Appearance
Polycrystalline diamond and cubic boron nitride are the hardest tool materials available for deep hole drilling — but applying each to the wrong workpiece material causes rapid tool failure and scrapped parts.
Overview
Deep hole drilling places extraordinary demands on cutting tool materials. The combination of continuous cutting edge engagement, high temperatures at the tool-chip interface, limited coolant access, and the difficulty of tool change at depth means that tool life and wear resistance directly determine process economics. In materials that are highly abrasive (high-silicon aluminium, metal matrix composites), extremely hard (hardened steels above 45 HRC, chilled cast iron), or chemically reactive (titanium, nickel alloys), conventional carbide tooling reaches its economic limit.
Superabrasive tool materials — polycrystalline diamond (PCD) and cubic boron nitride (CBN, also referred to as PCBN) — offer step-change improvements in these applications. PCD is approximately 50× harder than cemented carbide and provides 10–50× longer tool life in non-ferrous materials. CBN is the second-hardest known material and is thermally stable to 1,200°C, making it the superabrasive of choice for ferrous materials that destroy PCD.
However, these materials come with strict application boundaries, higher tool cost, and brittleness that demands rigid machine setups. Applying PCD to steel or CBN to aluminium produces rapid failure. Correct selection and parameter optimisation are essential to realise the economic benefits.
PCD Tooling Fundamentals
Composition and Manufacture
PCD is produced by sintering synthetic diamond particles (typically 2–30 µm grain size) with a metallic binder — usually cobalt — at pressures of 5–7 GPa and temperatures of 1,300–1,500°C. The result is a tough, wear-resistant composite with random diamond-to-diamond bonding throughout the structure.
PCD blanks are manufactured as a thin layer (0.5–2.0 mm) bonded to a cemented carbide substrate. The carbide backing provides impact resistance and enables brazing or clamping into tool bodies. For deep hole drilling tools, PCD-tipped inserts are brazed onto carbide shanks or mounted in indexable cartridges.
Key Properties
| Property | PCD | Carbide (Comparison) |
|---|---|---|
| Hardness (HV) | 7,000–8,000 | 1,300–1,800 |
| Thermal stability | ~700°C (graphitisation onset) | ~1,000°C |
| Compressive strength | 4–7 GPa | 4–6 GPa |
| Fracture toughness | 6–9 MPa·m¹/² | 10–18 MPa·m¹/² |
| Thermal conductivity | 500–600 W/m·K | 80–120 W/m·K |
The high thermal conductivity of PCD is a significant advantage in deep hole drilling — it conducts heat away from the cutting edge rapidly, reducing thermal damage to the workpiece and allowing higher cutting speeds.
Materials PCD Can Machine
PCD is effective in all non-ferrous materials where abrasion resistance is the primary wear mechanism:
- Aluminium alloys (especially high-silicon > 12 % Si)
- Copper and copper alloys (brass, bronze)
- Magnesium alloys
- Metal matrix composites (MMCs) — Al₂O₃ or SiC-reinforced aluminium
- Carbon-fibre reinforced polymers (CFRP)
- Glass-fibre reinforced plastics (GFRP)
- Graphite and carbon composites
- Ceramics (green and sintered)
- Wood and wood composites
Materials PCD Cannot Machine
PCD must never be used on ferrous materials (steel, cast iron, stainless steel). At temperatures above approximately 700°C, carbon from the diamond dissolves into the iron matrix, forming iron carbide. This chemical wear mechanism — graphitisation and diffusion — destroys the PCD cutting edge within seconds, regardless of coolant application.
The chemical affinity between carbon and iron is the fundamental constraint on PCD application. Even small amounts of ferrous contamination in an otherwise non-ferrous workpiece can cause accelerated edge wear.
CBN Tooling Fundamentals
Composition and Manufacture
Cubic boron nitride (CBN) is the second-hardest known material, with a hardness of 3,000–4,500 HV. It does not occur naturally; it is synthesised from hexagonal boron nitride (hBN) under high pressure and temperature. Polycrystalline CBN (PCBN) is produced by sintering CBN particles with a ceramic or metallic binder.
