Achieving H7 hole tolerance (±0.012mm typical) consistently requires the right reamer choice and process discipline. This guide shows the production-proven approach.
Reamer Materials
- HSS-Co — Cobalt-alloyed HSS, good for steel up to 30 HRC. Lowest cost.
- HSS-PM — Powder metallurgy HSS, finer grain, longer life than HSS-Co.
- Solid carbide — Best surface finish (Ra <0.4 µm achievable), 5–10× HSS life. Premium choice for production.
- Carbide-tipped (brazed) — Lower cost than solid carbide but less rigid; good for medium production.
Flute Geometry
Straight flute reamers work in through holes with continuous chip flow. Best for cast iron and short chips.
Spiral (helical) flute reamers — left-hand spiral for through holes (chips down), right-hand spiral for blind holes (chips up). Required for steel, stainless, aluminum.
Pre-Reaming Hole Size
Critical: the pre-drilled hole must be the right size — neither too tight (excess stock) nor too loose (lack of guidance). General rule:
- HSS reamer: pre-drill diameter = reamer dia − 0.4mm (for diameters 6–25mm)
- Carbide reamer: pre-drill diameter = reamer dia − 0.2mm
For H7 tolerance, ensure the pre-drilled hole is round and straight (drill with through-coolant carbide drill, not jobber HSS).
Speed and Feed
Reaming requires LOW speed and HIGH feed — opposite of drilling:
| Material | Carbide Vc (m/min) | Feed/rev (mm) |
|---|---|---|
| Mild steel | 40–60 | 0.40–0.60 |
| Stainless 304 | 15–25 | 0.30–0.45 |
| Cast iron | 30–50 | 0.50–0.80 |
| Aluminum | 80–120 | 0.50–0.90 |
Coolant Strategy
For H7 tolerance, use through-coolant reamers and flood at >7 bar. Sulfur-based cutting oil for steel reaming gives better finish than water-soluble emulsion. Mist coolant only for aluminum.
Quality Checks
Measure hole size with a plug gauge or air gauge — calipers give misleading readings on small holes. Check 3 axial positions (top, middle, bottom) for taper.
Browse Reamers
See the reamer catalog for current stock from Walter, OSG, YG-1, Sandvik.
