Gear Cutting Machine
Gear cutting machine is the collective name for the machine tools built to make gear teeth. Where the general machines this wiki treats — the milling machine, the machining centre — remove metal to arbitrary shapes, the gear cutting machine has a single demanding purpose: to generate teeth of an exact shape, spaced exactly round a wheel, so that one gear rolls against another smoothly and silently. The tooth shape is nearly always the involute, a curve chosen because two involute profiles roll together at a constant velocity whatever the centre distance, and the difficulty of cutting it is that a straight-sided cutter cannot simply be plunged in: the involute has to be generated, the cutter and the blank moving together in a controlled way that rolls the tooth form onto the wheel. That generation is the heart of the gear cutting machine, and it takes two classic forms — the hobbing machine and the gear shaping machine — with grinding machines finishing the teeth that must be hard and quiet. This entry sets out what these machines are and how each generates its teeth.
Cutting teeth by generation
Gear teeth are not milled one at a time like pockets and slots; they are generated, and the difference is the whole of the subject. When a gear tooth is generated, the cutter and the blank are driven in the relative motion of two gears meshing, and the tooth profile is rolled onto the blank as the cutting edges sweep through it — the cutter acts like a gear or a worm meshing with the wheel being cut, and its cutting edges, moving in that mesh, produce the involute by the geometry of the motion itself. The advantage of generation is accuracy and consistency: because every tooth is produced by the same rolling action, all the teeth come out identical and correctly spaced, and one set of cutter settings cuts any gear that meshes with the same basic rack or hob. Generation stands against form cutting, in which a cutter shaped like the tooth space is fed into each gap in turn, simpler and right for large gears and one-offs, but carrying the cutter’s own shape and wear into every tooth. The machines of this entry are generating machines, each arranging that motion in its own way.
The hobbing machine
The hobbing machine is the workhorse of gear cutting, and the fastest way to cut a gear in quantity. Its cutter is a hob: a screw-like tool whose gashed flanks form a spiral of cutting edges, like a worm with teeth. The hob is mounted on a spindle angled to the helix of the gear to be cut, and it is rotated in time with the gear blank, which turns on the machine’s table beneath; the ratio between the two rotations is set so that, as the hob turns one revolution, the blank advances by the number of teeth being cut — and as the hob is fed across the face of the blank, each of its gashed edges takes a cut and the rolling motion generates the involute round the whole wheel. The result is a continuous, overlapping cut that produces a finished gear in a fraction of the time a form cutter would take, which is why hobbing machines cut the great volume of the world’s gears — the spur gears, helical gears and worm wheels of gearboxes and drives. A hob is a precision tool, ground to a master form, and its wear governs the accuracy of every tooth it cuts.
The gear shaping machine
The gear shaping machine takes a different route to the same tooth, and it reaches the places the hob cannot. Its cutter is a shaper cutter: a short cylinder of gear-tooth form, sharpened on its end faces, mounted on a vertical spindle. The shaper cutter is driven up and down in a rapid reciprocating stroke, cutting on the downstroke, and the blank is fed slowly into mesh with it while both the cutter and the blank rotate in the ratio of two gears meshing — the cutter generating the teeth by the same rolling principle, but cutting with a straight reciprocating motion instead of the hob’s rotating spiral. Because the cutter approaches from the side rather than over the top, the gear shaper can cut a gear that the hob cannot reach: a gear close to a shoulder or a flange, where the hob’s travel across the face would collide, and above all an internal gear, a ring with teeth cut round its bore. The shaping machine is slower than the hobber, tooth by tooth, but its reach makes it essential wherever gears must sit against a step or inside a ring.
Grinding the teeth that must be quiet
The teeth a hob or shaper cuts are accurate, but they are cut in steel soft enough to machine, and many gears must be harder than that. The gear that carries load in a gearbox is usually cut soft and then hardened — the heat treatment this wiki describes distorts the teeth, moving them enough to spoil the accuracy of the cut — and a hardened gear that must run quiet and carry load needs its teeth ground afterwards. The gear grinding machine does this: it is the grinding machine specialised for tooth flanks, its abrasive wheel grinding the hardened teeth to the final involute and spacing, removing the distortion of hardening and delivering the accuracy and surface a quiet, long-lived gear demands. Gear grinding is the most expensive step in gear making, so it is reserved for the gears that justify it — transmission gears, instrument gears, gears whose noise and life depend on teeth true to a few micrometres. Between the cutting machine and the grinder, the tooth is born soft, hardened, and ground true.
Gear cutting in the modern shop
The gear cutting machine has followed the whole machining family into numerical control, and the modern CNC gear machine is as much a product of the craft this wiki describes as any machining operation. The ratio between hob and blank that once was set by change gears is now commanded by servos, so a machine sets up for a new gear by program in minutes; the feeds and the depth of cut are controlled precisely, the machines cut dry with the latest coatings or flooded with the right cutting fluid, and the same machine handles hobs, skiving cutters and grinding wheels for roughing and finishing in one setup. The gear that once was cut on a machine of its own is increasingly finished on a machining centre by gear milling and skiving, and cut fast in volume on dedicated hobbers and shapers that still make the millions of teeth the world turns with. The gear cutting machine remains what it has always been — the machine tool specialist that generates the involute — now driven by the same numerical control and the same pursuit of accuracy as every machine this wiki treats, making the teeth that mesh together and carry the load of every drive.