. Why: Thin walls vibrate and deflect while being cut.
Hole depth. Standard drilled holes up to about 4× the diameter; up to about 10× with special drilling. Why: Deep holes cause tool wander and chip-clearing problems.
Threads. Use standard thread sizes (M3 and larger are easiest) and a thread depth of about 1–1.5× the diameter. Why: Deeper engagement adds little strength but adds machining time.
Setups. Design so that most features can be reached from few sides. Why: Every extra setup adds cost and risks misalignment.
Tolerances. Mark only the dimensions that must be tight; leave the rest to the general tolerance. Why: Tolerances drive cost more than any other single factor.

Before you ask for a quote

Material and grade
Quantity
STEP file and PDF drawing
Critical tolerances and surfaces
Threads
Finish (anodize, passivation, plating)
Certificates or inspection reports needed

Common mistakes

Sharp inside corners
Very deep pockets and holes
Tight tolerances everywhere
Missing thread specification
Forgetting finish thickness on tight fits

Agents and scripts can check a design automatically with the check_design MCP tool or POST /api/design-rules. How to connect.

Request a quote Find manufacturers

Typical rules of thumb, not a guarantee. The manufacturer's own design guidelines override them: ask for them and check your design against both before ordering.