Appearance
A manufacturer of cemented carbide wire-drawing dies needs a 0.3 mm diameter coolant hole through a WC-Co (ISO K20, 92 HRA / 78 HRC equivalent) blank 18 mm long — an L/D ratio of 60:1. Conventional carbide and PCD drills cannot penetrate the material at this aspect ratio. EDM drilling with CuW electrodes achieves 0.8 mm³/min material removal rate with 8% electrode wear ratio but leaves a 15 µm recast layer requiring post-processing. Ultrasonic-assisted diamond grinding produces Ra 0.06 µm surface finish at only 0.3 mm³/min. Nanosecond laser drilling removes material at 2.5 mm³/min but generates 25 µm recast, 0.5° taper, and cannot reach beyond 10 mm without beam refocusing. The 18 mm through-hole requirement selects EDM drilling as the production method, delivering 12 holes per CuW electrode with orbital erosion for wear compensation.
Material Characteristics
Tungsten carbide (WC-Co) is a cemented carbide composite consisting of tungsten carbide particles bound by a cobalt metal matrix. It is among the hardest materials routinely machined in industrial applications.
Grades and Properties
| ISO Grade | WC Grain Size | Co Binder (%) | Hardness (HRA) | Hardness (HRC equiv.) | Fracture Toughness |
|---|---|---|---|---|---|
| K10 | 1–2 µm | 4–6 | 91–93 | 76–80 | 8–10 MPa·m¹⸍² |
| K20 | 1–3 µm | 6–8 | 90–92 | 74–78 | 10–12 MPa·m¹⸍² |
| K30 | 2–4 µm | 8–10 | 88–91 | 70–76 | 12–14 MPa·m¹⸍² |
| K40 | 3–5 µm | 10–13 | 86–89 | 66–72 | 14–17 MPa·m¹⸍² |
| Micro-grain | 0.2–0.5 µm | 6–10 | 92–94 | 78–82 | 7–9 MPa·m¹⸍² |
Higher cobalt content increases toughness but reduces hardness and wear resistance. Fine-grain grades offer higher hardness but are more difficult to machine by all methods.
Machinability Comparison
| Method | Relative Removal Rate | Surface Finish (Ra) | Typical L/D | Diameter Range |
|---|---|---|---|---|
| Conventional carbide drilling | Not feasible above 50 HRC | — | — | — |
| EDM drilling (sinker / hole poper) | 0.5–2.0 mm³/min | 0.8–3.0 µm | Up to 100:1 | 0.08–6 mm |
| Ultrasonic-assisted grinding | 0.1–0.5 mm³/min | 0.04–0.15 µm | Up to 30:1 | 0.1–10 mm |
| Laser drilling (ns / ps) | 1.0–5.0 mm³/min | 0.5–2.0 µm | Up to 30:1 | 0.01–2 mm |
| PCD drilling (limited use) | 0.1–0.3 mm³/min | 0.2–0.5 µm | Up to 10:1 | 1–6 mm |
Key Challenges for Deep Hole Drilling in WC
| Challenge | Cause | Effect |
|---|---|---|
| Extreme hardness | WC particles 1,800–2,400 HV | Conventional cutting edges cannot penetrate |
| Brittle fracture risk | Low fracture toughness (8–17 MPa·m¹⸍²) | Edge chipping, micro-cracking at hole entry/exit |
| High abrasive wear | WC particles abrade any tool material | Rapid tool wear in mechanical methods |
| Recast layer (EDM/laser) | Melted and resolidified WC-Co | Surface defects, reduced fatigue strength |
| Depth limitation | Beam divergence (laser), electrode wear (EDM), tool wear (ultrasonic) | Aspect ratio constrained by method |
Why Conventional Drilling Falls Short
Tungsten carbide at 90+ HRA is harder than any common cutting tool material:
- Carbide drills (1,500–1,800 HV) — the workpiece is harder than the tool. Flank wear is instantaneous.
- CBN drills (4,500 HV) — can cut WC in theory but CBN's brittleness and the high cutting forces cause edge chipping at small diameters. Limited to diameters above 3 mm and L/D below 5:1.
- PCD drills (8,000 HV) — polycrystalline diamond is harder than WC, but PCD drills are extremely brittle and cannot withstand the interrupted cutting and thrust forces of deep hole drilling. Chemical wear (graphitisation) also occurs at elevated temperatures.
