Best Drill Bit For Cast Iron: Reviews & Guide [Updated]

Last Update: March 30 2023 19PM

Drilling through cast iron can be a challenging task, as it requires a drill bit that is durable enough to withstand the hot temperatures and wear associated with the job. When looking for the best drill bit for cast iron, it's important to consider factors such as the material, point angle, size, flute design, and brand reputation.

In this guide, we will discuss these factors in more detail, helping you to make an informed decision when purchasing a drill bit for your next cast iron drilling project.

Understanding Cast Iron: Properties, Types, and Applications

Cast iron is a group of ferrous alloys that contains carbon and silicon as the primary alloying elements. It is known for its excellent casting properties, high strength, and wear resistance. Cast iron is a popular material used in various applications, from cooking utensils to automobile engine blocks. In this article, we will explore cast iron’s properties, types, and applications in detail.

Properties of Cast Iron

Cast iron is a unique material with several properties that make it ideal for various applications. Here are some of the primary properties of cast iron:

High Carbon Content

One of the defining features of cast iron is its high carbon content, typically between 2% and 4%. The high carbon content makes cast iron harder and more brittle than other materials. However, it also gives it its unique properties, such as wear resistance, strength, and durability.

High Melting Point

Cast iron has a high melting point, which makes it ideal for high-temperature applications. It can withstand temperatures of up to 1,200°C, making it suitable for applications requiring high-temperature resistance.

Good Casting Properties

Cast iron has excellent casting properties, which means it can be easily cast into complex shapes and structures. Its high fluidity and low shrinkage make it easy to mold and cast, making it ideal for intricate designs and shapes.

Excellent Wear Resistance

Cast iron has excellent wear resistance, making it suitable for applications requiring high abrasion resistance. Its wear resistance makes it ideal for engine components, gears, and other moving parts.

Types of Cast Iron

There are several cast iron types, each with unique properties and characteristics. Here are the most common types of cast iron:

Gray Cast Iron

Grey cast iron is the most common type of cast iron, accounting for over 80% of all cast iron production. It is known for its excellent casting properties, high strength, and wear resistance. Grey cast iron has a flake graphite microstructure, which gives it its characteristic grey color.

White Cast Iron

White cast iron is a complex and brittle form with a white-colored surface. It is produced by rapidly cooling molten cast iron, which leads to the formation of cementite. White cast iron has excellent wear resistance but is not as ductile as grey cast iron.

Ductile Iron

Ductile iron, also known as nodular cast iron, is a type of cast iron with graphite in the form of nodules instead of flakes. Ductile iron has excellent strength, ductility, and wear resistance, making it ideal for applications requiring high strength and toughness.

Malleable Iron

Malleable iron is a type of cast iron that has been heat-treated to convert its microstructure into a ductile and plastic form. Malleable iron has good ductility, toughness, and strength, making it ideal for applications requiring high strength and toughness.

Applications of Cast Iron

Cast iron has many applications, from cookware to heavy machinery. Here are some of the most common applications of cast iron:

Cookware

Cast iron cookware is popular due to its excellent heat retention and distribution properties. It is ideal for cooking stews, soups, and other slow-cooked meals.

Pipe and Fittings

Cast iron pipes and fittings are commonly used in plumbing and drainage applications due to their high strength and durability.

Engine Blocks

Cast iron engine blocks are used in automobiles and other heavy machinery for their excellent strength, durability, and wear resistance.

Machine Tool Structures

Cast iron is commonly used in machine tool structures due to its excellent vibration-damping properties. The high damping capacity of cast iron reduces vibration and noise in machine tools, making them more efficient and accurate.

Wind Turbine Components

Due to its excellent strength and durability, cast iron is used in wind turbine components such as hubs, mainframes, and gearboxes. Wind turbines operate in harsh environmental conditions, and cast iron components can withstand these conditions without suffering from fatigue or failure.

Railroad Components

Due to its excellent wear resistance and durability, cast iron is used in railroad components such as brake shoes, couplings, and axle boxes. These components are subjected to high loads and wear, and cast iron can withstand these conditions without failing or degrading.

Choosing the Right Drill Bit for the Job

Drilling cast iron can be challenging due to its hardness and brittleness. Choosing the right drill bit for the job is crucial to ensure a successful outcome. Here are some factors to consider when selecting the right drill bit for drilling cast iron:

Cast Iron Thickness vs Drill Bit Size

The thickness of the cast iron determines the size of the drill bit that should be used. A smaller drill bit should be used for thinner cast iron, while a larger drill bit should be used for thicker cast iron. Using the correct drill bit size can prevent the drill bit from breaking or wearing out quickly.

