Fillet Engineering: A Design Guide for Machined Parts

Q: What is the smallest internal fillet radius a CNC mill can produce?

In production practice, R0.5 is the sensible lower bound for milled internal corners: smaller corner-radius end mills deflect, wear quickly and drive up cycle time. R0.5 to R2 covers most pocket work. Where the design truly demands a sharper internal transition, the corner can be sunk or finished by EDM — Boyang Hardware runs 7 EDM machines for exactly this — but that is a secondary operation with its own lead time and cost.

Q: How much does a fillet reduce stress concentration?

Substantially, and non-linearly. For a shouldered round bar in bending, increasing the fillet ratio r/d from 0.05 to 0.15 drops the theoretical stress concentration factor from roughly 2.0 to about 1.3 — the benefit flattens out beyond that. The practical rule: use the largest radius the mating parts and envelope allow rather than a token R0.2, which barely moves the number.

Q: Should a fillet on my drawing match the tool radius exactly?

Better to specify the fillet as 'R equal to or greater than X' where the design allows. If the radius is dimensioned as exactly R1.0 and our standard tool is R0.8 or R1.2, we either buy a tool or slow down for no functional gain. Tolerance the radius with a plus range (e.g. R1.0 +0.5/0) and the part is easier to make with identical function.

Written from the machining floor · Boyang Hardware, Zhongshan, Guangdong, China

Direct answer Fillet engineering is the deliberate placement of radii at transitions to control stress, manufacturing cost and part behaviour. Three rules cover most machined parts: (1) put a fillet at every re-entrant corner that carries load — sharp internal corners are crack starters; (2) size internal radii to standard corner-radius end mills, with R0.5 the practical minimum and R1–R3 the sweet spot; (3) dimension radii with a plus tolerance (e.g. R1.0 +0.5/0) instead of an exact value, so the shop can use its standard tooling without negotiation.

R0.5practical minimum milled radius
R1–R3efficient radius range
7EDM machines for sharp corners
6grinding machines for seated radii
1–2 dquotation turnaround

Why fillets exist in engineering

A sharp internal corner forces stress lines to crowd: the local stress at a re-entrant corner can be two or more times the nominal stress in the part. A radius spreads the flow. The same geometry also helps in manufacture — a radius gives the cutter a path instead of a corner it cannot reach — and in service, where radii resist chipping and crack initiation. This is the mechanical justification behind the fillet side of the chamfer vs fillet comparison, and it is why cast and moulded parts (where we also work: die casting and injection moulding) are designed radiused throughout.

Choosing a radius: the selection table

Table 1 — Radius selection by design intent, from the drawing reviews we perform before quoting. Stress-concentration factors quoted are theoretical Kt values for a shouldered bar in bending.
Intent	Suggested radius	Notes
General stress relief at a shoulder (r/d ≈ 0.15, Kt ≈ 1.3)	R1.5–R3 on typical 10–20 mm shoulders	Largest radius the envelope allows
Light-duty corner, load mostly static	R0.5–R1	Standard corner-radius tooling covers it
Fatigue-loaded bracket or arm (7075 class parts)	R2+, polished transition if specified	See AL7075 machined parts
Cosmetic edge softening	R0.5–R1	Alternative: small chamfer, see round vs fillet corners
Bolt-head or bearing seat radius	Per mating standard	Ground to size on our grinding machines when critical
Deep, narrow pocket corner	R0.5 minimum, EDM fallback	Tool access governs; flagged at quotation

Tool-radius rules and DFM notes

Every internal fillet radius must be ≥ the radius of the end mill that cuts it. Specifying R0.8 when the pocket floor is cut with an R1 tool produces either a corner mismatch or a second, slower tool path.
Depth-to-radius matters: an R0.5 corner 60 mm deep means a slender 1 mm-diameter-class tool, low feeds, real deflection risk. If the corner is not functional, chamfer it instead.
Undercut and variable-radius fillets are 5-axis work — we run 3/4/5-axis milling across 54 machine tools, but variable radii still carry a programming and proving cost worth avoiding on simple parts.
For a full geometric comparison of radiused, angled and bevelled transitions, see chamfer vs fillet vs bevel.

Mating and seat radii

Fillet engineering does not stop at one part. A shoulder fillet larger than the chamfer on the mating shaft-bearing corner will not let the bearing seat down; a radius under a bolt head must be smaller than the washer's chamfer ID. The reliable habit: pull the mating standard (bearing catalogues list the maximum fillet radius rs per size) and dimension your radius against it. Where the seat is functionally critical — bearing fits, sealing shoulders — we finish those features by grinding and measure them on the CMM, within our 0.001 mm critical-feature capability.

Honest limits: we are machinists, not a certification body for your stress analysis — Kt values here are textbook guidance, and anything safety-critical should be verified by your own calculation or FEA. Also, "biggest radius possible" collides with real assemblies: envelope, wrench clearance and sealing geometry all cap the radius. We will happily sanity-check fillet callouts during the quotation (1–2 working days) and flag any radius that forces non-standard tooling, but the final sizing call belongs to your design team.

FAQs

What is the smallest internal fillet radius a CNC mill can produce?

R0.5 is the production-sensible minimum for milled internal corners; smaller radii mean fragile tools and slow passes. Sharper transitions are sunk or finished by wire EDM — we run 7 EDM machines — at extra cost and lead time.

How much does a fillet reduce stress concentration?

Raising the fillet ratio r/d from ~0.05 to ~0.15 on a shouldered bar in bending drops the theoretical Kt from about 2.0 to about 1.3, with diminishing returns beyond. Use the largest radius the envelope allows.

Should a fillet on my drawing match the tool radius exactly?

Prefer a plus tolerance, e.g. R1.0 +0.5/0. Exact radii force special tooling or slow paths for no functional gain; a one-sided range lets us use standard corner-radius mills.

Do you review fillets before quoting?

Yes — as part of drawing review during the 1–2 working day quotation we flag radii that do not match standard tooling, conflict with mating parts, or fall below the R0.5 practical limit.