Appearance
At 50 HRC, the cutting edge of a gun drill experiences compressive stress of approximately 3,000 MPa — enough to deform or chip the carbide edge within seconds if the geometry, coating, and coolant are not optimised. At 60 HRC, the cutting edge operates at the limit of what cemented carbide can withstand. The margin between cutting and catastrophic failure narrows to a few microns of edge preparation and a few degrees of clearance angle. Deep hole drilling of hardened steel is not about speed or productivity — it is about survival. Every parameter must be chosen to keep the cutting edge intact for the full length of the hole, because the cost of a broken tool in a hardened steel workpiece is almost always a scrapped part.
Why Hardened Steel Is Different
Cutting Forces vs. Hardness
| Hardness | Specific Cutting Force (kc) | Relative to Annealed (200 HB) | Comments |
|---|---|---|---|
| 200 HB (~20 HRC) | 2,200 N/mm² | 1.0× | Baseline — annealed steel |
| 300 HB (~30 HRC) | 2,600 N/mm² | 1.2× | Pre-hardened mould steel |
| 400 HB (~42 HRC) | 3,500 N/mm² | 1.6× | Hardened and tempered |
| 500 HB (~50 HRC) | 4,800 N/mm² | 2.2× | Full hardened tool steel |
| 600 HB (~57 HRC) | 6,000 N/mm² | 2.7× | High-hardness tool steel |
The Work Hardening Problem
Hardened steel work-hardens further during cutting, and the effect compounds with tool wear:
| Condition | Surface Hardness After Cutting | Effect on Drilling |
|---|---|---|
| Sharp tool, proper parameters | +2–5 HRC | Acceptable |
| Slightly worn tool | +5–10 HRC | Increased cutting forces |
| Dull tool or interrupted feed | +10–15 HRC | Tool will chip on re-entry |
| Dwell or rubbing at depth | +15–20 HRC | Tool destroyed — part likely scrapped |
Critical rule: Never interrupt the feed while cutting hardened steel. A dwell of even 0.1 seconds creates a work-hardened spot hard enough to chip the cutting edge when feed resumes. Continuous feed from entry to exit is mandatory.
Chip Formation
| Hardness Range | Chip Characteristic | Chip Breaking | Heat in Chip |
|---|---|---|---|
| 30–40 HRC | Continuous, flowing | Moderate | Distributed |
| 40–50 HRC | Segmented, saw-tooth | Good | Concentrated in segments |
| 50–60 HRC | Highly segmented, almost powdery | Excellent (natural) | Extreme — chip colour changes |
| > 60 HRC | Cracked, discontinuous | Natural | Intense — can reach 900°C |
At hardnesses above 50 HRC, the chip forms by a periodic cracking mechanism (adiabatic shear localisation). The chip is naturally segmented, which is favourable for evacuation — but each segment-forming event creates a spike in cutting temperature that stresses the cutting edge.
Tool Material and Coating Selection
Carbide Grade Selection
| Hardness | Recommended Grade | Cobalt % | Grain Size | Edge Preparation |
|---|---|---|---|---|
| 40–48 HRC | K15–K20 | 8–12% | Fine (0.5–1.0 µm) | 0.02–0.05 mm hone |
| 48–55 HRC | K20–K30 | 10–14% | Fine to medium | 0.03–0.06 mm T-land |
| 55–62 HRC | K25–K35 | 12–16% | Medium | 0.05–0.08 mm T-land |
| > 62 HRC | PCBN (recommended) or K40+ | N/A | N/A | 0.05–0.10 mm chamfer |
Higher cobalt content provides the toughness needed to resist edge chipping at the expense of wear resistance. The trade-off is acceptable because hardened steel drills are run at low cutting speeds where abrasive wear is not the dominant failure mode — edge chipping is.
Coating Selection
| Coating | Max Temperature | Why It Works for Hardened Steel | Limitation |
|---|---|---|---|
| TiAlN (PVD) | 850°C | Forms Al₂O₃ thermal barrier — reflects heat into chip | Limited toughness for interrupted cuts |
| AlTiN (PVD) | 900°C | Higher Al content — better oxidation resistance | Higher residual stress in coating |
| AlCrN (PVD) | 1,100°C | Cr improves hot hardness, corrosion resistance | Higher cost |
| TiSiN / AlTiSiN (nano) | 1,200°C | Nano-layered structure — extreme wear resistance | Limited availability, expensive |
| Uncoated carbide | — | Lower cost, sharper edge possible | Very short tool life above 45 HRC |
For gun drilling of hardened steel, AlTiN or AlCrN coatings provide the best balance of thermal protection and edge toughness. The coating must survive the high compressive stress at the cutting edge — a thick coating (> 3 µm) may spall off under the pressure. PVD coatings of 1–3 µm are preferred.