For deep holes (L/D above 10:1), none of these conventional cutting tools are viable for tungsten carbide. Non-conventional methods are required.
Tip: If you must drill tungsten carbide conventionally, use PCD-tipped tools with very low feeds (0.001–0.005 mm/rev) and speeds (5–10 m/min), and limit L/D to 5:1 maximum. This is only feasible for soft grades (K40 with 86 HRA or lower). For any deep hole in WC, select an EDM, laser, or ultrasonic method.
EDM Drilling
EDM (electrical discharge machining) drilling — also called hole popper drilling or fast-hole EDM — is the most established production method for deep holes in tungsten carbide.
Process Principle
A rotating tubular electrode (typically brass, copper, or CuW) erodes the workpiece through controlled electrical discharges in a dielectric fluid. The electrode does not contact the workpiece. Material is removed by melting and vaporisation at each spark.
Electrode Materials
| Electrode | Wear Ratio | Minimum Diameter | MRR | Surface Finish (Ra) | Cost |
|---|---|---|---|---|---|
| Brass | 15–25% | 0.2 mm | High | 1.5–3.0 µm | Low |
| Copper | 8–15% | 0.1 mm | Medium | 1.0–2.5 µm | Medium |
| CuW (copper-tungsten) | 2–10% | 0.08 mm | Medium | 0.8–2.0 µm | High |
| Graphite | 5–15% | 0.3 mm | High | 2.0–4.0 µm | Low |
| Tungsten | 1–5% | 0.05 mm | Low | 0.5–1.5 µm | Very high |
CuW electrodes offer the best balance of wear resistance and MRR for production WC drilling. Pure tungsten electrodes provide the lowest wear but are expensive and have lower MRR.
EDM Drilling Parameters (WC-Co, ISO K20)
| Parameter | Value | Notes |
|---|---|---|
| Discharge current | 2–8 A | Higher current increases MRR but also recast layer |
| Pulse duration (on-time) | 5–50 µs | Shorter pulses reduce recast, longer pulses increase MRR |
| Duty cycle | 40–70% | Higher duty cycle increases MRR but risks arcing |
| Voltage | 80–200 V | Higher voltage improves gap flushing |
| Dielectric | Deionised water or hydrocarbon oil | Oil gives better surface finish |
| Electrode rotation | 100–500 RPM | Improves flushing, reduces taper |
| Flushing pressure | 3–10 MPa | Internal through-electrode flushing |
| Orbital erosion | 0.005–0.020 mm (radial) | Compensates for electrode wear, improves roundness |
Achievable Results
| Metric | Typical Value | Optimised Value |
|---|---|---|
| MRR | 0.3–1.5 mm³/min | Up to 3.0 mm³/min |
| Electrode wear ratio | 5–15% | 2–8% (CuW, optimised) |
| Recast layer thickness | 10–30 µm | 5–15 µm (short pulse, low current) |
| Surface roughness (Ra) | 1.0–3.0 µm | 0.5–1.5 µm |
| Maximum L/D | 100:1 | 200:1 (very small diameters) |
| Diameter tolerance | ±0.005–0.020 mm | ±0.003 mm (with orbital) |
| Taper | 0.01–0.05 mm per 10 mm | <0.01 mm per 10 mm |
Tool Life
The electrode is consumed during the process. Typical electrode life:
| Electrode Type | Holes per Electrode (0.3 mm Ø, 18 mm deep) |
|---|---|
| Brass | 2–5 |
| Copper | 4–8 |
| CuW | 8–15 |
| Tungsten | 12–25 |
Electrode life depends strongly on discharge energy, flushing conditions, and WC cobalt content (higher cobalt increases MRR but also electrode wear).
Warning: The recast layer (also called white layer or resolidified layer) on EDM-drilled WC surfaces contains micro-cracks and tensile residual stresses. For high-fatigue applications (e.g., carbide dies, cutting tools), the recast layer must be removed by post-process polishing or ultrasonic honing. A 15 µm recast layer can reduce fatigue strength by 30–50% if left in place.
Ultrasonic-Assisted Drilling
Ultrasonic-assisted drilling (also called ultrasonic grinding or rotary ultrasonic machining) uses a diamond-impregnated tool vibrating at ultrasonic frequencies (20–60 kHz) with small amplitude (5–40 µm) to abrade the WC workpiece.
