DefinitionReaming 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.
Explained
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.
Key takeaways
- Reaming is a finishing operation that uses a multi-edged reamer to slightly enlarge a pre-drilled hole.
- It produces an accurately sized hole with a smoother wall than drilling alone.
- A reamer follows the existing hole, so it cannot correct position or straightness errors.
- Boring can correct location and straightness and is usually used for larger holes.
- For small holes under about 0.500 in (12.7 mm), reaming is the most accurate finishing method.
- Design for reaming by avoiding intersecting holes, drilling blind holes deeper, and only reaming holes that need tight tolerance.
Example
Related terms
References
- Standard Operating Procedures for Manual Milling Machines — UC Riverside Mechanical Engineering Department Machine Shop
- DFMA Tips — University of Florida, MAE Design and Manufacturing Laboratory