Edge Geometry
| Feature | Standard Steel | Hardened Steel (45–55 HRC) | Hardened Steel (> 55 HRC) |
|---|---|---|---|
| Point angle | 120–130° | 130–140° | 135–145° (stronger edge) |
| Rake angle | 0–5° positive | 0–2° positive (neutral preferred) | 0° to -3° negative |
| Edge preparation | 0.01–0.03 mm hone | 0.03–0.06 mm T-land | 0.05–0.10 mm chamfer |
| Primary clearance | 10–15° | 8–12° (reduced for edge support) | 6–10° |
| Secondary clearance | 20–25° | 15–20° | 12–18° |
| Margin width | 0.3–0.6 mm | 0.4–0.8 mm | 0.5–1.0 mm |
The T-land (a chamfered edge preparation) is essential for hardened steel. It replaces the sharp cutting edge with a small negative land that distributes the compressive load over a larger area, reducing the stress concentration that causes edge chipping.
Gun Drilling Parameters
Recommended Starting Parameters
| Diameter | Hardness | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure | Tool Material |
|---|---|---|---|---|---|
| 3–6 mm | 40–48 HRC | 40–60 | 0.005–0.015 | 120–180 bar | K15–K20 carbide |
| 3–6 mm | 48–55 HRC | 30–45 | 0.004–0.012 | 150–200 bar | K20–K30 carbide |
| 3–6 mm | 55–62 HRC | 20–35 | 0.003–0.010 | 150–200 bar | K25–K35 or PCBN |
| 6–12 mm | 40–48 HRC | 45–65 | 0.010–0.030 | 100–150 bar | K15–K20 carbide |
| 6–12 mm | 48–55 HRC | 30–50 | 0.008–0.025 | 120–180 bar | K20–K30 carbide |
| 6–12 mm | 55–62 HRC | 25–40 | 0.006–0.020 | 120–200 bar | K25–K35 or PCBN |
| 12–25 mm | 40–48 HRC | 40–60 | 0.020–0.050 | 80–120 bar | K15–K20 carbide |
| 12–25 mm | 48–55 HRC | 30–50 | 0.015–0.040 | 100–150 bar | K20–K30 carbide |
| 12–25 mm | 55–62 HRC | 20–35 | 0.012–0.030 | 100–150 bar | K25–K35 or PCBN |
Feed Rate Adjustment by L/D Ratio
| L/D Ratio | Feed Adjustment | Reason |
|---|---|---|
| < 10:1 | 100% (nominal) | Short hole, good chip evacuation |
| 10:1 – 30:1 | 80–90% | Increasing friction, chip packing risk |
| 30:1 – 50:1 | 65–80% | Significant friction, reduced torque margin |
| > 50:1 | 50–65% | Maximum safety margin for tool survival |
BTA Drilling Parameters
Recommended Starting Parameters
| Diameter | Hardness | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Flow | Coolant Pressure |
|---|---|---|---|---|---|
| 20–40 mm | 40–48 HRC | 40–60 | 0.04–0.10 | 200–350 L/min | 40–70 bar |
| 20–40 mm | 48–55 HRC | 30–45 | 0.03–0.08 | 200–350 L/min | 50–80 bar |
| 20–40 mm | 55–62 HRC | 20–30 | 0.02–0.06 | 200–350 L/min | 60–100 bar |
| 40–80 mm | 40–48 HRC | 35–55 | 0.06–0.15 | 300–500 L/min | 30–60 bar |
| 40–80 mm | 48–55 HRC | 25–40 | 0.05–0.12 | 300–500 L/min | 40–70 bar |
Insert Selection for BTA
| Feature | Recommendation for Hardened Steel |
|---|---|
| Insert grade | K20–K30 carbide with AlCrN coating, or PCBN |
| Chip breaker | B-type (stronger edge, larger corner radius) |
| Wiper edge | Required for surface finish |
| Insert clamping | Screw-clamp (not clamp-on-top) for rigidity |
| Number of inserts | Single for < 50 mm, two for larger |
Coolant Strategy
Requirements
| Parameter | Recommendation | Why |
|---|---|---|
| Pressure | 100–200 bar (gun drilling) | Ensures coolant reaches cutting edge through restricted clearance |
| Flow rate | Adequate for chip transport velocity | Segmented chips must be evacuated promptly |
| Filtration | ≤ 10 µm | Hardened steel fines recirculate and cause abrasive wear |
| Coolant type | EP oil with high sulphur content | Maximum lubricity at extreme contact pressures |
| Temperature | 20–30°C controlled | Thermal stability of process |
| Coolant on before cut | Mandatory | Thermal shock prevention |
| Coolant during retraction | Mandatory | Clears chips from bore |
WARNING
In hardened steel deep hole drilling, coolant starvation is the most common preventable cause of tool failure. If the coolant pressure drops below 80 bar during the cut, stop the spindle and retract immediately. Restarting without adequate coolant will destroy the tool within seconds — the cutting edge reaches 800°C without coolant and the carbide softens or cracks.