Process Principle
A metal-bonded diamond tool rotates while oscillating axially at ultrasonic frequency. Material removal occurs by micro-cracking and chipping at the WC surface — the diamond grits indent the brittle carbide, causing lateral crack propagation and material spalling.
Parameters
| Parameter | Value | Notes |
|---|---|---|
| Frequency | 20–40 kHz | Higher frequency increases MRR |
| Amplitude | 10–30 µm | Higher amplitude increases MRR but accelerates tool wear |
| Spindle speed | 3,000–10,000 RPM | Higher speed improves MRR |
| Feed rate | 0.1–1.0 mm/min | Pressure-controlled feed recommended |
| Coolant | Water-based emulsion, 5–40 bar | Flushes debris, cools the tool |
| Tool material | Metal-bonded diamond (bronze or nickel bond) | Bond hardness matched to WC grade |
| Diamond grit size | 30–100 µm | Finer grit gives better surface finish |
Tool Wear and Life
| Tool Type | Tool Life | Wear Mechanism |
|---|---|---|
| Bronze-bond diamond | 20–100 holes | Diamond pullout, bond erosion |
| Nickel-bond diamond | 50–200 holes | Diamond attrition, bond wear |
| Hybrid-bond diamond | 80–300 holes | Gradual diamond wear |
Tool wear is the primary limitation of ultrasonic drilling. As diamond grits are lost or become blunt, MRR drops and surface finish degrades.
Achievable Results
| Metric | Value |
|---|---|
| MRR | 0.1–0.5 mm³/min |
| Surface roughness (Ra) | 0.04–0.15 µm |
| Maximum L/D | 20:1–30:1 |
| Diameter range | 0.1–10 mm |
| Hole taper | 0.005–0.020 mm per 10 mm |
| Roundness | 0.002–0.008 mm |
Ultrasonic drilling produces the best surface finish of any WC deep hole drilling method but has the lowest MRR and limited depth capability.
Tip: Ultrasonic drilling is the preferred method when surface finish is critical and the hole depth is under 10× diameter. For wire-drawing dies, the 0.04–0.06 µm Ra achievable with ultrasonic grinding can eliminate the need for post-process polishing, offsetting the slower MRR.
Laser Drilling
Laser drilling uses focused laser pulses to ablate WC material through melting, vaporisation, and ejection of molten material.
Laser Types
| Laser Type | Wavelength | Pulse Duration | Typical MRR | Recast Layer | Best For |
|---|---|---|---|---|---|
| Nanosecond fibre | 1,064 nm | 10–200 ns | 2–5 mm³/min | 20–50 µm | Drilling speed, larger diameters |
| Picosecond | 1,064/532 nm | 5–50 ps | 0.5–2 mm³/min | 2–10 µm | Reduced recast, better quality |
| Femtosecond | 800/1,064 nm | 100–500 fs | 0.1–1 mm³/min | <1 µm | Maximum quality, micro holes |
| Green (532 nm) | 532 nm | ns–ps | 1–3 mm³/min | 10–30 µm | Higher absorption in WC |
Laser Drilling Parameters (Nanosecond, WC-Co)
| Parameter | Value | Notes |
|---|---|---|
| Power | 20–100 W | Higher power increases MRR |
| Pulse energy | 0.5–5 mJ | Determines material removal per pulse |
| Repetition rate | 20–200 kHz | Higher rate increases MRR |
| Spot size | 10–50 µm | Determines minimum hole diameter |
| Assist gas | Compressed air, N₂, or O₂ at 2–10 bar | Ejects molten material |
| Drilling strategy | Percussion or trepanning | Percussion for small Ø, trepanning for larger |
| Number of passes | 1–20 (trepanning) | Multiple passes improve taper and quality |
Achievable Results
| Metric | Nanosecond | Picosecond | Femtosecond |
|---|---|---|---|
| MRR | 1–5 mm³/min | 0.5–2 mm³/min | 0.1–1 mm³/min |
| Recast layer | 20–50 µm | 2–10 µm | <1 µm |
| Surface roughness (Ra) | 1.0–3.0 µm | 0.3–1.0 µm | 0.1–0.5 µm |
| Max L/D (percussion) | 10:1–20:1 | 10:1–15:1 | 5:1–10:1 |
| Max L/D (trepanning) | 20:1–30:1 | 15:1–25:1 | 10:1–15:1 |
| Taper | 0.2–1.0° | 0.1–0.5° | 0.05–0.2° |
| Heat-affected zone | 20–100 µm | 5–20 µm | <5 µm |
Depth Limitation
The primary limitation of laser drilling in WC is depth. As the hole deepens:
- Beam divergence reduces energy density at the bottom
- Ejected material re-deposits on hole walls
- Plasma shielding absorbs incoming laser energy
- Assist gas cannot effectively clear molten debris
These effects limit percussion drilling to approximately 10–15 mm in WC. Trepanning can extend this to 20–30 mm but with significant taper.
