Round vs Fillet Corners: Full Radius, Partial Radius or Chamfer?

Q: What is the difference between a round corner and a fillet corner?

A round corner (full radius or bullnose) replaces the entire end or edge with a semicircle — the part width equals twice the radius, and there is no flat left at all. A fillet is a partial arc that blends two faces at a corner while the faces themselves remain. A round end is a single feature covering the whole edge; a fillet is a transition. On a drawing, a plate with fully rounded ends is dimensioned by width (e.g. 12.0 wide, full R6 ends), while a filleted corner carries an R callout at the corner only.

Q: When is a fully rounded end worth the extra machining cost?

When the end itself does a job: sliding contact where a chamfer would dig in, parts that must locate by their rounded end, safety-critical hand contact, or wear parts where a sharp corner would chip. A full-radius end needs a bullnose (full-radius) end mill whose diameter equals the part width, so wide parts need large, expensive tools or slower contouring. For pure edge relief, a small chamfer remains cheaper.

Q: Do rounded corners matter for anodised or plated finishes?

Yes — a real advantage of radiused geometry. Anodising, zinc and chrome plating build up unevenly on sharp corners: thin at the point, prone to burning on external corners. Radiused corners accept a far more uniform coating, so parts destined for anodising or plating benefit from at least a small edge radius or generous chamfer before finishing.

Edge geometry guidance from the shop floor · Boyang Hardware, Zhongshan, Guangdong, China

Direct answer A round corner (full radius, "bullnose") replaces the whole end or edge with a semicircle: on a 12 mm-wide plate, fully rounded ends are R6 and no flat remains. A fillet is a partial arc blending two faces at a corner while the faces continue. Rule of thumb: full-round the end when the end does a job (sliding contact, locating, handling safety, wear life); fillet the corner when stress or cosmetics call for a blend; chamfer everything else. Full-round ends cost more to machine because tool diameter must match part width.

R = W/2full-round rule
Bullnosetool for full-radius ends
R0.5–R3typical fillet range
54machine tools on site
1–2 dquotation turnaround

Round vs fillet vs chamfer corner: comparison table

Table 1 — Corner treatment options as they appear on machined parts we produce.
Attribute	Full-round end (bullnose)	Fillet corner	Chamfer corner
Shape	Semicircle spanning full width	Arc blending two faces	Angled flat
Dimensioning	Width 12.0, ends full R6	R2 at corner	C0.5 / C1
Flat remaining on edge	None	Yes, between radii	Yes
Best for	Sliding ends, locators, hand tools, wear parts	Stressed corners, cosmetic softening, cast/moulded style	Edge break, assembly lead-in
Tooling	Bullnose mill Ø = part width, or contour	Corner-radius end mill R = callout	Chamfer mill / spot drill
Relative cost	Highest of the three	Moderate	Lowest
Stress benefit	Excellent at the end	Good, scales with radius	Minimal

The mechanical side of fillet sizing is covered properly in our fillet engineering guide; the flat chamfer option is compared in chamfer vs fillet.

Tooling implications of each choice

A full-round end locks the tool diameter to the part width: a 12 mm-wide part with fully rounded ends wants a Ø12 bullnose end mill in one pass, or an end-mill contour path for larger widths. Tool cost grows fast above Ø12, and very wide rounded ends are contoured on our 4-axis and 5-axis centres instead. Fillets are kinder: any corner-radius mill matching the callout works, and R1–R3 tooling is standard stock. This is the same tool-access logic that governs internal pocket corners — the R0.5 production floor and the EDM fallback are described in the fillet guide and in chamfer vs fillet vs bevel.

Corners and surface finishing

Radiused geometry is friendlier to post-processing than sharp corners. Anodising and plating deposits thin out on sharp points and can burn on sharp external corners; powder coating pulls back from sharp edges. A corner that is even slightly radiused or chamfered takes a noticeably more uniform finish — one reason machined parts that will be anodised (Type II or hard) almost always carry edge radii. Common finishes we arrange: anodising, hard anodising, zinc and chrome plating, powder coating, polishing and sand blasting.

How to call it out on a drawing

Full-round ends: dimension the width and note "ends full radius" — never a separate R smaller than half the width, which is geometrically impossible.
Fillets: R callout with a plus range where function allows, e.g. R2 +0.5/0.
Mixed parts: "break all sharp edges C0.3–C0.5 unless otherwise specified" covers remaining corners in one line.

Honest limits: a full-round end on a wide or thick part is genuine machining, not a free cosmetic upgrade — tooling, cycle time and inspection all rise with width, and on very wide parts we will quote contour machining instead. Also, full rounds remove the flat face some fixtures need for gripping and datum-ing, which can add set-up cost on second operations. If cost is the driver and the corner has no function, take the chamfer. We will say which applies when we quote your drawing within 1–2 working days.

FAQs

What is the difference between a round corner and a fillet corner?

A full-round end is a semicircle spanning the entire width — no flat remains; a fillet is a partial arc blending faces that continue. Round ends are dimensioned by width (full R = width/2); fillets carry an R callout at the corner.

When is a fully rounded end worth the extra machining cost?

When the end functions: sliding contact, locating, hand-contact safety, or wear life. Pure edge relief does not justify it — a C0.5 chamfer is cheaper.

Do rounded corners matter for anodised or plated finishes?

Yes. Coatings build unevenly on sharp corners; radiused or chamfered edges accept a much more uniform anodised or plated layer.