Reamers
Reamers are the multi-edge cutters that bring a drilled hole to its finished size and its finished surface, the tools that make the hole that must be right. The drill makes the hole, but a drilled hole is never quite to size nor quite smooth — its walls carry the marks and the wander of the drill, and its diameter is only as accurate as the drill that cut it. Where the hole must hold a pin, seat a bearing or take a precise fit, the shop finishes it with a reamer. A reamer is a rotary cutter with several straight or helical cutting edges running along its body, sized to the finished hole; it is run into a hole already drilled a little undersize, and its edges shave off that small remaining stock — a fraction of a millimetre — leaving the hole straight, smooth and to a size tolerance that a drilled hole never approaches. This entry sets out what a reamer is, how it cuts and how it is used.
What a reamer is
A reamer is a tool of finished size, and that is the first thing to understand about it: unlike a drill, which cuts a hole of its own making, a reamer is made to the size of the hole it finishes, its body a precise cylinder of that diameter. Along its length run the flutes — the grooves whose lands carry the cutting edges — usually six or more of them, far more than a drill’s two, so that each edge takes only a tiny cut as the reamer turns. At its front end a short chamfer leads into the cut, and behind the chamfer the full, round body of the reamer follows the hole, its edges cutting and its cylindrical lands steadying it. A reamer is made in high speed steel or with carbide edges for the hard or the long-running work, in the sizes of the holes it finishes, and its accuracy is the accuracy of its grinding: a good reamer is ground to its size within a few micrometres, and it carries that size into the hole.
How the reamer cuts
The reamer cuts by removing only the small stock left for it, and its many edges make its cut a light, even shave rather than a heavy bite. The hole is first drilled or bored to within a small allowance of finished size — a few tenths of a millimetre under, for the common hole — and then the reamer is run in. As it turns, each of its edges takes a thin, full-length cut round the hole, and the overlapping cuts shave the wall to the reamer’s diameter: because the tool is a precise cylinder and its edges all cut on that one circle, the hole is left round, straight and of the reamer’s own size. The cut is light by design — a reamer asked to remove a large allowance will chatter, cut oversize or break its edges, so the reaming is always the last, fine operation on a hole that has already been brought near size. Run at a modest speed and feed with a steady motion, the reamer leaves the smooth surface and the close size that make it the tool of the fitting hole.
The reamer family
Reamers come in the forms that the work calls for, and the family is read by flute and by use. The chucking reamer is the machine reamer, the straight tool held in the spindle or the tailstock of a machining centre, its shank gripped in a holder; it is the reamer of the production hole, run in on the machine that drilled the hole. Its flutes may be straight, the common form for the through hole, or spiral: a right-hand spiral reamer shears its cut and suits the through hole in steel, while a left-hand spiral reamer, its flutes twisting the other way, pushes its chips ahead and clears them from a blind hole, where a straight or right-hand reamer would pack chips at the bottom. The hand reamer, its shank squared for a wrench, is turned by hand to finish and true a hole in fitting work, enlarging and aligning by a few tenths at a time. And the expansion and adjustable reamers, their diameter adjustable by a screw or tapered plug, cover a small range of sizes from one tool. The choice follows the hole: the machine reamer for the machined hole, the spiral for the blind, the hand reamer for the fitter’s work.
The limits a reamer lives with
The reamer is a finishing tool, and its limits are the terms of its good work. It does not correct position: a reamer follows the hole it is put into, so if the drilled hole wanders or leans, the reamed hole wanders and leans with it — the reamer sizes and smooths a hole, it does not straighten it, and a hole that must be truly placed is drilled true first, or bored to position before it is reamed. It removes only a little: a reamer asked for much stock cuts badly, so the preparation — the drill, the bore — must bring the hole near its size. And it needs the alignment of its own path with the hole, which is why a rigid machine and a holder that runs true matter: a reamer running off the hole’s axis cuts oversize on one side, and the runout this wiki treats is the enemy of the accurate hole. Worked within these limits — the near-size hole, the true machine, the light cut — the reamer is one of the most dependable tools in the shop.
The reamer in the hole’s story
The reamer completes the making of the hole that this wiki’s drilling entry begins, and its place in the story is the last, precise act. The hole is drilled, then perhaps bored or milled to its near size and its true position, and then reamed to its finished diameter and surface — the operation that this wiki’s metrology entries will treat as the difference between a hole and a sized hole. The reamer’s tool is a cutting tool like any other, but one of finished size rather than of cutting geometry: its value is that it carries accuracy into the work, its precisely ground cylinder of edges shaving the drilled hole to size and leaving the smooth wall that a bearing seat, a dowel pin or a precision fit demands. For the holes that must be right, the reamer is the quiet, accurate last word — many small edges, each taking its light cut, finishing in seconds what a drill alone could never make.