6 Common Mistakes in CNC Machining Design (And How to Fix Them)

If you’re a CAD designer working on CNC machining parts, small design errors can lead to big problems: broken tools, wasted material, or parts that don’t fit. The good news? Most mistakes are easy to avoid once you know what to look for. This guide breaks down the 6 most common CNC design blunders, with real examples, data, and step-by-step fixes to save you time, money, and frustration.

1. Mistake 1: Designing Walls That Are Too Thin

Thin walls might seem like a way to save material—but they’re a disaster for CNC machining. Thin sections vibrate during cutting, break easily, or lose precision.

Why It’s a Problem:

Low-stiffness materials (like aluminum) are especially risky. For example, a 0.5 mm aluminum wall will twist or crack when a milling tool applies pressure. Even slightly thicker walls (0.6–0.7 mm) can warp, as the tool’s vibration bends the material.

Key Guidelines for Wall Thickness:

Real Example:

A designer created a 0.6 mm thick aluminum bracket for a drone. During milling, 9 out of 10 brackets broke because the tool’s vibration bent the thin walls. When they increased the thickness to 0.9 mm, the success rate jumped to 98%—and the bracket still weighed less than 5 grams (no extra material waste).

How to Fix It:

• Follow the H:T (Height to Thickness) ratio: Keep wall height no more than 5x its thickness (e.g., a 1 mm thick wall should be ≤5 mm tall).
• If thin walls are required (e.g., for a lightweight part), switch to sheet metal manufacturing instead of CNC machining—it’s cheaper and avoids vibration issues.

2. Mistake 2: Designing Features That Can’t Be Machined

CAD software lets you draw almost any shape—but CNC machines have physical limits. The most common mistake? Designing curved holes (holes that bend or twist through the part).

Why It’s a Problem:

CNC tools move in straight lines (along X/Y/Z axes) or fixed rotations (A/B/C axes). They can’t follow a curved path for holes—trying to do so will either break the tool or leave an uneven, unusable hole.

Real Example:

A medical device designer added a curved hole to a stainless steel sensor housing (to route wires). The CNC mill couldn’t cut the curve, so the team had to scrap 20 prototypes. They ended up redesigning the hole as two straight holes connected by a small channel—simple for the CNC to machine.

How to Fix It:

• Avoid curved holes entirely for CNC parts. Use straight holes, or split the path into multiple straight sections.
• If you must have a curved feature (e.g., for a custom pipe), use Electrical Discharge Machining (EDM) instead. EDM uses electrical sparks to cut complex shapes—no straight tool paths required.

3. Mistake 3: Overusing Tolerances

Tolerances (the allowed variation in part size) are important for mating parts (e.g., a lid that fits a box). But adding tight tolerances to every surface wastes time and money.

Why It’s a Problem:

• Tight tolerances (e.g., ±0.001 mm) require slower cutting speeds, special tools, and extra quality checks—all of which increase cost.
• Most CNC machines can’t even hit extremely tight tolerances. For example, a basic 3-axis mill has a maximum accuracy of ±0.01 mm—any tolerance tighter than that is impossible.

Tolerance Guidelines by Machine Type:

How to Fix It:

• Only add tight tolerances to mating surfaces (e.g., the hole where a bolt fits). Leave non-critical surfaces (e.g., the outer edge of a bracket) with loose or no tolerances.
• Don’t assign numerical dimensions (like radius or diameter) to surfaces that don’t need precision. For example, a decorative notch doesn’t need a tolerance—just a general size.

4. Mistake 4: Designing Unnecessary Aesthetic Features

It’s tempting to add complex shapes (e.g., 3D logos, curved edges) to make parts look nice—but these features often require extra machining time and material removal.

Why It’s a Problem:

• Aesthetic features like deep engravings or custom curves need 5-axis machining (more expensive than 3-axis) or multiple tool changes.
• Removing extra material for looks increases scrap—for example, a 1 kg aluminum block might become a 0.5 kg part with unnecessary cuts, wasting $10–$20 per part.

