CNC Machining Design Guidelines for Faster, Easier Manufacturing
At Rapid CNC Parts.com, our design guidelines are the foundation of a faster, smarter CNC experience. By applying these proven best practices, your parts are optimized for our AI-powered quoting and programming system — designed specifically for efficient aluminum machining. This is how we’re redefining CNC: standardized tooling, automated workflows, and design-driven manufacturability checks that reduce cost, eliminate delays, and accelerate production. The result? Precision aluminum parts, made fast, and made right here in the USA.
Design Guidelines:
Geometry and Manufacturability
Guidelines related to part shape simplicity, feature alignment, wall thickness, internal radii, hole design, chamfers, and accessibility—emphasizing ease of machining, reducing complexity, and improving tooling efficiency.
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.
Material and Specs - Optimization
Guidelines covering material selection, appropriate tolerance ranges, standard thread sizes, surface finishes, and avoiding over-engineering—focusing on cost reduction, consistent quality, and performance optimization.
Material Selection
Use common, machinable materials (aluminum, steel, plastics).
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.
Why Design Guidelines Matter for Instant CNC Machining
Designing for manufacturability isn’t just good practice—it’s essential for getting fast, affordable quotes from Rapid CNC Parts.com. Our system uses AI-powered analysis to check your part for common geometry, standard features, and optimal toolpaths. Parts that follow our guidelines are processed automatically, saving time and avoiding costly delays.
✅ Better surface finishes
✅ Lower machining time = lower cost
✅ Fewer programming hold-ups
✅ Instant online quoting — no manual reviews
💡 The more your part aligns with these principles, the faster we can machine it.
Features & Designs to Avoid
What doesn’t work well with the Rapid CNC Parts Instant Process
Our system is optimized to deliver precision CNC Machined custom aluminum parts fast—small the medium sized parts that use standard CNC machine tools and can be machined efficiently on a 3+2 setup mill.
To keep things fast, affordable, and reliable, avoid the following design elements:
Feature | Why to Avoid |
Parts larger than 6" in any direction with deep pockets | May exceed travel limits or require a manual quote |
Complex organic surfaces or full 5-axis contours | Slows down quoting and increases programming/setup time |
Tight internal radii (< 0.031") | Requires custom tooling; use standard fillets (≥ 0.0625") |
Deep narrow slots (>6× depth-to-width) | Difficult to machine without deflection |
Non-standard threads (e.g. NPT, Acme) | NPT and Acme threads often require additional specifications based on the application |
External threads | Require special setups and are not included in standard workflow |
Multiple setup parts (undercuts, compound angles) | May exceed 3+2 capabilities—keep features on accessible sides |
Tiny holes (< #6 or < 0.0625") | Risk of tool breakage and unsupported in instant quote |
Sharp internal corners | Round tools can’t cut zero-radius corners—always fillet |
Thin, tall walls | Can vibrate or deflect—see wall thickness guidelines |
💡 Optimizing for 3+2 Axis Machining:
Design features on accessible sides whenever possible. Avoid parts that require continuous tool tilting or simultaneous multi-axis motion unless absolutely necessary.
Contact Us for a manual quote if your project requires any of the noted features and we'll be happy to manually review and quote your part.
Rapid CNC Parts Design Guidelines Checklist
Features That Work Best with Our Instant Quote System
These features make it easy for our software to generate quotes instantly and route your part into production without human intervention.
(avoid deep recesses or features blocked by other geometry)
Need a Visual Reference?
Want to see these tips in action? Check out our Design for CNC Video Series on our YouTube channel where we walk through real custom machined parts and how to optimize them for fast, low-cost manufacturing.
What to do if your Part doesn’t fit the Guidelines
Not Sure If Your Part Will Work?
If your part includes complex geometry, unusual materials, or features not supported in our instant quote system, don’t worry — we can still help.
- Submit your file, click the "Ask for Help" link
- Our team can provide feedback or offer alternatives
- In many cases, minor edits can bring the part back into the automated workflow
70-80% of companies involved in product development currently use rapid prototyping methods during their design and development phases.
Learn more about how our services can help you in your development process.