DFM Guide · CNC Milling

Pockets. Holes.
Thin walls.
Mill it right.

CNC milling is incredibly versatile, but designing with the cutting tool in mind can dramatically reduce cost and improve quality. This guide covers the key considerations.

Free DFM Review CNC milling service

01 · Core principles

Four key principles for milled parts.

Internal radii

Maximize internal corner radius

All internal vertical corners get a radius from the round cutting tool. Larger radius = larger, more rigid tool = faster material removal = lower cost. Avoid sharp internal corners.

Pocket depth

Limit pocket depth to 4-6× tool diameter

Deeper than 6× tool diameter requires long slender tools that vibrate and cut slowly. For 10mm wide pocket, depth ≤ 50mm ideal.

Standard tools

Use standard end mill diameters

Design internal radii and hole sizes to match standard end mill and drill diameters (3, 4, 5, 6, 8, 10, 12, 16, 20mm). Avoid custom tooling.

Wall stiffness

Avoid tall thin walls

Tall thin walls vibrate during machining (chatter), causing poor surface finish and dimensional drift. Aspect ratio limit: height ≤ 4× thickness for aluminum, 3× for steel.

02 · Floor radii

Pocket floor corners matter too.

Standard square end mill: leaves sharp corner where floor meets wall. Stress concentration here for loaded features.

Designed-in floor radius: requires bull-nose or ball end mill. Reduces stress concentration. Slightly higher cost but better fatigue performance for cyclic-loaded parts.

Recommended floor radius: 0.5mm minimum, 1-2mm for fatigue-critical features. Specify ≥ 0.5mm for any pocket where stress matters.

03 · Hole DFM

Drilling and reaming rules.

4×

Standard depth

Depth up to 4× diameter standard. Beyond requires peck drill or gun drill (specialty cost).

2mm

Practical minimum

Below 2mm diameter, drills deflect and break. Use EDM or laser drilling instead.

±0.025

Reamed bore

For H7 fits, drill undersize then ream. Plan for reaming step in DFM.

45°

Chamfer entry

Specify chamfer at hole entry for fastener alignment and burr prevention.

1.5-2×

Thread depth

Thread depth 1.5-2× diameter for full strength. Longer adds cost without strength.

M3+

Min standard thread

Specify M3 or larger for production. M2 and smaller are fragile and slow.

04 · 5-axis advantages

When to specify 5-axis milling.

5-axis wins when

• Compound angle features impossible in 3-axis
• Multiple sides need machining (avoid setup repositioning)
• Curved surfaces with consistent finish
• Aerospace impellers, blades, complex geometry
• Reduce setups from 5-6 to 1-2 (faster + better tolerance)

3-axis adequate when

• Features all on one face
• Simple prismatic geometry
• Cost-sensitive parts
• Volumes where 5-axis premium not justified
• Standard pockets, holes, slots

FAQ

What internal corner radius minimum?

Specify internal radius equal to or greater than 1/3 of pocket depth. For 15mm deep pocket: R5 minimum. This allows 10mm tool to reach bottom — much faster than smaller tool. Smaller radius forces specialty tooling and slower cutting.

Material affects DFM rules?

Yes. Aluminum forgives more — thinner walls, deeper pockets, faster cutting. Stainless and titanium need stiffer tooling and slower speeds — keep walls thicker, pockets shallower. Plastics machinability varies by material.

Tolerance specification approach?

Default ISO 2768-m or ±0.1mm works for most features. Reserve tighter tolerances (±0.025) for bearing fits, mating surfaces, dowel pin holes. Never specify ±0.01 unless truly required — costs 5-10× more.

When does 5-axis cost more than multiple setups?

Generally 5-axis is faster and more accurate when geometry justifies it. For simple prismatic parts, 3-axis 1-2 setups is cheaper. Break-even: when part needs 4+ setups in 3-axis, 5-axis usually wins.

Surface finish realistic targets?

Standard milled finish: Ra 1.6-3.2 µm. Fine milling (CBN/diamond): Ra 0.4-0.8 µm. Below Ra 0.4 needs grinding or polishing. Specify per function — over-specified surface finish drives cost.

Free DFM review available?

Yes. Every quote includes DFM review at no extra cost. We identify cost-driving features and suggest design changes that maintain function while improving manufacturability. Often saves 20-40% on machining cost.