CNC Part Design Checklist: 10 Rules to Reduce Machining Cost

Before you send your CAD model out for a manufacturing quote, a quick Design for Manufacturability (DFM) review can save your company thousands of dollars and weeks of lead time.

CNC machining is incredibly versatile, but it is bound by the physical limits of spinning cutting tools. Every time your design forces a machinist to use a fragile tool, take microscopic cuts, or manually flip the part, your cost increases.

Run your next CAD model through this 10-point checklist to strip out unnecessary costs before the first chip is cut.

Pre-Design Questions

Before applying geometric rules, ask yourself these three fundamental questions to establish a baseline:

• What is the primary function of this part? (Does it bear heavy loads, or is it just a cosmetic enclosure?)
• Does it require an exotic material? (If standard Aluminum 6061 will work, do not specify Stainless Steel or Titanium.)
• What is my production volume? (Designing a complex part might make sense for a one-off prototype, but for 1,000 units, the design should be heavily optimized for speed.)

The 10-Point CNC Cost Reduction Checklist

Tolerances

1. Use standard shop tolerances for non-critical dimensions. Unless otherwise specified, default to a standard tolerance of ±0.1 mm (or ±0.125 mm). This allows manufacturers to use standard tooling at maximum feed rates.

2. Reserve tight tolerances exclusively for critical features. Only apply tight tolerances (e.g., ±0.025 mm) to areas that physically require them, such as bearing press fits, O-ring grooves, or critical alignment dowels.

3. Utilize a general tolerance note. Always include a note in your drawing’s title block stating: "Unless otherwise specified, all dimensions are ±0.1 mm." This prevents machinists from guessing your intent and over-engineering the entire part.

Feature Geometry

4. Maximize internal corner radii. A perfectly sharp internal corner is impossible to machine with a round end mill. Always design internal corner radii to be slightly larger than 1/3 of the pocket depth (e.g., a 12 mm deep pocket needs a radius of at least 4.2 mm).

5. Limit pocket depths to 4× their width. Deep, narrow pockets cause tool deflection, chatter, and poor surface finishes. Keep the depth-to-width ratio strictly under 4:1.

6. Maintain safe minimum wall thicknesses. Thin walls vibrate during machining, requiring painfully slow feed rates to prevent warping. Keep metal walls at least 0.8 mm thick, and plastic walls at least 1.5 mm thick. If your part requires complex, thin-walled structures or tight deep pocket paths, you can consult our engineering team to verify tool access and prevent component warping before quoting.

Holes and Threads

7. Stick to standard drill sizes. Always size your holes to match standard fractional, wire, letter, or metric drill bits. Non-standard holes require a machinist to slowly interpolate the hole with a smaller end mill.

8. Limit thread depths to 3× the diameter. The holding power of a screw is achieved in the first few threads. Tapping deeper than 2× to 3× the hole diameter exponentially increases the risk of tap breakage and adds zero functional strength.

Setups and Finishing

9. Design features on a single plane. Every time a part must be unclamped, rotated, and re-centered, you pay for manual labor (a "setup"). Try to design all holes, pockets, and critical features so they can be machined from a single top-down orientation.

10. Eliminate cosmetic text and complex finishes. Machining part numbers or company logos requires tiny tools and massive amounts of machine time. Stick to standard "as-machined" surface finishes (Ra 3.2 µm) and use laser engraving or silkscreening for text later.

This article is part of our comprehensive guide to How to Reduce CNC Machining Costs: A Guide to Tolerances, Features, and Design.