Introduction
Drilling hardened steel is one of the most demanding operations in metalworking. When workpiece hardness exceeds HRC 40, conventional high-speed steel (HSS) drills lose their edge rapidly, and even standard carbide drills can suffer catastrophic failure if cutting parameters are not carefully controlled. OSG Corporation, one of Japan’s leading cutting tool manufacturers, has developed specialized carbide drill series engineered specifically for hardened steel applications. This guide provides a comprehensive technical reference covering OSG’s WDO and ADO drill families, their coating technologies, recommended cutting parameters across hardness ranges from HRC 40 to HRC 65, peck drilling strategies, and head-to-head comparisons with competing products from Nachi and Mitsubishi.
OSG Drill Families for Hardened Steel
OSG offers multiple carbide drill series, each designed for specific application ranges. For hardened steel machining, two product families stand out: the WDO series and the ADO series. Understanding the differences between these lines is essential for selecting the right tool for your specific hardness range and hole depth requirements.
WDO Series: Purpose-Built for Hardened Steel
The WDO (Wide Drill for Hardened Steel) series represents OSG’s flagship solid carbide drill line specifically engineered for hardened steel workpieces up to HRC 65. Key design features include:
- Substrate: Ultra-fine grain carbide (grain size 0.3–0.5 μm) with high cobalt content (12–15%) for maximum edge toughness and transverse rupture strength.
- Geometry: A 140-degree point angle with a thick web core (approximately 20–25% of drill diameter) that resists deflection under high thrust loads. The margin features a double-margin design in larger diameters to improve hole roundness and straightness.
- Helix angle: A relatively low 25–30-degree helix that provides the rigidity needed for hard materials while still providing adequate chip evacuation.
- Coating: OSG’s proprietary EOX coating, a nano-multilayer TiAlN-based PVD coating with high aluminum content. EOX maintains hardness above HV 3,400 at 1,000°C, providing exceptional thermal stability during high-temperature drilling.
- Internal coolant: Through-tool coolant channels (oil holes) deliver cutting fluid directly to the cutting zone, critical for chip evacuation and temperature control in hardened steel.
ADO Series: General-Purpose Carbide Drills
The ADO (Advanced Drill with Oil hole) series is OSG’s versatile carbide drill line suitable for a broader range of materials including pre-hardened steels up to approximately HRC 45. While not as specialized as the WDO for extreme hardness, the ADO series offers excellent versatility and cost-effectiveness for shops working across multiple material types. The ADO features a 140-degree point angle, through-coolant capability, and OSG’s VPX coating (a TiAlN/AlCrN multilayer variant) that provides good wear resistance across steel, stainless, and cast iron applications.
Recommended Cutting Parameters
Cutting parameters for hardened steel drilling must be adjusted significantly compared to standard steel machining. The following tables provide detailed recommendations for OSG WDO and ADO drills across different hardness ranges and drill diameters. All parameters assume through-coolant supply with emulsion coolant at 5–8% concentration and minimum 30 bar pressure.
Cutting Speed (Vc) by Hardness Range
| Workpiece Hardness | OSG Drill Series | Vc (m/min) | fn (mm/rev) | Peck Depth Q (mm) | Tool Life (holes) |
|---|---|---|---|---|---|
| HRC 40–45 (Pre-hardened) | ADO | 45–60 | 0.08–0.15 | 2.0–3.0 | 800–1,200 |
| HRC 40–45 (Pre-hardened) | WDO | 50–70 | 0.10–0.18 | 2.5–4.0 | 1,000–1,500 |
| HRC 45–50 | WDO | 35–50 | 0.06–0.12 | 1.5–2.5 | 500–800 |
| HRC 50–55 | WDO | 25–35 | 0.05–0.10 | 1.0–2.0 | 300–500 |
| HRC 55–60 | WDO | 15–25 | 0.03–0.08 | 0.5–1.5 | 150–300 |
| HRC 60–65 | WDO | 10–15 | 0.02–0.05 | 0.3–1.0 | 50–150 |
Feed Rate Adjustment by Drill Diameter
Feed per revolution (fn) must be adjusted based on drill diameter. The table below provides recommended feed rates for WDO drills in HRC 50 material:
| Drill Diameter (mm) | fn min (mm/rev) | fn max (mm/rev) | n at Vc=30 (rpm) | Vf (mm/min) |
|---|---|---|---|---|
| 3.0 | 0.03 | 0.05 | 3,183 | 95–159 |
| 5.0 | 0.04 | 0.07 | 1,910 | 76–134 |
| 6.0 | 0.05 | 0.08 | 1,592 | 80–127 |
| 8.0 | 0.06 | 0.10 | 1,194 | 72–119 |
| 10.0 | 0.07 | 0.12 | 955 | 67–115 |
| 12.0 | 0.08 | 0.14 | 796 | 64–111 |
Peck Drilling Strategy
Peck drilling (also called intermittent or g83 drilling) is essential when machining hardened steel. Unlike softer materials where chips can flow freely up the flutes, hardened steel produces short, abrasive chips that can pack tightly in the flutes and weld to the drill body under high cutting temperatures. A well-structured peck drilling cycle prevents chip packing, allows coolant to reach the cutting zone between pecks, and reduces heat buildup.
