What are common design mistakes that increase CNC part cost?

Overly Tight Tolerances and Unnecessary Precision

One of the most frequent cost drivers is specifying tighter tolerances than functionally required. For instance, a ±0.01 mm tolerance may demand additional setups, slower feed rates, and specialized processes such as CNC grinding or EDM machining. During CNC milling or CNC turning, maintaining unnecessary micro-level accuracy increases tool wear and inspection time. Proper DFM guidance ensures that tolerances are based on actual fit and function, rather than arbitrary precision expectations.

Complex Geometry and Hard-to-Reach Features

Designs featuring deep cavities, thin walls, or sharp internal corners often require multiple setups or custom tooling. Instead of using basic CNC machining operations, such geometries may necessitate advanced multi-axis machining or custom electrodes for EDM. Each additional setup adds cost and risk. Simplifying angles, adding fillets, and standardizing radii can cut machining time by 20–40% without compromising strength or performance.

Improper Material Selection

Choosing difficult-to-machine materials when not required for the intended application significantly raises cost. For example, selecting Inconel 718 or Ti-6Al-4V for non-critical parts leads to longer machining cycles and higher tool consumption. More cost-effective alloys, such as Aluminum 6061-T6 or Brass C360, often deliver sufficient mechanical properties and machinability. Similarly, for corrosion resistance, SUS304 stainless steel can replace higher-cost grades, such as SUS316L, in many environments.

Ignoring Surface Treatment Requirements in the Design Stage

Designers often overlook surface finishing needs until after machining, leading to costly rework. Adding allowances for coatings, such as anodizing or electropolishing, ensures that final dimensions remain within tolerance. When post-treatment is not accounted for, components may require remachining or resizing to ensure proper fit. Similarly, finishes like powder coating and chrome plating should be planned within the original CAD model for efficient production.

Lack of Standardization Across Industries

Industry-specific standards often dictate economical design parameters. In aerospace and aviation, missing chamfering or undercut details delay inspection and approval cycles. Automotive programs require standardized hole sizes and thread depths for automation compatibility. For medical devices, complex free-form surfaces may inflate both tooling and validation costs. Aligning geometry with industry norms enables faster quoting, tooling reuse, and reduced programming time—directly cutting per-part costs.