Face vs Peripheral Milling

Face milling and peripheral milling are the two ways a milling cutter can engage the work, and between them they cover the milling that this wiki treats as an operation. A milling cutter is a rotating tool with teeth on its end, its side, or both, and the two methods differ in which of those teeth does the cutting and what surface they leave. Face milling cuts with the cutter’s end — its face — sweeping a flat surface that lies across the cutter, perpendicular to the spindle: the tool that opens and finishes the top of a block or the face of a part. Peripheral milling cuts with the cutter’s side — its periphery, the teeth along its circumference — cutting surfaces that run parallel to the cutter’s axis: the slots, the walls, the shoulders and the profiles that a milling cutter makes with its flutes. Both are milling, both use rotating multi-tooth cutters, but the surface they make is set by which edge they cut with. This entry sets out the two methods and when each is used.

Face milling: cutting with the end

Face milling is the method of the flat surface, and it is the milling that a face mill does. The cutter is mounted with its axis perpendicular to the surface to be cut, and its teeth — the inserts round its face — sweep that surface as the cutter is fed across it; because the teeth cut across the full width of the cutter in a plane, the surface they leave is flat and perpendicular to the spindle, a smooth plane across the top of the work. Face milling is the way a block of stock is opened to a clean, flat reference, the way a casting’s face is machined to its seat, the way a wide surface is finished flat and square — and it is the high-metal-removal method, the cutter sweeping a wide path and its many teeth sharing the cut, removing the top of the work in passes. The face-milling cutter is often a large indexable body carrying carbide inserts, and its finish, its flatness and its squareness to the machine’s axis are the qualities that make the flat face that parts are located from.

Peripheral milling: cutting with the side

Peripheral milling is the method of the slot, the wall and the shoulder, and it is the milling that an end mill or a slab mill does with its circumference. The cutter’s axis runs parallel to the surface it cuts, and the teeth on its side — the flutes of an end mill, or the teeth round a plain cylindrical slab mill — cut the surface as the cutter rotates and is fed along it; the surface they leave is parallel to the cutter’s axis, the vertical wall of a slot, the side of a shoulder, the straight side of a pocket. Peripheral milling cuts the features that face milling cannot: the slot cut into the top of the work, whose walls and floor are made by the cutter’s side and end together; the square shoulder that steps the work down; the keyway, the T-slot, the dovetail; and the profile, when the cutter is driven round the outline of a part, its side cutting the shape. The accuracy of peripheral milling is the accuracy of the cutter’s diameter and its runout, which is why the width of a slot and the squareness of a wall are the qualities this method controls.

The surface each leaves

The two methods leave surfaces in different attitudes to the machine, and the difference is the key to choosing between them. Face milling leaves a surface perpendicular to the spindle axis — a horizontal plane on the vertical mill, across the top of the work — so it is the method of the flat face that must be flat and smooth and true to the machine. Peripheral milling leaves a surface parallel to the spindle axis — vertical walls and the sides of features — so it is the method of the straight side that must be straight and square and of the right width. The same cutter can often do both — an end mill cuts with its end as well as its side — but the two methods are distinct in their nature and their finish: face milling’s flat sweep gives a broad, smooth plane, while peripheral milling’s side cut gives a straight, accurate wall, and each feature of a part is milled by whichever method makes the surface it needs.

When each is preferred

The choice between the methods follows the shape of the part and the economy of the cut. Face milling is preferred for the broad, flat surface: it removes metal fast, its wide sweep and many teeth cutting across the face with a good finish and a flat result, so the top of a block, the face of a casting or the wide land is face-milled to take the metal off quickly and leave the reference surface true. Peripheral milling is preferred for the feature that is defined by its sides: the slot must be its width, the shoulder must be square, the profile must be its shape, and those are cut by the cutter’s periphery, the depth of the feature limited by the cutter’s flute length and its width by its diameter. In practice a part is milled by both — its faces opened and finished by face milling, its slots, steps and profiles cut by peripheral milling — and the two methods share the machine, the speeds and the feeds that the milling operation allows.

The two methods on the machine

Face milling and peripheral milling are the two halves of what the machining centre does with a rotating cutter, and they belong to the same family of operations. Both are interrupted cutting — each tooth cuts, then leaves the work, then cuts again — with the chip load, the speeds and the feeds set by the cutter and the material; both need a rigid machine and a true-held cutter, for the flatness of the face and the straightness of the wall both come from the truth of the spindle and the holder. The modern shop uses the two without always naming them: the face mill sweeping the top of the part, the end mill slotting its features, each doing the part of the milling that its cutting edge is made for. Between them the face and the periphery of the milling cutter make every surface that milling makes: the flat face, cut by the end; the straight wall, cut by the side.