Drill Bits For Hardened Steel

Drilling Hardened Steel: A Comprehensive Guide to Choosing the Right Drill Bits

Drilling hardened steel presents a unique challenge for even experienced machinists. The extreme hardness of the material leads to rapid wear and tear on standard drill bits, often resulting in broken bits, damaged workpieces, and frustratingly slow progress. This comprehensive guide delves into the specifics of drilling hardened steel, explaining the complexities involved and offering a detailed breakdown of the drill bits specifically designed for this demanding task. Understanding the properties of hardened steel and the mechanisms of different drill bit types is crucial to achieving successful and efficient drilling.

Understanding the Challenges of Hardened Steel

Hardened steel, typically possessing a Rockwell hardness of 50 HRC or higher, possesses exceptional resistance to deformation. This high hardness significantly increases the friction and cutting forces during drilling, leading to several key challenges:

• Rapid Bit Wear: The extreme hardness of the material causes the cutting edges of standard drill bits to wear down quickly, leading to reduced accuracy and increased breakage.
• Workpiece Damage: The high forces involved can cause the workpiece to crack or chip, especially if improper drilling techniques are used. This is especially true with thinner pieces of hardened steel.
• Heat Generation: Friction generates significant heat, which can further exacerbate bit wear and potentially anneal (soften) the surrounding workpiece material, compromising its structural integrity.
• Material Build-up: Hardened steel can exhibit a tendency to weld itself onto the cutting edges of the drill bit, further hindering the drilling process.

Types of Drill Bits for Hardened Steel

Several specialized drill bit types are designed to overcome these challenges. Each type utilizes specific materials and geometries to improve durability, reduce friction, and enhance cutting performance.

1. Carbide Drill Bits:

• Material: These bits are manufactured from tungsten carbide, a remarkably hard and wear-resistant material. This makes them ideal for tackling the extreme hardness of hardened steel.
• Geometry: Carbide drill bits often feature a unique geometry, such as a high helix angle, designed to promote chip evacuation and reduce friction. They may also have increased rake angles to improve the cutting action and reduce the heat generated. Many incorporate a special coating to further enhance performance and longevity.
• Advantages: Exceptional wear resistance, longevity, and ability to maintain accuracy even during extended use.
• Disadvantages: More expensive than high-speed steel bits. They are also more brittle and can chip if subjected to excessive force or improper handling.

2. Cobalt Drill Bits:

• Material: These bits are made from high-speed steel (HSS) with a significant cobalt content (typically 5-8%). Cobalt enhances the hardness and red hardness (ability to maintain hardness at elevated temperatures) of the bit.
• Geometry: Similar to standard HSS bits in basic geometry but with superior hardness and wear resistance.
• Advantages: A good balance between cost and performance. They offer significantly improved durability compared to standard HSS bits when drilling hardened steel.
• Disadvantages: Still less durable than carbide bits, especially for very hard materials or large-diameter holes.

3. Solid Carbide Drill Bits:

These represent the highest end of carbide tooling. They differ from carbide-tipped bits, which have a carbide insert brazed onto a steel shank. Solid carbide drills are a single piece of carbide, making them significantly stronger and more resistant to breakage, especially at larger diameters.

4. PCD (Polycrystalline Diamond) Drill Bits:

• Material: Made from a composite of ultra-hard diamond particles, PCD drill bits are incredibly durable and capable of drilling even the toughest hardened steels.
• Geometry: Similar in design to carbide bits but with a unique cutting edge configuration that is particularly effective at removing material.
• Advantages: Exceptional wear resistance, enabling very high-speed drilling and extended tool life. Ideal for applications demanding very precise holes.
• Disadvantages: Significantly more expensive than carbide or cobalt bits, making them suitable for high-volume production or applications requiring utmost precision.

5. CBN (Cubic Boron Nitride) Drill Bits:

• Material: CBN is another extremely hard material, second only to diamond in hardness. These bits are suitable for drilling the most difficult-to-machine hardened steels.
• Geometry: Similar to carbide and PCD drills, with variations in geometry to optimize chip evacuation and cutting performance based on the material and the application.
• Advantages: Exceptional wear resistance, especially at high temperatures. Often used for drilling very hard or abrasive materials.
• Disadvantages: Extremely expensive and specialized, primarily used in industrial applications.

