Reaming Basics: Drilling Performance

Reaming is a sizing and finishing operation performed on a pre-existing hole using a rotating cutting tool called a reamer that is fed into the hole to slightly enlarge and improve its geometric accuracy and surface finish. The goal is to produce holes with extreme precision in size, roundness, straightness and surface finish.

Unlike drilling, which creates the initial hole, reaming removes only a small amount of material (typically 2-5% of the hole diameter) to bring the hole to exact specifications. This process is essential when applications demand tight tolerances, such as dowel pin holes, bearing seats or precision shaft fittings.

The 3 Main Hole Making Operations

Three primary methods for creating precision holes in machining: drilling, boring and reaming. Each serves a distinct purpose in the hole-making process.

Drilling creates the initial hole by removing material with a rotating drill bit. It’s the fastest method but produces relatively rough surfaces with limited accuracy. Drills typically have two or three cutting edges and are designed for material removal rather than precision sizing. However, modern carbide drills produce decent quality holes, which are good enough in most cases. When a hole demands higher quality, boring and reaming come into play.

Boring uses single-point cutters to enlarge existing holes and improve geometric accuracy. This method offers superior control over hole geometry and can correct location errors from previous operations. Boring excels when position accuracy is critical. Boring tools can also be adjusted; thus a single tool can serve a wide range of diameters.

Reaming provides the highest precision for hole diameter, roundness and surface finish by removing minimal material from a drilled hole. However, reamers follow the existing hole path and cannot fix positional errors. They will “copy the mistake” if the pre-drilled hole is misaligned.

Drilling is primarily a hole-making operation, not a sizing operation. Boring offers the best control over geometry and true position, with the ability to correct location errors. Reaming delivers the tightest size and surface finish but cannot correct location errors.

Basic Reamer Structure and Cutting Action

Understanding how a reamer cuts is fundamental to using it effectively. Unlike endmills that cut with multiple edges along their length, reamers primarily rely on small chamfers at the tool’s tip to perform cutting. These chamfered cutting edges do all the work.

The face of a reamer cannot cut, and the outer edges (flutes) are not cutting either. Their critical task is to guide the tool through the hole, using the already reamed section as a drill bushing to aid alignment. As the reamer advances, the chamfered edges remove material while the body follows the path established by the previously cut portion.

This design makes the pre-drilled hole critical. Since reamers follow the existing hole, starting with a quality drill and proper hole size is essential. The 3% rule of thumb recommends leaving 2-5% diameter stock allowance after drilling for the reamer to remove. Too much allowance overworks the cutting chamfers, degrading surface finish. Too little causes the tool to rub instead of cutting cleanly, generating heat and accelerating wear.

Types of Reamers and Their Applications

Reamers fall into two main categories: hand reamers and chucking reamers.