Appearance
Cutting tool material selection is one of the most consequential decisions in deep hole drilling. The tool material directly affects achievable cutting speed, hole quality, tool life, and cost per hole. Unlike conventional machining where tool changes are quick, replacing a gun drill or BTA head mid-cycle is time-consuming and expensive. Selecting the right tool material for the workpiece material and production volume is essential for process reliability and profitability.
Tool Material Properties Overview
Hardness, Toughness, and Temperature Resistance
| Material | Hardness (HV) | Max Operating Temp | Relative Toughness | Cost Index |
|---|---|---|---|---|
| HSS (M2, M42) | 800–900 | 600°C | High | 1× (lowest) |
| Cemented carbide (WC-Co) | 1,200–1,800 | 900–1,000°C | Medium | 3–5× |
| Cermet | 1,300–1,800 | 800–1,000°C | Medium-low | 4–6× |
| Ceramics (Al₂O₃, Si₃N₄) | 1,800–3,000 | 1,200+°C | Very low | 6–10× |
| CBN (PCBN) | 3,000–4,500 | 1,000–1,400°C | Very low | 15–30× |
| PCD | 7,000–10,000 | 700–800°C | Low | 10–20× |
The fundamental trade-off in tool materials is hardness versus toughness. HSS is the toughest but least hard, making it suitable for interrupted cuts and flexible setups. PCD and CBN are the hardest but most brittle, requiring rigid machines and stable cutting conditions.
Typical Cutting Speeds by Material
| Tool Material | Steel (m/min) | Stainless (m/min) | Cast Iron (m/min) | Aluminum (m/min) |
|---|---|---|---|---|
| HSS | 20–40 | 15–25 | 25–50 | 60–120 |
| Carbide (uncoated) | 80–150 | 50–100 | 100–200 | 200–500 |
| Carbide (coated) | 120–250 | 80–160 | 150–300 | 300–800 |
| CBN | 150–300 | — | 200–500 | — |
| PCD | — | — | — | 500–1,500 |
| Ceramics | 300–600 | — | 400–800 | — |
TIP
These speed ranges are for general reference. Deep hole drilling typically requires speeds 20–40% lower than conventional turning or milling due to the extended tool engagement, reduced coolant access at the cutting edge, and the need to manage chip formation over long lengths.
High-Speed Steel (HSS)
Composition and Properties
HSS tools are made from tool steel alloys containing tungsten, molybdenum, chromium, and vanadium. Common grades include M2 (general purpose), M42 (cobalt HSS, higher hot hardness), and T15 (high vanadium, wear resistant).
| Grade | Composition | Hardness | Best For |
|---|---|---|---|
| M2 | W-6Mo-5Cr-4V-2 | HRC 62–64 | General purpose drilling |
| M42 | Co-8Mo-10Cr-5V-2 | HRC 64–66 | Higher speed, harder materials |
| T15 | W-12Co-5V-4Cr | HRC 64–66 | Abrasive wear resistance |
| ASP (PM) | Powder metallurgy grades | HRC 64–68 | Superior edge retention |
Applications in Deep Hole Drilling
| Application | Suitability | Rationale |
|---|---|---|
| Small-diameter gun drilling (< 3 mm) | Good | High toughness resists breakage |
| Low-volume production | Excellent | Low cost per tool, easy resharpening |
| Soft materials (aluminum, brass) | Good | Adequate wear at low speeds |
| Manual or older machines | Excellent | Forgiving of vibration and misalignment |
| High-volume production | Poor | Short tool life compared to carbide |
Advantages
- Lowest material cost
- High toughness — resists chipping and breakage
- Easy to grind, re-sharpen, and modify
- Forgiving in unstable cutting conditions
- Wide availability
Disadvantages
- Low hot hardness — loses cutting edge above 600°C
- Poor wear resistance at high speeds
- Limited to lower cutting speeds (20–60 m/min on steel)
- Higher cost per hole in production volumes
Best for: Prototype and low-volume work, small diameters (< 3 mm), flexible setups, and materials where carbide would chip.
Cemented Carbide
Tungsten carbide is the dominant tool material for deep hole drilling, accounting for an estimated 80–90% of all gun drills and BTA heads in production use.
