CNC Machining Design Guidelines for Faster, Easier Manufacturing

Simplify Geometry and Part Size

Use basic, machinable geometry and avoid unnecessary complexity. Our Instant Quote system is best suited for small to medium-sized parts designed for efficient aluminum machining with standard tooling.

• Best fit for the automated workflow:
• Parts roughly within a 6" cube
• Parts larger than approximately 0.5" overall
• Features accessible from standard machining directions

Parts outside this range may still be possible, but they may require manual review.

Very Small Parts and Features

Our Instant Quote workflow is optimized for parts large enough to be machined efficiently with standard tooling and workholding. Very small parts — especially those under approximately 1" cube or containing extremely small features — may require specialized tools, higher spindle speeds, and alternate machining strategies. These parts may require manual review and may not be supported in the standard process.

Corner Radii and Fillets

Add internal fillets to corners whenever possible. Sharp internal corners cannot be machined with standard round cutting tools.

Best practice: Use internal radii of 0.0625" (1/16") or larger. Larger radii generally improve tool access, reduce machining time, and produce better surface finish.

Hole Design

Use standard hole sizes and practical hole depths whenever possible. Through-holes are generally preferred over blind holes because they improve chip evacuation and reduce cycle time.

Best practices:

• Maximum drilled hole depth: 6× diameter
• Prefer standard drill sizes to reduce tool changes
• Use through-holes where possible
• Avoid overlapping holes or overly complex hole geometry
• Model holes as standard holes in your 3D CAD file
• Do not model threads in your 3D file; use pilot holes only for threaded features

Wall Thickness

Avoid thin, unsupported walls. Thin features can vibrate, deflect, or distort during machining, especially as wall height increases.

Recommended minimum wall thickness for aluminum:

• Walls under 0.5" tall: 0.040" minimum
• Walls from 0.5" to 1.0" tall: 0.060" or greater
• Walls over 1.0" tall: 0.080" or greater

Also keep in mind:

• Minimum hole diameter: 0.0625" (1/16")
• Very small engraved or internal features under 0.030" wide may not machine cleanly

💡 Tip: Thicker walls generally improve rigidity, dimensional accuracy, and surface finish.

Tool Access and Feature Orientation

Design parts so tools can easily reach the features being machined. Features that are blocked, buried deep in pockets, or placed on difficult angles often require more setups and may fall outside the automated workflow.

Best practices:

• Minimize or avoid undercuts
• Use chamfers or edge breaks where appropriate for safety and handling
• Align holes, slots, and major features to common planes or axes
• Keep features on accessible faces whenever possible
• Avoid excessively deep pockets or narrow recesses that restrict tool access

Tolerances

Specify only the tolerances that are functionally necessary. Tighter tolerances increase cost and may require manual review, additional inspection, or secondary operations.

Unless otherwise specified, Rapid CNC Parts.com machines parts to general tolerances of ±0.005" (±0.127 mm), which is suitable for many mechanical parts, fixtures, and functional prototypes.

Standard tolerances (default):

• Linear dimensions: ±0.005"
• Hole diameters: ±0.003"
• Flatness / straightness: 0.003"–0.005" typical

If no tolerance is specified, our standard tolerance range will be applied by default.

Tolerance Note for Multi-Setup Machining

Unless otherwise specified, standard tolerances apply to features machined within the same setup. Features machined on different faces, sides, or orientations may have greater positional variation due to re-fixturing and part reorientation. If alignment, true position, or dimensional relationships between features across multiple setups are critical, they should be clearly identified on the drawing and may require manual review.

Threads

Use standard thread sizes whenever possible. Threaded features are typically added after machining using taps or thread mills.

Supported standard internal threads:

• UNC, UNF, and Metric
• Typical supported sizes: #6-32 to 1/2-13 or M3 to M12
• Recommended thread depth: no more than 2× diameter

For CAD files:

• Model threaded holes as pilot holes only in the 3D file
• Add thread size and depth in a 2D drawing or part note
  • Example: M6 × 1.0 – 10 mm deep

Avoid:

• Threads too close to edges
• External threads
• Tapered threads such as NPT
• Very small threads below M3 or #4
• Threads on angled or difficult-to-access surfaces

These thread types may require manual review.

Avoid Over-Engineering

Design complexity should serve a purpose. Features that do not improve function, fit, or performance can increase cost, extend lead times, and add unnecessary manufacturing risk.

⚠️ Common Signs of Over-Engineering 

• Tight tolerances on non-critical features
• Complex internal geometry with no functional benefit
• Multiple finishes or materials without a clear reason
• Over-specified threads, chamfers, or cosmetic details
• Aesthetic features that significantly increase machining time

✅ Best Practices

• Start simple and add complexity only where needed
• Use standard radii, threads, and hole sizes
• Prioritize manufacturability early in the design process
• Focus precision where it matters most

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

Clear documentation helps avoid delays and ensures your part is quoted and machined correctly. Include material, tolerances, thread callouts, surface finish requirements, and any critical notes in your drawing or part documentation.