The cutter material defines the process limits. Carbide runs faster than HSS but is more brittle. CBN cuts hardened steel but costs 10× more than carbide. PCD machines aluminum for hundreds of metres but cannot cut steel. Matching the cutter material to the workpiece material and production volume is the most consequential tool selection decision in deep hole drilling.
Cutter Material Overview
Material Properties Comparison
| Property | HSS | Carbide (WC-Co) | CBN (PCBN) | PCD |
|---|
| Hardness (HV) | 800–900 | 1,300–2,000 | 3,500–4,500 | 6,000–8,000 |
| Toughness (MPa·m¹/²) | 15–25 | 8–15 | 4–8 | 6–10 |
| Transverse rupture strength (MPa) | 3,000–4,500 | 1,500–3,000 | 800–1,200 | 1,000–1,500 |
| Thermal stability (°C) | 500–600 | 800–900 | 1,000–1,200 | 600–700 |
| Wear resistance | Low | High | Very high | Highest |
| Chemical reactivity with steel | Low | Low | Low (inert) | High (reacts with Fe) |
| Relative cost (per cutting edge) | 1× | 3–8× | 20–50× | 15–40× |
| Typical cutting speed (m/min in steel) | 15–30 | 60–150 | 100–250 | N/A (not for steel) |
Application Range
| Cutter Material | Best Material Match | Avoid | Typical Tool Types |
|---|
| HSS | Low-carbon steel, aluminum, brass | Hard materials > 30 HRC | Gun drills (small diameter) |
| Carbide | Most materials — steel, stainless, cast iron, aluminum | Interrupted cuts (brittle), high-temp alloys | Gun drills, BTA drill heads |
| CBN | Hardened steel (> 45 HRC), cast iron (hard), tool steel | Soft steel (chemical wear), aluminum | BTA drill head inserts, reamers |
| PCD | Aluminum (high Si), composites, plastics, non-ferrous | Steel (chemical reaction), titanium | Reamers, special drills |
HSS (High-Speed Steel)
Applications in Deep Hole Drilling
| Application | Typical Size Range | Advantage | Limitation |
|---|
| Small-diameter gun drills (< 5 mm) | 1–5 mm | Lower cost than carbide at small sizes | Lower speed capability |
| Prototype or low-volume production | Any | Low tool cost, easy to modify | Short tool life |
| Materials requiring high toughness | Any | Higher toughness than carbide | Not for production volumes |
| Regrind-friendly applications | Any | Easy to regrind with conventional wheels | Lower wear resistance |
HSS Grades for Deep Hole Drilling
| Grade | Composition | Hardness | Best For |
|---|
| M2 | W-Mo-V | 64–66 HRC | General purpose |
| M42 (8% Co) | Co-W-Mo-V | 66–68 HRC | Higher hardness, better wear resistance |
| T15 | W-V-Co | 65–67 HRC | Abrasive materials |
| ASP (powder metallurgy) | Various | 66–69 HRC | Improved toughness over conventional HSS |
Carbide (Tungsten Carbide)
Carbide Grades for Deep Hole Drilling
| Grade Group | Grain Size (µm) | Cobalt % | Hardness (HRA) | Best For |
|---|
| Micrograin (K10–K20) | 0.5–0.8 | 6–8% | 91–93 | General steel drilling |
| Sub-micrograin (K20–K30) | 0.2–0.5 | 8–11% | 92–94 | Stainless, titanium |
| Nanograin (K05–K15) | < 0.2 | 5–7% | 93–95 | High-speed finishing |
| Coarse grain (K30–K40) | 1.0–3.0 | 10–14% | 89–91 | Interrupted cuts, heavy feeds |
Carbide Selection by Material
| Workpiece Material | Recommended Grade | Cobalt % | Grain Size | Coating |
|---|
| Low-carbon steel | K10–K20 | 6–8% | Micrograin | TiN or TiCN |
| Alloy steel (4140, 4340) | K15–K25 | 7–9% | Micrograin | TiCN or TiAlN |
| Stainless steel (304, 316) | K20–K30 | 9–11% | Sub-micrograin | TiAlN |
| Stainless (precipitation-hardened) | K25–K35 | 10–12% | Sub-micrograin | TiAlN or AlTiN |
| Cast iron | K05–K15 | 5–7% | Micrograin | TiN or none |
| Aluminum (wrought) | K10–K20 | 6–8% | Micrograin | None or TiN |
| Aluminum (high Si) | K15–K25 | 7–9% | Micrograin | PCD (preferred) or diamond-coated |
| Titanium | K25–K35 | 10–12% | Sub-micrograin | AlTiN or AlCrN |
| Inconel / superalloys | K30–K40 | 11–14% | Sub-micrograin | AlCrN or AlTiN |
CBN (Cubic Boron Nitride)
Applications in Deep Hole Drilling
| Application | Typical Hardness | Speed Advantage vs Carbide | Tool Life vs Carbide |
|---|
| Hardened steel boring/reaming | 45–65 HRC | 2–3× | 5–20× |
| Hard cast iron (chilled, Ni-hard) | 400–600 HB | 2–4× | 10–30× |
| Powder metal steels | 35–50 HRC | 1.5–2× | 3–8× |
| Tool steel (D2, H13, etc.) | 45–60 HRC | 2–3× | 5–15× |
CBN Grade Selection
| CBN Grade | CBN Content | Binder | Hardness | Best For |
|---|
| High CBN (85–95%) | 85–95% | Ceramic or metallic | Very high | Hardened steel, continuous cut |
| Medium CBN (65–80%) | 65–80% | Ceramic | High | Hard cast iron, mixed cut |
| Low CBN (40–60%) | 40–60% | Carbide or ceramic | Moderate | Interrupted cuts, tough conditions |
PCD (Polycrystalline Diamond)
Applications in Deep Hole Drilling
| Application | Material | Tool Life vs Carbide | Surface Finish | Limitation |