PCBN grades vary in CBN content (typically 40–95 %) and grain size (0.5–15 µm):
- High-CBN grades (80–95 %): Coarse grain, high wear resistance, for continuous cutting of hardened cast iron
- Low-CBN grades (40–65 %): Fine grain with ceramic binder, lower thermal conductivity but higher edge strength, for hardened steel finishing
- Coated PCBN: TiAlN or TiCN coatings applied to PCBN for reduced crater wear and improved surface finish
Key Properties
| Property | CBN | PCD (Comparison) |
|---|---|---|
| Hardness (HV) | 3,000–4,500 | 7,000–8,000 |
| Thermal stability | 1,200–1,300°C | ~700°C |
| Chemical reactivity with iron | Inert — no reaction | Reacts — forms iron carbide |
| Thermal conductivity | 100–200 W/m·K | 500–600 W/m·K |
| Fracture toughness | 4–7 MPa·m¹/² | 6–9 MPa·m¹/² |
CBN's chemical inertness with respect to iron is its defining advantage. While diamond dissolves into iron at cutting temperatures, CBN remains stable. This enables machining of ferrous materials at high cutting speeds that would destroy carbide through crater wear.
Materials CBN Can Machine
- Hardened steels (45–70 HRC) — bearing steels, tool steels, die steels
- Chilled cast iron and high-chromium cast iron
- Pearlitic grey cast iron
- Powder metallurgy steels
- Nickel-based superalloys (finishing operations)
- Hard-facing alloys and weld overlay materials
Materials CBN Cannot Machine
CBN is ineffective in soft, ductile materials. Below approximately 45 HRC, the workpiece material causes rapid edge build-up and micro-chipping rather than clean cutting. CBN should not be used for:
- Low-carbon and mild steels
- Aluminium and other non-ferrous materials
- Annealed or normalized steels
Material Suitability Comparison
| Material | Recommended Superabrasive | Alternative | Rationale |
|---|---|---|---|
| Aluminium, Si < 12 % | PCD (fine grain) | Carbide | PCD tool life 20–50× longer; mirror finish possible |
| Aluminium, Si > 12 % | PCD (coarse grain) | Carbide (limited life) | Highly abrasive — PCD essential for production volumes |
| Copper and brass | PCD (fine grain) | Carbide | PCD prevents edge build-up, improves finish |
| CFRP / GFRP | PCD | Carbide (high wear) | Abrasive fibres destroy carbide edges |
| Metal matrix composites | PCD (coarse grain) | None viable | Only PCD has sufficient abrasion resistance |
| Hardened steel (45–65 HRC) | CBN (low-content grade) | Carbide (low speed) | CBN enables high-speed finish machining |
| Hardened steel (65–70 HRC) | CBN (high-content grade) | Ceramic | CBN provides better surface finish |
| Pearlitic grey cast iron | CBN (high-content grade) | Carbide | CBN enables cutting speeds > 600 m/min |
| Chilled cast iron | CBN (high-content grade) | Carbide (rapid wear) | CBN tool life 5–10× carbide |
| Nickel superalloys | CBN (finishing only) | Carbide or ceramic | CBN limited to light cuts; carbide for roughing |
Cutting Parameters for Deep Hole Drilling
Deep hole drilling imposes specific constraints on cutting parameters: coolant delivery and chip evacuation limit the maximum speed and feed, and tool deflection limits depth of cut. The following tables give starting parameters for PCD and CBN tooling in deep hole drilling applications.
PCD Parameters for Deep Hole Drilling (Non-Ferrous)
| Material | Cutting Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) | Coolant |
|---|---|---|---|---|
| Aluminium, Si < 12 % | 500–1,500 | 0.05–0.20 | 0.1–2.0 | Flood or through-tool |
| Aluminium, Si > 12 % | 300–800 | 0.05–0.15 | 0.1–1.5 | Through-tool mandatory |
| Copper alloys | 200–600 | 0.05–0.15 | 0.1–1.0 | Flood |
| CFRP | 100–400 | 0.02–0.10 | 0.1–0.5 | Dry or MQL |
| MMC (Al-SiC) | 100–300 | 0.02–0.08 | 0.1–0.5 | Through-tool mandatory |
PCD deep hole drills typically use lower speeds than PCD turning tools to manage heat generation at depth. Through-tool coolant is strongly recommended for all deep hole drilling with PCD, as the brittleness of the cutting edge makes thermal shock a risk with intermittent coolant.
CBN Parameters for Deep Hole Drilling (Hard Materials)
| Material | Cutting Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) | Coolant |
|---|---|---|---|---|
| Hardened steel (< 55 HRC) | 80–150 | 0.05–0.15 | 0.1–0.5 | MQL or dry |
| Hardened steel (55–65 HRC) | 60–120 | 0.05–0.10 | 0.1–0.3 | MQL or dry |
| Chilled cast iron | 40–100 | 0.05–0.15 | 0.1–1.0 | Dry recommended |
| Pearlitic grey cast iron | 300–600 | 0.10–0.30 | 0.1–2.0 | Dry (preferred) or MQL |
| Nickel superalloys (finish) | 100–200 | 0.05–0.10 | 0.1–0.3 | High-pressure coolant |
CBN is often used dry or with MQL in deep hole applications because the material maintains hardness at high temperature, and coolant-induced thermal shock can cause edge fracture. When coolant is used, it must be applied consistently — never intermittent.
