Cutting Tools

§1The cutting wedge

A cutting tool is a hardened wedge. Driven into the workpiece, it shears a layer of metal ahead of it into a chip that flows up the tool face — cutting is controlled shearing, not scraping.

Three things must be true for it to work: the tool must be much harder than the work (the materials section covers how tool steels and carbides achieve that), it must present the right angles to shear cleanly rather than rub, and it must survive the heat and force of the cut long enough to be economic. Those three — hardness, geometry and life — organise this whole section. This page sets out the geometry and the speed-and-life relationships; the pages that follow apply them to each tool type, from drills to broaches.

§2Rake and clearance

Two angles define the wedge. The rake angle sets how the chip is sheared; the clearance (relief) angle keeps the tool from rubbing the surface it has just cut.

The rake angle is measured on the face the chip flows over. A large positive rake shears easily, cuts with low force and gives a good finish, but leaves a thin, weak edge that chips under shock — so it suits ductile materials and lighter machines. A negative rake presents a strong, blunt edge that survives interrupted cuts and hard materials, at the cost of higher cutting force; carbide and ceramic tools often run negative rake for exactly this strength. The clearance angle is the small angle (typically 5–10°) beneath the cutting edge that lifts the tool flank clear of the freshly cut surface; too little and the flank rubs, generating heat and wear, too much and the edge is left unsupported and fragile. Every tool geometry in this section is a particular choice of these two angles.

§3The tool signature

A single-point turning tool is fully specified by a short ordered list of angles — its signature — so that any tool can be described, ground and reproduced unambiguously.

The signature names, in a fixed order, the back rake, side rake, end relief, side relief, end cutting-edge angle, side cutting-edge angle and nose radius. The two rakes control chip flow and force; the two reliefs prevent rubbing; the two cutting-edge angles set how the edge leads into the work and distribute the load along it; and the nose radius rounds the tip, which strengthens it and improves finish (a larger radius gives a smoother surface but raises cutting force and chatter tendency). The signature is to a cutting tool what the four-digit code is to a steel — a compact, complete specification — and the same seven-element idea underlies the more specialised tools that follow.

§4Cutting speed and spindle speed

Cutting speed is how fast the cutting edge passes the work, in metres per minute — a property of the material and tool. Spindle speed, in rev/min, is what the machine is set to, and one converts to the other through the diameter.

§5Material removal rate

How fast metal is actually cut away — the material removal rate — is the product of the three cutting parameters, and it sets both productivity and the load on the machine.

Raising any of the three lifts the removal rate and the productivity, but each has a cost: more speed shortens tool life sharply (§6), more feed roughens the finish, and more depth raises the cutting force and the power demanded of the machine. Roughing therefore uses heavy feed and depth at modest speed to shift metal; finishing uses light feed and depth for surface quality. The art of setting up a cut is choosing the three to remove metal as fast as the tool life, finish and machine power allow — a balance, not a maximum.

§6Tool life — the Taylor law

The single most important trade-off in machining: cutting speed and tool life are inversely and steeply related, captured by Taylor’s tool-life equation.

§7Quick reference

The working core of the page on one card rack.