Reaming, Boring & Hole Quality

A drilled hole is near-net, not accurate. Reaming finishes and sizes a hole to a fixed diameter; boring corrects position and roundness. Choosing between them — and leaving the right stock — decides whether the hole meets its tolerance and finish.

Concept

Reaming passes a multi-tooth reamer through a drilled hole to size it to a fixed diameter and improve finish. It follows the existing hole's alignment — it cannot move the hole's center. Boring uses a single-point boring bar on a lathe or machining center to cut the hole to a chosen diameter, correcting position and roundness from the previous operation. Drilling → boring → (reaming) is the usual hole-making ladder.

Why It Matters

The two processes solve different problems. If a drilled hole is roughly right in position but needs a closer diameter and better finish, ream it. If the hole wandered off center or is not round, boring is the only way to correct it — reaming would just polish the mislocated hole. Leaving too much stock for reaming loads the reamer and makes it wander; leaving too little leaves drill marks. The allowance is the whole trick.

How It Works

Reaming vs boring

Stock allowance

Reaming removes only a small amount — typical radial stock ~0.1–0.3 mm for a medium drill size. More than that overloads the reamer; less leaves drill marks. Boring removes a deliberate rough then finish amount to control size.

Worked Example

A drawing calls for a Ø10 H7 hole with a good finish. Sequence: drill Ø9.7 mm, then ream to Ø10. Radial stock left for reaming is (10−9.7)/2 = 0.15 mm — a healthy small allowance. If the hole must also be precisely located relative to a datum, bore it after drilling instead, because reaming cannot move the center. Boring depth is limited by the bar overhang logic from page 16.

Reaming vs boring

Both finish a hole, but they do different jobs. Reaming sizes and finishes an already straight, aligned hole, removing only a small stock — it cannot correct a wandering or mismatched hole. Boring uses a single-point tool to both correct position/geometry and size, but needs a boring bar whose overhang must stay short to avoid chatter. Choose reaming when the drilled hole is aligned and you only need accuracy and finish; choose boring when position or roundness must be set. In a sequence (page 34), you often bore first to fix the hole, then ream for the final finish. Leave reaming stock modest (around 0.1–0.15 mm radially for a small hole): too little and the ream rubs instead of cuts, too much and it pushes the tool and wears fast.

Why reaming cannot fix a wandering hole

A reamer has several teeth that all touch the wall at once, so it is pulled into the hole that already exists rather than pushed onto a new center. If the drill wandered 0.2 mm off the datum, the reamer faithfully follows that 0.2 mm path — it will produce a beautiful, round, well-finished hole in exactly the wrong place. This is the single most important reason the hole-making ladder matters: position and alignment are set by the drill (and by spot drilling / boring), while reaming only guarantees diameter and finish. So when a drawing tolerates hole position tightly, the process is drill to open up, then bore to correct the center and roundness, and only then ream for the final H7 size and surface. Try to ream a misaligned hole and you have spent a good reamer to polish a reject. The decision on page 34 is built on exactly this principle: ream when position is already good, bore when it is not.

Why the allowance is radial, not double

When the drill comes out at Ø9.7 and the target is Ø10, the stock removed on each side is (10−9.7)/2 = 0.15 mm — that is the radial depth of cut the reamer's teeth take. It is easy to mistakenly treat the full 0.3 mm diameter difference as "the stock" and think it is larger than it is, but each cutting edge only engages 0.15 mm. This distinction matters when scaling the allowance: a small hole wants a smaller radial layer (around 0.1–0.15 mm), a larger hole can carry a bit more, but the number you keep in the head is the radial one because that is what loads each tooth. Leave too much and the multi-tooth reamer is forced to cut like a boring tool, pushing off the hole it is following and wandering; leave too little and the edge rubs on the drill's residual surface instead of shearing, burning and over-sizing. The 0.15 mm figure on the locked Ø10 example is therefore the per-side layer, and the same division-by-two logic applies whenever you compute reaming stock from a drill-to-final-diameter difference.

Why reaming feed and coolant deserve attention

Because reaming removes so little stock, it is easy to treat it as a "just run it through" operation, but two small settings control whether the hole comes out at size or oversized. The reamer feeds much slower than a drill — it is a finishing tool, and forcing a high feed pushes the multi-tooth cutter off its guided path and worsens the size; yet feeding too slowly makes the teeth rub instead of shear, which work-hardens gummy material and burns the edge. Coolant matters for the same reason: it flushes the tiny chips out of the hole so they are not dragged back across the finished wall, and it keeps the reamer cool enough that it does not expand and cut oversize. This is why reaming is usually run at a modest speed (low heat is the priority, not MRR) with good, directed coolant, and why a reamed hole that trends large across a batch is diagnosed first as reamer wear or rising temperature — not as the machine drifting. Treat the reamer like the finishing gauge it is, not like a drill that happens to cut a little smoother.

