Chamfer vs Fillet - Cost-Effective Edge Finishing

Exterior chamfer (C), interior fillet (R), stress concentration prevention, cost-saving design

When designing edge treatment—radius (R, fillet) or chamfer (C)—novices choose aesthetics. Experts consider toolpath mechanics. This choice can double machining time and cost.

1. External Edges: Chamfer (C) is Cheaper

For external corners, prefer chamfer (C).

Reason: Chamfering uses standard chamfer mills or end mills in one pass. External radii require specialized corner rounding cutters or dozens of ball end mill passes (3D machining).
Practical Tip: Unless hand contact or aesthetics demand otherwise, C0.5-C1 chamfers are most economical for external edges.

2. Internal Corners: Radius (R) is Mandatory

End mills are cylindrical rotating tools. Sharp internal corners are physically impossible without expensive processes like EDM.

Practical Tip: Internal corners need R larger than tool radius. For Ø10mm end mill, use R5.5+ for smooth toolpath (exact R5 causes tool chatter and marks).

3. Functional Consideration: Stress Concentration

Beyond cost, consider strength. Load-bearing step features require radii.

Stress Concentration: Sharp corners and chamfers concentrate stress, causing cracks. For cyclic loading, use gradual fillets (R) to distribute stress.

4. Chamfers for Assembly, Radii to Avoid Interference

Assembly Lead-In: Large chamfers on bolt holes and shaft ends guide smooth insertion.
Interference Avoidance: If mating part has R corner, design larger C to clear it.

💡 Expert Advice
"Stop defaulting to R3 on all edges. When quotes come back shockingly high, check for unnecessary external radii. Remember: external=C, internal=R. This formula alone earns you 'clean drawing' praise from shop supervisors."

Chamfer vs Fillet — US Practice

US drawings use 45° chamfers as the default unless angle is specified — common practice on shaft ends, hole entries, and edge breaks. ASME Y14.5 governs notation; ISO 13715 covers edge condition specs more explicitly. For fatigue-critical fillet radii, AISC and AGMA design guides specify minimum r/d ratios (≥0.1 typical) — undersized fillets are the most common stress concentration error in US fabrication shops.

Inspection

Chamfers and fillets are typically not dimensioned for general parts — visual inspection against the first article suffices. For aerospace and pressure vessel applications, ASME PCC-2 and AWS D17.1 specify inspection methods. CMM probing of fillet radius is rare; profile gauges and shadow projectors (Mitutoyo PJ series) handle production inspection.