Appearance
Deep hole drilling of hardened steels and engineering ceramics is not simply conventional deep hole drilling with reduced parameters. At hardnesses above 45 HRC, chip formation changes fundamentally — the chip transitions from a ductile shear to a segmented, brittle fracture mode. At hardnesses above 55 HRC, carbide tooling loses economic viability and CBN becomes necessary. For ceramics, conventional cutting is impossible and machining relies entirely on diamond abrasion or ultrasonic fracture mechanisms.
Overview
The difficulty of deep hole drilling increases non-linearly with material hardness. This article divides hard-material deep hole drilling into three regimes:
| Regime | Hardness Range | Primary Tool Material | Material Examples |
|---|---|---|---|
| Moderately hard | 45–55 HRC | Coated carbide | H13, 4140 Q&T, 4340 Q&T |
| Hard | 55–65 HRC | CBN (PCBN) or coated carbide (limited) | D2, M2, 60-62 HRC bearing steels |
| Extreme (ceramics) | N/A (non-metallic) | Diamond (PCD, electroplated, brazed) | Al₂O₃, SiC, Si₃N₄, ZrO₂, CMCs |
Each regime requires different tool materials, cutting parameters, coolant strategies, and machine capabilities.
Hardened Steel: 45–55 HRC
In this range, coated carbide tooling remains viable with appropriate parameter adjustments.
Cutting Parameters
| Material | Hardness | Tool Material | Cutting Speed (m/min) | Feed Rate (mm/rev) | Coolant Pressure |
|---|---|---|---|---|---|
| H13 tool steel | 48–52 HRC | AlTiN-coated carbide | 20–40 | 0.02–0.10 | 35–70 bar |
| 4140 Q&T | 45–50 HRC | TiAlN-coated carbide | 30–50 | 0.03–0.12 | 35–70 bar |
| 4340 Q&T | 45–50 HRC | TiAlN-coated carbide | 25–45 | 0.03–0.10 | 35–70 bar |
For BTA drilling of quenched and tempered alloy steel (275–350 HB, approximately 28–38 HRC), ISCAR's Tri-DEEP system recommends:
| Condition | Cutting Speed (m/min) | Feed Rate (mm/rev) |
|---|---|---|
| Low alloy, Q&T 275–350 HB | 50–100 | 0.03–0.12 |
| High alloy/tool steel, Q&T 325 HB | 50–100 | 0.03–0.13 |
For gun drilling in this hardness range, feed rates must be reduced to approximately 1/10 of conventional drilling: 0.005–0.025 mm/rev for diameters under 20 mm.
Chip Formation
At hardnesses above 45 HRC, the chip formation mechanism transitions from continuous shear to segmented chip formation (also called saw-tooth chip or adiabatic shear banding). This has three consequences for deep hole drilling:
- Chip fragmentation improves — segmented chips break into small pieces more readily, reducing chip evacuation problems
- Cutting force oscillations increase — the cyclical chip formation generates vibration that can affect hole straightness
- Temperature at the cutting edge increases — adiabatic shear concentrates heat in a narrow band
Tool Life Expectations
Research on drilling AISI H13 at 48–52 HRC with AlTiN-coated carbide achieved up to 210 holes at 30 m/min with 0.1 mm/rev feed and 70 bar coolant pressure. The primary failure mode was chipping at the outer corner — a consequence of the interrupted nature of segmented chip formation.
Warning: Tool life drops sharply above 52 HRC. D2 at 60–62 HRC produced only 6–9 holes under similar conditions. If production volumes require drilling D2 or similar tool steels at full hardness, consider CBN tooling or EDM hole-making instead.
Hardened Steel: 55–65 HRC and CBN Tooling
Above 55 HRC, carbide tooling becomes uneconomical for production deep hole drilling. Cubic boron nitride (CBN, also called PCBN) is the preferred tool material.
