A reamer is a precision rotary cutting tool that enlarges and finishes pre-drilled holes to exact diameters and smooth surfaces, but it cannot create a hole from scratch.
If you’re working with metal and need a hole that’s exactly the right size—round, smooth, and within a hair’s width of perfect—you start with a drill bit but finish with a reamer. This tool removes a thin layer of material (typically 2–4% of the hole diameter) to achieve tolerances that drilling alone cannot deliver. It’s the difference between a hole that’s “close enough” and one that fits a bearing, pin, or shaft with no play.
How a Reamer Works
A reamer has multiple cutting edges arranged along its body. When rotated and fed into an existing hole, these edges shave off a controlled amount of material. The cutting end is ground with a 45° taper that guides the tool smoothly into the hole, preventing binding or misalignment. Standard reamers achieve tolerances of ±0.005–0.010 mm, which corresponds to H7 or H8 fit grades—sufficient for most precision applications. Solid carbide reamers can reach ±0.005 mm (H6) for tighter fits.
The surface finish produced by machine reaming typically falls between Ra 0.8 and 1.6 µm, which is the range required for smooth-running mechanical assemblies. The tool cannot originate a hole—it only finishes one that already exists.
Common Types of Reamers
| Type | Best Use | Key Feature |
|---|---|---|
| Machine Reamer | CNC production and high-volume manufacturing | Fixed diameter, high rigidity, precision to H7/H8 |
| Hand Reamer | Manual finishing and repair work | Longer chamfer taper for self-alignment |
| Adjustable Reamer | Maintenance and non-critical work | Variable diameter, fine-tunable |
| Taper Reamer | Conical holes such as Morse taper sockets | Conical cutting profile |
| Shell Reamer | Large-diameter holes (≥20–30 mm) | Replaceable cutting heads, modular design |
| Spiral Groove Reamer | Deep holes and viscous materials | Helical flutes prevent binding and improve chip evacuation |
| Extrusion Reamer | Chipless finishing for improved surface finish | Cold extrusion deforms material rather than cutting it |
Flute configuration—straight versus helical—matters. Helical (spiral) flutes bridge discontinuities like keyways without catching, making them the better choice for holes that pass through interrupted surfaces. Straight flutes are simpler and more rigid for standard through-holes.
How to Ream a Hole Correctly
Start by drilling a pilot hole 2–4% smaller than the final reamed diameter—refer to standard reaming charts for the exact bit size. Align the reamer perfectly with the hole axis before starting; misalignment produces tapered or oval holes. On a machine, set the cutting speed to half to two-thirds of the drilling speed, and increase the feed rate to 200–300% of the drilling rate. Apply cutting oil or coolant generously—dry reaming creates oversized, rough holes and shortens tool life.
Rotate the reamer forward only; running it backward dulls the cutting edges immediately. For blind holes, drill the pilot deeper than the final depth and use a quill stop to prevent the reamer from hitting the bottom, which can flex the tool and enlarge the hole unevenly.
Common Mistakes and Safety
The most frequent errors are running the reamer backward, removing more than 10% of the material, and ignoring coolant. Each causes immediate tool damage or a ruined hole. Secure the tool rigidly with minimal overhang to prevent chatter. A reamer needs a pre-drilled hole—it cannot create one. These tools are used in drill presses, lathes, milling machines, and CNC equipment across aerospace, automotive, and general manufacturing settings.
FAQs
What’s the difference between reaming and boring?
Boring enlarges a hole with a single-point cutting tool and can correct location errors. Reaming uses multiple cutting edges to refine size and finish but cannot adjust the hole’s position. Boring comes first when alignment matters; reaming follows for final sizing and surface quality.
Can you ream a hole by hand without a machine?
Yes. Hand reamers have a longer tapered lead that helps self-align as you turn the tool clockwise with steady pressure. Manual reaming is common for repair and maintenance work where access or setup time limits machine use, though the precision is lower than machine reaming.
Why does the pilot hole need to be smaller than the reamer?
The reamer removes only a thin layer—typically 2–4% of the hole diameter. If the pilot hole is already at final size, the reamer has nothing to cut and cannot improve roundness or tolerance. If it’s too large, the reamer may chatter or produce an oversized hole. The correct undersize is critical for achieving H7/H8 fits.
References & Sources
- Wikipedia. “Reamer.” Technical overview of reamer types, tolerances, and applications.
- Britannica. “Reamer.” Encyclopedia entry on reamer function and design.
- Kennametal. “Reaming Tool Basics.” Industry guide on reaming parameters and best practices.