Design rules that
halve machining cost.
Or double it.
Every CNC drawing decision — internal corner radius, pocket depth, thread spec, tolerance callout — either helps or hurts manufacturability. These rules are what separate a $50 part from a $200 one.
The single biggest DFM rule.
Square internal corners require EDM. Rounded internal corners are milled. The cost difference: 5-10×. This one rule accounts for more preventable CNC cost than any other.
Internal radius ≥ 1/3 pocket depth
For 15mm deep pocket, use R5 minimum. Allows 10mm tool to reach bottom. Larger radius = larger tool = faster roughing = lower cost.
Radius > tool radius + clearance
Corner radius should be larger than any practical tool's radius (typically 3mm minimum for 6mm endmill). Tiny radii force specialty tooling.
Square internal corners
Requires sinker EDM or broaching. Minimum 2× cost, often 5-10× for small corners. Re-evaluate whether you truly need it.
Dog-bone corners
Need sharp corner for mating feature? Add small overlap cutout ("dog bone") — tool clears corner, mating part still fits flush.
Minimum wall thickness.
| Material | Absolute min | Practical min | Notes |
|---|---|---|---|
| Aluminum 6061 | 0.5 mm | 0.8 mm | Vibrates below 0.8 mm |
| Steel 1018/1045 | 0.8 mm | 1.0 mm | Work hardening at thin sections |
| Stainless 304/316L | 1.0 mm | 1.2 mm | Work hardens aggressively |
| Plastic (Delrin/PEEK) | 0.5 mm | 1.0 mm | Flex during cutting |
| Titanium Gr.5 | 1.0 mm | 1.5 mm | Springs back during cutting |
Hole DFM rules.
Standard drill depth
Up to 4× diameter with twist drill. 10mm hole → 40mm deep standard.
Deep hole territory
Peck drilling or gun drilling. 20-50% cost premium. Specify only if needed.
Specialty gun drilling
Only economical for hydraulic cylinders, long shaft bores. Specialty process.
Thread depth
Thread 1.5-2× diameter for full strength. Longer = more cost, same strength.
Bearing fit hole
H7 tolerance needs reaming or boring after drilling. Specify on drawings.
Practical minimum
Below 2mm, drills deflect and break. Use EDM or laser drilling for smaller.
Think about tool reach.
If a tool can't reach a feature, it can't be machined. Obvious, but constantly violated. Review designs for tool access before sending to quote.
Good access
- • Features machined from outside surfaces — easy tool access
- • Pockets with generous entry clearance
- • Holes perpendicular to surface
- • Features on planar surfaces accessible from above
- • Part fixtures on one side, machine other 5 sides
Poor access
- • Internal features only reachable through small opening
- • Pockets with overhangs or undercuts (require specialty tools)
- • Holes at compound angles without 5-axis machine
- • Features requiring tool extension > 4× tool diameter
- • Parts requiring 6+ setups for full machining
FAQ
What is the minimum wall thickness for CNC machined parts?
Aluminum: 0.5 mm for small parts, 0.8 mm practical minimum. Steel: 0.8 mm minimum, 1.0 mm preferred. Walls below these minimums vibrate during machining, produce poor surface finish, and risk breakage. For very thin walls (below 0.3 mm), specialty techniques like wire EDM or chemical etching produce better results than conventional milling.
Why do internal corners need radii?
Square internal corners require EDM — 5-10× more expensive than standard milling. Milling an internal corner requires the tool to change direction, leaving a radius equal to tool radius. Standard practice: specify internal radius 1.5× the depth of feature (allows large tool to reach bottom). For 10mm deep feature, specify R5 internal corner — easy to machine. Avoiding sharp internal corners is the single biggest cost-saving DFM rule.
How deep can holes be drilled?
Standard drilling: depth up to 4× diameter with twist drill. Beyond that: gun drilling or peck drilling required. For 10mm hole, 40mm depth standard; 80mm depth requires specialty. Tighter tolerance on deep holes requires reaming or boring — slower. Rule of thumb: if hole depth > 5× diameter, cost increases significantly.
When to use bosses vs pockets?
Pockets (removing material) are often cheaper than bosses (adding material, then machining around them). A boss 10mm tall on a 100×100mm plate requires removing 2,000 mm² around it; a pocket 10mm deep removes only pocket volume. Start with billet stock size of final bosses; machine pockets from there. This DFM decision alone can halve machining time.
Best practices for threaded holes?
Thread depth: 1.5-2× diameter for standard strength (longer doesn't add strength, just cost). Chamfer entry for easier fastener alignment. Use standard tap sizes only — metric M3-M12 and UNC/UNF 4-40 to 1/2-13 most common. Avoid: blind threads in deep pockets (can't reach), very small threads (M1.6 and below — fragile), non-standard sizes (require custom tap).
What tolerance should I specify for CNC features?
Default ISO 2768-m or ±0.1 mm works for 90% of features. Reserve tighter tolerance for: bearing fits (±0.025 mm H7), mating surfaces, dowel pin holes. Never specify ±0.01 mm unless actually needed — costs 5-10× more. Flag truly critical dimensions clearly; let the rest ride on general tolerance class.