Choosing the Right Drill Bit

Selecting the appropriate drill bit for hardened steel requires careful consideration of several factors:

• Hardness of the Steel: The harder the steel, the more robust the drill bit needs to be. For extremely hard steels, PCD or CBN bits might be necessary.
• Diameter of the Hole: Larger diameter holes require more robust drill bits to prevent bending or breakage. Solid carbide drills are often preferred for larger holes.
• Depth of the Hole: Deep holes require a drill bit with good chip evacuation capabilities to prevent overheating and bit clogging.
• Quantity of Holes: For high-volume production, the cost-effectiveness of the drill bit becomes a crucial factor.

Drilling Techniques and Best Practices

Even with the right drill bit, proper technique is essential for successful drilling in hardened steel. Here are some key best practices:

• Use Cutting Fluid: Employ a high-quality cutting fluid specifically designed for hardened steel. This helps to lubricate the cutting edges, reduce friction, and control heat generation. Keep the cutting fluid flowing liberally throughout the process.
• Low Speed, High Feed: Contrary to intuition, drilling hardened steel is generally more effective at lower speeds and higher feed rates. This reduces heat build-up and improves chip evacuation. Experiment to find the optimal combination for your specific setup and material.
• Light Pressure: Avoid excessive pressure on the drill bit. Excessive force will accelerate wear and increase the risk of breakage. Let the bit do the work.
• Sharp Drill Bits: Start with sharp drill bits. A dull bit will generate excessive heat, leading to accelerated wear and breakage.
• Pilot Hole (if necessary): For larger holes, it's often beneficial to start with a smaller pilot hole. This reduces the initial stress on the drill bit and aids in guiding the larger drill bit accurately.
• Multiple Passes (if necessary): Consider multiple passes with slightly increasing drill bit sizes, especially for deeper holes, to reduce the strain on the drill bit in any one pass.
• Proper Clamping: Ensure the workpiece is securely clamped to prevent movement during drilling.
• Regular Inspection: Regularly inspect the drill bit for signs of wear or damage and replace it as needed.

Scientific Explanation of the Drilling Process

The process of drilling involves the removal of material from the workpiece via shearing and plastic deformation. In hardened steel, the high strength and hardness significantly resist this deformation. The cutting edges of the drill bit need to overcome immense forces, leading to significant wear if not made of a sufficiently durable material. The shear angle and rake angle of the drill bit significantly influence its effectiveness and ability to remove material. The rake angle affects the cutting force and the shear angle influences the chip formation and its evacuation from the hole. The heat generated during drilling is a function of friction between the drill bit and the workpiece, and improper chip evacuation can lead to a build-up of heat, accelerating wear and potentially damaging the workpiece.

Frequently Asked Questions (FAQ)

Q: Can I use a regular HSS drill bit to drill hardened steel?

A: While you can attempt this, it is highly discouraged. Regular HSS drill bits will wear down extremely quickly and are prone to breakage. The effort and expense wasted on dull bits far outweighs the cost difference in using the correct tooling.

Q: What is the best cutting fluid for hardened steel?

A: A high-quality cutting oil or soluble oil specifically designed for hardened steel is recommended. The exact choice will depend on your specific setup and preferences but look for a fluid that is highly lubricative and provides excellent heat dissipation.

Q: How can I tell if my drill bit is dull?

A: Signs of a dull drill bit include increased cutting forces, increased heat generation, uneven hole finish, and a significant reduction in drilling speed. Inspect the cutting edges for any signs of wear or chipping.

Q: What is the difference between a carbide-tipped and a solid carbide drill bit?

A: A carbide-tipped bit has a carbide cutting edge brazed onto a steel shank. A solid carbide bit is a single piece of carbide, making it significantly stronger and less prone to breakage, particularly for larger-diameter holes.

Q: Can I drill hardened steel without a drill press?

A: While possible, it is highly challenging and not recommended. A drill press provides the necessary stability and control to prevent the drill bit from wandering, ensuring a straight and accurate hole. Hand-held drills lack the necessary control for this challenging task, especially with hardened steel.

Conclusion

Drilling hardened steel requires specialized drill bits and techniques to achieve successful results. By understanding the challenges presented by this demanding material and selecting the appropriate drill bit – considering factors like material hardness, hole diameter, and depth – you can significantly improve your chances of success. Remember to employ proper drilling techniques, utilizing cutting fluid and appropriate speeds and feeds, to ensure both the longevity of your drill bits and the integrity of your workpiece. Investing in high-quality drill bits specifically designed for hardened steel is an investment in efficiency and avoids costly mistakes and wasted time. With the right knowledge and tools, drilling hardened steel becomes a manageable and even satisfying machining task.