End Mills & Ball Nose Cutters

End mills are the rotating cutters that do the bulk of the work on a machining centre, the tools that cut with their end and with the teeth along their sides, slotting, profiling, pocketing and surfacing metal by the hour. An end mill is not a drill, which cuts only with its tip to make a hole; it is the tool of milling, cutting sideways and in, sweeping a shape across the work as the machine feeds it. And an end mill is not one tool but a family, its members differing in the shape of their cutting end and in the number of their flutes: the square-end mill for the sharp shoulder and the straight wall, the ball-nose cutter for the sculpted 3D surface, the corner-radius end mill between them for strength. This entry sets out what an end mill is, what the main types do, and how to read the cutter a job calls for.

The parts of an end mill

An end mill is a cylinder of solid tool steel or carbide with cutting teeth cut into it, and its working parts are its end and its flutes. The flutes are the helical grooves that spiral up the body, their sharp edges the teeth that cut; each flute, as it spins, shears a slice of metal off the work, and the groove behind the edge carries the chip away. At the bottom, the end teeth cut across the face of the tool — a square-end mill cutting flat-bottomed slots and square shoulders, and when the end teeth meet at the centre the mill is centre-cutting, able to plunge straight down into the work like a drill to start a slot or a pocket. The tool’s size is the diameter of the cylinder — the 6, 10, 12 or 20 millimetre that names it — and its shank is the plain upper end that the holder grips. The holder that grips the shank, the spindle speed and the feed, the rigidity of the whole assembly, decide how well the teeth can cut.

The three ends

The end mill family is named by the shape ground on its cutting end, and the three common shapes are the square, the ball and the corner radius. The square-end mill — the flat-bottomed, straight-sided cutter, also called the flat end mill — is the generalist: it cuts square shoulders, straight walls and flat-bottomed slots, and its sharp ninety-degree corner leaves the crisp edge that many parts call for. The ball-nose cutter (the ball end mill) has a hemispherical end, its whole nose a continuous radius the diameter of the tool; it leaves no corner and no flat, so it is the tool of the sculpted and the contoured, as the next section describes. Between them stands the corner-radius end mill (the bull-nose cutter), a square mill whose sharp corner is blended into a small radius; the radius strengthens the corner, which would otherwise be the first part of a square mill to chip under load, so the corner-radius tool is the favoured roughing mill for steel, cutting hard where a square corner would break. Choosing among them is choosing the shape the part leaves behind.

The ball nose for 3D

The ball-nose cutter earns its place on the machines that cut free-form surfaces, and its working is worth understanding. Because its end is a hemisphere, the ball nose presents a radius to the work from every angle, so it can reach into a concave form and sweep over a convex one without the sharp corner of a square mill gouging the surface — the tool of the mould, the die and the sculpted part. The ball nose cuts with a point contact rather than a full-diameter sweep, so to cover a surface the machine feeds it over the work in parallel passes, and the stepover — the sideways distance between passes — sets the finish: the smaller the stepover, the finer the scalloped ridges left between passes, down to the smooth surface that this wiki treats under surface finish. The trade of the ball nose is that most of its cutting happens near its slow-moving centre, so fine 3D finishing is slow work; the shop balances the stepover, the speed and the time to get the surface it needs.

Flutes and their bargain

Alongside the shape of the end, the number of flutes — the teeth — sets the character of the cut, and the count is a bargain between chip space and finish. A two-flute end mill has large, open gullets between its two teeth, so it swallows the big, thick chips of a slotting cut and clears them from deep slots, and its wide centre lets it plunge well; it is the cutter of the deep slot and the soft material. A four-flute mill has twice the teeth on the same circle, so each tooth takes a thinner bite at the same feed — the finer finish, the steadier cut and the higher feed on steel — but its gullets are smaller, so it cannot swallow the heavy chips of a deep slot and it clears less well. Three-flute mills sit between, and the five-or-more-flute mills, their teeth crowded for the finest finish at high feed, are the cutters of the finishing pass on aluminium and steel. The shop reads the count against the job: the open two-flute for the deep slot in soft metal, the many-fluted for the fine finish on the feeds and speeds the cut allows.

End mills at work

The end mill does its work in the grip of the spindle, and how it is held and run decides what it can do. The cutter sits in its holder at the end of the spindle, running at the speed and fed at the rate that the material and the tool call for — the feeds and speeds that this wiki treats — with the rigidity of the machine and the truth of the runout determining whether the cut is smooth and the finish fine. Its teeth cut in one of two directions: conventional milling, the cutter climbing against the feed, the safe and simple way; or climb milling, the cutter sweeping with the feed, the better finish on a rigid machine, as the milling entry describes. In use the end mill is the most flexible tool the cutting-tool family offers, and the machinist’s rack holds the set of them — the square ends, the ball noses, the corner radii in the sizes and flute counts the work has taught them to keep — reaching for the cutter whose shape and teeth match the shape and the finish the part demands. From the straight slot to the sculpted mould, the end mill is the cutter that shapes metal by sweeping across it, its end and its flutes cutting the part out of the solid.