Warning: Laser drilling of WC produces a recast layer containing micro-cracks and a heat-affected zone (HAZ) with altered cobalt distribution. The HAZ in WC-Co shows cobalt depletion near the surface (cobalt evaporation during melting), which reduces surface hardness and corrosion resistance. Post-processing (polishing, honing) is required for most applications.
Method Comparison
Head-to-Head Comparison
| Criterion | EDM Drilling | Ultrasonic Grinding | Laser Drilling |
|---|---|---|---|
| Material removal rate | Medium (0.5–2 mm³/min) | Low (0.1–0.5 mm³/min) | High (1–5 mm³/min) |
| Surface finish (Ra) | 0.8–3.0 µm | 0.04–0.15 µm | 0.3–3.0 µm |
| Maximum L/D | 100:1+ | 30:1 | 30:1 |
| Minimum diameter | 0.05 mm | 0.1 mm | 0.01 mm |
| Recast layer / damage | 5–30 µm recast | None (mechanical) | 1–50 µm + HAZ |
| Taper | Low (orbital correction) | Low–moderate | Moderate–high |
| Tool / electrode wear | Consumes electrode | Consumes diamond tool | No tool wear (optical) |
| Equipment cost | Medium (£50k–150k) | Medium (£40k–100k) | High (£150k–500k) |
| Operating cost | Medium (electrodes, dielectric) | Medium (diamond tools) | Low (electricity, gas) |
| Post-processing needed | Yes (recast removal) | No | Yes (recast + HAZ removal) |
Selection Guide by Application
| Application | Recommended Method | Rationale |
|---|---|---|
| Wire-drawing die coolant holes | EDM drilling | Best L/D, proven in production, orbital erosion compensates wear |
| Carbide cutting tool coolant holes | EDM drilling or laser | EDM for deeper holes, laser for faster through-holes under 10 mm |
| Carbide mould / die vent holes | EDM drilling | Reliable process for small diameters at high L/D |
| Fuel injector nozzle holes (carbide) | Laser drilling (ps/fs) | Smallest diameters, minimal recast with ps/fs |
| Medical carbide implant holes | Ultrasonic grinding | Best surface finish, no recast layer |
| Carbide wear part cooling channels | EDM drilling | Deep holes, production volume |
| Micro holes (<0.1 mm) in WC | Laser drilling | Only method that can achieve sub-50 µm diameters |
| Prototype / low-volume WC parts | EDM drilling | Lower equipment investment, flexible |
| High-volume WC production | EDM drilling or laser | EDM for deep holes, laser for shallow holes with speed |
Troubleshooting
| Problem | Likely Cause | Correction |
|---|---|---|
| EDM — electrode breaks in hole | Discharge energy too high or flushing insufficient | Reduce current, increase flushing pressure, check dielectric condition |
| EDM — excessive recast layer | Pulse duration too long or current too high | Reduce on-time and current, use deionised water dielectric |
| EDM — electrode wear too high | Wrong electrode material or polarity | Switch to CuW or tungsten electrode, check polarity (electrode negative) |
| EDM — hole taper exceeds tolerance | Electrode wear not compensated | Implement orbital erosion routine |
| Ultrasonic — MRR too low | Diamond grit worn or bond too hard | Replace tool, check if bond hardness matches WC grade |
| Ultrasonic — tool binds in hole | Swarf not cleared or hole taper | Increase coolant pressure, reduce feed rate |
| Ultrasonic — poor surface finish | Diamond grit too coarse or tool worn | Use finer grit tool (30–50 µm), replace tool |
| Laser — hole does not penetrate | Beam defocused at depth or power insufficient | Increase power, refocus at intermediate depth, use trepanning |
| Laser — excessive taper | Too many pulses at entry or beam divergence | Reduce pulse count at entry, use shorter focal length lens |
| Laser — recast layer too thick | Pulse duration too long or gas pressure insufficient | Switch to ps laser, increase assist gas pressure |
| All methods — burst at exit | Material thin at breakthrough | Reduce power/energy in final 10% of hole depth |
FAQ
Can tungsten carbide be deep hole drilled with conventional gun drills?