CBN Tool Selection
| CBN Grade | CBN Content | Edge Preparation | Application | Cutting Speed (m/min) | Feed (mm/rev) |
|---|---|---|---|---|---|
| High CBN (IB90, MB835) | 85–95% | Heavy chamfer 0.13 × 35° | Heavy interrupted cuts, roughing | 30–120 | 0.10–0.30 |
| Medium CBN (IB20HC, CB7135) | 65–80% | Medium chamfer 0.13 × 25° | General purpose, light interruptions | 100–200 | 0.05–0.25 |
| Low CBN (IB10HC, CB7125) | 45–55% | Light chamfer 0.13 × 15° | Continuous finishing cuts | 150–350 | 0.05–0.20 |
Application to Deep Hole Drilling
CBN is most commonly used for boring pre-existing holes in hardened steel rather than for drilling from solid. The typical application sequence is:
- Drill near-net-shape hole in annealed or pre-hardened condition with carbide
- Heat treat to final hardness (55–65 HRC)
- Finish bore with CBN tooling to final size and surface finish
This sequence exploits carbide's lower cost for rough material removal and CBN's ability to hold tight tolerances in hard materials.
For CBN boring of hardened steel in deep hole applications:
| Application | Material | Hardness | Cutting Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) |
|---|---|---|---|---|---|
| Rough boring | Tool steel | 55–62 HRC | 60–120 | 0.10–0.25 | 1.0–2.5 |
| Finish boring | Tool steel | 55–62 HRC | 100–180 | 0.05–0.15 | 0.1–0.5 |
| Continuous boring | Case-hardened steel | 60–62 HRC | 150–200 | 0.05–0.10 | 0.1–0.3 |
Coolant Strategy for CBN
CBN tooling is sensitive to thermal shock. In deep hole boring applications:
- Oil-based coolant preferred — provides consistent lubrication without thermal cycling
- Water-miscible coolant acceptable at concentrations above 10%
- Avoid intermittent coolant flow — thermal cycling between hot cutting edge and cool coolant can cause micro-cracking of the CBN layer
- Minimum pressure 10–20 bar for chip evacuation; higher (35–70 bar) for deeper bores
Engineering Ceramics
Engineering ceramics — alumina (Al₂O₃), silicon carbide (SiC), silicon nitride (Si₃N₄), zirconia (ZrO₂), and ceramic matrix composites (CMCs) — cannot be drilled by conventional cutting. Their high hardness (1,500–2,500 HV) and low fracture toughness require abrasive machining methods.
Drilling Methods
| Method | Material Removal Mechanism | Typical Applications | Hole Quality |
|---|---|---|---|
| Rotary ultrasonic drilling | Diamond abrasion + ultrasonic fracture | SiC, Al₂O₃, CMCs | IT 7–10, Ra 0.5–2.0 μm |
| Diamond core drilling | Diamond abrasion under pressure | Al₂O₃, Si₃N₄, ZrO₂ | IT 10–12, Ra 1.5–3.0 μm |
| Laser drilling | Thermal ablation/ vaporisation | Thin ceramics, small holes | HAZ concerns |
| EDM drilling | Electrical discharge erosion | Conductive ceramics (SiC, B₄C) | Recast layer |
Diamond Core Drilling Parameters
| Material | Tool Type | Spindle Speed (r/min) | Feed Rate (mm/min) | Pressure / Load | Coolant |
|---|---|---|---|---|---|
| Al₂O₃ (99%) | Sintered diamond core drill | 700–2,000 | 10–30 | 500–800 N | Water-based flood |
| SiC | Electroplated diamond core drill | 2,500–2,700 | 15–25 | 780–820 N | Water-based flood |
| Si₃N₄ | Brazed diamond thin-wall drill | 700–900 | 10–20 | 500–705 N | Water-based flood |
| Cf/SiC CMC | Brazed PCD core drill | 5,000 | 30 | Light pressure | Water-based flood |
Rotary Ultrasonic Drilling
Rotary ultrasonic machining (RUM) combines diamond abrasion with high-frequency vibration (20–80 kHz) to improve material removal rate and hole quality in ceramics:
| Benefit vs Conventional Diamond Drilling | Improvement |
|---|---|
| Surface roughness | 28–45% improvement |
| Hole circularity | 8–31% improvement |
| Tool life | 2–3× longer |
| Edge chipping | Significantly reduced |
| Material removal rate | 30–50% higher |
The ultrasonic vibration creates a micro-impact action that fractures the ceramic ahead of the diamond abrasive grains, reducing the specific cutting energy and the load on individual diamond particles.
