One arc apart
Set a bullnose and a square end mill of the same diameter side by side and the difference is a single arc: the square meets its face and periphery at a sharp 90-degree corner; the bullnose - the trade's other name is corner radius end mill - puts a radius there instead. Everything practical about the choice follows from that arc. The sharp corner is geometry the part often demands and the tool least enjoys providing. The radius is geometry the tool enjoys and the part must tolerate. Buying well means knowing which of those statements applies to each feature on each part, and writing the answer per line item.
What the square tool is for
The square end mill is the general-purpose rectangle of milling: slots with vertical walls, steps, shoulders, pockets whose internal corners the drawing wants sharp, and every profile where a 90-degree intersection is the specification. Where the corner is a dimension, the square tool is not optional - no radiused tool can leave a corner it does not have. The cost lives at the tool: a sharp corner is a stressed point where two cutting edges meet at zero radius, force concentrates, and carbide - hard but brittle - chips first. In soft materials and lighter duty that cost stays manageable; in harder alloys it arrives early, which is where the comparison below gets interesting.
What the bullnose buys
The radius spreads the same cutting force over a curved contact instead of a point, and the corner stops being the fragile tip that fails first. Bullnose tools resist chipping, hold their geometry longer, and so run longer in harder materials - steel, stainless, titanium, the alloys where sharp corners round and chip in short order. This is the standard trade description of the corner radius family, and it is why heavy roughing, long production runs and hard-alloy work default to it. The deeper dive on the mechanism sits in the note on corner radius benefits; the family's spec lines are on the bull nose selection page.
The part pays for the radius too
The arc transfers to the workpiece as surely as the sharp corner does: every internal corner a bullnose cuts comes out filleted at the tool's radius. That is either fine or fatal depending on the drawing, and no amount of machine skill changes it. Programs split cleanly along that line. Where the print demands sharp internal corners, the bullnose may still rough the pocket, but a square tool - or a small-radius tool within the corner tolerance - finishes the corners. Where the print specifies or tolerates a fillet, the bullnose runs everything and banks the durability. Mold and die work standardized on generous internal radii decades ago for exactly this arithmetic; the mold and die note shows the pairing in practice.
| Aspect | Square end mill | Bullnose / corner radius |
|---|---|---|
| Corner | Sharp 90-degree intersection | Radiused; value confirmed per spec |
| Corner strength | Fragile point; forces concentrate | Force spread over an arc |
| Hard alloys | Corner wear and chipping arrive early | Geometry holds; longer runs between changes |
| Internal corners on the part | Sharp, as drawn | Fillet matching the tool radius |
| Typical role | Sharp corners, steps, general milling | Heavy cuts, long runs, harder materials |
Floor finish near the walls
One difference buyers meet only at the machine: near a pocket wall, the bullnose touches the floor with its arc rather than its flat face, so floor finish close to walls behaves differently than a square tool's - and stepover planning for floor finishing adapts accordingly. Neither is better across the board; they finish differently, and programs that know it plan for it. If a customer's process qualifies floor finish by a fixed recipe, the recipe follows the geometry actually specified - one more reason the corner line on the sheet should be explicit rather than discovered by the operator.
Which one a range needs
For a catalog or a shelf, this is not either-or; the two families divide the work. Square tools cover the general-purpose volume and every sharp-corner feature. Bullnose tools cover hard alloys, heavy roughing and the growing share of parts that already specify corner radii to suit modern tooling. A range planned by material pairs them deliberately - square plus bullnose in the common diameters, with radius values selected to match the corner tolerances the market's drawings actually carry. Radius values, quantities and packing are all per-specification lines; the desk quotes them itemized through the quote form, with route lead times per the MOQ and lead time reference.
Choosing radius values across a range
A range does not need every radius value; it needs the values its market's drawings carry. The working method is to sample the corner callouts across a representative stack of customer drawings, sort them, and spec two or three radii per common diameter that cover the bulk - one small radius for parts that only just tolerate a fillet, one mid value for mold work, and whatever the local industry standard favors. Fewer, well-chosen values beat a dense ladder of SKUs that nobody orders, because shelf space and reorder discipline both cost. The values are stated per line item and confirmed per spec at quotation, which keeps the catalog honest: a radius printed on a box is a measured geometry, checked at inspection like a diameter. And when a customer's drawing calls a value off the shelf, the answer is a quotation rather than a substitution - a near-miss radius saves nothing and settles nothing. Packaging follows suit: the radius value goes on the box label beside the diameter, because a mixed pick bin cannot tell a radius by feel.
What to write on the spec sheet
The corner line names the geometry and the value: "square" for the sharp family; "corner radius, value per spec" for the bullnose, with the number filled from the drawing or the agreed reference. If the program mixes both on the same pockets - bullnose roughing, square corner finishing - write the pairing into the line items so the boxes reach the right machines. The corner line is short, but it is the line that decides tool life in hard alloys and the corners your customer measures; give it the same precision as the diameter.