Carbide is the standard for gun drilling, but it is not always the best choice. In specific applications — high-silicon aluminum, hardened steels, and abrasive composites — PCD and CBN gun drills can outlast carbide by 10-50x.
The catch is that these tools are expensive, brittle, and require a rigid setup. Here is what I have learned about when they work and when they do not.
What PCD and CBN Are
PCD (polycrystalline diamond) and CBN (cubic boron nitride) are superhard materials. PCD is the second-hardest known material after natural diamond. CBN is the second-hardest after PCD.
| Material | Hardness (HV) | Max Operating Temp | Chemical Limitation |
|---|---|---|---|
| PCD | 7000-8000 | 700°C | Reacts with iron — not for steel |
| CBN | 3000-4000 | 1400°C | Stable with iron — good for hardened steel |
| Carbide | 1500-2000 | 900°C | — |
The key difference: PCD cannot be used on steel because the carbon in the diamond reacts with iron at cutting temperatures. CBN does not have this limitation and is suitable for hardened steels and cast irons.
I cover the standard tool material choice in Carbide vs HSS Gun Drills.
Where PCD Gun Drills Work
PCD gun drills excel in non-ferrous materials where tool life is the limiting factor:
| Application | Tool Life vs Carbide | Why |
|---|---|---|
| High-silicon aluminum (>5% Si) | 20-50x longer | Carbide wears rapidly from silicon particles |
| Carbon fiber composites | 10-30x longer | PCD resists the abrasive carbon fibers |
| Copper and brass | 10-20x longer | PCD maintains a sharper edge longer |
| Ceramic-filled plastics | 15-40x longer | Extreme abrasion resistance |
The cost of a PCD gun drill is typically 5-10x that of a comparable carbide drill. The tool life improvement must justify the premium.
In high-silicon aluminum (A390, 390 alloy), I have seen PCD gun drills produce 10,000+ holes per edge compared to 200-400 for carbide. At that volume, the per-hole cost of PCD is a fraction of carbide.
Where CBN Gun Drills Work
CBN gun drills are used for hardened steels and wear-resistant cast irons:
| Application | Typical Hardness | Tool Life vs Carbide |
|---|---|---|
| Hardened tool steel (D2, A2) | 58-62 HRC | 5-15x longer |
| Hardened die steel (H13) | 48-55 HRC | 8-20x longer |
| White cast iron | 55-65 HRC | 10-25x longer |
| Hardfacing alloys (stellitic) | 45-60 HRC | 5-10x longer |
CBN is chemically stable with iron at high temperatures, which makes it suitable for steels where carbide fails by crater wear or plastic deformation.
I cover tool steel drilling in Deep Hole Drilling Tool Steels.
When Superhard Tooling Does Not Work
PCD and CBN are brittle. They chip and fracture under conditions that carbide handles without issue:
| Condition | Why Superhard Tooling Fails |
|---|---|
| Interrupted cuts (cross holes) | The impact shock chips the edge |
| Vibration or chatter | The brittle edge fractures |
| Setup with excessive runout (>0.005mm) | Uneven load chips the edge |
| Low-rigidity machines | Vibration transmits to the tool |
| Variable stock allowance | Sudden load changes crack the edge |
If your machine has spindle runout over 0.005mm TIR, or if the workpiece is not rigidly supported, superhard tooling will fail faster than carbide. The rigidity requirements are significantly higher.
I cover runout measurement in Gun Drill Runout Effects.
Parameter Adjustments
PCD and CBN gun drills need different parameters than carbide:
| Parameter | vs Carbide |
|---|---|
| Cutting speed | 1.5-3x higher |
| Feed rate | 0.5-0.8x of carbide |
| Coolant pressure | Same or higher |
Higher cutting speed takes advantage of the superhard material’s heat resistance. Lower feed rate reduces the mechanical load on the brittle cutting edge.
Cost Justification
The decision to use PCD or CBN comes down to cost per hole:
| Factor | Carbide | PCD/CBN |
|---|---|---|
| Tool cost per drill | $50-200 | $500-2000 |
| Tool life (holes) | 200-500 | 2000-10000 |
| Cost per hole (tool only) | $0.10-1.00 | $0.05-0.25 |
| Regrinds possible | 5-8 | 3-5 |
The crossover point where PCD/CBN becomes cheaper per hole is typically at 1000-2000 holes total production volume.
I cover cost analysis in Cost Per Hole Deep Hole Drilling.
Key Takeaways
- PCD gun drills work in non-ferrous materials (aluminum, composites, brass) and can outlast carbide by 20-50x in high-silicon aluminum.
- CBN gun drills work in hardened steels (58+ HRC) and can outlast carbide by 5-20x.
- Superhard tooling requires rigid setups with runout under 0.005mm. Brittle edges chip easily.
- Cost per hole is the deciding factor. At volumes over 1000-2000 holes, PCD/CBN is often cheaper.
- Do not use PCD on steel — the diamond reacts with iron at cutting temperatures.
- Increase cutting speed by 1.5-3x over carbide. Reduce feed slightly to protect the brittle edge.