Real Example:

A consumer electronics brand added a 3D engraved logo to the back of an aluminum phone case. The logo required 2 extra tool changes and 15 minutes of machining time per case. When they switched to electropolishing (a post-processing step that smooths the surface) and a simple printed logo, they cut production time by 25% and saved $5 per case.

How to Fix It:

• Ask: “Is this feature necessary for function?” If not, skip it.
• Use post-processing for aesthetics: Electropolishing (smooths surfaces), anodizing (adds color), or laser engraving (fast, cheap logos) are better than machining complex shapes.

5. Mistake 5: Designing Too Deep Cavities

Cavities (hollowed-out sections) are useful for lightweight parts—but CNC tools have a limited cutting length. Too-deep cavities cause tool failure or poor quality.

Why It’s a Problem:

• Milling tools work best when cavities are 2–3x the tool’s diameter. For example, a 15 mm tool can safely cut cavities up to 35 mm deep (2.3x its diameter).
• Deeper cavities lead to:
  1. Tool deflection: The tool bends under pressure, leaving uneven walls.
  2. Chip buildup: Debris gets stuck in the cavity, scratching the part.
  3. Tool breakage: The tool protrudes too far from its holder and snaps.

Real Example:

A designer created a 50 mm deep cavity in a plastic part using a 15 mm tool (3.3x the tool’s diameter). The tool deflected, making the cavity walls 2 mm thicker on one side—rendering the part useless. They fixed it by reducing the cavity depth to 35 mm and adding a small ledge (no loss in functionality).

How to Fix It:

• Follow the tool diameter rule: Keep cavity depth ≤3x the tool’s diameter.
• For deeper cavities:
  • Use a longer tool holder (to reach the bottom without bending).
  • Cut in small increments (1–2 mm at a time) to reduce tool stress.
  • Use high-pressure coolant to flush out chips.

6. Mistake 6: Designing No Radius for Internal Corners

CNC milling tools are cylindrical—they can’t cut sharp internal corners. Designing sharp edges forces the machine to use smaller tools (slower, more expensive) or leaves uneven corners.

Why It’s a Problem:

• A sharp internal corner requires a tool with a tiny diameter (e.g., 1 mm) to fit into the corner. Small tools cut slowly and break easily.
• Even if you design a sharp corner, the CNC will automatically leave a small radius (equal to the tool’s radius)—so your part won’t match the CAD design.

Internal Corner Radius Guidelines:

How to Fix It:

• Add a corner radius to all internal corners in your CAD design. Make the radius slightly larger than the tool’s radius (e.g., a 6 mm tool needs a 3.5 mm corner radius).
• If you need a sharp edge (e.g., for a part that fits into a rectangular slot), design an undercut (a small notch) instead. Undercuts let the tool reach the corner without leaving a radius.

7. Yigu Technology’s Perspective on CNC Design Mistakes

At Yigu Technology, we’ve seen every one of these mistakes delay projects or waste money. The biggest takeaway? Design for manufacturability first. Before finalizing a CAD model, ask: Can a CNC machine actually cut this? Is this tolerance necessary? Small tweaks—like adding a corner radius or simplifying a curved hole—save time and avoid rework. We often work with designers early on to review plans, catching issues before they become costly problems. By aligning design with CNC capabilities, you’ll get parts that work right the first time.

8. FAQ: Common CNC Design Questions

Q1: What’s the thinnest wall I can safely design for CNC aluminum parts?

Stick to a minimum of 0.8 mm for aluminum (6061). Thinner walls (0.6 mm or less) will vibrate and break during machining. If you need something thinner, use sheet metal instead.

Q2: Can I use EDM for all impossible-to-machine features?

EDM works for complex shapes like curved holes or sharp internal corners, but it’s slower and more expensive than CNC. Use EDM only when CNC is impossible—for most parts, simplifying the design (e.g., straight holes) is better.

Q3: Do I need to add tolerances to every surface of my CNC part?

No! Only add tight tolerances to mating surfaces (e.g., holes for bolts). Non-critical surfaces (e.g., the top of a bracket) can have loose tolerances or no tolerances at all—this cuts machining time and cost.