Best Drill Bits For Hardened Steel Bolt

Drilling through hardened steel bolts can be one of the most challenging tasks in metalworking. When you encounter a broken bolt, a seized fastener, or need to modify hardened steel components, having the right drill bit makes all the difference between success and frustration. Standard drill bits often snap, dull quickly, or simply can’t penetrate the hardened surface. In this comprehensive guide, we’ll explore the most effective drill bits specifically designed to tackle hardened steel bolts, helping you save time, money, and unnecessary headaches.

1. Cobalt Drill Bits

Cobalt drill bits are consistently rated as the top performers when it comes to drilling hardened steel bolts. These bits contain between 5-8% cobalt blended with high-speed steel (HSS), creating an alloy that maintains its hardness even at high temperatures.

Cobalt bits excel in drilling through hardened steel because they resist heat better than standard HSS bits. This heat resistance is crucial when drilling through hardened steel bolts, as the friction generated can quickly dull lesser quality bits.

When using cobalt bits on hardened steel:
– Run at slower speeds (500-1000 RPM)
– Apply consistent pressure
– Use cutting oil to extend bit life and improve cutting performance

The M35 grade (containing 5% cobalt) is suitable for most hardened steel applications, while the more expensive M42 grade (containing 8% cobalt) offers superior performance for extremely hard materials.

2. Carbide Drill Bits

When faced with extremely hardened steel bolts, solid carbide drill bits are often the ultimate solution. These bits are made from tungsten carbide, which is significantly harder than cobalt steel.

Carbide bits can drill through materials with hardness ratings up to 65 HRC (Rockwell C scale), making them ideal for the toughest hardened steel bolts, including grade 12.9 bolts and hardened fasteners.

The primary drawbacks of carbide bits are their brittleness and cost:
– They require precise technique to prevent breakage
– They’re significantly more expensive than other options
– They need to be run at higher speeds but lower feed rates

For extracting broken hardened bolts, stub-length carbide bits provide extra rigidity and are less prone to breaking during the high-stress operation.

3. Titanium-Coated HSS Bits

Titanium-coated high-speed steel bits represent a middle-ground option for drilling hardened steel bolts. These bits feature a titanium nitride (TiN) coating applied to standard HSS bits, which increases surface hardness and reduces friction.

While not as durable as cobalt or carbide for hardened steel applications, they offer several advantages:
– More affordable than cobalt or carbide options
– The titanium coating reduces heat buildup
– They last 3-6 times longer than standard HSS bits when used properly

These bits work best on moderately hardened steel bolts rather than fully hardened fasteners. They’re a good choice for occasional use or when working with bolts that aren’t fully hardened to maximum specifications.

4. Specialty Left-Hand Drill Bits

When dealing specifically with broken hardened steel bolts, left-hand drill bits deserve special consideration. These bits rotate counterclockwise (opposite to standard bits) and can sometimes extract the broken bolt while drilling.

The counterclockwise rotation tends to “unscrew” the broken bolt as you drill, potentially saving you from needing to use separate extraction tools. Left-hand bits are available in cobalt and carbide varieties for maximum effectiveness on hardened steel.

For best results with left-hand bits:
– Start with a center punch to create a starting point
– Use slower speeds than you would with standard bits
– Apply cutting oil liberally
– Be prepared to switch to a standard bit if extraction doesn’t occur

5. Step Drill Bits for Special Applications

While not typically the first choice for hardened steel, high-quality cobalt step drill bits can be effective for specific applications involving hardened steel bolts, particularly when you need to enlarge an existing hole or create a tapered opening.

The stepped design provides additional stability and reduces chatter when drilling, which is particularly important when working with hardened materials. Premium cobalt step bits can handle hardened steel in situations where:
– You’re enlarging an existing pilot hole
– You need to create a countersink in one operation
– You’re working with thinner hardened steel components

Comparison Table of Drill Bits for Hardened Steel Bolts

6. Essential Techniques for Drilling Hardened Steel Bolts

Even the best drill bits will fail if not used correctly on hardened steel. These techniques are crucial for success:

1. Start with a center punch: Create a dimple to prevent the bit from wandering.

2. Use cutting fluid: A quality cutting oil or lubricant reduces heat and extends bit life dramatically.

3. Drill at the right speed : Slower speeds (500-1000 RPM) prevent overheating and premature dulling.

4. Apply consistent pressure: Too much pressure can break bits; too little allows the bit to rub and overheat.

5. Start with a smaller pilot hole: Begin with a smaller bit (1/8″ or 3/16″) before moving to your final size.

6. Take breaks during drilling: Allow the bit and material to cool periodically during extended drilling.

7. Consider annealing: For extremely hard bolts, consider using a torch to heat and slowly cool the bolt to reduce hardness before drilling.

7. When to Use Specialized Extraction Methods

Sometimes, even the best drill bits aren’t the right solution for hardened steel bolts. Alternative methods to consider include:

– EDM (Electrical Discharge Machining): For extremely hard bolts in critical applications
– Screw extractors: After drilling a pilot hole in the hardened bolt
– Welding method: Welding a nut to the broken bolt to provide extraction leverage
– Chemical solutions: Specialized chemicals that can dissolve or weaken specific types of hardened steel

These alternatives may be necessary when dealing with bolts hardened beyond the capabilities of even carbide drill bits or in situations where precision is absolutely critical.

Conclusion

When tackling hardened steel bolts, cobalt and carbide drill bits stand out as the most effective options, with cobalt offering the best balance of performance and value for most applications. Carbide provides superior cutting ability for the most challenging scenarios but requires careful handling due to its brittleness.

Remember that successful drilling through hardened steel isn’t just about having the right bit—proper technique, including appropriate speed, pressure, and lubrication, is equally important. For occasional use, titanium-coated bits provide a cost-effective alternative, while specialized left-hand bits can serve double-duty by potentially extracting broken bolts during the drilling process.

Investing in quality bits specifically designed for hardened steel will save you time, frustration, and ultimately money by reducing the number of broken or dulled bits and ensuring successful completion of your metalworking projects.