No — conventional gun drills (carbide or HSS) cannot cut tungsten carbide at 90+ HRA. The workpiece is harder than the cutting tool material. For very soft grades (K40, 86 HRA), PCD-tipped gun drills can achieve shallow holes (L/D <5:1) at very low parameters.
What is the best method for deep holes (L/D >30:1) in tungsten carbide?
EDM drilling is the only production-proven method for deep holes above L/D 30:1 in WC. With CuW electrodes and orbital erosion compensation, L/D ratios of 100:1 are routinely achievable.
What surface finish can I expect from EDM drilling WC?
Typically Ra 1.0–3.0 µm for standard parameters, improving to 0.5–1.5 µm with optimised short-pulse, low-current settings. Post-processing is required for surface finish below 0.5 µm.
Does laser drilling produce a recast layer in tungsten carbide?
Yes — nanosecond laser drilling produces a 20–50 µm recast layer with micro-cracks and cobalt depletion. Picosecond lasers reduce this to 2–10 µm. Femtosecond lasers produce negligible recast (<1 µm) but at significantly lower MRR.
What is the minimum hole diameter achievable in tungsten carbide?
Laser drilling can produce holes as small as 10 µm diameter. EDM drilling can reach 50–80 µm with tungsten electrodes. Ultrasonic grinding is limited to approximately 100 µm minimum.
Which method gives the best surface finish for WC deep holes?
Ultrasonic-assisted diamond grinding produces the best surface finish at Ra 0.04–0.15 µm — equivalent to fine grinding or lapping. This can eliminate the need for post-process polishing.
How deep can a laser drill in tungsten carbide?
Percussion laser drilling is limited to approximately 10–15 mm depth in WC. Trepanning can extend to 20–30 mm. Beyond these depths, beam divergence and debris evacuation become limiting factors.
What is the tool life for EDM drilling of WC?
Electrode life depends on the electrode material. CuW electrodes typically produce 8–15 holes per electrode for a 0.3 mm diameter hole 18 mm deep. Tungsten electrodes can achieve 12–25 holes. Higher cobalt content in the WC grade improves MRR but increases electrode wear.
Is post-processing always required after EDM or laser drilling WC?
For most applications, yes. The recast layer from EDM (5–30 µm) or laser (1–50 µm) contains micro-cracks and tensile residual stresses that reduce fatigue strength. For high-stress applications (cutting tools, dies, structural components), the recast layer must be removed by polishing, honing, or ultrasonic finishing.
How does cobalt content affect machinability in WC?
Higher cobalt content improves electrical conductivity (beneficial for EDM), increases fracture toughness (reduces edge chipping), and improves MRR in all methods. However, higher cobalt reduces hardness and wear resistance. For EDM drilling, WC with 10–13% cobalt (K30–K40) drills faster and with less electrode wear than low-cobalt grades (K10–K20).
Summary
Tungsten carbide deep hole drilling requires non-conventional methods — conventional cutting tools cannot penetrate material at 90+ HRA:
- EDM drilling — the established production method for deep holes (L/D up to 100:1+). CuW electrodes with orbital erosion compensation deliver 0.5–2 mm³/min MRR, Ra 0.8–3.0 µm finish, and 8–15 holes per electrode. Recast layer (5–30 µm) requires post-processing for fatigue-critical applications.
- Ultrasonic-assisted diamond grinding — the best surface finish (Ra 0.04–0.15 µm) but lowest MRR and limited L/D (30:1). Preferred when surface quality eliminates the need for post-processing.
- Laser drilling — highest MRR (1–5 mm³/min) and smallest diameter capability (10 µm) but limited depth (30:1 maximum) and significant recast/HAZ with nanosecond lasers. Picosecond and femtosecond lasers reduce damage at higher equipment cost.
- Selection — EDM for deep holes and production volume, ultrasonic for surface quality, laser for micro holes and shallow holes requiring speed.
- The wire-drawing die manufacturer in the opening scenario selected EDM drilling with CuW electrodes for the 0.3 mm × 18 mm (L/D 60:1) coolant hole, achieving 12 holes per electrode with orbital erosion compensation, followed by recast removal via ultrasonic honing.