RPM vs Drill Bit Material

The drill’s revolutions per minute (RPM) also plays a significant role in selecting the right drill bit material. A drill bit with a softer material, such as high-speed steel (HSS), can be used for lower RPM. In comparison, a drill bit made of a more rigid material, such as cobalt or carbide, is ideal for higher RPM. Using the wrong drill bit material can lead to premature wear or failure of the drill bit.

Angle vs Material

The angle of the drill bit is also an essential factor to consider when drilling cast iron. A smaller angle is ideal for softer materials, while a larger angle is better suited for more rigid materials. A larger angle is recommended for drilling cast iron to prevent the drill bit from wandering or skipping.

Flute Shape vs Material

The flute shape of the drill bit can also affect its performance when drilling cast iron. A twist drill bit with a parabolic flute shape is ideal for drilling cast iron. It can remove chips more efficiently and reduce heat buildup. Using a drill bit with the wrong flute shape can lead to clogging, overheating, and premature wear.

Coolant/Lubricant vs Material

Coolant or lubricant when drilling cast iron can help reduce friction and heat buildup, making the process smoother and more efficient. Water-based coolants are recommended for softer materials, while oil-based lubricants are ideal for more complex materials. Using the wrong coolant or lubricant can affect the drill bit’s performance and lead to premature wear or failure.

Best drill bit for cast iron Reviews

Hymnorq Metric M35 Cobalt Steel Extremely Heat Resistant Twist Drill Bits

The Hymnorq Metric M35 Cobalt Steel Twist Drill Bits are 13 extremely heat-resistant pieces designed to cut through hard metals like stainless steel and cast iron. The bits feature a 135-degree angle requiring less force to remove metal than lower angles like 90° and 118°.

The self-centering split point of the bits allows for direct drilling to the size needed without any pre-drilling. The heavy-duty, fully ground body construction and high-flow spiral design also contribute to the fastest removal rate and are ideal for thick materials.

The straight shank of the bits is suitable for a standard drill chuck and reduces the likelihood of “walking” or “wandering” during drilling. The end of the shank is chamfered to allow for easy and firm holding. Additionally, the unique bronze SST heat and surface treatment makes the bits more durable and able to withstand high temperatures and stress during drilling.

Highlighted Features:

• Manufactured from M35 cobalt steel, with molybdenum and cobalt added to create a hardened alloy, measuring Rockwell hardness up to 67, extremely heat resistant. 
• For much faster cutting and longer life span, with up to twelve times when compared with an average HSS drill bit.
• The 135° Quick-Cut points are self-centering and penetrate quickly with less pressure. Will not “walk” or “wander”. 
• Straight shank allows for firm holding and accurate centering, fit for standard drill chucks and regular round collet chucks.
• The set includes 13 PCs of M35 cobalt steel twist drill bits: 1.5mm, 2mm, 2.5mm, 3mm, 3.2mm, 3.5mm, 4mm, 4.5mm, 4.8mm, 5mm, 5.5mm, 6mm, 6.5mm. Well organized in a plastic indexed storage case for easy transport to your worksite.
• They successfully cut through hard metals such as stainless steel and cast iron. However, they can be used for softer materials, too. 
• Not for hardened steel / high carbon steel. Try to adjust the RPM when it fails to penetrate.

Metric M42 8% Cobalt Twist Drill Bits Set for Stainless Steel and Hard Metal

This drill bit set is made from premium cobalt, which costs $80,000 per ton. The cobalt content, specifically 8%, gives the high-speed steel added heat and wear resistance compared to generic high-speed steel. The 3-step design tip allows direct drilling with less force and eliminates “walking” across the work surface.

These drill bits are ideal for drilling through hard metals like stainless steel and materials like hardwood, aluminum, copper, and plastic. The 3-step design also allows for accurate centering, making it suitable for precision woodworking jobs that require precise drilling.

Furthermore, the cobalt content in the design makes the drill bits perform even better when drilling through hardwood. This premium cobalt drill bit set is an excellent investment for those who frequently work with rugged materials and require precise and reliable drilling performance.

Highlighted Features:

• Made of M42 cobalt steel with 8% cobalt content
• It has a hardness of 68-70 HRC on the Rockwell hardness scale
• Provides excellent heat, wear, and chip resistance, resulting in a long tool life
• Specifically designed for cutting through stainless steel, hard metals, aluminum, and plastic
• Capable of running three to five times faster than other drill bits
• Although it is more expensive, the cost per hole is lower because it can produce more holes
• The sharp bit edge can cut through metal material easily, making long strips of iron chips
• Includes 19 pieces of 135-degree split point drill bits.

Factors to Consider when Drilling Cast Iron

Drilling cast iron can be challenging due to its hardness and brittleness. It requires specific tools and techniques to ensure a successful outcome. Here are some factors to consider when drilling cast iron.

Cast Iron Thickness

The thickness of the cast iron affects the drilling process. Thicker cast iron requires more power and a slower drilling speed than thinner cast iron. It’s essential to adjust the drilling speed and feed rate according to the thickness of the cast iron to ensure the best results.

