Milling Process
Milling is the machining process in which a rotating cutter with several cutting edges is fed against a workpiece to remove material. Unlike turning, where the work spins and one edge cuts continuously, in milling the cutter spins and the work is fed past it, so the operation is not tied to round stock — it machines flats, slots, pockets, holes, shoulders and sculpted forms alike. This wiki’s entry on the milling machine covers the machine; this one is about the process itself: how the interrupted cut works, the two geometric families it divides into, and the parameters that govern it.
The interrupted cut
The single fact that shapes milling is that each cutter tooth cuts only part of the time. A tooth enters the work, takes its chip, leaves, and does not cut again until the cutter has rotated round to the work once more. Cutting is therefore a rapid sequence of small impacts rather than a steady push, and that one difference explains most of milling’s character. The impacts make milling harder on the tool than turning — each entry is a shock that a brittle edge may not survive, which is why milling tooling and grades are chosen with toughness and edge strength in mind. The fluctuating forces also tend to set the machine vibrating, so chatter, not steady force, is usually what limits how deep and fast a milling cut can go. And because the tooth is out of the cut between engagements, it cools briefly each revolution — a cycle of heating and cooling that is gentle on a well-run carbide edge but punishing on one pushed too hard.
Chip formation in milling is correspondingly special. Each tooth cuts a chip that starts thin and thickens, or starts thick and thins, depending on direction — the subject of this wiki’s entry on climb versus conventional milling — and the average chip is much thinner than the feed would suggest, a phenomenon called chip thinning that matters whenever a small cutter takes a shallow cut.
Two families: peripheral and face
Milling operations divide by where on the cutter the cutting edges do the work. In peripheral milling — also called plain or slab milling — the tool axis lies parallel to the surface being cut and the edges on the cutter’s outside diameter do the cutting. It is the family of slots, keyways, side cuts and the long flat surfaces cut by a horizontal-spindle slab mill: the width and depth of the cut are set by the cutter, and the finished surface is generated by the side of the cutter rather than its end. In face milling the cutter axis is perpendicular to the surface, and the teeth on the cutter’s end and periphery do the work; the cutter is larger than the feature and wipes a flat surface as it passes. Face milling is the standard way to produce a flat, square face quickly and is the operation behind the big indexable face mills found on machining centres.
Everything in between is end milling: a cutter smaller than the work is driven like a rotary file to follow any path — slotting a groove, pocketing a cavity by sweeping side to side, profiling a contour, or plunging and interpolating a hole. End milling is where the three-axis machining centre earns its versatility, because the same small cutter and the machine’s moving table can trace two-dimensional paths and, with depth control, three-dimensional forms. Peripheral and face milling are the classic textbook pair; in a modern shop most milling is end milling, and the same interrupted-cut mechanics govern all of them.
The parameters of a milling cut
Because the cutting is done by a rotating cutter, milling parameters are expressed differently from turning’s. The cutting speed belongs to the cutter’s outside diameter and sets the spindle speed; a small cutter must spin far faster than a large one to run the same surface speed. The feed is set per tooth — the chip each edge takes, called the chip load — and the table feed rate is the product of spindle speed, number of teeth and feed per tooth. Two depths define every cut: the axial depth (how deep the cutter engages along its axis) and the radial or width of cut (how much of the cutter’s diameter engages sideways). Roughing takes the biggest depths the machine and workholding can hold; finishing takes light depths and feeds tuned to the required surface. Chip thinning, mentioned above, means a small radial engagement produces an unexpectedly thin chip, so feeds are often raised to keep the tooth actually cutting rather than rubbing — a subtlety handled by the cutting-data logic of feeds and speeds.
What milling does well
Milling’s strength is geometry. Because it does not depend on the workpiece rotating, it machines the prismatic world — blocks, plates, housings, brackets, moulds and dies — and because the cutter can be moved in two dimensions and stepped in the third, it produces shapes turning cannot: flat and curved faces, slots, pockets, drilled and tapped hole patterns, bosses, threads, and three-dimensional contours. A single setup on a milling machine can reach several sides of a part and chain every operation — facing, slotting, pocketing, drilling, tapping — before the part is moved, which is why the machining centre is the general-purpose machine of production. Milling removes metal at high rates on flat and broad work, and its finish is governed by feed, geometry and the same stability limits as every process.
Milling has its own limits. It is interrupted cutting, so it is harder on tools and more prone to chatter than turning; it does not naturally produce the concentric roundness that turning gives a body of rotation; and its surfaces are made by a moving cutter, so accuracy depends on the machine’s positioning truth rather than on a fixed axis of rotation. Round parts with concentric features still belong to turning, prismatic parts belong to milling, and parts that are both belong to a machine that can do both — the same division this wiki draws between the milling machine and the turning machine. Within milling itself, the direction of the cut is decided by climb versus conventional milling, the cut data is set under feeds and speeds, and the chips it makes are explained by chip formation — all part of the practice of CNC machining.