Drill Bits For Hardened Steel

Drilling Hardened Steel: A Comprehensive Guide to Drill Bits and Techniques

Drilling hardened steel presents a unique challenge for even experienced machinists. The extreme hardness of the material necessitates specialized drill bits and techniques to avoid breakage, excessive wear, and inaccurate holes. This comprehensive guide will delve into the world of drill bits designed for hardened steel, examining their composition, application, and the crucial techniques needed for successful drilling. Understanding these factors is key to achieving clean, precise holes in this demanding material.

Understanding the Challenges of Hardened Steel

Hardened steel, by its very nature, is resistant to deformation. This hardness, while desirable for many applications, makes it incredibly difficult to penetrate with standard high-speed steel (HSS) drill bits. Attempting to drill hardened steel with the wrong tools will inevitably lead to:

• Drill bit breakage: The extreme pressure and friction generated can cause the drill bit to shatter or fracture.
• Excessive wear: The hardened steel will quickly dull standard drill bits, rendering them useless.
• Poor hole quality: The resulting holes will often be rough, oversized, or otherwise imperfect.
• Heat damage: Friction generates significant heat, which can compromise the workpiece's hardness or even cause warping.

Types of Drill Bits for Hardened Steel

Several specialized drill bit types are designed to tackle the challenges of hardened steel. Each type possesses unique characteristics making it suitable for specific applications and material properties. Let's explore the most prominent options:

1. Carbide Drill Bits

Carbide drill bits are arguably the most popular choice for drilling hardened steel. They are manufactured from tungsten carbide, an exceptionally hard and wear-resistant material. Their superior hardness allows them to withstand the pressures encountered when drilling hardened steel, resulting in longer tool life and improved hole quality.

• Advantages: Exceptional hardness and wear resistance, longer lifespan compared to HSS bits, produces cleaner holes.
• Disadvantages: More expensive than HSS bits, can be brittle and prone to chipping if used improperly, require higher speeds and feeds compared to cobalt drills.
• Applications: Ideal for general-purpose drilling of hardened steel, suitable for various thicknesses and hole sizes.

2. Cobalt Drill Bits

Cobalt drill bits are a variation of high-speed steel (HSS) drill bits containing a higher percentage of cobalt (typically 5-8%). This addition significantly enhances the bit's hardness, wear resistance, and red hardness (the ability to retain hardness at high temperatures). Cobalt bits offer a balance between cost and performance, making them a viable alternative to pure carbide bits for many applications.

• Advantages: Good balance between cost and performance, higher wear resistance compared to standard HSS bits, capable of handling some hardened steels effectively.
• Disadvantages: Not as durable as carbide bits, may require specialized lubricants for optimal performance, less effective on extremely hard steels.
• Applications: Suitable for drilling moderately hardened steel, a cost-effective option for less demanding applications.

3. Solid Carbide Drill Bits

These are a more advanced form of carbide drill bits, distinguished by their solid carbide construction. Unlike brazed carbide tips, these bits are made entirely of carbide, leading to even greater strength and wear resistance. They are the preferred choice for drilling very hard, deep holes in hardened steel.

• Advantages: Exceptional strength and wear resistance, ideal for deep hole drilling in hardened steel, significantly longer lifespan.
• Disadvantages: Very expensive, require highly precise machining setup, can be brittle and susceptible to breakage if mishandled.
• Applications: Best suited for high-precision, deep hole drilling operations in extremely hard steels.

4. Polycrystalline Cubic Boron Nitride (PCBN) Drill Bits

PCBN drill bits are the most advanced and expensive option. PCBN is an ultra-hard material, second only to diamond in hardness. These bits are specifically designed for drilling the hardest and toughest materials, including hardened steels with exceptionally high hardness values.

• Advantages: Unmatched hardness and wear resistance, capable of drilling the toughest hardened steels, minimal wear and tear.
• Disadvantages: Extremely expensive, requires specialized equipment and expertise, delicate and prone to damage if mishandled.
• Applications: Ideal for drilling the hardest and most challenging hardened steels, especially in aerospace and other high-precision industries.

