Understanding Reaming: A Machinist’s Perspective

Reaming is a precision machining operation used to slightly enlarge and true up a hole, typically following a drilling or boring process. From a machinist’s standpoint, reaming isn't about making a hole—it's about finishing one. The purpose is to achieve a highly accurate diameter with superior surface finish and concentricity. Unlike drilling, which is fast but imprecise, or boring, which is flexible but slow, reaming occupies a niche where consistency, speed, and precision must align.

What Reaming Is

A reamer is a multi-fluted cutting tool designed to remove a very small amount of material—usually between 0.005” and 0.015” for inch reamers (or 0.1–0.3 mm for metric). It doesn’t correct a badly located or tapered hole; it simply follows the existing geometry and removes high spots to size and smooth the wall. The flutes help evacuate chips while maintaining alignment with the pre-drilled hole.

Reamers come in various styles—straight flute, spiral flute, chucking, hand, and adjustable—but all require a pre-existing hole slightly undersized relative to the reamer’s diameter. They are typically used on drill presses, milling machines, or in CNC operations, always with attention to speed, feed, and lubrication to avoid chatter or galling.

Why Reaming Matters

Reaming is essential when a hole needs to meet tight tolerances on diameter—usually ±0.0005” or better—and possess a smooth finish for functional or assembly reasons. Common applications include:

Press-fit or slip-fit components: bushings, dowel pins, bearings.

Hydraulic or pneumatic parts: where leaks due to surface imperfections are unacceptable.

Alignment-critical assemblies: precision tooling, jigs, or fixtures.

Reaming provides better hole geometry than drilling alone: rounder, straighter, smoother, and more consistent. Surface finishes in reamed holes can approach 32 µin Ra or better, compared to ~125 µin Ra for drilled holes.

When Reaming Is and Isn’t Necessary

Reaming is necessary when:

Tolerances are tighter than what drilling can reliably achieve.

Surface finish or circularity is critical for part performance.

The production environment demands repeatable, low-variability holes without custom boring setups.

It is not necessary when:

The hole is purely clearance or non-critical in location or size.

Tolerances are loose enough to be satisfied by a twist drill alone.

You’re already boring to final size on a CNC mill or lathe.

The material is difficult to ream cleanly (e.g., gummy aluminum without proper tooling).

In many high-precision shops, reaming complements other operations. For instance, a hole might be spot drilled, drilled undersize, then reamed. But in other cases—especially in prototype work or low-volume jobs—boring is favored due to its adjustability. Reamers, being fixed in size, aren’t suited for correction, only refinement.

Conclusion

Reaming, in a machinist’s toolkit, is about finishing what drilling starts. It’s not a magic bullet; it can’t fix misalignment or gross errors. But when used correctly, it delivers the kind of consistent, high-quality holes that enable precision assemblies to function without play, wear, or leakage. Like all machining processes, its value lies in knowing when not to use it as much as when to reach for it.