Drill, Bore, Ream, Oh Why!?

Drilling, boring, and then reaming is the proper order of operations when machining a precise hole. Whether you are working on a manual mill or a CNC milling center, this process can produce a more accurate hole size, location, circularity, and finish.

Why Not Just Drill?

A drill can make a clean-looking hole, especially after spot drilling, but it may not reliably hold tight diameter or circularity tolerances. A drill can wander as it cuts, changing the location or straightness of the hole. Manufacturing differences or uneven cutting lips can also cause the resulting hole to be oversized or slightly egg-shaped.

Bore for Circularity and Alignment

A boring bar comes next when a round, accurately positioned hole is required. Because the boring tool is comparatively rigid and cuts along a controlled path, it can correct some of the location, straightness, and circularity errors left by drilling.

Boring establishes the hole geometry before the finishing operation. This matters because a reamer generally follows the path of the existing hole rather than correcting a crooked or misplaced one.

Ream for Size and Finish

Reaming is the final operation when an accurate diameter and improved surface finish are needed. Only a small amount of material—typically several thousandths of an inch, depending on the hole size, material, and tooling—should remain after boring.

A good reamer can hold a much tighter size tolerance than a standard drill and can produce a better finish than boring alone. When the drilled and bored hole is properly prepared, reaming may hold tolerances measured in tenths of a thousandth of an inch.

Recommended Operation Order

1. Drill to remove most of the material and create the initial hole.
2. Bore to establish location, straightness, and circularity.
3. Ream to achieve the final diameter and surface finish.