Carbide Grades
| ISO Class | Composition | Hardness | Toughness | Application |
|---|---|---|---|---|
| K10–K20 | WC + 6% Co | HRA 88–90 | Medium | Cast iron, non-ferrous, composites |
| K30–K40 | WC + 10% Co | HRA 86–88 | Medium-high | Steel, stainless, interrupted cuts |
| P10–P20 | WC + TiC + Co | HRA 89–91 | Low | Steel finishing |
| P30–P40 | WC + TiC + Co | HRA 87–89 | Medium | Steel roughing |
| M10–M20 | WC + TiC + Co | HRA 88–90 | Medium | Stainless steel |
| M30–M40 | WC + alloy binder | HRA 86–88 | Medium-high | Stainless, tough materials |
Micro-Grain and Ultra-Fine Carbide
Grain size significantly affects performance:
| Grain Type | Grain Size | Hardness | Toughness | Application |
|---|---|---|---|---|
| Coarse | 2–5 μm | Lower | Highest | Heavy interrupted cuts |
| Medium | 1–2 μm | Moderate | High | General purpose |
| Fine | 0.5–1 μm | High | Moderate | Precision drilling |
| Micro-grain | 0.2–0.5 μm | Very high | Medium | High-speed, abrasive materials |
| Ultra-fine | < 0.2 μm | Highest | Low | Maximum wear resistance |
Coated Carbide
Coatings significantly extend tool life by reducing friction and providing thermal and chemical barriers.
| Coating | Application | Temperature Limit | Benefits |
|---|---|---|---|
| TiN | General purpose | 600°C | Low friction, visual wear indicator |
| TiCN | Steel, cast iron | 800°C | Higher hardness than TiN |
| TiAlN | High-temperature alloys | 900°C | Excellent oxidation resistance |
| AlTiN | Hardened materials | 900°C | Better heat resistance than TiAlN |
| TiSiN | Very high temperature | 1,100°C | Maximum thermal protection |
| AlCrN | Corrosive environments | 1,100°C | Chemical resistance + hardness |
| Diamond (CVD) | Non-ferrous, composites | 700°C | Extreme wear resistance |
Applications in Deep Hole Drilling
| Application | Suitability | Rationale |
|---|---|---|
| Production gun drilling | Excellent | Best balance of cost, life, and performance |
| BTA drilling | Excellent | Carbide cutting edges and guide pads |
| High-speed drilling | Excellent | Thermal stability at high speeds |
| Stainless steel | Good | Requires sharp edge + coating |
| Titanium alloys | Good | Requires sharp edge + high coolant pressure |
| Superalloys (Inconel) | Good | Requires micro-grain grade + AlTiN coating |
| Interrupted cuts | Poor | Carbide is brittle compared to HSS |
Best for: The default choice for most deep hole drilling applications. Use uncoated micro-grain carbide for general work and coated carbide for higher speeds and difficult materials.
CBN (Cubic Boron Nitride)
CBN (also called PCBN — polycrystalline CBN) is the second-hardest known material and offers exceptional wear resistance when machining ferrous materials.
Properties
| Property | Typical Value |
|---|---|
| Hardness | 3,000–4,500 HV |
| Thermal stability | Up to 1,000–1,400°C |
| Chemical reactivity | Inert with ferrous materials |
| Toughness | Low — requires rigid setup |
| Relative cost | 15–30× cemented carbide |
CBN Grades
| Grade Type | CBN Content | Binder | Application |
|---|---|---|---|
| High CBN (> 85%) | 85–95% | Ceramic | Cast iron, hardened steels (HRC 50+) |
| Medium CBN (65–85%) | 65–85% | Ceramic + metallic | Hardened steel finishing |
| Low CBN (40–60%) | 40–60% | Ceramic | Hardened steel semi-finishing |
Applications in Deep Hole Drilling
| Application | Suitability | Rationale |
|---|---|---|
| Hardened steel (HRC 50+) | Excellent | 10–50× carbide tool life |
| Cast iron | Excellent | High-speed finishing |
| Powder metal components | Good | Resists abrasive wear |
| Soft steel | Poor | Not cost-effective |
| Aluminum | Not suitable | Chemical reaction risk |
| Titanium / Superalloys | Limited | Better options with coated carbide |
Advantages
- Exceptional wear resistance on hardened steels
- Maintains hardness at high temperatures
- Long tool life reduces changeover frequency
- Excellent surface finish capability
Disadvantages
- Very high cost
- Brittle — requires rigid, stable setup
- Limited to ferrous materials
- Difficult to regrind or modify
- Requires high cutting speeds to be economical
Best for: High-volume production drilling of hardened steels (HRC 45+) and cast iron where carbide tool life is inadequate and the higher cost can be justified by reduced downtime.
PCD (Polycrystalline Diamond)
PCD is the hardest tool material available, offering unmatched wear resistance on non-ferrous materials.
