CNC milling

In the realm of manufacturing and metalworking, CNC (Computer Numerical Control) milling stands out as one of the most vital processes. At the heart of this technique lies a variety of tools, each designed for specific tasks and materials. This guide dives deep into understanding CNC milling tools, particularly focusing on end mills and twist drills.

Understanding CNC Milling Tools

CNC milling is the process where a material, often metal, is cut into a desired shape and size with precision by a computer-controlled machine. The milling machine accomplishes this by removing material using rotating cutting tools.

To fully grasp the importance of these tools, it’s essential to understand the factors that influence their choice and application:

  1. Material to be milled: Depending on the material’s hardness, toughness, and other properties, specific tools are better suited for the task.
  2. Type of operation: Different operations, such as roughing, finishing, or drilling, require different types of tools.
  3. Machine capabilities: The machine’s power, torque, and speed will influence the kind of tool to be used.

With this foundation, let’s delve into some of the most commonly used CNC milling tools.

End Mills

End mills are versatile tools used for both roughing and finishing operations. They can be recognized by their fluted design, which can range from two to several flutes, depending on the specific design and purpose. Here’s what to know about them:

  • Types: There are various types of end mills – flat, ball-nose, and corner radius being some of the most common. Each type is tailored for specific tasks. For instance, ball-nose mills are perfect for 3D contour work, while flat ones are suitable for slotting and pocketing.
  • Material: End mills are made from different materials, with high-speed steel (HSS) and carbide being the most popular. Carbide end mills are harder and more heat-resistant than HSS, allowing for faster milling at the cost of being more brittle.
  • Coatings: To enhance performance and longevity, many end mills are coated with materials like Titanium Nitride (TiN) that reduce friction and heat.

Twist Drills

Twist drills are probably among the first images that come to mind when someone mentions drilling. They are characterized by their helical flutes running up the body and a pointed tip.

  • Point Angle: The angle of the point determines many of the twist drill’s properties. A standard point angle is about 118°, suitable for many metals. However, harder materials might require a flatter angle, like 135°.
  • Flute Design: The flute’s primary function is to channel away the chips produced during the drilling process. A wide flute is better for softer materials as it provides more space for the chips, while a narrower flute is ideal for harder materials.
  • Material & Coating: Like end mills, twist drills can be made from HSS or carbide. They can also be coated with materials like TiN to increase heat resistance and reduce friction.

Face Mills

Distinguished by their wide cutting diameter and multiple replaceable carbide or high-speed steel cutting inserts positioned on the face, they are designed to cut along the face of a workpiece, hence the name. The process with face mills is typically horizontal, skimming off layers from the surface to achieve a flat, fine finish. One of the standout features of face mills is their ability to offer a higher feed rate due to the multiple cutting edges engaged with the material. The choice of insert material and geometry is paramount in face milling operations. Depending on the workpiece’s material, certain inserts can maximize tool life, reduce chatter, and achieve that perfectly smooth finish so sought after in many industries.

Ball Nose Mills

Ball nose mills, often known as ball end mills, are distinctly characterized by their hemispherical tips. These tools are the go-to options when the machining task calls for creating contoured shapes and intricate 3D profiles. The rounded tip ensures smooth tool engagement with the workpiece, reducing the occurrence of sharp corners and edges which might otherwise result in tool breakage or uneven cuts. As the ball nose mill traverses the workpiece, it creates a scallop pattern. The size of the scallop can be controlled by the step-over distance, with smaller step-over distances yielding finer finishes at the expense of a longer machining time. Ball nose mills are particularly useful in mold-making, aerospace, automotive, and any industry where complex geometric profiles are a requirement. When selecting a ball nose mill, the material of the tool, flute count, and coating can significantly impact its performance and longevity.

