Reaming

Reaming is the finishing operation that brings a drilled hole to an accurate diameter and a smooth surface, using a reamer — a multi-edged cutting tool that removes a small, controlled amount of material from a hole that drilling has already made. Drilling makes a hole quickly but not precisely; reaming is the cheap, fast way to finish it to the class of accuracy a dowel pin, a bearing or a press fit demands. The essential thing to understand about reaming is what it can and cannot do. A reamer corrects the hole’s size, roundness and finish — and that is all. It cannot change where the hole is, because it has no way to steer itself: it follows the hole that is already there, smoothing it to size without moving its position. That single limitation defines how reaming is used and what must be true of the hole before the reamer ever goes in.

How a reamer cuts

A reamer looks like a drill with straight flutes, but it cuts on a different principle. A drill tears its way in on the end, the chisel edge displacing metal at the centre; a reamer has no chisel edge and instead carries several cutting edges around its circumference, all cutting simultaneously as it turns. The actual cutting is done by the chamfered lead at the front, which takes the small allowance off the hole wall, and the long, straight body behind it does not cut at all — it guides and stabilises the tool, burnishing the surface the lead has cut and holding the tool on the hole’s axis. Because several edges share the work and the body steadies the cut, a reamer removes its light stock evenly all round, leaving a hole that is rounder, straighter and smoother than the drilled hole it started from. And because it is guided by the existing hole, it must enter that hole on its axis — the reason reamers are made with long shanks that can flex a little, and the reason a machine reamer is often held in a floating holder that lets the tool align itself with the hole rather than forcing the hole to align with the spindle.

What reaming buys you

The gain over drilling is real and measurable. A drilled hole typically repeats within a few hundredths of a millimetre of its nominal diameter, with a surface finish that is workmanlike but not fine. Reaming moves the hole into the region where close fits live: diameters held to a few thousandths of a millimetre — the order of an ISO H7 fit — and a surface roughness a fraction of a drilled hole’s, down to the single-digit micrometre class. That is the difference between a hole a pin will rattle in and a hole a pin must be pressed into, and it is why reaming is the standard finish for dowel and locating-pin holes, bearing and bushing seats, and every hole whose fit is part of the design. The price is a second operation, but a fast one: a reamer removes so little material that it takes only a moment, and on a machining centre the ream simply follows the drill in the same program, each hole drilled and reamed in turn before the part moves.

The allowance is everything

A reamer works only within a narrow band of stock, and the amount left for it — the reaming allowance — is the most important decision in the process. Too little allowance and the reamer does not cut; its edges rub and burnish the wall instead, the tool wears quickly, the finish smears, and the hole may come out undersize or with a glazed surface no pin will fit properly. Too much allowance and the reamer is asked to cut like a drill: the load climbs, the tool chatters, the entrance bells out oversize, and the hole is ruined. The working numbers are well established — the pilot hole is drilled a small percentage smaller than the reamer diameter, of the order of a few percent on smaller holes and somewhat less on larger ones, which on a 10 mm hole means leaving roughly a tenth of a millimetre of stock on the diameter for the reamer to take. When in doubt the error is made on the small side, because a reamer that finds too little stock at worst rubs, while one that finds too much scrapes the hole. Good reaming practice therefore starts with the drill: a good, true pilot hole of the right size is more than half the reaming job, and reaming is never used to rescue a poor one.

Speeds, feeds and finish

Reaming is gentle work, and its cutting data reflect that. The cutting speed is kept low — on the order of half the drilling speed for the same material — because a fast reamer runs hot, picks up built-up edge and chatters, all of which wreck the very finish reaming exists to produce. The feed, by contrast, is generous, because a reamer’s several edges share the chip and each edge needs enough feed to actually cut rather than slide: too timid a feed makes every edge rub, and a rubbing reamer glazes the hole and dulls itself. The edge must take its small bite cleanly, and coolant or cutting fluid is used to wash the tiny chips away and keep the cut cool, since heat at the sizing margins is what swells the reamer and opens the hole oversize. Two further habits belong with the process. The reamer is aligned with the hole, not the reverse — by a floating holder on a machine, or by hand, where a hand reamer is turned with a tap wrench and follows the hole entirely. And a test hole is reamed before production, because the size a reamer actually cuts depends on material, alignment and coolant, not just on the diameter stamped on the tool; a reamer can cut a few thousandths of a millimetre oversize or undersize for reasons that only the first hole reveals.

Reaming in the job flow

Reaming belongs to the finishing end of the holemaking chain that begins with drilling: drill the hole undersize, then ream it to the drawing size. It is the natural choice when the hole must hold a close fit but the drilled hole’s position is already acceptable — and it is precisely when position is not acceptable that reaming must be passed over in favour of a process that can move the hole, such as boring. On a machining centre the sequence is unremarkable and cheap: the drill and the reamer sit in the magazine, each hole is drilled and reamed in the same setup, and the fit-critical holes come off the machine finished. Speeds and feeds follow the usual discipline, and the cut is planned under the same feeds and speeds logic as every operation within CNC machining, most often on the spindle of a milling machine — reaming being, in the end, the quiet operation that turns a machined hole into an engineered one.