Cutting Tool

Cutting tool is the hardened edge that does the actual work of machining — the part of the machine that meets the metal and removes it, chip by chip, in the shearing this wiki treats under chip formation. Every CNC machine this site describes is, at bottom, a machine for holding a cutting tool against the work and moving it in a controlled path: the machine gives the motion and the rigidity, but the tool enters the material, shears the chip and leaves the surface. A cutting tool is a wedge of material harder than the work, forced into it at a controlled angle and depth; the edge presses into the metal, the metal ahead is sheared away as a chip, and the edge passes on leaving the new surface. The variety of machining — turning, milling, drilling, threading — is variety in the shape and motion of that wedge, and the craft of machining is, in the end, presenting a good edge to the work. This entry sets out what a cutting tool is, what it is made of, and how its edge does the cutting.

The tool at the cut

The tool earns its name from what happens at its edge, and the event is the same for every tool. As the tool is fed into the work, its edge penetrates the metal; the material just ahead is compressed, shears along a plane running from the edge to the surface, and slides up the tool’s face as the chip, the waste this wiki describes under chip types. The tool does not scrape or tear; it shears, and the efficiency and finish of the cut are set by how cleanly that shear happens. Three things at the tool govern it: the edge, hard enough to cut the work and sharp enough to shear it cleanly; the rake face, up which the chip slides, whose angle decides how the chip forms and how much force the cut needs; and the flank below the edge, whose clearance lets the edge enter without rubbing. Around these the cutting forces build and the heat is generated, and the tool must withstand both — which is why it is never a simple thing but an edge of the right material ground to the right angles.

Single-point and multi-point

Tools divide first by how many edges they carry, and the division follows the machine. The single-point tool is the tool of the lathe: one edge on a shank, presented to the rotating work, cutting one surface at a time — the turning tool this wiki treats under turning, its edge shaped for roughing, finishing or threading. The multi-point tool carries many edges at once, and it is the tool of the mill and the drill: the milling cutter with teeth round its body, each taking its share of the cut in turn as it rotates; the drill with its two edges and its chisel; the tap, the reamer, the broach. The distinction shapes the cut — the single-point tool cuts continuously and can be shaped freely, the multi-point tool cuts in interrupted bites and spreads the work over many edges — but the physics at each edge is the same wedge shearing the same metal.

What a tool is made of

The material of the edge decides what the tool can do. The tool must be harder than the work — much harder, to cut it at speed — and keep that hardness when the cut heats it, and be tough enough not to shatter under load. The high-speed steel of the earlier machine is tough and cheap, ground to complex forms, still the material of drills and form tools; the carbide that dominates modern machining is far harder and faster, its edge cutting steels and irons at speeds that would destroy HSS, in the indexable insert and the solid carbide end mill. Beyond carbide lie the edges for the work that defeats it: ceramics and cermets for the hard, hot cuts; cubic boron nitride for the hardened steel; polycrystalline diamond for aluminium and the abrasive non-metals; and the coatings laid on carbide to make it harder, slicker and more heat-resistant still. Each material is a bargain of hardness against toughness.

The geometry of the edge

The same material, ground differently, is a different tool, and the geometry that this wiki treats under tool geometry and angles is the second half of its making. The rake angle decides how the chip forms — how easily it flows, how much force the cut needs, how strong the edge must be; the clearance under the edge lets it cut without rubbing; the lead of the edge shapes where the cut begins; and the nose radius blends the edge and governs the finish it leaves. A tool that cuts well is one whose geometry matches its material and its work — a sharp, positive-rake edge for the soft and the fine, a robust negative-rake edge of carbide or ceramic for the hard and the interrupted. Geometry is also what the tool loses as it wears: the sharp edge rounds, the rake craters, and the tool that sheared cleanly begins to rub and tear.

The life of the tool

No cutting tool cuts for ever, and the way it ends is the subject this wiki treats under tool wear. The edge wears by the mechanisms any edge wears by — the flank wears back as it rubs the new surface, the rake craters as the hot chip flows over it, the edge notches where scale or a hard skin strikes it, and at the last the edge may chip or break. The machinist reads that wear, changes the tool or indexes the insert at the right moment, and manages the tool’s life by the speeds and feeds at which it cuts and the coolant that keeps the cut from destroying the edge. Much of machining’s craft is getting the most from that edge — changed, indexed and replaced so the machine keeps cutting and the parts come true.

The tool in the machine

The cutting tool is the smallest and the most important part of the machine, and every entry in this wiki’s tooling group is an entry about some form of it. It is held by its holder, driven by the spindle, moved by the axes; but the tool is where machining happens — the edge that meets the metal. When the machinist sets a job — turning a shaft, milling a pocket, drilling a hole — the choice that matters most is the tool: its material, its geometry, its edge. This entry is the anchor of that group, the definition of the cutting tool itself; the entries that follow — from the materials tools are made of to the wear that ends them — are the anatomy of the little wedge of hard material that, driven by the machine and guided by the program, turns a block of metal into a part.