Our Design Guidelines

Simplify Geometry and Part Size

Use basic shapes and avoid overly complex features. 

Our instant quoting system is optimized for parts that fit within a 6" cube down to about 0.5" in size. While parts outside of this range may still be possible, they’ll likely require manual review and may not be supported by the Rapid CNC Parts.com Instant Process at this time.

Corner Radii and Fillets

Add radii to internal corners (avoid sharp corners).

Best practice: Use 0.0625" (1/16") or larger internal fillets to improve surface finish and reduce cost

Diagram Below: 

A sharp corner (not machinable with round tools), A small internal radius (~0.031"), and A large internal radius (~0.125") — ideal for faster machining and better surface finish.

Hole Design

Favor standard hole sizes and depths; prefer through-holes to blind holes. Please do not model holes in your the .STP or .STEP file, these are not supported.

Maximum depth for drilled holes: 6× diameter (e.g., a 0.25" drill = 1.5" max depth)

Prefer standard drill sizes for faster quoting and reduced tool changes

Use through-holes where possible; they evacuate chips better and reduce cycle time

Avoid overlapping holes—they confuse toolpath generation and may result in bad geometry

Wall Thickness

Avoid thin walls; maintain sufficient thickness for stability.

Recommended minimum wall thickness: 0.040" (1.0 mm) for aluminum parts

Minimum hole diameter: 0.0625" (1/16")

Small internal features: Avoid text or engraving smaller than 0.030" wide

Wall thickness isn’t one-size-fits-all—it depends on how tall the wall is. Taller walls need to be thicker to remain rigid during machining.

• Short walls (under 0.5" tall): minimum thickness of 0.040"
• Medium walls (0.5" to 1.0" tall): aim for ≥ 0.060"
• Tall walls (over 1.0" tall): use ≥ 0.080" to prevent vibration and deflection

💡 Tip: Thicker walls improve surface finish and dimensional accuracy, especially on unsupported features.

Draft and Undercuts

Minimize or avoid undercuts; slight draft angles help in deep pockets.

Chamfers & Edge Breaks

Use chamfers or rounded edges for safer handling.

Accessibility

Design for tool access, minimizing obstructions and deep recesses.

Feature Alignment

Align holes, slots, and features along common axes or planes.

Part Size & Depth

Consider machine and tool limitations; avoid excessively deep pockets. 

Tolerances

Specify standard, functionally acceptable tolerances; tighter tolerances increase cost and may require manual quoting.

Precision starts with clear communication.

Unless otherwise specified, all CNC parts at Rapid CNC Parts.com are machined to general tolerances of ±0.005" (±0.127mm). This works well for most mechanical parts, fixtures, and functional prototypes.

✅ Standard Tolerances (Default)

Linear dimensions: ±0.005"

Hole diameters: ±0.003"

Flatness, straightness: 0.003"–0.005" typical for standard setups

No tolerance specified? We'll apply our standard tolerance range by default.

Surface Finish

Specify standard surface finishes unless critical to performance.

Threads

Choose standard thread sizes; avoid custom threads when possible.

• Standard internal threads only (UNC, UNF, Metric)
• Threads are typically cut after CNC machining using taps or thread mills
• Supported sizes range from #6-32 to 1/2-13 (or M3 to M12)
• Thread depth should not exceed 2× diameter for best results
• Model threaded holes as pilot holes only in your 3D CAD file
• Add thread callouts and depth in your 2D drawing or part notes (e.g., “M6 x 1.0 – 10mm deep”)
• Avoid thread starts too close to edges—maintain at least 2× diameter of material around threads
• Unsupported or Manual-Review Thread Types:
• • Threaded bosses or external threads
  • Tapered threads (e.g., NPT)
  • Very small threads (under M3 or #4)
  • Threads on angled surfaces or complex geometry

Avoid Over-Engineering

Simplify designs by removing unnecessary features or overly stringent requirements.

We get it—you want your part to be strong, precise, and look awesome. But adding complexity that doesn’t serve a clear function can lead to higher costs, longer lead times, and unnecessary risk during manufacturing.

⚠️ Common Signs of Over-Engineering

• Ultra-tight tolerances on non-critical features
• Complex internal pockets with multiple toolpaths that serve no purpose
• Using multiple surface finishes or materials without clear performance benefits
• Over-specifying threads, chamfers, or edge treatments just for appearance
• Adding aesthetic features that increase cycle time without functional gain

✅ Best Practices

• Start simple. Build complexity only where it directly improves function or fit
• Use standard features (fillet radii, thread sizes, hole patterns) whenever possible
• Design with machining in mind—straightforward parts run faster and cost less
• Ask: “Would this feature still matter if no one ever saw it?”

💡 Pro Tip: Smart design isn’t about doing more—it’s about doing only what’s needed, really well.

Communication & Documentation

Clearly detail materials, tolerances, finishes, and special instructions on engineering drawings.