Why a reamer is its own gauge

A reamer cuts to its own exact diameter: unlike a boring bar, where you program any size and dial it in, a reamer produces the diameter it physically is. That makes the tool itself a measuring instrument, and it has two consequences. First, a worn reamer that has lost a few hundredths will produce a hole a few hundredths small, so the trend across a batch points at the tool — not at the program, which never changes the diameter. Second, because the hole follows the reamer, holding H7 means choosing the right reamer tolerance and keeping it sharp and cool; if the reamer expands from heat or rubs from too-light stock, the hole comes out oversize even though the tool was nominally correct. This is why reaming speed is deliberately lower than drilling: heat both expands the cutter and accelerates wear, and a reamer that finishes hot cuts slightly large. It also explains why the stock, feed and coolant on this page are all tuned together — each exists to keep the reamer running at its true, sharp diameter rather than rubbing or growing. When a reamed hole drifts, check the reamer condition and temperature before touching the program.

Why boring is limited by the bar, not the tool

Boring can cut any programmed diameter and correct position, but it reaches into the hole on a cantilever bar (page 16), and that bar — not the insert — sets the limit. The deeper and thinner the bore, the more the bar deflects under the cutting force, and deflection on an internal cut shows up as a tapered or lobed hole and chatter, even with a perfectly sized boring insert. This is why a deep, small bore often cannot be "bored accurately" at all: the L/D of the bar exceeds what a steel shank can support, and the answer is a shorter, thicker or carbide/damped bar rather than a different insert. It also shapes the sequence: because boring is the position-correcting step but is the rigidity-weakest step, you do as much of the position correction as the hole depth allows, then hand off to the reamer for the final size. When a bored hole comes out tapered, the diagnostic first stop is bar overhang and clamping length (page 16), not the boring insert grade. The insert does the cutting; the bar decides whether the cut stays true.

Why spotting first protects both drill and reamer

The alignment that reaming cannot fix is set right at the start of the hole-making ladder, which is why a spot drill or centre drill often precedes the twist drill. Without a prepared start, the tip of a drill wanders on the curved or rough entry surface, and that initial walk becomes the hole's off-center error that later boring must correct. A short, rigid spot drill creates a precise starting cone that guides the twist drill onto the intended axis, so the hole comes out closer to position from the first cut. This is a cheap, zero-risk step that improves every downstream operation: the drill follows the cone, the bore has less position error to remove, and the reamer finishes a hole that was aligned from the start. When a batch of holes shows a systematic position offset, the cause is rarely the reamer — it is often a soft spot where the drill walked at entry because nothing guided it. Spotting is the hole-making equivalent of facing and centering on a lathe: it sets the datum before the real cut, and it pays back in reduced correction downstream.

Why hole quality is more than diameter

A drawing's hole callout usually lists diameter and finish, but a quality hole also depends on roundness, straightness and the absence of a torn or work-hardened wall — and these are set by the process sequence, not by the final reamer. A drilled hole that is then bored straightens and rounds the wall; reaming to size after that leaves a surface that has been sheared rather than torn. Skip the boring correction and the reamer polishes a hole that is out-of-round in the wrong place, which matters for a bearing bore even if the diameter reads on the gauge. This is why page 34 treats the hole as a system: the drill opens it, the bore corrects geometry, the reamer sizes and finishes, and each step inherits the quality of the one before it. A single "reamer choice" cannot repair a poor drilled core. When a hole fails inspection on roundness or position rather than diameter, look upstream at whether the prior operation actually did its job — the reamer was only ever going to finish what the drill and bore left behind.

Reaming and boring are sizing steps that assume a honest pre-hole. The reamer follows the hole it is given, so position and straightness are earned upstream; its job is to finish the diameter and surface with a light, even stock. When a finished hole is lobed or oversized, look first at the pre-hole and the stock left, not only at the reamer.

A reamer on a wrong setup wobbles like any other tool, but the error it produces is subtler: a slightly lobed, oversized hole that still gauges near size. Inspect the pre-hole roundness before blaming the reamer, because the tool will faithfully reproduce whatever shape it is fed.

Common Mistakes

• Reaming a mislocated hole to fix position. Reaming follows the hole. Fix: bore first to correct position.
• Leaving too much stock for reaming. The reamer overloads and wanders. Fix: keep the small allowance.
• Expecting reaming on a deep slender bore. Bar rigidity limits it. Fix: control overhang and rigidity first.

Remember that the reamer only sizes what the pre-hole hands it: position and straightness are set upstream, and the last station is judged on diameter and finish. If the hole trends oversized, look at stock and edge wear first; if it is off location, the cause is never in the reamer itself.

Reaming follows an aligned pre-hole and only sizes and finishes it; position is set upstream. Leave a small, even stock confirmed against the reamer maker, run directed coolant to flush the small chips, and check diameter with an internal gauge while position goes on a CMM or dedicated fixture.

When reamed diameter drifts across a batch, separate tool wear from stock variation: a growing oversized trend points to wear, while a jump after a new pre-drill batch points to the upstream hole size. Keep both the pre-drill diameter and the ream size logged so the two do not blur together.

Practice