Boring Bars & Heads

Boring bars and boring heads are the tools that do the boring this wiki treats as an operation — the single-point cutting tools that enlarge and true an existing hole from the inside. Where the drill makes the hole and the reamer sizes and smooths it, the boring tool does something neither can: it cuts the hole with one adjustable point on a fixed path, so it can correct the hole’s position, straightness and diameter, not merely its size and surface. A boring bar is the slender single-point tool that reaches into a hole, used in the lathe to bore a rotating part; a boring head is the adjustable cutter that sweeps a diameter, used in the machining centre to bore a stationary part. Both carry the same kind of carbide cutting insert, and both earn their place by the accuracy they bring to the hole. This entry sets out the two tools and how they work.

The boring bar

The boring bar is the tool of the lathe and of the single-point bore, a bar with a cutting edge at its end that reaches into a hole to cut its wall. In the lathe — the machine where the work rotates — the boring bar is clamped in the toolpost or the turret and fed along the axis, its cutting point bearing on the wall of the hole in the turning work, enlarging the hole as the part spins round it; this is how the turned part’s bore is sized and trued in the same setup as its outside. The bar itself is a round or square steel shank, long enough to reach the bottom of the hole, and at its nose it carries the cutting edge — a brazed or clamped insert, often set at an angle to reach into the bore. The boring bar’s whole character follows from its shape: it is a long, slender cantilever reaching into a hole, and its stiffness, its reach and its overhang decide how well it can cut, as the section on rigidity explains.

The boring head

The boring head is the tool of the machining centre and the milling machine, the adjustable cutter that bores a hole in work that does not turn. A boring head is a body that mounts in the spindle, carrying a slide that can be moved off-centre by a screw or a vernier; a cutting tool — a small boring bar or a direct insert — is set in the slide, and the head is set so that the tool’s point sweeps the circle of the hole’s diameter. To bore, the head spins while the machine feeds it along the hole’s axis, the single point cutting the wall to the swept diameter; because the slide is adjustable, the head can be set to cut a hole of a precise size, and adjusted again by a fraction of a millimetre to take a finishing cut. The boring head is the tool of the CNC bore that must be exact — a bore set with the head, measured, and adjusted until the swept diameter is the size the hole must be.

Rigidity and reach

The boring tool works from inside the hole, and that fact governs everything about it. The boring bar is a cantilever: it is held at one end and reaches unsupported into the hole, so its stiffness falls as its overhang grows, and a bar that reaches too far for its diameter will flex, vibrate and chatter — the runout and the instability that this wiki treats as the enemy of accuracy. The boring-tool user therefore chooses the shortest, stiffest bar that will reach, and works with the light, steady cuts that a slender bar can take; the modern bar is made of solid carbide or of steel with a carbide core to raise its stiffness, and the adjustable heads and the fine-boring tools hold their setting so that the swept diameter repeats. Cutting inside a hole also hides the cut from view, so the bore is measured as it is made — the metrology that confirms the size that the tool is set to cut.

Boring against reaming

The boring tool and the reamer both finish holes, and the difference between them is the reason a shop chooses one or the other. The reamer sizes and smooths a hole, but it follows the hole it is put into: it makes the drilled hole its own diameter and surface, yet it cannot straighten a leaning hole or move a misplaced one. The boring tool cuts its own truth: because it is a single point on a path the machine controls, boring can correct the position and the straightness of a hole, cutting it true to the machine’s axis and to the datum that the part is set up to — the operation that the boring entry describes. The trade is speed: reaming cuts a finished hole in one pass, while boring is set, cut, measured and adjusted. So the shop reams the ordinary sized hole and bores the hole that must be true to position and straight — using the boring head in the machining centre and the boring bar in the lathe.

The boring tools in the machine

The boring bar and the boring head are the quiet specialists of the accurate hole, and their place in the shop follows the part. In the turning operation, the boring bar in the turret bores the part’s bore in the same setup as its outside, so the wall thickness and the concentricity hold as the part is turned; in the machining centre, the boring head and the fine-boring tools set the bores that must be round, straight and placed, cutting the bearing seat or the precision bore to the size the drawing calls. Both carry the inserts that this wiki’s tooling entries describe, run at the speeds and feeds their cut allows, and both are used within the one rule that governs all boring: the shortest rigid tool that reaches, the light steady cut, the true machine — and the measurement that confirms the hole is the size, the place and the straightness that the part demands.