Micro Reamers: Achieving Tight Hole Size Control
Reamers exist because drills cannot hold the tolerance. This guide covers spiral versus straight reamers, taper reamers, runout sensitivity and how to split the material removal between drill and reamer.
A reamer removes a thin layer to finish a hole that has already been drilled. Its value is precision, not removal rate — and its behaviour is dominated by runout, cutting edge condition and how much of the material removal you have left for it to take.
Split the work correctly
Standard practice is to drill at roughly 95% of final diameter and leave the reamer the remaining 5%. If you drill to final diameter there is nothing to remove and the reamer rubs. If you leave too much, the reamer deflects and you lose the tolerance you were after.
Spiral versus straight
Spiral reamers evacuate chip actively, which makes them suitable for deeper holes and materials that pack. They are also more sensitive to runout than straight reamers, because the helical entry amplifies any wobble.
- Spiral, solid carbide — general finishing, deeper holes, chip evacuation.
- Straight, HSS — shallow holes, softer materials, fixtures with limited runout control.
- Taper — where bore form and consistency of shape matter more than speed.
Runout is the limiting factor
A reamer with high runout does not just make a rounder hole — it makes the hole non-round, and it damages the cutting edge on every pass. We measure runout at the cutting end of our reamers, typically ≤ 3 µm, because that is where the tool meets the workpiece. Shank runout is irrelevant to the hole.
Matching sets
Where a process requires bores across stations, matched sets of reamers hold the diameter consistent from station to station. This is a specification question, not a tool question: tell us the tolerance and the process and we will supply to it.
What we need
Final diameter and tolerance, drilled diameter, hole depth, material, spindle speed and feed, and how the part is held. If you have a reamer that is not holding tolerance, send it — the failure mode is usually diagnostic.
Does this apply to your part?
Send the drawing and we will tell you, not guess.
Related notes.
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August 23, 2026What we make.
Micro drills from 0.03 mm, in standard, helical, step, left-hand and aluminium-grade geometry. Every entry is a live, shipped configuration.
End mills from 0.1 mm across single and double flute, helical and straight, ball and flat geometry, with aluminium- and acrylic-dedicated lines.
Single and double flute chamfer mills, ball-tip finishing cutters, flat-bottom finishing cutters and extended-reach versions for deep features.