Tool Geometry and Edge Preparation
Superabrasive tools require different edge geometries than carbide because of their higher hardness but lower fracture toughness.
Rake Angle
| Tool Material | Rake Angle | Reason |
|---|---|---|
| PCD (non-ferrous) | Positive (5°–10°) | Reduces cutting forces; sharp edge for clean cut |
| CBN (hardened steel) | Negative (0° to –7°) | Strengthens cutting edge; impacts compressive stress |
Edge Preparation
PCD edges are typically prepared with a small hone radius (0.005–0.020 mm) to eliminate micro-notches from the grinding process. CBN edges require more substantial edge preparation:
| CBN Edge Type | Application | Preparation |
|---|---|---|
| Sharp | Finish machining, continuous cut | 0.01–0.03 mm hone |
| Chamfered | Interrupted cut, roughing | 0.05–0.15 mm × 20°–30° chamfer |
| Heavy chamfer | Heavy roughing, scale | 0.15–0.30 mm × 30° |
Grain Size Selection
PCD grain size affects edge quality and wear resistance:
- Coarse grain (10–30 µm): Maximum wear resistance for highly abrasive materials (high-Si Al, MMC, CFRP)
- Medium grain (5–10 µm): General-purpose, good balance of wear and edge quality
- Fine grain (2–5 µm): Best surface finish for copper, brass, wood, plastics
CBN grain size selection:
- Coarse grain (3–15 µm): High-CBN content grades for cast iron, interrupted cuts
- Fine grain (0.5–3 µm): Low-CBN content grades for hardened steel finishing
Tool Wear Mechanisms
Superabrasive tools fail through fundamentally different mechanisms than carbide, and understanding these mechanisms is essential for troubleshooting and parameter optimisation.
PCD Wear Mechanisms
| Mechanism | Cause | Prevention |
|---|---|---|
| Graphitisation | Temperature > 700°C; carbon reverts to graphite | Reduce speed; improve coolant delivery |
| Diffusion wear | Chemical reaction with iron-group metals | Do not use PCD on ferrous materials |
| Micro-chipping | Mechanical shock, vibration, interrupted cut | Improve machine rigidity; use finer grain |
| Abrasive wear | Hard particles in workpiece (Si, Al₂O₃) | Switch to coarser grain PCD grade |
| Thermal fatigue | Intermittent coolant application | Consistent coolant flow; MQL or dry as alternative |
CBN Wear Mechanisms
| Mechanism | Cause | Prevention |
|---|---|---|
| Crater wear | Chemical dissolution of CBN grains in workpiece chip | Reduce speed; use coated grade |
| Flank wear | Abrasion by hard carbide particles in workpiece | Use higher CBN content grade |
| Notch wear | Work-hardened layer at depth of cut line | Reduce depth of cut below case depth |
| Chipping | Mechanical overload, interrupted cut | Use negative rake; chamfered edge |
| Thermal cracking | Intermittent coolant | Consistent coolant or dry machining |
Tool Life Expectations
In correctly matched applications:
- PCD in aluminium (Si < 12 %): 20–50× carbide tool life
- PCD in high-Si aluminium: 10–30× carbide tool life
- PCD in MMC: Indispensable — carbide fails in minutes
- CBN in hardened steel: 5–10× carbide tool life at 2–3× higher cutting speed
- CBN in chilled cast iron: 10–20× carbide tool life
Application Selection Guide
| Deep Hole Application | Recommended Tooling | Key Parameters | Expected Benefit |
|---|---|---|---|
| Gun drilling Al-Si alloy (12–18 % Si) | PCD-tipped gun drill | 300–600 m/min, 0.05–0.12 mm/rev, through-coolant | 30× tool life vs. carbide |
| BTA drilling Al-MMC (SiC reinforced) | PCD-indexable inserts | 100–250 m/min, 0.03–0.08 mm/rev, high-pressure coolant | Only viable production solution |
| Gun drilling hardened bearing steel (60 HRC) | CBN-tipped gun drill | 60–100 m/min, 0.03–0.08 mm/rev, MQL | 8× tool life vs. carbide |
| BTA drilling pearlitic cast iron | CBN-indexable inserts | 400–600 m/min, 0.15–0.30 mm/rev, dry | 15× tool life, 3× speed increase |
| Drilling CFRP/aluminium stack | PCD-tipped drill | 200–400 m/min, 0.03–0.08 mm/rev, peck cycle | Clean hole, no delamination |
| Deep reaming hardened die steel (55 HRC) | CBN-tipped reamer | 80–120 m/min, 0.05–0.10 mm/rev | IT7 tolerance, mirror finish |
Decision Framework
When selecting between PCD and CBN for a deep hole drilling application:
- Identify workpiece material: Ferrous → CBN. Non-ferrous → PCD
- Assess hardness: Below 45 HRC → evaluate carbide or coated carbide; above 45 HRC → CBN
- Check abrasiveness: High-Si Al, MMC, CFRP → PCD regardless of other factors
- Evaluate machine rigidity: Superabrasives require rigid, vibration-free setups. If machine condition is poor, carbide or cermet may be more reliable
- Calculate breakeven volume: The higher tool cost of PCD/CBN must be offset by tool life and productivity gains. For very low volumes, carbide is often more economical
FAQ
Can PCD be used for drilling steel?
