§1Sizing, not shaping
A reamer is a multi-edged tool that follows an existing hole, shaving a thin, even layer from its wall to bring it to an exact diameter and a fine finish. It refines a hole; it cannot make or move one.
A drilled hole is rarely good enough on its own: it runs a little oversize, its walls are rough, and it may be slightly out of round or bell-mouthed at entry. A reamer, cutting on many edges around its full circumference, corrects the size and finish — turning a rough drilled hole into a precise one fit for a dowel, a bearing or a close fit. The key limitation flows from how it works: a reamer follows the hole it is given, so it corrects size and finish but not location — a hole drilled off-position is reamed to size in the wrong place. Everything else on this page follows from that: reaming is a light finishing operation that depends entirely on the hole beneath it being nearly right.
Contents§2The reaming allowance
The stock left for the reamer — the reaming allowance — is small and matters greatly: too little and the reamer rubs instead of cutting, too much and it cuts poorly, oversize and rough.
For a finished hole of 12 mm, a typical allowance is about 0.2 mm on the diameter, so the hole is drilled 11.8 mm and the reamer removes 0.1 mm per side. For a 20 mm hole the allowance grows to roughly 0.3 mm — 19.7 mm drilled, 0.15 mm per side — since larger holes take a little more. The band is narrow deliberately: below about 0.1 mm the reamer’s edges skid and burnish rather than cut, glazing the hole and dulling the tool, while much above the allowance the reamer is forced to act like a drill, chattering and cutting oversize. The right allowance keeps every edge cutting a genuine, thin chip.
§3Hand and machine reamers
Reamers divide by how they are driven, and the difference shows in the lead — the tapered start that guides the reamer into the hole.
A hand reamer is turned slowly by hand for the most accurate work; it has a long, gentle lead taper at its tip so it starts straight and true into the hole with only hand pressure, and a square drive end for a tap wrench. A machine (chucking) reamer is driven under power in a drill press, lathe or mill; it has a short 45° lead chamfer that does the cutting quickly and a straight or tapered shank to suit the machine. Machine reaming is faster and the norm for production; hand reaming is reserved for the finest fits and for finishing on the bench. Both share the same body of straight or spiral flutes behind the lead.
Contents§4Speed and feed
A reamer runs slower and feeds faster than the drill that preceded it — the opposite instinct to most finishing operations, and essential to a good result.
The cutting speed for reaming is low, commonly about two-thirds of the drilling speed for the same material, because a high speed causes the reamer to chatter and burnish, spoiling the finish and size. The feed, by contrast, is relatively high — several times the drill’s feed per revolution — so that each edge takes a definite chip rather than rubbing. Low speed, high feed is the reaming rule, and it is why a reamer is never simply run at the drill’s settings. Generous cutting fluid is important too: it flushes chips clear of the flutes and cools the light cut, both of which protect the finish.
Contents§5Flutes and finish
A reamer’s many flutes are what let it cut so lightly and finish so well, and their form is chosen for the hole and material.
Because a reamer carries six, eight or more cutting edges, each removes only a fraction of the already-small allowance, which is why the finish is so fine — the cut is shared many ways. The flutes are usually spaced slightly unevenly around the tool, a deliberate irregularity that breaks up the rhythm of the cut and prevents the chatter marks that even spacing would leave. Straight flutes suit most through-holes; a left-hand spiral pushes chips ahead and forward, useful in through-holes, while a right-hand spiral pulls them back out, preferred for holes with a keyway or interruption that a straight flute would catch on. More edges and the right flute form together give reaming its characteristic accuracy and smoothness.
Contents§6Getting an accurate hole
A good reamed hole is the product of the whole sequence before it, not the reamer alone.
The chain runs: drill on-position and reasonably straight, leaving the correct small allowance; for the most precise holes, bore or use a stub drill first to correct any drift, since the reamer will not; then ream at low speed and firm feed with plenty of fluid, letting the reamer follow the hole. Because the reamer cannot fix location or straightness, any error in size, roundness or position that must be corrected has to be dealt with before reaming — by boring — while the reamer is trusted only for the final size and finish. Respect that division of labour and a reamer delivers a precise, smooth hole every time; ask it to correct a bad hole and it will faithfully reproduce the fault at final size.
Contents§7Quick reference
The working core of the page on one card rack.
Role
size + finish, not location
follows the hole
Allowance
~0.2 mm dia (12 mm hole)
→ 0.1 mm per side
Types
hand (long lead) · machine (45° chamfer)
Speed/feed
low speed (~⅔ drill)
high feed, lots of fluid
Flutes
many, unevenly spaced
against chatter