What Is a Reaming?

Reaming is a machining operation that slightly enlarges an existing drilled hole with a multi-edged cutting tool called a reamer, bringing it to a precise diameter with a smooth finish. It fine-tunes the hole's size and finish, but it follows the drilled hole, so it cannot correct the hole's position or straightness.

In short: drill to make the hole, ream to make it the right size.

What Is Reaming in Machining?

A reamer looks a little like an end mill: several cutting edges arranged around a central shaft. It is run into a hole that has already been drilled slightly undersize and removes only a thin layer from the wall. Because it takes such a light cut, and cuts with many edges at once, it leaves a hole that is accurately sized and smoother than a drilled one.

The limitation comes from the same light cut. The reamer is guided by the hole it enters, so if the drill wandered or ran crooked, the reamed hole will be crooked in the same place. Reaming sets diameter and finish; it does not set location.

That makes reaming the usual choice for small holes that a pin, shaft or bushing must fit closely, such as dowel-pin holes that locate one part on another. For holes under about 0.500 in (12.7 mm), reaming is the most accurate way to finish the hole.

Reaming vs Boring vs Drilling

The three are often done in sequence, and each does a different job.

	Drilling	Boring	Reaming
What it does	Makes a hole in solid material	Enlarges an existing hole with a single-point tool	Slightly enlarges a drilled hole to a precise size
Tool	Twist drill	Boring bar or adjustable boring head	Multi-edged reamer
Corrects position or straightness?	No — drills can wander	Yes	No — follows the existing hole
Recommended diameter tolerance, 1/4–1/2 in (6–13 mm) hole	+0.006 / −0.001 in (+0.15 / −0.025 mm)	Set by adjusting the tool	0.001 in (0.025 mm)
Typical use	Clearance holes, tap drills, rough holes	Larger holes (usually over 0.500 in / 12.7 mm) and bores that must be on location	Small precision holes and pin fits
Relative cost	Lowest	Highest of the three	Low

When a small hole must be both on position and on size, the common sequence is drill, then bore or interpolate with an end mill to true up the location, then ream to final size.

How to Design Holes for Reaming

• Don't rely on reaming to fix position or alignment. If a hole's true position is critical, allow for boring or end-mill interpolation first.
• Keep reamed holes clear of intersecting drilled holes. Interrupted cuts risk breaking the reamer.
• Drill blind holes deeper than the reamed depth so chips have somewhere to go.
• Only ream the holes that need it. Recommended reamed-hole tolerances grow with size: about 0.0005 in (0.013 mm) up to 1/4 in (6 mm) diameter, 0.002 in (0.05 mm) at 1–2 in (25–50 mm) and 0.003 in (0.08 mm) at 2–4 in (50–100 mm). A drilled tolerance is cheaper wherever the fit allows it.

NICE Rapid's CNC machining service includes CNC drilling of precision holes to specified diameters and depths, with precision tolerances to ±0.01 mm and critical dimensions verified by CMM or manual gauging.

Frequently Asked Questions

What is reaming?

Reaming is a finishing operation that runs a multi-edged reamer through a drilled hole to enlarge it slightly to an accurate diameter with a smooth wall. It is used where a pin, shaft or bushing must fit the hole closely.

What is the difference between reaming and boring?

Boring enlarges a hole with a single-point tool and can correct the hole's position and straightness; it is usually used for larger holes and costs more. Reaming uses a multi-edged tool that follows the existing hole, so it improves size and finish but not location.

Why is a hole reamed after drilling?

A drill makes the hole quickly but leaves a rougher wall and looser diameter. Reaming removes a thin layer to bring the diameter within a tight tolerance and improve the finish, which drilling alone cannot do.

Related Services at NICE Rapid

• CNC Machining — in-house milling, turning and drilling, with CMM inspection of critical dimensions.
• Request a Quote — send a drawing with your hole tolerances and our engineers will confirm how each hole will be produced.

Sources

1. UC Riverside Mechanical Engineering Department Machine Shop, Standard Operating Procedures for Manual Milling Machines (rev. V5), §17 "Drilling, Boring & Reaming" — reaming slightly enlarges a pre-existing hole to a tightly toleranced diameter; reamer has several cutting edges around a central shaft; always done after drilling; follows a crooked hole and should not be relied on to correct location; boring uses a single-point tool and is usually used above 0.500 in; reaming is the most accurate finish for holes under 0.500 in — me.ucr.edu
2. University of Florida, MAE Design and Manufacturing Laboratory, DFMA Tips — design for reaming (don't rely on reaming for position, use a boring bar or end mill; avoid intersecting drilled and reamed holes; drill blind holes deeper), recommended tolerance tables for drilled and reamed holes, boring is more expensive than drilling or reaming — web.mae.ufl.edu