Practical Limitations
| Limitation | Impact on Deep Hole Drilling |
|---|---|
| Maximum L/D ratio | Typically < 10:1 for diamond core drills; > 20:1 difficult |
| Hole diameter minimum | 0.5 mm with ultrasonic assistance; 3 mm with conventional core drills |
| Tool wear | Diamond tool wear is significant — predict tool life and plan change intervals |
| Surface damage | Micro-cracking at entry and exit edges is common |
| Cost | Diamond tooling cost per hole is 10–100× carbide tooling per hole in steel |
| Machine requirements | RUM requires ultrasonic spindle capable of 20–80 kHz vibration |
Machine Requirements for Hard Material Deep Hole Drilling
Drilling hard materials places demands on the machine tool that exceed those for conventional deep hole drilling:
| Requirement | Reason |
|---|---|
| High spindle torque at low RPM | Hard materials require low cutting speeds (15–50 m/min) but high torque to maintain feed |
| High coolant pressure capability | Minimum 70 bar (1,000 PSI) for hardened steel; up to 200 bar for some gun drilling of high-strength alloys |
| Rigid machine frame | Segmented chip formation generates force oscillations that can excite machine vibrations |
| Low runout spindle | Tool runout that is acceptable in soft materials causes premature edge chipping in hard materials |
| Through-spindle coolant | Essential for gun drilling and BTA — cannot rely on external coolant for deep holes |
| Thermal stability | Long cycle times in hard materials generate heat that can affect bore accuracy |
Tool Material Selection Guide
| Material to Drill | Hardness | Primary Tool | Secondary Tool | Coolant |
|---|---|---|---|---|
| Low-alloy steel | 45–50 HRC | Coated carbide (TiAlN) | Carbide + AlCrN coating | Oil or emulsion, 35–70 bar |
| Tool steel (H13) | 48–52 HRC | Coated carbide (TiAlN) | CBN (for finishing) | Oil preferred, 35–70 bar |
| Tool steel (D2) | 58–62 HRC | CBN (for boring) | Carbide (very low parameters) | Oil only for CBN |
| Bearing steel | 60–65 HRC | CBN | — | Oil only |
| Alumina (Al₂O₃) | 1,500 HV | Brazed diamond core drill | RUM diamond | Water-based flood |
| Silicon carbide (SiC) | 2,500 HV | Electroplated diamond + RUM | Laser (thin sections) | Water-based flood |
| Silicon nitride (Si₃N₄) | 1,600 HV | Brazed diamond thin wall | RUM diamond | Water-based flood |
| CMC (SiC/SiC, C/SiC) | 1,500+ HV | Brazed PCD core drill + RUM | Laser | Water-based flood |
Process Planning for Hard Material Deep Hole Drilling
Recommended Sequence for Hardened Steel Components
- Rough machine in annealed or pre-hardened condition (carbide tooling)
- Deep hole drill in soft condition (conventional BTA or gun drilling parameters)
- Heat treat to final hardness (quench and temper)
- Finish bore with CBN tooling (if final bore tolerance requires it)
- Inspect — bore diameter, straightness, surface finish in hardened condition
This sequence minimises the amount of material removed in the hard condition, where material removal rates are low and tool costs are high.
When to Drill in Hardened Condition
Drilling in the hardened condition is justified when:
- Heat treatment distortion of a pre-drilled bore would exceed tolerance limits
- The component geometry prevents pre-drilling (e.g., thin walls that would distort during heat treatment)
- The quantity is small and CBN tooling cost is acceptable
- The material is pre-hardened (e.g., mould steel supplied at 48–52 HRC)
Summary
| Material | Tool | Speed (m/min) | Feed (mm/rev) | Pressure | Key Limitation |
|---|---|---|---|---|---|
| Steel 45–55 HRC | Coated carbide | 20–50 | 0.02–0.12 | 35–70 bar | Tool life drops above 52 HRC |
| Steel 55–65 HRC | CBN | 60–200 | 0.05–0.30 | 10–70 bar | Thermal shock sensitivity |
| Al₂O₃ ceramic | Diamond core drill | 700–2,000 RPM | 0.005–0.015 | Flood coolant | Tool wear, L/D < 10:1 |
| SiC ceramic | Diamond + RUM | 2,500–5,000 RPM | 0.005–0.010 | Flood coolant | Edge chipping at exit |
| CMC | PCD core drill + RUM | 5,000 RPM | 0.006 | Flood coolant | Delamination, fibre pull-out |
FAQ
What is the practical hardness limit for carbide gun drilling of steel?
