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Choosing CNC machining gets your part made - but how you design it decides whether it ships on time, on budget, and within tolerance. This CNC machining design guide shares 8 practical DFM (design for manufacturing) rules our engineers apply on every project at KAIAO, so your drawings avoid the costly mistakes that cause scrap, rework, or late deliveries.
Table of Contents
A part can use the perfect alloy and still fail on the shop floor if the geometry fights the process. DFM (design for manufacturing) is the practice of shaping a part so it machines cleanly, holds tolerance, and costs what you expect. Our CNC machining services include a DFM check on every CAD file before cutting metal - because fixing a drawing is free, while fixing a scraped batch is not. Good DFM typically cuts lead time and cost 10-30% on a first article, and it is the single biggest lever on whether a supplier can hold your tolerance consistently. When you evaluate shops, ask how they handle DFM; the answer separates a true manufacturing partner from a quoting broker. See our notes on how to choose a CNC manufacturer.
Thin walls vibrate under the cutter, trap heat, and warp after machining. As a rule of thumb, keep walls at or above the minimums below; go thinner only when function demands it and you accept tighter tolerances on flatness.
Material | Minimum practical wall | Why it matters |
Aluminum (6061/7075) | 0.8 mm | Soft; thin walls chatter and bend |
Stainless / steel | 1.0 mm | Work-hardens; deflects under load |
Titanium | 1.2 mm | Low conductivity; heat builds fast |
Engineering plastics (POM/PEEK) | 1.5 mm | Spring back and warp after release |
If you need a thinner feature, design it as a local rib rather than a full wall, and expect the shop to confirm flatness after finish. Ribs concentrate stiffness where it counts without the whole envelope going flexible.
For enclosures and brackets, a 0.6 mm wall is sometimes required. In that case specify a tolerance on flatness (not just thickness) and plan for a fixture that supports the part during cutting. Tell your supplier early - this is exactly the kind of note a DFM check is meant to catch.
Tighter tolerance means more setups, more inspection, and higher scrap. Standard CNC holds plus or minus 0.01-0.05 mm comfortably; pushing to plus or minus 0.005 mm or better adds cost fast and often buys nothing. Tolerances belong on the features that actually mate or seal - not across the whole drawing.
Tolerance band | Typical cost impact | Use it for |
plus or minus 0.10 mm (ISO 2768-m) | Baseline | Non-critical, cosmetic edges |
plus or minus 0.05 mm | +0-10% | General fits |
plus or minus 0.01 mm | +20-40% | Bearing seats, pin holes |
plus or minus 0.005 mm | +50% or more | Metrology references only |
A full reference on achievable numbers is in our CNC machining tolerances guide. Rule of thumb: if a feature never touches another part, leave it at the general standard and save the budget for the one that does.
A rotating end mill cannot cut a perfect inside corner - its tip is round. Every internal corner needs a radius at least as large as the tool. Designing a 0.5 mm radius into a 6 mm pocket forces a tiny tool, slow passes, and breakage risk. Open the radius to match a standard tool (for example 3 mm or 5 mm) and cycle time drops sharply.
For outside corners you can leave them sharp if the application needs it, but inside corners must be relieved. Where a sharp inside edge is truly required, plan for EDM or hand finishing and budget the extra time.
Blind holes deeper than about 4 times the diameter get difficult: chips pack at the bottom, drills wander, and tolerances open up. Through-holes and shallow blind holes are cheaper and more accurate. Keep hole sizes to standard drill series so the shop is not grinding a custom tool.
Specify standard thread forms (M, UNC/UNF) and avoid threads smaller than M2 in metal unless function demands it - tiny threads strip easily and need special taps. For threaded features in plastic, use inserts rather than cut threads; the holding power is far better. Note thread depth: a thread engaged 1.5 times the diameter already carries most of the load, so deeper rarely helps.
Pocket depth should stay under about 10 times the end mill diameter for side walls; deeper pockets need long tools that deflect. Cavities with drafted walls release cleanly, while straight deep walls may need extra clearance.
Undercuts - features a straight tool cannot reach - are the big cost driver. A simple undercut can force a 4th- or 5th-axis index, a special form tool, or wire EDM. 5-axis CNC machining reaches many undercuts in one setup, but the geometry still has to be designed for it. Where you can, replace an undercut with a two-piece assembly or a drafted wall; you keep the function and drop the special operation.
Engraved text smaller than about 1.5 mm tall becomes unreadable after anodizing, and fine logos may not hold paint. Keep marks simple, at least 2 mm tall, and place them on a flat, accessible face. If the part will be anodized or coated, engrave before the finish so the mark reads against the color; laser etch after coating only if the contrast is acceptable.
Material drives everything above: a hard titanium wall needs more thickness than aluminum, and a thin plastic tab may need a steel insert. Pick the material for function first, then design the geometry to suit it - our CNC machining materials guide maps common choices to their machining behavior.
Finish matters too. Anodizing adds 5-25 micrometers per side; if a bore must stay at a pressed fit, machine undersize and let the coating bring it home, or mask the bore. Powder coat and paint add even more, so design clearances with the finish in mind rather than discovering the bind after coating.
Strong CNC parts are designed, not just drawn. Hold walls and corners to tool-friendly sizes, tolerance only what mates, keep holes shallow and standard, and avoid undercuts wherever function allows. Run a DFM pass before release - it is the cheapest insurance against scrap and delay. Use this CNC machining design guide as a checklist, and send us the CAD when you are ready for a review.
What is the minimum wall thickness for CNC machined parts? A: About 0.8 mm in aluminum and 1.0 mm in steel as a practical floor; plastics need 1.5 mm. Thinner is possible with ribs and fixture support, but expect tighter flatness tolerances.
How tight a tolerance can CNC machining hold? A: Standard machines hold plus or minus 0.01-0.05 mm reliably. Plus or minus 0.005 mm is achievable but adds significant cost and is rarely needed outside metrology features.
Do inside corners have to be rounded? A: Yes. A round end mill cannot cut a sharp inside corner, so every internal corner needs a radius at least as large as the cutter. Open the radius to a standard tool size to save time.
How deep can a CNC hole be? A: Through and shallow blind holes are cheapest. Keep blind holes under roughly 4 times diameter; deeper holes pack chips, drift off axis, and open tolerances.
What is an undercut and why does it cost more? A: An undercut is a feature a straight tool cannot reach. Clearing it may need a 4th/5th-axis index, a form tool, or EDM - each adding setup, time, and cost. Redesigning it out is usually the best move.
Should I tolerance every dimension on the drawing? A: No. Tolerance only the features that mate, seal, or locate. Leave the rest at the general standard (for example ISO 2768-m). Over-tolerancing raises cost without improving the part.