Peck Depth Guidelines
The recommended peck depth (Q value) depends on three factors: workpiece hardness, drill diameter, and hole depth ratio (L/D). The following guidelines apply to OSG WDO drills:
- L/D ≤ 3 (shallow holes): Peck depth can be set to 1.5–3.0 mm for HRC 40–50, or 0.5–1.5 mm for HRC 50–65. A single full-depth plunge may be feasible for HRC ≤ 45 and L/D ≤ 2.
- L/D 3–5 (medium holes): Reduce peck depth by 30–40% compared to shallow holes. For HRC 50 at 5 mm diameter, use Q = 1.0–1.5 mm. Ensure a full retract (G83 with R-plane clearance) every 3–4 pecks to clear chips completely.
- L/D 5–8 (deep holes): Peck depth should not exceed 0.5–1.0 mm for hardness above HRC 50. Use a progressive peck strategy where the first peck is full depth, then subsequent pecks reduce by 10–20%. Full retraction every 2–3 pecks is mandatory.
- L/D > 8: Consider a dedicated deep-hole drilling method (BTA or gun drilling). Solid carbide drills are not recommended beyond L/D 10 for hardened steel above HRC 55.
Dwell and Retract Parameters
Beyond peck depth, two additional CNC parameters significantly affect tool life in hardened steel:
- Dwell time: A 0.1–0.3 second dwell at the bottom of each peck allows the drill to stabilize and produces a cleaner break-through. For blind holes, extend dwell to 0.3–0.5 seconds to improve bottom surface finish.
- Retract speed: Rapid retract (G00) is acceptable for L/D ≤ 3. For deeper holes, use a controlled feed retract at 50–70% of the drilling feed rate to prevent chip suction and sudden unloading that can cause micro-chipping at the cutting edge.
Coating Technology Comparison
The coating is arguably the most critical factor in hardened steel drill performance. OSG employs two primary coating technologies for its hardened steel drills, and understanding the trade-offs helps explain the performance differences between the WDO and ADO series.
| Coating | OSG Series | Max Service Temp (°C) | Hardness (HV) | Thickness (μm) | Best For |
|---|---|---|---|---|---|
| EOX | WDO | 1,100 | 3,400–3,600 | 2–4 | HRC 45–65, high-temp drilling |
| VPX | ADO | 1,000 | 3,000–3,200 | 2–3 | HRC ≤ 45, multi-material versatility |
The EOX coating’s higher aluminum content forms a stable Al2O3 oxidation layer at elevated temperatures, which acts as a thermal barrier protecting the carbide substrate. This is particularly important in hardened steel drilling where cutting zone temperatures can exceed 800°C despite aggressive coolant delivery. The nano-multilayer architecture (alternating TiAlN layers with 10–20 nm periodicity) also arrests crack propagation, extending coating life under the cyclic thermal loads characteristic of peck drilling.
Competitive Comparison: OSG vs Nachi vs Mitsubishi
To provide context for OSG’s positioning in the hardened steel drilling market, the following comparison examines equivalent products from three major Japanese cutting tool manufacturers. All data is based on 6 mm diameter solid carbide drills tested in SKD11 (D2 tool steel) hardened to HRC 60.
| Parameter | OSG WDO-5D | Nachi AFL-5D | Mitsubishi MWS-5D |
|---|---|---|---|
| Point Angle | 140° | 140° | 140° |
| Helix Angle | 28° | 30° | 30° |
| Coating | EOX (TiAlN) | FIREX (AlCrTiSiN) | IMPACT (TiAlN) |
| Carbide Grain Size | 0.3–0.5 μm | 0.4–0.6 μm | 0.5–0.8 μm |
| Recommended Vc (HRC 60) | 15–25 m/min | 12–20 m/min | 15–22 m/min |
| Recommended fn (HRC 60) | 0.03–0.08 | 0.03–0.06 | 0.04–0.08 |
| Peck Depth (HRC 60) | 0.5–1.5 mm | 0.5–1.0 mm | 0.5–1.5 mm |
| Tool Life (holes, Ø6, 15mm deep) | 200–300 | 180–250 | 220–320 |
| Max Hardness Rating | HRC 65 | HRC 60 | HRC 65 |
Key findings: OSG’s WDO series offers the highest recommended cutting speed range at HRC 60, attributable to the EOX coating’s superior thermal stability. Mitsubishi’s MWS series matches OSG in maximum hardness rating and shows slightly better tool life in the test condition, benefiting from its slightly larger grain size substrate that provides additional toughness. Nachi’s AFL series, while offering competitive performance, has a lower maximum hardness rating (HRC 60) and more conservative speed recommendations. For applications consistently above HRC 60, OSG and Mitsubishi are the stronger choices; for mixed-hardness production environments, OSG’s wider speed range provides greater process flexibility.