Properties
| Property | Typical Value |
|---|---|
| Hardness | 7,000–10,000 HV |
| Thermal stability | Up to 700–800°C |
| Wear resistance | 50–100× carbide on aluminum |
| Toughness | Low — sensitive to impact |
| Relative cost | 10–20× cemented carbide |
Applications in Deep Hole Drilling
| Application | Suitability | Rationale |
|---|---|---|
| Aluminum alloys | Excellent | 50–100× carbide life, mirror finish |
| Copper and brass | Excellent | Long life, excellent finish |
| Composites (CFRP, GFRP) | Excellent | Abrasive wear resistance |
| Plastics and graphite | Excellent | Very long tool life |
| Steel and iron | Not suitable | Chemical reaction graphitizes diamond |
| Titanium | Not suitable | Chemical reaction |
WARNING
PCD must never be used to machine steel, iron, or other ferrous materials. The carbon in the diamond reacts chemically with the iron at cutting temperatures, causing rapid graphitization and tool failure within seconds. PCD is strictly limited to non-ferrous materials. For hardened ferrous materials, use CBN instead.
Advantages
- Extremely long tool life on non-ferrous materials
- Produces mirror-like surface finishes
- Maintains size over long production runs
- Reduces machine downtime for tool changes
- High thermal conductivity
Disadvantages
- Cannot machine ferrous materials
- High initial cost
- Brittle — requires stable cutting conditions
- Requires diamond grinding wheels for resharpening
- Limited edge geometry options
Best for: High-volume production drilling of aluminum, copper, brass, composites, plastics, and graphite. Particularly common in aerospace and automotive aluminum component production.
Ceramic Tool Materials
Ceramics offer high hardness and temperature resistance but are rarely used for deep hole drilling due to their extreme brittleness.
| Material | Hardness | Max Temp | Application |
|---|---|---|---|
| Alumina (Al₂O₃) | 1,800–2,400 HV | 1,200°C | Cast iron finishing |
| Silicon nitride (Si₃N₄) | 1,800–2,200 HV | 1,200°C | Cast iron roughing |
| Whisker-reinforced (Al₂O₃ + SiC) | 2,000–2,800 HV | 1,300°C | Superalloys |
| Mixed ceramic (Al₂O₃ + TiC) | 2,200–3,000 HV | 1,200°C | Hardened steel |
Limited use in deep hole drilling: Ceramics are extremely sensitive to shock, vibration, and temperature fluctuations — all of which are present in deep hole drilling. They may be used for finishing operations in BTA drilling of cast iron where conditions are stable, but are generally not suitable for gun drilling or standard BTA drilling.
Material Selection by Workpiece
Quick Reference Matrix
| Workpiece Material | Recommended Tool Material | Alternative | Reason |
|---|---|---|---|
| Carbon steel (< HRC 40) | Micro-grain carbide (coated) | Carbide (uncoated) | Best balance of cost and life |
| Alloy steel (< HRC 40) | Micro-grain carbide (TiAlN) | Carbide (uncoated) | Good wear resistance |
| Hardened steel (HRC 45–60) | CBN | Ultra-fine carbide | CBN for production; carbide for shorter runs |
| Stainless steel (300 series) | Micro-grain carbide (TiCN or TiAlN) | Powder metal HSS | Carbide for production; HSS for low volume |
| Stainless (400 series / PH grades) | Micro-grain carbide (AlTiN) | CBN (for hardened) | Coated carbide for standard; CBN for hardened |
| Cast iron (gray) | Carbide (K10–K20) | CBN (for high speed) | Carbide for standard; CBN for high volume |
| Ductile iron | Carbide (K20–K30) | CBN | Carbide for toughness; CBN for finish |
| Aluminum (cast, wrought) | PCD | Micro-grain carbide | PCD for production; carbide for short runs |
| Aluminum (high silicon > 12%) | PCD | Diamond-coated carbide | PCD essential for abrasive Si |
| Titanium alloys | Micro-grain carbide (sharp edge, AlTiN) | — | Carbide only practical option |
| Inconel / Superalloys | Micro-grain carbide (AlTiN, sharp edge) | — | Carbide only; CBN possible for finishing |
| Copper / Brass | PCD | Carbide (K10) | PCD for finish and life |
| CFRP / Composites | PCD | Diamond-coated carbide | PCD essential for abrasive wear |
| Plastics | Carbide (K10, sharp edge) | PCD | Carbide for general; PCD for high volume |
| Graphite / Carbon | PCD | Diamond-coated carbide | Abrasive wear requires diamond |
Volume-Based Selection
| Production Volume | Recommended Material | Rationale |
|---|---|---|
| Prototype (1–10 holes) | HSS or standard carbide | Lowest tool cost |
| Low (10–500 holes) | Coated carbide | Balance of cost and performance |
| Medium (500–5,000 holes) | Premium coated carbide | Optimized tool life reduces changeovers |
| High (5,000–50,000 holes) | PCD (non-ferrous) or CBN (ferrous) | Maximum tool life, minimum downtime |
| Very high (> 50,000 holes) | PCD or CBN | Automated production demands max life |
Cost-Per-Hole Analysis
When selecting tool materials, consider total cost per hole, not just tool purchase price:
Example: Drilling 10,000 holes in steel
| Tool Material | Tool Cost | Holes per Tool | Tools Required | Tool Cost per Hole | Changeover Cost per Hole | Total per Hole |
|---|---|---|---|---|---|---|
| HSS | $50 | 50 | 200 | $1.00 | $0.20 | $1.20 |
| Carbide (standard) | $150 | 500 | 20 | $0.30 | $0.08 | $0.38 |
| Carbide (coated) | $200 | 1,500 | 7 | $0.13 | $0.03 | $0.16 |
| CBN | $2,000 | 10,000 | 1 | $0.20 | $0.01 | $0.21 |
Coated carbide offers the lowest cost per hole for this scenario. CBN becomes competitive when changeover cost or downtime is a significant factor.