Tool Life Expectations
| Hardness | Tool Material | Holes per Regrind (10 mm × 200 mm hole) | Failure Mode |
|---|---|---|---|
| 40–48 HRC | K15–K20 carbide | 50–200 | Flank wear |
| 48–55 HRC | K20–K30 carbide | 20–80 | Edge chipping |
| 55–62 HRC | K25–K35 carbide | 5–25 | Edge chipping or fracture |
| 55–62 HRC | PCBN | 50–200 | Flank wear (slower progression) |
| > 62 HRC | PCBN | 20–100 | Flank wear |
Tool Replacement Criteria
| Criterion | Limit | Action |
|---|---|---|
| Flank wear (carbide) | 0.12–0.18 mm | Regrind at lower limit |
| Flank wear (PCBN) | 0.15–0.25 mm | Replace or regrind |
| Edge chipping (any) | > 0.05 mm | Replace immediately |
| Surface finish Ra | > 0.8 µm | Check tool, replace if worn |
| Hole diameter change | > 0.01 mm from nominal | Check tool wear |
| Coolant pressure increase | > 15% from baseline | Indicates chip packing or wear |
| Spindle load increase | > 20% from baseline | Indicates edge wear |
Surface Finish and Hole Quality
Achievable Quality
| Hardness | Gun Drilling Ra | BTA Drilling Ra | Diameter Tolerance |
|---|---|---|---|
| 40–48 HRC | 0.4–0.8 µm | 0.8–1.6 µm | H8–H9 |
| 48–55 HRC | 0.3–0.6 µm | 0.6–1.2 µm | H8 |
| 55–62 HRC | 0.2–0.5 µm | 0.5–1.0 µm | H7–H8 |
Harder materials produce better surface finish because the guide pads burnish the bore wall more effectively and the segmented chip does not score the surface. The trade-off is shorter tool life.
Common Defects
| Defect | Cause | Fix |
|---|---|---|
| Edge chipping on entry | Feed too high at entry, or no chamfer | Reduce entry feed, add entry chamfer |
| Tool breakage at depth | Chip packing from insufficient pressure | Increase coolant pressure, check chip breaker |
| Rough surface finish | Worn cutting edge, incipient chipping | Replace tool at lower wear threshold |
| Oversize hole | Tool deflection from high cutting forces | Reduce feed, increase rigidity |
| Chatter / rifling marks | Insufficient damping, tool deflection | Adjust speed, check guide pad condition |
| Burr at exit | Feed too high at breakthrough | Reduce feed in last 2–3 mm |
| Heat checking / thermal cracks | Interrupted coolant flow | Ensure constant coolant flow, preheat if needed |
FAQ
Q: What is the maximum hardness that can be gun drilled? Conventional carbide gun drills can drill up to approximately 60–62 HRC with appropriate grades and parameters. Above 62 HRC, PCBN-tipped gun drills are required for production quantities. With carbide, tool life above 60 HRC is very short (5–25 holes per regrind).
Q: What cutting speed is recommended for gun drilling hardened steel at 50 HRC? For carbide gun drills at 48–55 HRC, recommended cutting speed is 30–50 m/min depending on diameter. This is approximately 40–50% of the speed used for annealed steel (80–120 m/min).
Q: Why does hardened steel require a different edge preparation than soft steel? The cutting edge in hardened steel experiences compressive stress of 3,000–6,000 MPa — enough to chip a sharp edge. A T-land or chamfered edge preparation (0.03–0.08 mm) distributes this load over a larger area, preventing edge chipping. The land is typically at a negative angle of -5° to -15°.
Q: What tool coating is best for deep hole drilling hardened steel? AlCrN (aluminium chromium nitride) PVD coating provides the best combination of hot hardness (up to 1,100°C), wear resistance, and toughness for hardened steel deep hole drilling. For slightly lower hardness ranges (40–50 HRC), TiAlN is cost-effective and adequate.
Q: How does work hardening affect hardened steel deep hole drilling? Hardened steel work-hardens further during cutting — a worn tool can increase surface hardness by 10–20 HRC. The most critical rule is to never interrupt the feed. A dwell of even 0.1 seconds creates a hardened spot that will chip the tool on re-entry.
Q: What chip form is expected when drilling hardened steel? Hardened steel produces naturally segmented, saw-tooth chips due to adiabatic shear localisation. The chips are short and easily evacuated — which is an advantage for deep hole drilling. The chip colour (straw to blue) indicates cutting temperature.
Q: Can BTA drilling be used for hardened steel? Yes, BTA drilling is effective for hardened steel in diameters above 20 mm. Cutting speeds are 20–45 m/min depending on hardness. Insert selection should favour tougher grades with reinforced edges and AlCrN coating.
Q: What coolant pressure is needed for gun drilling hardened steel? Minimum 100 bar (1,500 PSI), recommended 120–200 bar for diameters under 12 mm. Hardened steel generates more heat per unit volume of material removed, and the higher pressure is needed to ensure the coolant reaches the cutting edge and evacuates the segmented chips.
Q: How does hardened steel drilling affect hole quality? Harder materials generally produce better surface finish because the guide pads burnish the bore wall more effectively. Ra values of 0.2–0.5 µm are achievable in the 55–62 HRC range. However, straightness can be more difficult to maintain because of higher cutting forces causing tool deflection.
Q: Is PCBN worth the cost for hardened steel gun drilling? For production volumes above 50–100 holes per month in material above 55 HRC, PCBN gun drills provide lower cost per hole due to 3–10× longer tool life compared to carbide. For short runs or prototype work, coated carbide with proper edge preparation is more economical.