RPM (Revolutions Per Minute)

The RPM or speed of the drill bit is critical when drilling cast iron. It’s essential to use a drill bit with a low RPM to prevent overheating and damage to the drill bit. The recommended RPM for drilling cast iron is between 300 and 500 RPM.

Drill Bit Angle

The angle of the drill bit affects the drilling process. Using a drill bit with the correct angle is essential to ensure the best results. A 118-degree angle is the most common angle for drilling cast iron. However, a 135-degree angle is recommended for harder cast iron.

Flute Shape

The shape of the flute on the drill bit affects the drilling process. A drill bit with a twisted flute is the most common for drilling cast iron. However, a straight flute is recommended for softer cast iron to prevent chip clogging.

Coolant/Lubricant

Using a coolant or lubricant when drilling cast iron is crucial to prevent overheating and prolong the life of the drill bit. Using a water-soluble coolant or lubricant is recommended when drilling cast iron.

Clamping/Work Holding

Clamping or work holding is essential when drilling cast iron. It prevents the cast iron from moving and ensures accurate drilling. Using a vise or clamp to hold the cast iron in place during drilling securely is recommended.

Drill Bit Maintenance and Care

In addition to choosing the right drill bit for the job, it’s also essential to properly maintain and care for your drill bits to ensure they perform at their best. Here are some critical factors to consider for drill bit maintenance and care when drilling cast iron:

Storage

Proper storage is essential to maintaining the quality and sharpness of your drill bits. Keep your drill bits dry, cool, and clean to prevent rust and corrosion. A dedicated storage case or drawer with individual compartments for each drill bit can help protect them from damage.

Sharpening

Over time, drill bits can become dull or damaged, leading to poor performance and potentially damaging the drilled material. Sharpening your drill bits is essential to keep them working effectively regularly. Use a sharpening tool or grinding wheel explicitly designed for drill bits and follow the manufacturer’s instructions carefully.

Cleaning

After each use, cleaning your drill bits is essential to remove any debris or material buildup that can affect their performance. Use a brush or compressed air to remove any chips or dust, and wipe them down with a clean cloth or towel. Avoid harsh chemicals or solvents that damage the drill bit surface or coating.

Properly storing, sharpening, and cleaning your drill bits can extend their lifespan and ensure they perform at their best when drilling cast iron or any other material.

Tips and Tricks

In addition to choosing the right drill bit and considering the factors discussed earlier, here are some additional tips and tricks to keep in mind when drilling cast iron:

Preparing the Cast Iron

Before drilling, it’s essential to prepare the cast iron surface properly. Clean any debris or rust using a wire brush, and mark the drilling location using a center punch or drill bit starter. This will help ensure a clean and accurate hole.

Proper Drilling Technique

When drilling cast iron, it’s essential to use the correct drilling technique to avoid damaging the material or drill bit. Start with a slow speed and gradually increase to the desired RPM. Use light pressure and avoid forcing the drill bit, which can cause it to overheat or break. Use a pecking motion, drilling a short distance at a time and lifting the bit out to remove debris.

Safety Precautions

Drilling cast iron can produce sharp chips and debris, so taking proper safety precautions is essential. Wear eye protection, gloves, and a face mask to prevent injury from flying debris. Use a drill press or secure the material in a vise to prevent it from moving during drilling. And always read and follow the manufacturer’s safety guidelines and instructions for your drill bit and equipment.

By following these tips and tricks, you can confidently drill cast iron and achieve excellent results.

Frequently Asked Questions (FAQs)

What is the best drill bit for cast iron?

The best drill bit for cast iron depends on the specific job and the thickness of the material. High-speed steel (HSS) drill bits work well for thinner cast iron. In contrast, cobalt steel or carbide-tipped bits are recommended for thicker material.

How do I know what size drill bit to use for cast iron?

The drill bit size needed for cast iron will depend on the hole diameter you need to drill. Consult a drill bit size chart or measure the hole diameter you need and choose a bit slightly more extensive than that.

Can I use a regular drill bit on cast iron?

While regular drill bits can be used on thinner cast iron, they are not recommended for thicker material. Cobalt steel or carbide-tipped bits are better suited for drilling through thick cast iron.

What is the best RPM for drilling cast iron?

The best RPM for drilling cast iron varies depending on the thickness of the material and the size of the drill bit being used. Consult the manufacturer’s guidelines for your drill bit and adjust the RPM accordingly. Generally, a slower speed is better for thicker cast iron and more significant drill bits.

Do I need to use a coolant or lubricant when drilling cast iron?

Using a coolant or lubricant when drilling cast iron can help extend the drill bit’s life and improve the hole’s quality. A cutting oil or water-soluble coolant is recommended for best results. Consult the manufacturer’s guidelines for your specific drill bit and material.

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