Precision Reaming

H7 tolerance.
±0.005 mm.
Standard bore finishing.

Reaming is the standard finishing process for precision hole diameters. After drilling roughs the bore, reaming brings it to final H7 tolerance (±0.005 mm typical), improves surface finish to Ra 0.4 µm, and corrects minor geometric errors.

Reaming Quote CNC milling

±0.005 mm Ra 0.4 µm H7 / H6 2–50 mm

01 · What it is

How Reaming works.

Reaming is a hole-finishing process that uses a multi-flute cutting tool to enlarge a previously drilled hole to a precise final diameter. The reamer removes 0.1–0.3 mm of material from the bore, correcting diameter, surface finish, and roundness simultaneously.

Standard workflow: drill slightly undersized hole (e.g., 9.8 mm drill for a 10 mm reamed bore), then pass reamer through to achieve final 10 H7 (±0.007/0.000 mm) diameter. The reamer's multiple flutes produce a cleaner surface than drilling alone, and the tight diameter control of H7 enables precision bearing press-fits and dowel pin locations.

Reamers come in standard sizes matched to common fit classes: H7 (standard precision), H6 (high precision), H8 (loose precision). Adjustable reamers handle custom sizes. Reaming is often overlooked in DFM discussions but matters critically for assemblies requiring precision hole diameters.

02 · Specifications

Capability specs.

Standard tolerance

Standard reaming tolerance class. +0.015/0.000 mm on 10 mm bore typical

±0.005 mm

Diameter tolerance

Typical reamed bore precision. H6 achievable on precision work

Ra 0.4 µm

Surface finish

Standard reamed bore finish. Better than drilled (Ra 3.2 µm)

2–50 mm

Diameter range

Common reaming sizes. Below 2 mm: specialty micro-reaming. Above 50 mm: boring preferred

0.1–0.3 mm

Stock removal

Typical drill-to-ream material removal. Larger stock requires roughing pass

Straight / helical

Reamer geometry

Straight flute for blind holes, helical for through-holes and better chip flow

Any material

Compatibility

Steel, stainless, aluminum, brass, plastics — any machinable material

Fast

Production speed

Reaming adds only 5–10 seconds per hole in production — minimal cycle time impact

03 · Applications

Where Reaming excels.

Bearing press-fit holes

Precision bore for bearing outer-race press-fit per ISO 286 H7/p6 or H7/s6

Dowel pin locations

Precision dowel pin bores for repeatable assembly positioning

Precision bushings

Bronze or Oilite bushing bores — tight tolerance for running fits

Fluid passages

Hydraulic valve bodies, fluid connectors — precision bore diameter for orifices

Hydraulic fittings

Precision bore for O-ring boss fittings, hydraulic connector interfaces

Valve bodies

Precision valve stem bores — tight clearance for proper valve function

Precision mechanical

General mechanical assemblies requiring H7 or better bore tolerance

Production applications

High-volume production where reaming adds minimal cycle time per part

Combined operations

Drill-and-ream combinations on CNC mill or lathe — common sequence

04 · When not to use it

Not suitable for:

Every process has its limits. Being honest about where Reaming isn\'t the right answer saves time and money.

Large stock removal — reaming removes 0.1–0.3 mm, not mm
Very tight tolerances (±0.001 mm) — grinding or honing needed instead
Non-round bores — reaming is round only, follows existing drilled bore path
Very shallow holes — reamer needs engagement depth to self-align
Bores much larger than standard reamer sizes (> 50 mm) — boring preferred

FAQ

Reaming questions.

Drilled holes: ±0.05–0.1 mm diameter tolerance, Ra 3.2 µm finish, potentially not perfectly straight. Reamed holes: ±0.005 mm tolerance, Ra 0.4 µm finish, improves straightness. For precision bearing press-fits (H7/p6 requires ±0.015 mm on hole + ±0.010 on shaft), drilling alone cannot achieve. Drill first to rough, then ream to final. Adds about 10 seconds per hole in production.

H7: standard precision, ±0.015 mm on 10 mm bore. Adequate for most bearing fits, dowel pins, general precision. H6: high precision, ±0.010 mm on 10 mm bore. For precision instrument bearings, tight tolerance assemblies. H8: loose precision, ±0.022 mm on 10 mm bore. For situations where exact fit doesn't matter. Default specification: H7 unless specific reason for tighter or looser.

Partially. Reamer follows the existing drilled hole — it cannot correct gross miss-location (more than 0.1 mm). Reaming corrects: diameter, roundness, surface finish. It does NOT correct: hole position, angular alignment, off-axis drift. For precision hole locations, use precision drilling or interpolation cutting first, then ream.

Through-holes: helical-flute reamers preferred, chip evacuation through the hole, higher cutting speeds possible. Blind holes: straight-flute reamers required (helical would pull chips back into reamer), slower cutting, more careful chip management. Blind holes also need relief at the bottom — typically undercut or larger undercut below the reamed depth to accommodate chip accumulation.

When drawings specify H7 or tighter tolerance on hole diameters: yes, automatically included. When drawings don't specify tolerance: standard CNC drilling (±0.05 mm) used unless we identify precision fits requiring reaming. Always explicit on quote. Reaming operation adds minimal cost — typically included within hole-making cycle time on production parts.

Below 2 mm diameter: specialty micro-reaming (0.5–2 mm range) requires precision tooling and slower cutting. Tolerances achievable: ±0.003 mm. For very small precision holes, micro-reaming is the finishing process of choice — drilling alone cannot maintain adequate tolerance at these small diameters. Cost higher per hole due to specialty tooling and careful handling.