Selecting the Right Drill Bit for Hardened Steel

Choosing the appropriate drill bit depends on several factors:

• Hardness of the steel: The Rockwell hardness scale (HRC) is used to measure the hardness of steel. Higher HRC values indicate harder materials. For very high HRC values (above 60), PCBN or solid carbide bits are recommended. Moderately hardened steels (HRC 45-60) can often be drilled with cobalt or carbide bits.
• Hole size and depth: For deep holes, solid carbide or PCBN bits are often preferred due to their superior strength and resistance to deflection.
• Accuracy requirements: For high-precision applications, solid carbide or PCBN bits are crucial to ensure accuracy and surface finish.
• Budget: Carbide bits are generally more expensive than cobalt or HSS bits. The budget will influence the type of drill bit that can be chosen.

Drilling Techniques for Hardened Steel

Proper technique is paramount when drilling hardened steel. Improper techniques can lead to drill bit breakage, poor hole quality, and workpiece damage. Here's a breakdown of crucial techniques:

• Use cutting fluid: Cutting fluid, also known as cutting oil or coolant, is essential for lubricating the drill bit and dissipating heat. This prevents the bit from overheating and helps maintain a clean cut. Soluble oils are a popular choice for hardened steel drilling.
• Slow speed, light feed: Use a significantly slower rotational speed and feed rate than when drilling softer materials. This minimizes heat generation and prevents the drill bit from overloading. Start with a lower speed than you might expect and gradually increase if necessary.
• Pilot hole: Drilling a smaller pilot hole before drilling the full-size hole can be beneficial. It provides a starting point for the larger drill bit and reduces the initial impact force.
• Multiple passes: For deeper holes, it's often advisable to use multiple passes with progressively larger drill bits to reduce the stress on the drill bit and maintain precision.
• Sharp drill bits: Use sharp drill bits to reduce friction and wear. A dull bit will generate excessive heat, leading to premature failure.
• Secure workpiece: Ensure the workpiece is securely clamped or held in a vice to prevent movement during drilling. Any movement can lead to inaccurate holes or damage to the drill bit.
• Proper machine setup: A correctly configured machine is critical, ensuring the drill press is properly aligned and the chuck is tight.

Frequently Asked Questions (FAQ)

Q: Can I drill hardened steel with a standard HSS drill bit?

A: While possible for very shallow holes and less hardened steels, it is generally not recommended. Standard HSS bits will likely break or become dull very quickly.

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

A: Soluble oils are a popular choice, as they provide good lubrication and heat dissipation. Other options exist, and choosing the right one depends on the specific application and material.

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

A: A dull drill bit will often show signs of wear, such as a rounded cutting edge or chipped teeth. It may also vibrate excessively during drilling.

Q: What is the difference between brazed and solid carbide drill bits?

A: Brazed carbide bits have a carbide tip brazed onto a steel shank. Solid carbide bits are entirely made of carbide, resulting in greater strength and durability.

Q: What is the importance of using a slow feed rate?

A: A slow feed rate helps reduce the pressure on the drill bit, thus minimizing the risk of breakage and generating less heat.

Q: Can I use a hand drill to drill hardened steel?

A: It's possible for very small holes in relatively thin pieces, but it's extremely challenging and not recommended. A drill press is essential for accuracy, control, and reduced risk of accidents.

Conclusion

Drilling hardened steel successfully requires careful consideration of the drill bit material, its application, and the drilling technique. Choosing the right drill bit, using appropriate cutting fluid, and employing the correct speed and feed rates are crucial for achieving clean, accurate holes while minimizing the risk of drill bit breakage and damage to the workpiece. By understanding the challenges and following the techniques outlined above, you can confidently tackle this demanding material. Remember, safety should always be your top priority. Wear appropriate safety gear, including safety glasses and hearing protection, throughout the drilling process. If unsure about any aspect, consult a professional machinist or refer to the manufacturer's instructions for the specific drill bit and machine being used.