Example: Drilling 10,000 holes in aluminum
| Tool Material | Tool Cost | Holes per Tool | Tool Cost per Hole | Total per Hole |
|---|---|---|---|---|
| Carbide | $150 | 500 | $0.30 | $0.36 |
| PCD | $800 | 15,000 | $0.05 | $0.07 |
PCD is clearly the most economical choice for high-volume aluminum drilling despite the higher initial cost.
FAQ
Q: What is the most common cutting tool material for deep hole drilling? Cemented tungsten carbide (WC-Co) is the dominant material, used in approximately 80–90% of gun drills and BTA heads. It offers the best balance of hardness, toughness, and cost for most workpiece materials.
Q: When should I use HSS instead of carbide for deep hole drilling? Use HSS for small-diameter gun drills (< 3 mm), prototype or low-volume work, on older or less rigid machines, or when drilling materials prone to chipping carbide (such as certain bronzes or composites). HSS is more forgiving of vibration and misalignment.
Q: Can PCD be used for drilling steel? No. PCD reacts chemically with iron at cutting temperatures, causing rapid diamond graphitization and tool failure. PCD is strictly limited to non-ferrous materials including aluminum, copper, brass, composites, plastics, and graphite. For hardened steels, use CBN instead.
Q: What tool material is best for drilling hardened steel (HRC 50+)? CBN (PCBN) offers the longest tool life for hardened steel deep hole drilling, typically 10–50× that of carbide. However, CBN requires rigid machine setups and high cutting speeds to be economical. For shorter runs or less rigid setups, ultra-fine grain carbide is a practical alternative.
Q: What carbide grade is recommended for gun drilling stainless steel? Micro-grain or submicron carbide (ISO K30–K40 or M30–M40) with TiAlN or TiCN coating. The fine grain size provides a sharp cutting edge, while the coating reduces friction and built-up edge formation. High coolant pressure (100+ bar) is essential.
Q: What is the best tool material for drilling aluminum deep holes? PCD is the best choice for production volumes, offering 50–100× the tool life of carbide with superior surface finish. For short runs, micro-grain carbide (K10–K20) with a sharp, polished edge is adequate.
Q: Are ceramic tools used in deep hole drilling? Rarely. Ceramics are extremely brittle and sensitive to vibration, making them unsuitable for deep hole drilling where tool stability is inherently limited. They may be used for finishing operations in BTA drilling of cast iron under very stable conditions.
Q: How does coating affect tool performance in deep hole drilling? Coatings reduce friction (lower cutting forces and heat), provide thermal barriers (protecting the carbide substrate), and increase wear resistance. TiAlN and AlTiN are the most common coatings for deep hole drilling tools, effective up to 900°C. Coated carbide typically lasts 2–5× longer than uncoated carbide.
Q: What is the difference between micro-grain and standard carbide? Micro-grain carbide has grain sizes of 0.2–0.5 μm versus 1–5 μm for standard carbide. The finer grain structure provides higher hardness and wear resistance while maintaining adequate toughness, making it the preferred grade for precision deep hole drilling tools.
Q: How should I choose between CBN and coated carbide for drilling cast iron? For standard gray cast iron at moderate speeds, coated carbide (K10–K20) is the most economical choice. For high-speed production or abrasive cast iron grades, CBN offers significantly longer tool life. Consider cost per hole rather than initial tool cost.