Hardened steel presents a significant challenge for drilling. Its high hardness, often exceeding 60 Rockwell C, makes it resistant to conventional drilling techniques. This comprehensive guide explores the specialized tools, techniques, and considerations required to successfully drill through hardened steel.
Understanding the Challenges of Drilling Hardened Steel
The extreme hardness of hardened steel leads to several difficulties:
- Rapid Tool Wear: Standard high-speed steel (HSS) drills will quickly dull or break when drilling hardened steel, leading to inefficient drilling and potentially damaged workpieces.
- High Temperatures: The friction generated during drilling generates significant heat, which can further degrade the drill bit and potentially anneal (soften) the surrounding steel, compromising the workpiece's integrity.
- Workpiece Cracking: The stress caused by drilling can induce cracks in the hardened steel, especially if the drilling process is not carefully controlled.
- Material Buildup: The material being drilled can build up on the drill bit, hindering its cutting action and leading to poor surface finish.
Specialized Drills for Hardened Steel
To overcome these challenges, specialized drill bits are necessary:
1. Carbide Drills:
- Material: Made from tungsten carbide, these drills offer significantly greater hardness and wear resistance than HSS drills.
- Applications: Ideal for drilling smaller diameter holes in hardened steel. Different tip geometries are available to optimize performance for various applications.
- Considerations: Carbide drills are more brittle than HSS drills and are more susceptible to chipping, particularly at higher speeds.
2. Cobalt Steel Drills:
- Material: HSS drills with a high cobalt content (typically 5-8%). Cobalt increases the drill's hardness, wear resistance, and red hardness (ability to withstand high temperatures).
- Applications: Effective for drilling medium-sized holes in hardened steel, offering a balance between cost and performance.
- Considerations: While more durable than standard HSS, cobalt steel drills still wear faster than carbide drills when working with hardened steel.
3. Polycrystalline Diamond (PCD) Drills:
- Material: Made from extremely hard polycrystalline diamond, these drills offer the ultimate in hardness and wear resistance.
- Applications: Ideal for drilling large diameter holes or for high-volume production applications where tool life is critical.
- Considerations: PCD drills are very expensive and require specialized drilling equipment.
4. Cubic Boron Nitride (CBN) Drills:
- Material: CBN is another ultra-hard material second only to diamond in hardness.
- Applications: Excellent for drilling very hard and abrasive materials, including hardened steel. Often used in CNC machining.
- Considerations: CBN drills are also expensive and need specialized machines for effective use.
Techniques for Drilling Hardened Steel
Beyond the drill bit itself, proper techniques are crucial for successful drilling:
- Low Speed, High Feed: Use a low rotational speed (RPM) and a higher feed rate to minimize heat generation and wear on the drill bit.
- Coolant: Employ a suitable coolant (e.g., soluble oil) to lubricate the drill bit and dissipate heat. This significantly extends drill life and prevents workpiece damage.
- Pilot Hole: For larger diameter holes, it's often beneficial to pre-drill a smaller pilot hole to guide the main drill bit.
- Multiple Passes: For deeper holes, consider making multiple passes with a slightly smaller diameter drill to reduce the load on the bit and prevent breakage.
- Sharp Drill: Using a dull bit will increase the risk of breakage and produce a poor-quality hole. Ensure the drill is sharp or replace it as needed.
- Proper Clamping: Secure the workpiece firmly to prevent movement during drilling. Vibration can increase the risk of drill breakage.
Conclusion: Selecting the Right Approach
Drilling hardened steel requires careful consideration of the material's properties and the available tools and techniques. Choosing the right drill bit, employing proper drilling techniques, and using coolant are key factors in achieving successful and efficient drilling while maintaining the integrity of the workpiece. When in doubt, consult a professional machinist for guidance on complex projects.
