Reaming is a finishing operation that precisely enlarges and smooths a hole that has already been drilled. It doesn’t create a new hole. Instead, a multi-edged cutting tool called a reamer shaves off a tiny amount of material, typically 0.1 to 0.3 mm per pass, to bring the hole to an exact diameter with a very smooth interior surface. If drilling is the rough cut, reaming is the polish.
Why Drilling Alone Isn’t Enough
A standard drill bit removes material quickly, but it leaves behind a hole that’s only approximately the right size. A drilled hole has a typical tolerance of ±0.1 to 0.3 mm, meaning the actual diameter can vary by that much from what you intended. The interior walls are relatively rough, with tool marks and small burrs left behind from the cutting process.
For many applications, that’s perfectly fine. A hole for a wood screw or a bolt with clearance doesn’t need to be precise. But when a hole needs to accept a dowel pin with a press fit, guide a shaft, or align two parts with zero slop, that level of imprecision causes problems. That’s where reaming comes in. A reamed hole holds a tolerance of ±0.005 to 0.02 mm, which is roughly 10 to 20 times more precise than drilling alone. The surface finish improves dramatically too: a drilled hole typically measures 125 to 250 micro-inches Ra (a standard roughness measurement), while a reamed hole comes in at 16 to 32 micro-inches Ra.
How the Process Works
You always drill first, then ream. The drill creates a hole slightly smaller than the final target diameter, leaving just enough material for the reamer to remove. How much material to leave depends on the hole size and the material you’re cutting. For a half-inch hole in steel, the standard recommendation is to drill about 0.014 inches undersized. For softer metals like aluminum, you’d leave slightly more, around 0.016 inches. Harder materials like tool steel need less allowance, closer to 0.011 inches.
The reamer then enters the existing hole and shaves the walls evenly. Because it follows the path of the pre-drilled hole rather than cutting a new one, alignment is critical. If the drilled hole is off-center or angled, the reamer will follow that same path. Reaming fixes surface quality and diameter, not hole position.
Reaming runs at slower speeds than drilling. The tool turns more slowly and feeds into the work at a controlled, steady rate. Coolant or cutting fluid is important for carrying away the fine chips and preventing the material from welding onto the cutting edges, a problem machinists call “built-up edge” that ruins the surface finish.
How Reaming Compares to Boring
Boring is another method for refining holes, and people often confuse it with reaming. The key difference is that boring uses a single-point cutting tool on an adjustable bar, which can correct the position and alignment of a hole. If your drilled hole ended up slightly off-center, boring can fix that. Reaming cannot.
Boring achieves tolerances of ±0.05 to 0.1 mm with a surface finish of 63 to 125 micro-inches Ra. That’s better than drilling but not as precise or smooth as reaming. In practice, the two operations serve different purposes: boring corrects geometry, reaming perfects size and finish. For holes that need the highest precision, a common production sequence is to drill, then bore to correct alignment, then ream as the final step.
Types of Reamers
Hand reamers are designed for manual use, turned with a tap wrench. They have a slight taper at the tip to help them start in the hole, and most have spiral or twisted flutes that clear chips more effectively than straight-fluted designs. Straight flutes can catch on any interruption in the hole wall, like a keyway or slot, causing the tool to lock up. If you’re reaming by hand, a spiral-flute reamer is the safer choice.
Machine reamers mount in a drill press, milling machine, or lathe. They cut faster and more consistently than hand reamers, but they’re sensitive to alignment between the machine spindle and the hole. A floating reamer holder helps solve this problem. It mounts the reamer on an independent bearing system that lets the tool shift slightly off the machine’s axis and follow the existing hole rather than being forced into a potentially misaligned path.
Shell reamers are designed for larger holes, generally three-quarters of an inch and above. Instead of being a single solid tool, they’re separate cutting tips that attach to a reusable shank. This makes them cheaper to replace since the cutting portion uses less material and the shank lasts through many tip changes.
Common Problems and Their Causes
An oversized hole is the most frustrating reaming defect because you can’t add material back. The usual culprits are spindle runout (wobble in the machine), excessive speed or feed rate, or a pre-drilled hole that isn’t aligned with the reamer’s path. Even too much oil in your coolant mix can cause the hole to come out oversize.
Poor surface finish typically comes from inadequate coolant, which leads to material building up on the cutting edges. It can also result from a rough pre-drilled hole. The reamer can only improve the surface so much; if the starting hole is badly torn up, the finish won’t be clean.
Chatter marks, a pattern of fine ridges inside the bore, point to vibration during cutting. This happens when the tool isn’t clamped firmly enough, when the reaming allowance is too small (leaving too little material for the tool to bite into), or when the feed rate is too low. Counterintuitively, feeding too slowly can cause worse results than feeding at the recommended rate, because the tool skips and bounces instead of cutting smoothly.
When Reaming Is the Right Choice
Any time you need a hole to meet a tight dimensional specification, reaming is the standard approach. Pin holes, bushing bores, holes for precision dowels, hydraulic valve bodies, and any application where two parts need to align precisely through shared holes are all common uses. In production machining, the drill-expand-ream sequence is one of the most widely used methods for medium-sized holes that need high accuracy.
For quick workshop projects where fit doesn’t matter much, reaming is unnecessary overhead. But if you’ve ever tried to press a dowel pin into a drilled hole and found it either rattling loose or impossible to insert, a reamer is the tool that solves both problems by giving you exactly the diameter you specified, with walls smooth enough for the pin to slide or press in as intended.