Reaming is a finishing operation — it improves surface finish, diameter accuracy, and roundness in an existing hole. What it doesn't do is fix a badly positioned hole, correct severe taper, or remove the damage left by a dull drill running hot. Those problems belong to the drill, and if the drill doesn't do its job, the reamer can't compensate.
Never drill to size. Leave material for the reamer to remove. The standard recommendation is 0.005"–0.015" on the diameter depending on hole size, with larger holes leaving more material.
Leaving too little stock (under 0.003" on diameter) causes the reamer to rub rather than cut. Rubbing work-hardens the hole wall, shortens reamer life dramatically, and often produces a hole that's still out-of-round because the reamer couldn't correct the existing geometry. Leaving too much stock (over 0.020" on diameter in most cases) overloads the reamer cutting edges, generates excessive heat, and produces a rough finish.
For a finished hole of 0.500", drill to approximately 0.485"–0.492". The exact number depends on material — softer materials spring back less after drilling, harder materials more.
A reamer follows the existing hole. It cannot correct a hole that's been drilled off-axis, because the reamer enters the existing bore and tends to track it rather than correct it. This matters most when a drill has been allowed to walk before engagement, when the drill point is asymmetric due to geometry error or runout, or when the workpiece moved during drilling.
The solution is correct drilling practice, not reamer selection. A pilot hole (spotting drill before the full drill) prevents walking. A sharp, symmetrically ground drill with minimal runout produces a round hole the reamer can improve. Tight workholding prevents movement.
A reamer removes only a small amount of material with a light shearing action. It can significantly improve finish from what the drill left, but it can't remove the deep scratches, tears, or work-hardened layer left by a badly dull drill.
If the drilled hole has been worked by a dull drill — especially one rubbing at the cutting edge — the hole wall will have a compacted, hardened surface. The reamer entering that surface encounters material that's harder than the bulk stock, heats up faster, and the resulting finish may not meet spec even with correct reaming parameters. If reaming results don't match what the reamer should be capable of, pull the drill and check its condition first.
A split-point drill walks less and engages more aggressively. In pre-reaming applications where you're drilling to a tight undersize tolerance, the walk resistance of a split-point is valuable. A conventional point has a larger chisel edge that pushes before it cuts, increasing the risk of walking on entry.
After drilling and before reaming, check the drilled hole with a bore gauge or plug gauge to confirm you're in the correct undersize range. This one check prevents the frustration of reaming a hole that was either over-drilled (no stock for the reamer) or severely under-drilled (reamer overloaded).
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