30-Second Summary: Hardened steel deep hole drilling requires extremely high rigidity. Vc is far lower than for conventional steels. PCBN guide pads and specialized hard-turning tool grades are recommended for HRC 55+. Never continue with a worn tool — secondary quenching and cracking of the workpiece surface will result. Monitor spindle power; stop and inspect if fluctuations exceed 15%.
Classification by Hardness
| Hardness Range | Vc m/min | Tool Material | Feed (D=10mm ref) | Notes |
|---|---|---|---|---|
| HRC 40-48 | 30-45 | Fine-grain carbide (K10-K20) | 0.02-0.05 | Standard deep hole drills usable with reduced parameters |
| HRC 48-55 | 20-35 | Carbide / CBN-coated carbide | 0.015-0.04 | Reduce feed, increase coolant pressure to ≥80 bar |
| HRC 55-65 | 15-25 | PCBN inserts + PCBN guide pads | 0.01-0.03 | PCBN required. Strictly limit Vc. Expect tool life of 50-200 holes depending on depth |
Key Points
- Machine rigidity: Machine system rigidity must be adequate (heavy-duty machine recommended). Spindle runout should be below 0.005 mm for HRC 55+ materials.
- Entry: Feed in slowly at the start (50% of target feed for the first 2-3 mm) to avoid impact edge chipping from the hard surface.
- Power monitoring: Monitor spindle power; stop and inspect if fluctuations exceed 15%. Gradual power increase indicates normal tool wear; sudden spikes indicate edge chipping or chip clogging.
- Pre-drilling: For through-holes, consider pre-drilling a smaller pilot hole (60-70% of final diameter) in the annealed condition before heat treatment, then finish with a PCBN tool in the hardened state.
Tooling Recommendations by Hardness
- HRC 40-48: Fine-grain carbide with TiAlN or AlTiN coating. Standard gundrill or BTA head geometry with reduced cutting angles (negative or neutral rake for edge strength).
- HRC 48-55: Submicron-grain carbide with AlTiCrN superlattice coating, or CBN-coated carbide inserts. Guide pad material should be carbide or CBN-coated.
- HRC 55-65: PCBN (polycrystalline cubic boron nitride) inserts and guide pads. The PCBN grade should have a high CBN content (>85%) with ceramic binder for maximum wear resistance. Expect 3-5× higher tool cost vs. carbide but 10-20× longer tool life in this hardness range.
⚠️ Note: In hardened steel deep hole drilling, do not continue using a worn tool. Continuing with a worn tool generates excessive heat, which can cause secondary quenching (re-hardening) and cracking of the workpiece surface. This heat-affected zone (HAZ) can reach 0.1-0.3 mm deep and may require removal in subsequent operations.
💡 Coolant strategy: For hardened steels, emulsion coolant at 60-100 bar is generally preferred over oil for better heat dissipation. Ensure coolant reaches the cutting zone — verify that guide pad coolant orifices are not blocked by debris, as guide pad lubrication is critical at high hardness.