Specialty CNC Milling Tools

The world of CNC milling doesn’t just stop at the standard tool types. As industries evolve and machining tasks become more intricate, the demand for specialty CNC milling tools rises. For instance, there are dovetail cutters specifically designed to create dovetail slots or tapered end mills that create angled surfaces and edges. Thread mills, which are crafted to produce internal and external threads, exemplify the precision and specialization of CNC tooling. There are also lollipop cutters, which have an undercutting capability, making them ideal for tasks that require machining from below a workpiece.

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Thread Mills

Thread mills are uniquely designed tools intended to produce internal and external thread profiles on a workpiece. Unlike traditional taps and dies which are pushed or rotated into a pre-drilled hole, thread mills utilize a CNC machine’s ability to move in three dimensions, allowing for the precise creation of thread patterns. They are often cylindrical in shape, featuring cutting edges along their sides and sometimes at the tip, allowing them to cut threads for both blind and through holes. The key advantage of using thread mills is their flexibility. A single tool can produce different thread sizes by merely adjusting the CNC programming, reducing tool inventory and offering cost-efficiency. Moreover, because they cut threads using a milling process, thread mills reduce the risk of tool breakage, especially in harder materials, offering longer tool life and reduced downtime.

Slitting Saws and Woodruff Key Cutters

Both slitting saws and Woodruff key cutters are indispensable in specific CNC machining applications.

Slitting Saws are thin, circular cutting tools equipped with a series of sharp teeth on their periphery. These are used for making precise cuts, slits, or grooves in a workpiece. They are especially useful for cutting narrow slots in applications where a standard end mill might produce a cut that’s too wide. Due to their thin nature, it’s crucial to ensure they are run at appropriate speeds and feeds to prevent tool deflection or breakage.

Woodruff Key Cutters, on the other hand, are designed to produce the unique Woodruff keyseat shape, a semi-circular pocket that allows for the placement of Woodruff keys. These keys are used to lock gears, pulleys, and other components onto a shaft securely. The unique design of the cutter allows it to produce a slot that has a curved bottom, ensuring a snug fit for the Woodruff key. As with other specialty tools, the accuracy and efficiency they offer for their specific tasks are unparalleled.

Material Selection for CNC Milling Tools

Choosing the right material for a CNC milling tool is as critical as selecting the tool itself. The material directly influences tool longevity, cutting efficiency, and the finish of the machined part.

High-Speed Steel (HSS): HSS tools are generally more affordable and are perfectly suitable for many standard milling tasks, especially on softer materials like wood, plastics, and some metals. They are often praised for their toughness, which allows them to withstand the rigors of milling without chipping easily.

Carbide: This is a popular choice for many CNC milling applications, especially in industrial settings. Carbide tools, which are typically tungsten carbide, offer superior hardness and heat resistance compared to HSS. This means they can maintain sharpness longer, run at higher speeds, and excel in tougher materials like hardened steels or titanium. However, they tend to be more brittle, so they require careful handling to prevent breakage.

Coatings: Beyond the base material, many high-end milling tools receive coatings that further enhance their capabilities. Common coatings include Titanium Nitride (TiN), which offers increased hardness and heat resistance. There’s also Titanium Carbonitride (TiCN) which provides better wear resistance, and Titanium Aluminum Nitride (TiAlN) that excels in high-temperature applications.

Carbon Steel

Carbon Steel is one of the earliest and most widely used materials in the world of tooling. As the name suggests, its primary alloying element is carbon, and its properties change significantly based on its carbon content. The higher the carbon content, the harder the steel becomes, which can be advantageous for certain tools. However, the increased hardness comes with reduced ductility.

For machining and cutting tools, high-carbon steel was traditionally favored for its ability to retain sharpness. Its affordability made it an attractive choice for a broad range of applications. Nonetheless, while carbon steel tools are adequate for softer materials and less rigorous tasks, they don’t hold up well under high temperatures or when cutting harder materials. Their susceptibility to wear and losing temper in high heat scenarios can reduce tool life and efficiency.