Carbide gun drilling becomes uneconomical above approximately 55 HRC for production quantities. Between 45 and 52 HRC, coated carbide tooling can be used with reduced parameters (cutting speed 20–40 m/min, feed 0.02–0.10 mm/rev) and high-pressure coolant (35–70 bar). Above 52 HRC, tool life drops sharply — D2 at 60 HRC produced only 6–9 holes in documented testing.
Can I use CBN tooling for drilling from solid in hardened steel?
CBN is not typically used for drilling from solid. It is used for boring existing holes in hardened steel. The typical sequence is: rough drill in soft condition with carbide, heat treat to 55–65 HRC, then finish bore with CBN. Drilling from solid in hardened material is best done with coated carbide (limited tool life) or by EDM hole-making.
What coolant is recommended for CBN deep hole boring?
Oil-based coolant is preferred for CBN tooling to provide consistent lubrication and avoid thermal shock. If water-miscible coolant must be used, maintain concentration above 10% and ensure continuous coolant flow — never start or stop coolant flow while the tool is engaged with the workpiece.
How do I drill deep holes in engineering ceramics?
Engineering ceramics require diamond tooling and either rotary ultrasonic machining or precision diamond core drilling. Rotary ultrasonic drilling (RUM) is the most effective method for deep holes in ceramics, improving surface finish by 28–45%, reducing edge chipping, and extending tool life by 2–3× compared to conventional diamond drilling.
What feed rate should I use for gun drilling hardened tool steel?
For gun drilling hardened tool steel (45–55 HRC), start with 0.005–0.025 mm/rev for diameters under 20 mm. This is approximately 1/10 of the feed rate used for conventional twist drilling. The low feed is necessary because the gun drill's single cutting edge and V-shaped support geometry cannot tolerate the cutting forces associated with higher feed rates in hard materials.
What is rotary ultrasonic drilling and how does it work?
Rotary ultrasonic machining combines diamond abrasive drilling with high-frequency vibration (20–80 kHz) applied to the tool. The vibration creates micro-impacts that fracture the ceramic material ahead of the diamond grains, reducing cutting forces, improving material removal rate, and producing better surface finish than conventional diamond drilling alone.
Can I drill ceramics with standard carbide tooling?
No. Standard carbide tooling cannot cut engineering ceramics. Ceramics have hardness values of 1,500–2,500 HV — far exceeding carbide (1,500–1,800 HV). Diamond tooling is required. The only exception is green-state (unfired) ceramics, which can be machined with carbide before sintering.
How does chip formation differ in hardened steel deep hole drilling?
Above 45 HRC, chip formation transitions from continuous shear to segmented (saw-tooth) chip formation. The chips are smaller and more fragmented — which aids chip evacuation — but the cyclical cutting force generates vibration that can affect hole straightness and accelerates edge chipping through micro-impact loading.
What is the maximum L/D ratio for diamond core drilling of ceramics?
The practical maximum L/D ratio for diamond core drilling of ceramics is approximately 10:1. Beyond this, tool deflection, coolant delivery difficulty, and chip/slurry evacuation problems make the process unreliable. For deeper holes in ceramics, consider alternate methods (EDM for conductive ceramics, laser drilling, or multi-step drilling with progressively longer tools).
Should I drill hardened steel in the soft or hardened condition?
Drill in the soft (annealed or pre-hardened) condition whenever possible. The recommended sequence is: rough drill with carbide in soft condition, heat treat to final hardness, finish bore with CBN. Drilling in the hardened condition increases tool cost by 5–10× and reduces material removal rate by 50–80%. The exception is pre-hardened materials (e.g., mould steel supplied at 48–52 HRC) where drilling in the supplied condition is unavoidable.