TOOLS / MACHINING

Reaming allowance and pre-hole calculator

Work out the drill size to use before reaming so the reamer leaves a round hole on size, instead of drilling to finished size and getting a rough, oversize hole.

Drill / ream pre-hole diameter24.70 mm

Stock left for reaming: 0.30 mm on diameter · 0.15 mm per side.

Enter the allowance from your reamer maker's table for the hole size and material. Drilling to finished size leaves no stock and gives a rough, oversize hole; too much stock overloads the reamer.

Formula

Pre-hole diameter = finished hole diameter − reaming allowance (on diameter). Stock per side = allowance ÷ 2.

Standard / source

Sandvik and Kennametal holemaking guidance and Machinery's Handbook reaming allowances. There is no single ISO value: allowances vary with hole size, material and reamer type, which is why this tool asks you to enter the allowance from your reamer maker's table rather than hard-coding one. Typical shop practice is roughly 0.1–0.4 mm on diameter for small holes, more for large holes.

Worked example

Finished hole 25.00 mm, allowance 0.30 mm on diameter: pre-hole = 25.00 − 0.30 = 24.70 mm, with 0.15 mm stock per side. These are the calculator's default inputs, so it shows 24.70 mm on load.

Safety note

Planning estimate for hole preparation. For a dowel, bearing bore or pressure part, confirm the finished tolerance, surface finish and reamer maker's allowance against the drawing before cutting.