What's the Difference Between Reaming and Boring for Hole Finishing?

Reaming uses a fixed-diameter multi-flute tool to clean up a hole that's already close to size, and it's fast but locked to whatever diameter the reamer is ground to. Boring uses a single adjustable cutting edge, and it's slower per hole but can hit any diameter within its range, correct location error from the previous operation, and finish holes too large for a standard reamer.

Why the two exist for the same job

Both operations come after a drill or a rough bore has already removed the bulk of the material. Neither one is meant to remove much stock. A reamer typically takes off a few thousandths of an inch per side, roughly 0.05-0.15 mm, guided by the flutes already engaged with the hole wall.

That guidance is exactly what makes reaming fast and repeatable: the tool follows the existing hole, cleans up the surface finish, and brings the diameter to a tight tolerance in one pass without needing the machine to compensate for anything.

It's the right call for standard hole sizes in a production run where you're cutting the same diameter over and over.

A boring bar doesn't follow anything. It cuts on its own geometric path, set by the spindle or by an adjustable head, which means it corrects whatever error was left by the drill or rough bore rather than just following it. Boring can true up a hole that's slightly off-location or off-axis, and it can hit odd or oversized diameters a standard reamer doesn't come in. It's also inherently slower, because you're typically taking lighter cuts with a single point rather than distributing the cut across several flutes at once.

Where each one actually wins

Reaming wins on straightforward, standard-size holes where the location was already good coming out of the drill or rough bore: dowel pin holes, bushing bores, anything with a common nominal size and a tight but standard tolerance. It's a repeatable, low-drama operation once the reamer is dialed in, and tool cost per hole is usually lower than a boring setup.

Boring wins when the hole is large, non-standard, needs to correct positional error, or needs a specific finish that a reamer's fixed geometry can't reach, particularly in larger bores where a standard reamer either doesn't exist in that size or would be expensive to have made. Boring is also the better choice on interrupted cuts or holes with a keyway or slot already in them, where a reamer's flutes can catch and chatter.

The practical call

If the hole is a common size, the location coming out of the previous operation is already good, and you're running more than a handful of parts, ream it. If the hole is large, oversized, needs the axis corrected, or the tolerance calls for a specific single-point finish, bore it.

On a lot of jobs the two aren't competing at all. You rough with a drill, semi-finish with a bore to true up location and size, then ream the last few thousandths for finish. Which combination makes sense depends on hole size, tolerance, and how forgiving the upstream operation's accuracy already is.