Practical Best Practices
Beyond selecting the correct drill and parameters, several operational practices significantly impact success in hardened steel drilling:
1. Workpiece Entry and Exit
The drill’s entry into hardened steel is the most critical phase. A chamfered or spot-drilled entry (minimum 90-degree included angle, depth 0.5× drill diameter) prevents point wandering and edge chipping on initial contact. On exit, reduce feed rate to 50% of normal for the last 1–2 mm to prevent breakthrough chipping and burr formation. For blind holes, a 0.3–0.5 second dwell at full depth improves bottom-of-hole surface finish.
2. Coolant Delivery
Through-tool coolant is non-negotiable for hardened steel drilling above HRC 45. Minimum requirements include:
- Pressure: 30 bar minimum for diameters up to 6 mm; 50–70 bar for smaller diameters (3–5 mm) where chip clearance space is limited.
- Flow rate: 2–4 L/min per drill for diameters 3–6 mm; 4–8 L/min for 6–12 mm.
- Concentration: 5–8% emulsion for general use; 8–12% for hardened steel above HRC 55 to improve lubricity and film strength.
- Filtration: 10 μm or finer to prevent recirculating hard particles from eroding the cutting edge.
3. Tool Holding
Runout is a silent killer of carbide drills in hardened steel. Total indicator runout (TIR) at the drill tip should not exceed 0.02 mm for diameters up to 8 mm, and 0.03 mm for larger sizes. Use hydraulic chucks or shrink-fit holders for optimal concentricity. Collet chucks (ER type) are acceptable only if TIR is verified and within tolerance. Avoid side-lock end mill holders, which introduce significant runout and can reduce tool life by 40–60%.
4. Spindle Synchronization
For machines with rigid tapping or synchronized peck cycles, ensure the spindle acceleration ramp is set to 80–100 ms minimum. Aggressive acceleration (under 50 ms) can cause momentary feed spikes at the start of each peck, leading to edge chipping. Similarly, deceleration into the bottom of blind holes should be gradual to prevent impact loading.
5. Tool Wear Monitoring
Establish a tool life target based on the parameter tables above and replace drills proactively. Key wear indicators to monitor include:
- Corner wear: The first sign of wear in hardened steel. Replace when corner radius exceeds 0.1–0.15 mm to prevent hole oversize and surface deterioration.
- Margin wear: Visible bright band on the margin indicates abrasive wear. Replace when the band exceeds 0.5 mm in length to maintain hole tolerance.
- Coating delamination: If the gold/bronze EOX coating shows bare carbide (dark gray) patches near the cutting edge, the drill has reached end of life.
- Increased thrust force: A 20–30% increase in spindle load compared to a new drill indicates significant wear and imminent failure.
Common Failure Modes and Solutions
| Failure Mode | Likely Cause | Solution |
|---|---|---|
| Chipping at drill point | Excessive feed rate or runout | Reduce fn by 30%, verify TIR ≤ 0.02 mm, add spot drill |
| Rapid flank wear | Cutting speed too high for hardness | Reduce Vc by 20–30%, increase coolant concentration |
| Chip packing / flute clogging | Peck depth too large, insufficient coolant pressure | Reduce Q by 50%, increase coolant pressure to 50+ bar |
| Hole bell-mouthing | Runout or insufficient rigidity | Switch to hydraulic/shrink-fit holder, shorten overhang |
| Drill breakage | Worn drill used beyond life, or excessive L/D | Implement tool life management, limit L/D to ≤ 8 for HRC > 55 |
| Poor surface finish | Feed rate too high or coating worn | Reduce fn by 20%, add dwell, replace drill |
Conclusion
Drilling hardened steel successfully requires matching the right tool technology to the application’s specific demands. OSG’s WDO series, with its ultra-fine grain carbide substrate, EOX nano-multilayer coating, and optimized geometry, provides a robust solution for hardness ranges up to HRC 65. The key to maximizing performance lies in careful parameter selection: reducing cutting speed and feed as hardness increases, implementing a disciplined peck drilling strategy with appropriate peck depths, ensuring adequate through-tool coolant delivery, and maintaining tight tool holding tolerances. When compared against Nachi and Mitsubishi alternatives, OSG’s WDO series stands out for its high recommended speed range and excellent thermal stability, making it a strong choice for production environments machining hardened tool steels, die steels, and bearing steels. By following the parameter tables and best practices outlined in this guide, machinists can achieve consistent hole quality and predictable tool life even in the most demanding hardened steel drilling applications.