|---|
| Reaming aluminum (high Si) | Al > 12% Si | 20–50× | Ra 0.1–0.2 µm | Cannot run on steel parts |
| Boring composites | CFRP, GFRP | 50–100× | Excellent | Edge chipping risk |
| Finishing non-ferrous metals | Brass, bronze, copper | 20–40× | Ra 0.05–0.1 µm | High cost |
| Plastics and ceramics | Engineering plastics | 50–100× | Excellent | Brittle — avoid interrupted cuts |
PCD Limitations
| Limitation | Reason | Workaround |
|---|
| Cannot cut steel | Chemical reaction — carbon dissolves in iron at cutting temperature | Use carbide or CBN for steel |
| Brittle edge — no interrupted cuts | PCD is very hard but not tough | Use high-CBN or carbide for interrupted cuts |
| High temperature sensitivity | Diamond graphitizes at > 700°C | Adequate cooling essential |
| Difficult to regrind | Diamond grinding wheels required | Send to specialist service |
| High initial cost | PCD layer is expensive | Cost per hole must justify investment |
Cost per Hole Analysis
Material Cost Comparison
| Cutter Material | Tool Cost per Edge | Holes per Edge (Steel, typical) | Cost per Hole |
|---|
| HSS (small gun drill) | $15–$30 | 50–150 | $0.10–$0.60 |
| Carbide (standard grade) | $50–$200 | 300–800 | $0.06–$0.67 |
| Carbide (premium grade) | $80–$300 | 500–1,500 | $0.05–$0.60 |
| CBN insert (brazed) | $40–$100 | 2,000–10,000 | $0.004–$0.05 |
| PCD insert (brazed) | $50–$150 | 5,000–50,000 | $0.001–$0.03 |
Application-Specific Recommendation
| Part Type | Material | Annual Volume | Recommended Cutter | Justification |
|---|
| Hydraulic cylinder | 1026 steel | > 10,000 holes | Carbide (micrograin) | Best balance of cost and life |
| Hydraulic cylinder | 1026 steel | < 1,000 holes | HSS or standard carbide | Lower tool cost justified |
| Automotive component | Hardened steel (55 HRC) | > 5,000 holes | CBN (brazed insert) | Long life offsets higher cost |
| Aerospace actuator | Titanium | > 1,000 holes | Carbide (sub-micrograin, AlTiN) | Only practical choice |
| Aluminum engine block | AlSi12 | > 50,000 holes | PCD (reamer) | Extremely long life, excellent finish |
| Structural tube | Low-carbon steel | > 20,000 holes | Carbide (coated) | Reliable, consistent, economical |
FAQ
What is the best cutter material for deep hole drilling in steel?
Carbide (tungsten carbide with 6–10% cobalt binder, micrograin grade) is the best general-purpose cutter material for deep hole drilling in steel. It offers the best balance of hardness, toughness, wear resistance, and cost. For hardened steel (> 45 HRC), CBN provides significantly longer tool life. For low-volume production in soft steel, HSS is an economical choice.
When should I use CBN instead of carbide for deep hole drilling?
Use CBN when: workpiece hardness exceeds 45 HRC, carbide tool life is unacceptably short (less than 50 holes per edge), cutting speeds above 100 m/min are needed for productivity, or the material is difficult-to-machine cast iron (chilled iron, Ni-hard). CBN typically provides 5–20× the tool life of carbide in these applications, offsetting its higher cost.
Can PCD be used for deep hole drilling in steel?
No — PCD chemically reacts with steel at cutting temperatures. The carbon in the diamond dissolves into the iron matrix, causing rapid tool wear. PCD is only suitable for non-ferrous materials: aluminum, copper, brass, bronze, composites, plastics, and ceramics. For steel deep hole drilling, use carbide or CBN.
How do I choose between micrograin and sub-micrograin carbide?
Choose micrograin carbide (0.5–0.8 µm grain size, 6–8% cobalt) for general steel drilling — it offers the best balance of wear resistance and toughness. Choose sub-micrograin carbide (0.2–0.5 µm, 9–11% cobalt) for stainless steel and titanium — the finer grain provides better edge sharpness for gummy materials, and the higher cobalt content provides the toughness needed to resist notch wear.
Does cutter material affect achievable surface finish?
Yes — cutter material directly affects achievable surface finish. Finer-grained cutter materials (sub-micrograin carbide, CBN, PCD) can be polished to sharper edges, producing better surface finishes. PCD produces the finest surface finishes (Ra 0.05–0.1 µm on aluminum) but can only be used on non-ferrous materials. For steel, polished carbide or CBN produces the best finishes (Ra 0.1–0.4 µm).
Cutter material selection is a trade-off between hardness, toughness, wear resistance, and cost. Match the cutter material to the workpiece material and production volume — not to what is already in the tool crib. The right material choice reduces cost per hole and improves process stability. This article reflects industry practice as of 2026.