No. PCD reacts chemically with iron at cutting temperatures above ~700°C, causing rapid graphitisation and diffusion wear. Even small ferrous inclusions in the workpiece cause accelerated edge deterioration. CBN is the correct superabrasive for ferrous materials.
What is the cost premium for PCD and CBN tooling compared to carbide?
PCD tooling typically costs 5–15× more than equivalent carbide tooling. CBN costs 3–10× more than carbide. The cost is offset by 10–50× longer tool life and the ability to run at higher cutting speeds. For production volumes exceeding several hundred parts, superabrasive tooling is usually more economical on a cost-per-hole basis.
Does CBN require coolant?
CBN is often used dry, particularly in hardened steel and cast iron applications, because the material maintains its hardness at elevated temperatures and thermal-shock resistance is limited. When coolant is used, it must be applied consistently — intermittent coolant causes thermal cracking. MQL is a common compromise.
PCD vs CBN — which is harder?
PCD is harder (7,000–8,000 HV vs. 3,000–4,500 HV for CBN). However, PCD is limited to non-ferrous materials. For ferrous materials, CBN is the only superabrasive option, and its hardness is still 2–3× that of carbide.
What grain size should I choose for PCD in high-silicon aluminium?
Coarse grain PCD (10–30 µm) provides maximum abrasion resistance for high-silicon aluminium. The coarse diamond particles present a more wear-resistant surface to the abrasive silicon particles in the workpiece. Surface finish will be slightly rougher than with fine grain, but tool life is significantly extended.
Can PCD or CBN be reground?
Yes, but regrinding requires diamond grinding wheels (for PCD) or CBN grinding wheels (for CBN) and specialised equipment. Not all tool grinding shops have this capability. The regrinding cost is typically 20–40 % of a new tool. PCD can be reground 3–6 times depending on the original layer thickness.
What causes edge chipping in CBN deep hole drills?
Edge chipping in CBN is most often caused by interrupted cuts (cross-holes, keyways), insufficient machine rigidity, or incorrect edge preparation. Solutions include: switching to a chamfered edge, reducing feed, improving workpiece support, or using a tougher CBN grade with higher binder content.
Is there a superabrasive solution for drilling titanium alloys?
Titanium alloys fall in a difficult region. PCD is unsuitable because titanium reacts with carbon. CBN is marginal because titanium alloys are typically below 45 HRC (35–42 HRC typical for Ti-6Al-4V), where CBN performs poorly. The best current solution is high-performance coated carbide with optimised geometry and generous coolant delivery.
Summary
PCD and CBN superabrasive tooling offer transformative tool life and productivity improvements in deep hole drilling applications where they are correctly matched to the workpiece material.
PCD is the premier choice for non-ferrous materials — particularly high-silicon aluminium, metal matrix composites, and carbon-fibre composites — where it delivers 10–50× the tool life of carbide. Its fundamental limitation is chemical reactivity with iron, making it unusable for any ferrous workpiece.
CBN is the superabrasive of choice for hard ferrous materials — hardened steels above 45 HRC, chilled cast iron, and pearlitic grey cast iron — where it enables cutting speeds 2–3× higher than carbide with 5–20× longer tool life. CBN is not suitable for soft materials below 45 HRC or non-ferrous alloys.
Successful application of superabrasive tooling in deep hole drilling requires: correct material-tool matching, rigid machine setup, appropriate edge preparation, consistent coolant strategy, and attention to the specific wear mechanisms that limit tool life. Where these conditions are met, the cost-per-hole advantage of PCD and CBN over carbide is substantial and well-documented across production applications.