Deep Hole Drilling GuidePractical technical reference
⚠️ H-GROUP · HRC 40+ · HIGH RIGIDITY

Hardened Steel (HRC 40+)

Requires extremely high rigidity and a very small process window. Vc is a fraction of conventional steels, PCBN tooling takes over above ~HRC 55, and never continue with a worn tool — secondary quenching and workpiece cracking follow. Above ~HRC 65, drilling stops being practical: switch to grinding or EDM.

3Hardness TiersHRC 40–65
15–45m/min VcBy hardness
60–100barEmulsion coolant
PCBNToolingHRC 55+ requirement
CLASSIFICATION

Hardness & Parameters

HardnessVc (m/min)Tool MaterialFeed (D=10mm)
HRC 40–4830–45Fine-grain carbide K10–K20, TiAlN/AlTiN0.02–0.05
HRC 48–5520–35Submicron carbide + AlTiCrN or CBN-coated0.015–0.04
HRC 55–6515–25PCBN inserts + PCBN guide pads (>85% CBN)0.01–0.03
💡 PCBN economics: 3–5× higher tool cost vs carbide but 10–20× longer tool life in the HRC 55–65 band. Expect 50–200 holes per edge depending on depth and diameter. PCBN runs 3,000–4,000 HV with thermal stability above 1,000°C — that is what carries the heat.
CUTTING DATA

Feed by Diameter (Hardened & Tool Steels)

Drill diameter (mm)Feed (mm/rev)
2.50–2.990.004–0.006
3.00–3.490.005–0.007
5.00–5.990.013–0.018
8.00–8.990.020–0.027
10.00–11.990.025–0.038
14.00–15.990.035–0.050
20.00–23.990.046–0.069
28.00–31.990.059–0.085
✅ Start low, raise feed before speed: Begin at the conservative end, prove stable chip evacuation, then increase feed before cutting speed — hardened steels are unforgiving of rubbing. Reduce feed at hole start, at exit, and when crossing interruptions.
SPECS

Key Data

40–65
HRC
Practical drilling range
15–45
m/min
Vc range
0.004–0.085
mm/rev
Feed range by diameter
<0.005
mm
Spindle runout (HRC 55+)
Emulsion
Coolant
60–100 bar preferred
>85%
CBN content
PCBN grade for HRC 55+
TOOLING

Tooling by Hardness Tier

TierEdge & guide padsCoating / grade
HRC 40–48Fine-grain carbide gundrill or BTA headTiAlN or AlTiN; negative/neutral rake for edge strength
HRC 48–55Submicron carbide; carbide or CBN-coated padsAlTiCrN superlattice or CBN-coated inserts
HRC 55–65PCBN inserts + PCBN guide pads>85% CBN, ceramic binder
⚠️ The HRC 65 wall: beyond ~HRC 65, chip formation collapses and tool wear is catastrophic — switch to grinding or EDM. Do not force a drill into material that belongs on a grinder.
RIGIDITY

Machine & Setup Requirements

💡 Pre-drilling strategy: for through-holes, drill a pilot hole at 60–70% of final diameter in the annealed condition, then finish with the PCBN tool in the hardened state. This avoids the worst of the hard skin at full diameter.
PROCESS CONTROL

Entry & Power Monitoring

1
Slow entry

Feed at 50% of target for the first 2–3 mm to avoid impact chipping on the hard surface.

2
Watch spindle power

Gradual increase = normal wear. Sudden spike = edge chipping or chip clogging. Stop if fluctuations exceed 15%.

3
Verify guide-pad orifices

Blocked coolant orifices starve the pads — catastrophic at high hardness. Clear them before every run.

4
Retract on any doubt

A worn tool in hardened steel is not a judgment call — it is a scrapped part. Retract and inspect.

HEAT & INTEGRITY

Why a Worn Tool Is Not Allowed

🔥 Secondary quenching: a worn edge generates enough heat to re-harden the surface (secondary quenching), which cracks the workpiece. The heat-affected zone can reach 0.1–0.3 mm deep — deep enough to require removal in a later operation. For a finished bearing bore or a tool-steel cavity, that is a write-off.
💡 Coolant choice: emulsion at 60–100 bar beats oil for heat dissipation in hardened steel. Coolant must actually reach the cut — verify orifice flow before production, not after the first burn mark.
PROS & CONS

Strengths & Limitations

✅ Advantages

  • Hole made in hardened state keeps material properties — no distortion from re-heat-treating after machining
  • PCBN delivers 10–20× carbide tool life in the HRC 55–65 band
  • Deep holes possible to ~100:1 L/D with correct tooling
  • Guide-pad burnishing improves bore finish in-process

⚠ Limitations

  • Very small process window — rigidity and chip evacuation are everything
  • Vc 15–45 m/min means slow cycle times
  • PCBN tool cost 3–5× carbide; impractical beyond HRC 65
  • Worn tools cause secondary quenching and part scrap
APPLICATIONS

Where Hardened Deep Holes Are Drilled

🔧 Die & MoldCooling and ejector holes in hardened tool-steel mold plates (D2, H13 at HRC 50+)
🛡️ Aerospace ActuatorsThrough-bores in hardened 300M / 4340M landing gear components
🚘 Hydraulic CylindersHardened chrome-plated cylinder barrels with precision bores
🔗 Oil & Gas ComponentsBores in hardened wear-resistant downhole tooling
⚙️ Spindles & ShaftsAxial oil passages in induction-hardened transmission shafts
🎯 FirearmsBarrels in heat-treated alloy steel where post-machining hardness is specified
METHOD SELECTION

Which Method for Hardened Bores

📦
D < 20mm→ Gundrill + PCBN
⚖
D 20–80mm→ BTA, PCBN pads
⚡
HRC > 65→ Grind / EDM
🔗
Anncaled pilot→ Pre-drill 60–70% first
SAFETY

Key Safety Points

⚠️ High-pressure coolant: 60–100 bar lines are lethal if disconnected under pressure. Relieve at the pump before maintenance; use whip-checks on every high-pressure hose.
🔥 Hot chip & heat hazard: hardened steel generates intense local heat. Chips are hot and razor-sharp — use dedicated hooks, never hands.
🛡️ Tool failure in the bore: a broken PCBN tool in a hardened bore is extremely difficult to recover. Monitor load and pressure with automated retract, and have an approved recovery procedure before production.
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