How to Match Fillet Radius of CNC Parts with Post-Processing? Avoid Coating Peeling

Created on 10.14

For mechanical engineers, design engineers, and procurement managers in medical devices, robotics & automation, and automotive R&D, coating peeling on CNC parts is a costly frustration. A peeled coating doesn’t just ruin the part’s appearance—it compromises critical functions: medical device housings lose sterility, robot arm components become vulnerable to corrosion, and automotive prototype parts fail durability tests. One of the most overlooked causes? Mismatched fillet radius and post-processing. Get this pairing wrong, and even high-quality coatings (like powder coating or anodizing) will lift off. Below’s how to match them correctly, plus how Marigold Rapid (www.marigold-rapid.com) ensures your CNC parts stay coated and compliant.​

Why Fillet Radius Matters for Coating Adhesion​

Fillet radii (the rounded edges on CNC parts) affect how coatings bond during post-processing. Sharp corners or too-small radii create “thin spots” in the coating—paint or powder can’t fully cover the edge, leading to peeling. Too-large radii, meanwhile, waste material and may disrupt part fit (e.g., a too-big fillet on a medical sensor mount might prevent proper assembly). The key is to align the radius with your chosen post-processing method.​

3 Common Post-Processing Methods: Match Them to Fillet Radii​

1. Powder Coating (for Robotics & Automotive Parts)​

Powder coating is tough and resistant to impact—ideal for robot chassis or automotive prototype frames. But it requires slightly larger fillets to ensure even coverage:​

Recommended fillet radius: 1.5–3mm

Why: Powder particles are thicker than liquid paint; a 1.5mm+ radius lets powder flow into edges without gaps. For example, a robot arm bracket with a 1mm fillet will likely peel at the corners, while a 2mm radius keeps the coating intact.

Pro tip: Avoid radii larger than 3mm for tight-fit parts (e.g., automotive dashboard components) to prevent assembly issues.

2. Anodizing (for Medical & High-Precision Parts)​

Anodizing creates a thin, corrosion-resistant layer—perfect for medical device enclosures (e.g., diagnostic tool casings) or aluminum robot sensors. It works best with smaller, precise fillets:​

Recommended fillet radius: 1–2mm

Why: Anodizing is a chemical process that penetrates the metal surface; a 1mm+ radius avoids sharp edges that trap acid (causing uneven coating) but stays small enough for precision fits. A medical instrument handle with a 0.8mm fillet may peel, while a 1.2mm radius ensures uniform anodization.

3. Electrophoretic Coating (for Complex Medical/Automotive Parts)​

Electrophoretic (e-coat) coating is ideal for parts with intricate shapes—like medical valve housings or automotive engine components—since it covers hard-to-reach areas. It needs balanced fillets:​

Recommended fillet radius: 1–2.5mm

Why: E-coat uses an electric current to deposit paint; a 1mm minimum radius prevents “edge effects” (thin coating at sharp corners), while 2.5mm max keeps the part’s geometry functional.

2 Quick Tips to Avoid Coating Peeling​

Confirm Post-Processing Early: Choose your coating method before finalizing fillet radii. Our DFM team at Marigold Rapid reviews designs upfront—we’ll suggest a 2mm radius for a powder-coated robot part, or 1.5mm for an anodized medical component—so you don’t have to redesign.

Test with Prototypes: For critical parts (e.g., orthopedic tool housings), order a small-batch prototype first. We produce 5–10 test parts with your chosen radius and coating, letting you check adhesion before full production.

How Marigold Rapid Ensures Perfect Fillet & Coating Matching​

We don’t just machine CNC parts—we optimize them for post-processing:​

One-stop service: We handle both CNC machining (with precise fillet cutting) and post-processing (powder coating, anodizing, e-coat) in-house. This eliminates miscommunication between separate shops and ensures radii match coatings.

DFM expertise: Our English-speaking engineers review your CAD files for free, flagging mismatches (e.g., a 0.8mm fillet for powder coating) and suggesting fixes within 24 hours (our fast response keeps your R&D on track).

Compliance for medical parts: Backed by SGS ISO 9001/13485, our anodized medical parts meet sterility standards—no peeling coatings, no regulatory risks.

For teams tired of coating peeling ruining their CNC parts, Marigold Rapid is your solution. Contact us today to design parts with fillets that match your post-processing—no rework, no wasted time.