Reaming After Drilling: How to Prep the Hole First

Reaming gets a reputation for being easy. Drop a reamer in, spin it slow, pull it out — done. But experienced machinists know the truth: reaming only works as well as the hole going into it. A bad drilled hole produces a bad reamed hole, and no amount of reamer precision compensates for a sloppy setup upstream.

This post covers everything you need to do between the drill and the reamer to actually hit your tolerance.

Why the Drilled Hole Matters So Much

A reamer is a finishing tool. It removes a small, controlled amount of material — typically 0.003″–0.015″ on diameter — and it follows the existing hole geometry. That last part is the critical constraint: a reamer corrects surface finish, not position or straightness.

If the drilled hole is off-location, oversized, bell-mouthed, or drifted, the reamed hole inherits those problems. The reamer will track the existing path. Only boring corrects position; reaming refines finish and size.

The Reaming Allowance Rule

The standard pre-ream allowance for HSS reamers in steel:

Stay inside this range. Too little material and the reamer rubs without cutting, work-hardens the bore, and dulls fast. Too much and you're asking the reamer to act like a drill — it won't, and the finish suffers.

Drill Selection for Pre-Reaming

Don't just grab whatever drill is 0.010″ under. Think about the geometry:

• Point angle: A 118° drill is fine for most materials. Use 135° for harder steels — less thrust, less deflection, better hole straightness.
• Split point: Use split-point drills before reaming whenever possible. They self-center, reduce walking, and leave a more concentric hole. A conventional chisel edge leaves a rougher entry cone that can deflect the reamer tip.
• Runout: Check your chuck TIR before drilling the pre-ream hole. If your drill is running 0.003″ out, you're already inside your allowance and the reamer has nothing to clean up on one side.

Spot Drilling First

For any precision reamed hole, spot drill before the pre-ream drill. This gives the drill a clean, centered start and minimizes walk — especially on inclined surfaces or mill scale. A 90° spot drill is the standard; match the spot diameter to the drill web, not the flute diameter.

Skip the spot drill and you risk a drifted entry that no reamer can fix.

Speed and Feed for the Pre-Ream Drill

Don't rush the pre-ream pass. Treat it like it matters — because it does:

• Run the drill at normal speed but reduce feed rate by 20–25% vs. production drilling
• Use cutting fluid — you want a clean, cool hole wall, not a work-hardened surface
• Peck if the hole is deeper than 3× diameter — chip packing in the pre-ream hole will show up as chatter in the reamed bore
• Deburr the entry and exit of the drilled hole before reaming — burrs cause bell-mouth and surface marks

Checking the Pre-Ream Hole

Before the reamer ever touches the work, measure the drilled hole:

• Plug gauge or inside mike: Confirm you're in your allowance range. If the drill ran oversized (common with dull drills), you may have no stock left for the reamer.
• Bore gauge check for roundness: On tight H7 fits, take two readings 90° apart. A lobed hole means the drill was chattering or the setup was loose.
• Visual: Torn surface, bright spots, heavy chatter marks — regrind or replace the drill before reaming. You're not cleaning that up with a reamer pass.

Reamer Speed and Lubrication

Once the hole is prepped correctly, ream at 50–65% of your normal drilling speed for that material. Reaming is a slow operation by design — slower speed means better finish and longer reamer life.

Lubrication is non-negotiable. Flood coolant in a machining center; cutting oil by brush or drip for drill press work. Dry reaming leaves a torn surface even if everything else was perfect.

What Happens When You Skip the Prep

Common failure modes when pre-ream work is skipped:

• Bell-mouth entry: Reamer deflects into the oversized entry cone, producing a tapered bore
• Size drift: Reamer follows drifted hole path, final location is off even if diameter is correct
• Chatter finish: Chip packing in pre-ream hole translates to vibration during reaming
• Oversize: Too much allowance forces the reamer to act like a drill; built-up edge and tearing
• Undersize: Too little allowance — reamer rubs, work-hardens bore, produces wrong size anyway

Summary Checklist

1. Spot drill first (90°, centered)
2. Pre-ream drill with correct allowance (table above)
3. Use split-point drill, check TIR before drilling
4. Peck if depth > 3×D, use cutting fluid throughout
5. Deburr entry and exit of drilled hole
6. Measure drilled hole — confirm you're in allowance range
7. Ream at 50–65% of drill speed, forward only, flooded

The reamer gets the credit, but the drill sets it up. Do the prep work and reaming is straightforward. Skip it and you'll be wondering why you can't hold H7.