High-Speed Steel (HSS)

High-Speed Steel (HSS) emerged as a significant advancement over traditional carbon steel. It’s an alloy of steel that contains elements like tungsten, molybdenum, chromium, and sometimes vanadium. What distinguishes HSS from carbon steel is its ability to retain hardness at high temperatures, a property that’s crucial for cutting tools.

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The name “High-Speed Steel” derives from its capability to operate at faster cutting speeds compared to carbon steel tools without losing temper. HSS tools are not only harder and more heat resistant but are also tougher than carbide, making them less prone to chipping. They are particularly favored for drilling, milling, and turning softer metals and alloys. However, in applications where maximum hardness and heat resistance are required, other materials like carbide may be more suitable.


Carbide, specifically Tungsten Carbide, is a game-changer in the tooling industry. It’s a composite material made from equal parts of tungsten and carbon atoms. In its most basic form, it’s about twice as strong as steel and is extremely hard, ranking about 9 on the Mohs scale, which is just below the hardness of diamond.

Carbide cutting tools are preferred in scenarios demanding high wear resistance and precision. Because of their hardness, they can maintain a sharp cutting edge for much longer than HSS tools. Additionally, their heat resistance allows them to operate at higher speeds, which can significantly reduce machining time. Carbide tools are especially favored for machining hard and abrasive materials. However, their hardness comes with a trade-off: they’re more brittle than HSS, which means they can be more susceptible to breaking under sudden stress or if not handled correctly.

Factors to Consider When Choosing CNC Milling Tools

The intricacies of CNC milling necessitate an in-depth understanding of not just the machinery but the very tools that interact with the workpiece. Choosing the right tool can be the difference between an efficient, precise operation and wasted time, material, and resources. When it comes to the selection of CNC milling tools, several factors play a pivotal role:

Workpiece Material

Arguably one of the most vital considerations, the workpiece material, dictates the type of cutting tool required. Different materials have varying hardness, abrasiveness, thermal properties, and more, all of which can influence tool selection:

  • Soft Materials (like plastics or soft metals): While many tool materials can effectively machine soft workpieces, HSS might often be preferred due to its toughness and the reduced risk of tool breakage.
  • Hard Materials (like hardened steels or titanium): These require tools that can withstand the stress and heat generated during the milling process. Carbide tools, due to their hardness and heat resistance, are typically favored for such applications.
  • Abrasive Materials: Materials that are abrasive wear down tools rapidly. Thus, tools with harder materials and possibly coatings are essential to prolong tool life.

Machining Operation

The specific machining operation being performed is another paramount factor:

  • Roughing vs. Finishing: Roughing operations aim to remove large amounts of material quickly, which might require tools with a robust design and more flutes. Finishing operations, conversely, are about achieving a smooth surface finish, and tools with fewer flutes and specific geometries might be more suitable.
  • Slotting, Peripheral Milling, and Face Milling: Each of these operations places different demands on the tool. For example, slotting operations might benefit from tools with fewer flutes to evacuate chips more efficiently, while face milling might require a tool with a larger cutting diameter and multiple cutting edges.
  • Drilling vs. Milling: Twist drills and end mills have different designs and functions. While drills are made to create holes, end mills are designed to cut sideways.

Tool Geometry

Tool geometry encompasses several attributes, each influencing how the tool interacts with the workpiece:

  • Flute Count: Tools can have a varied number of flutes (the grooves or valleys on the tool). Fewer flutes allow for better chip evacuation, essential for materials like aluminum. In contrast, tools with more flutes can achieve a finer surface finish.
  • Helix Angle: This refers to the spiral design of the flutes. A high helix angle (like 45°) is better for chip evacuation and is often used for softer materials. A lower helix angle provides a more axial force and might be preferred for harder materials.
  • Cutting Edge Radius and Tool Nose Radius: These factors influence the finish of the machined part. A sharper edge is more aggressive but might wear out faster or be more fragile, while a larger nose radius can provide a smoother finish.
  • Coatings: Many high-end milling tools come with coatings that can enhance the tool’s properties. For instance, a Titanium Nitride (TiN) coating can increase hardness and heat resistance.