Broaching

Broaching is the machining process that cuts a finished profile in one pass: a long, toothed tool — the broach — is pushed or pulled through, or across, the work, and the whole form is cut complete in a single stroke. Where most cutting processes move one cutting edge over the work again and again, broaching spreads the cut over many teeth on one tool, each tooth slightly higher than the one before, It is how the internal keyway, the spline, the square hole and the shaped external profile are made in quantity. The processes this wiki treats — milling, turning, grinding — cut a feature by many passes; broaching cuts it in one, and this entry sets out the broach, the two kinds of broaching and the work they do.

The broach and its teeth

The broach is a tool like no other, and its form is the whole secret of the process. It is a long bar of high-speed steel or carbide, round for internal broaching and flat and rectangular for surface broaching, and along its length it carries a line of cutting teeth. The teeth rise gradually: each tooth stands a little prouder of the tool’s body than the one before it, by the small amount called the rise per tooth, so that the first teeth take only a sliver of metal and the cut deepens tooth by tooth along the tool. Between the teeth lie the chip gullets, the spaces that carry the chips away, and the teeth themselves are arranged in stages — a run of roughing teeth that removes the bulk of the metal, then semi-finishing teeth, and finally a few finishing teeth, all at the same final height, which shear the surface to size as the last of them pass. Because each tooth takes only a small chip, the force at any instant is modest; because the rise is built into the tool, the machine only draws it through once.

Internal broaching

The best-known broaching cuts profiles inside a hole, and it works by drawing the broach through the work. Internal broaching begins with a starting hole: the bore must already exist, cored in a casting, drilled or bored, so that the broach’s pilot — its leading, un-toothed section — can enter and align the tool. The work is then held on a faceplate or in the machine’s fixture, and the broach is pulled through: in a pull broach machine the drawhead grips the broach’s rear end and draws the whole tool through the hole in one smooth stroke, its teeth progressively enlarging the bore until the last finishing teeth leave the profile at its final size. In a single pass a round starting hole becomes a keyway, a spline, a square or polygonal hole, a serration or an internal gear-tooth form, cut complete and accurate. The commonest internal broaching of all is the keyway: a keyway broach is pushed through a hub or gear bore, guided by a bushing and a key in the bore, and the smaller keyways are often push-broached through an arbor press rather than a dedicated machine. One stroke, one finished slot — that is internal broaching.

Surface broaching

Broaching also cuts profiles on the outside of a part, and surface broaching works by moving the tool along the face. The part is clamped in a fixture beneath the machine, and a flat broach is drawn across its top surface, its rising teeth cutting deeper with each tooth until the profile is complete — cutting a flat, a dovetail, a half-round groove, a slot or an intricate form across the face of the part. Because the whole force of the cut is taken in that one pass, the workholding must hold the part firmly and rigidly against the stroke. In the continuous broaching machine, the parts are carried on a chain or conveyor past a stationary broach, each part broached as it travels under the tool, so that the machine cuts parts continuously rather than one stroke at a time — the most productive broaching of all. Surface broaching is how the contoured root of a turbine blade is cut, how the dovetail of a slide is finished, how flat faces and grooves are cut across parts in high volume.

The broaching machine

The machines that do this work are of a few forms, and each is built round the single straight stroke. The horizontal broaching machine is the machine of the long internal pull: the work is held at one end, the broach is fed through the starting hole, and the drawhead pulls it through on a long horizontal stroke, often a metre or more, cutting the whole profile as it travels. The vertical broaching machine works down: the broach is pushed down through the work, or a surface broach is drawn down past the part, the vertical stroke making a compact machine for the shorter cuts. All of them share the principle that the accuracy of the cut is set by the tool and by the straightness of the stroke rather than by a controlled path, and none of them is the machining centre — broaching needs the long, straight, powerful stroke of a dedicated machine.

When broaching is the right process

Broaching is chosen for the work that suits its one great compromise: an expensive tool for a cheap per-part cut. A broach is made for one profile and costs far more than a milling cutter, so broaching pays when enough parts share that profile to spread the cost of the tool — which is why it is the process of the parts made by the thousand: the gears and hubs with their keyways and splines, the connecting rods, the pulleys, the automotive and aerospace parts whose internal forms must be identical. Set against that cost, the process is fast — a part is broached in the seconds of one stroke — and consistent, each part cut by the same unvarying teeth until the broach dulls, when it is reground on its rake faces and returned to the cut. Broaching works on metal that is soft enough to cut, so it is usually done before hardening; for a profile in already-hard steel the spark-erosion processes take over. And for the very small quantity — a single keyway, a one-off slot — the milling machine that cuts one slot in a few minutes may well be the cheaper choice than a made-to-order broach.

Broaching in the shop

Broaching is the single-pass process that cuts a whole profile at once, and its place in the shop follows from that. It is not the everyday machining centre process, which cuts with rotating tools; it is the work of the dedicated broaching machine and the high-volume part whose form must be cut identically, leaving a surface often good enough to use as-cut, as this wiki’s surface finish entry describes. The broach is costly and the machine dedicated, but where quantities justify the tool, no process cuts an internal profile faster or more repeatably: one stroke, one finished form, again and again.