How to Reduce CNC Machining Costs During Product Design

6 min read

Written by Guntis Endziņš — Mechanical Engineer & Founder, Guenzo Engineering

By the time the supplier opens your STEP file, most of the cost is already locked in. These six rules consistently lower CNC quotes by 20–50%.

1. Don't over-tolerance

A general tolerance of ±0.1 mm is fine for most features. Reserve tight tolerances (±0.01 mm) for bearing seats, sliding fits and sealing surfaces.

2. Internal corners larger than the end mill

Internal corners must match a real end mill radius. Common shop tools are 1, 2, 3, 6 mm. A 1.5 mm internal radius forces a custom tool and slow cuts.

3. Limit pocket depth

Pockets deeper than 4x tool diameter need long-reach tools, slower feeds and produce chatter. Shallow the pocket or split the part.

4. Avoid deep narrow slots

A 50 mm deep, 4 mm wide slot is expensive in any material. Consider EDM, sheet metal substitution or splitting into two parts.

5. Minimize setups

Each time the part is flipped, the shop loses 15–30 minutes and adds a stack-up error. Designs reachable from one or two setups quote dramatically faster.

6. Pick a sensible material

6061-T6 aluminum machines fast and cheap. 304 stainless is 3–4x. Tool steels are 5–10x. If the part does not need corrosion or high strength, aluminum is usually right.

Example

A bracket originally designed in 304 stainless with ±0.02 mm general tolerance, 1.5 mm internal radii and three setups quoted at €180 each (50 off). Re-designed in 6061 with ±0.1 mm general tolerance, 3 mm radii and two setups: €72 each. Same function, 60% cheaper.

Common mistakes

• Using "default" CAD fillets without thinking about tool size.
• Specifying mirror polish where bead blast would do.
• Designing thin walls below 0.8 mm in aluminum (they vibrate during cutting).