Facing
Facing is the turning operation that cuts the end of a rotating workpiece square and flat, the first cut that most turned parts receive. A bar of stock comes from the saw or the bar feeder with an end that is anything but true — ragged, out of square, and of no use as a reference. Facing makes that end a true one: the work spins in the lathe or the turning centre, and a single-point tool is fed across the end, cutting a fresh, flat surface perpendicular to the axis of rotation. The faced end becomes the datum of the part — the surface from which its length is measured and against which it is later held, chucked or located — which is why facing is the operation that a turned part meets first, and why a faced end that is not square and flat spoils everything that follows. This entry sets out how facing is done, what makes a good face and where facing is used.
How a face is cut
Facing is cutting across a rotating end, and the geometry of it is simple to picture. The work is held in the chuck or on a faceplate, spinning about its axis; the tool is set with its cutting edge at centre height — level with the axis — so that the tool cuts cleanly on both sides of centre; and the tool is fed across the end of the work, from the outside diameter towards the centre, or from the centre outwards. As the tool moves across, it shears a thin layer from the whole face, and because the face is rotating about a true axis, the surface the tool leaves is flat and perpendicular to that axis: a plane at right angles to the axis of rotation, which is exactly the square, true end that a datum needs. The depth of the cut is the thickness of the layer taken from the face, and the feed is the rate at which the tool moves across it; the cutting speed is the surface speed of the work where the tool cuts, which falls towards the centre as the radius shrinks, so the cutting conditions change across the face.
Feeding in and out
The direction of the facing feed is a matter of practice, and the choice turns on the finish and the centre. Feeding from the outside in — from the diameter to the centre — is the common way: the tool starts at the OD, where the surface speed is highest and the cutting is easiest, and cuts towards the centre. As it nears the centre the surface speed falls to nothing, and if the tool cuts right to the axis it will meet the dead centre of the work, where the metal is not moving past the edge, and push a small pip or leave a rough nub at the centre of the face; the practised machinist feeds the tool short of the centre and breaks the last sliver, or angles the tool off as the centre approaches, leaving a clean face. Feeding from the centre out avoids the problem — the tool starts at or past the centre and moves outward into metal that is moving faster — but it needs the tool to reach the centre first, and it tends to push the work against the chuck rather than into it. The good facing cut, whichever way it runs, leaves a flat face with no pip at its centre.
The finish of the face
A faced end is judged by its flatness and its finish, and the two are made by the cutting, not by accident. The flatness of the face is set by the trueness of the machine and the setup: the face is flat because the tool path is straight and the axis is true, so a spindle that runs true and a tool held rigidly cut a flat face, while a machine out of true, a worn chuck or a tool that flexes under the cut leaves a face that is high at one edge or dished across its width. The finish of the face is set by the tool and the feed: the tool’s nose radius and its feed across the face leave the fine helical ridges that the surface finish of the face measures, so the finishing facing cut uses a fine feed and a tool with a generous nose radius, cutting a light skim to leave the smooth, flat face that a datum must have. The face that carries a part’s length measurement, its location or its seal is finished by facing as carefully as any bore or journal.
Facing in the shop
Facing is everywhere in the turning shop, and it appears in the operations around it. A turned part is usually faced first, its ragged saw end cut square so that the length can be measured and the part can be held against a true face; it is faced again before parting off or after, to give each end its final squareness; and a casting or forging is faced to clean up its surface and bring its face to the datum from which the rest is machined. Facing is also the operation that establishes the length: the faced end is the zero from which the turning operations measure, so the facing cut is made first and the rest of the part’s features are cut relative to it. On the milling side, the same idea of cutting a flat end face is done by face milling — a rotating cutter sweeping the face rather than a single-point tool — and the two are the turning and milling answers to the same need for a square, true surface.
The facing tool
The tool that faces is the same single-point cutting tool that turns, and it is chosen for the facing cut’s special conditions. It is held in the toolpost or the turret at centre height, its geometry — the rake and clearance — ground or set for the material it cuts, its edge carrying the carbide insert or the high-speed-steel edge of the turning tool. Facing differs from turning in one respect that the tool must suit: the tool cuts across the end rather than along the work, so its approach angle and its nose are set so that the trailing edge of the tool does not drag on the newly cut face as it feeds across, and its feed and speed are set to the face’s changing surface speed, as the feeds and speeds entry describes. Set right, the facing tool cuts the flat, smooth, square end that the rest of the part depends on — the modest first operation that makes every turned part’s length and its location true.