Beginner’s Guide to Milling Tools: Unlocking Your Creativity and Precision

Types of milling tools mainly include end mills, face mills, and slab mills. Typical milling operations include flat milling, chamfering or edge milling, contour milling, slot milling, cavity milling, keyway milling, and more.

Common designs of end mills include square end mills, which can perform vertical plunge milling or side milling similar to drill bits. Their cutting edges extend to the center of the end mill, allowing for plunge milling operations. Some three-flute end mills can also perform plunge milling. Both two-flute and three-flute end mills are suitable for machining non-ferrous metals. Ball end mills can be used for milling round-bottomed slots or edges with arc shapes.

Compared to two-flute end mills, four-flute precision end mills with square heads provide smoother milling and can maintain sharpness for a longer time, but they cannot perform plunge milling. These end mills are best suited for cutting ferrous metals. Compared to four-flute precision end mills, four-flute roughing end mills can remove metal at a faster rate with smaller machine power, tool deflection, and vibration. Additionally, their chip breakers can produce smaller and more controllable chips.

The helical flutes of end mills reduce machining vibrations. During machining, one or more points of the helical flutes are in contact with the workpiece, continuously cutting the metal material. This allows the machine tool to bear a continuous load, unlike when using straight flute tools, which create impact loads each time the tool contacts or disengages from the workpiece. Although manufacturing helical flute tools is more complex and costly than manufacturing straight flute tools, the reduction in machining vibrations and improvement in surface finish make them worthwhile.

Solid carbide end mills come in various types and specifications. They can be classified as right-hand cutters and left-hand cutters. Left-hand cut end mills are used when the cutting force direction needs to be changed to avoid pressure on the workpiece. Moreover, end mills have different flute lengths, including short, regular, long, and extended flutes. End mills can also have tapered designs, usually with taper angles of 0.5°, 1°, 1.5°, 2°, 3°, 5°, 7°, 10°, 15°, 20°, and 25°. These tapered end mills are particularly useful in mold making to provide draft angles for easy demolding of molded products.

Modular end mills utilize both end cutting and side cutting to provide excellent surface finish. The commonly used diameter range for milling cutters is 30-150mm, with slotting depths of 13-20mm. Vertical lift table milling machines are limited to using smaller-sized modular end mills due to power and rigidity constraints. Larger diameter modular end mills can be used for milling aluminum and plastics.

Introduction to Milling Tools Face mills are used for machining larger surfaces and rapidly removing a large amount of metal material. The diameter range of insert-type face mills is typically 13-75mm. When machining on a vertical lift table milling machine, due to insufficient machine rigidity and resulting vibrations, such machines are limited to using small-diameter face mills. Face mills are better suited for large single-column (single-arm) milling machines. When milling softer materials such as aluminum and plastics, larger diameter face mills can be used without experiencing vibrations.

Slab mills are typically used in horizontal machining centers and are mounted on core arbors supported at both ends. Horizontal end face milling machines can provide

The types of milling cutters mainly include end mills, face mills, and slab mills, among others. Typical milling operations include face milling, chamfering or edge milling, contour milling, slot milling, cavity milling, keyway milling, and more.

Some common types of end mills include:

1. Square end mills: These have straight cutting edges and are suitable for vertical plunging or side milling. They can perform plunge milling due to their cutting edges extending to the center of the end mill. Some three-flute end mills can also perform plunge milling. Both two-flute and three-flute end mills are suitable for machining non-ferrous metals. Ball nose end mills with two cutting edges can be used for milling grooves with circular bottoms or curved edges.
2. Compared to two-flute end mills, four-flute precision end mills with square heads provide smoother milling and can maintain sharpness for a longer time, but they cannot perform plunge milling. These end mills are best suited for cutting ferrous metals. Compared to four-flute precision end mills, four-flute roughing end mills can remove metal at a faster rate with smaller machine power, tool deformation, and vibration. They also have a chip breaker that produces finer and better-controlled chips.

The helical flute of an end mill reduces machining vibrations. During the operation, one or more points of the helical flute remain in contact with the workpiece, continuously cutting the metal material. This allows the machine tool to withstand a continuous load, unlike with straight flute tools where there is an impact load each time the tool makes contact or disengages from the workpiece. Although manufacturing helical flute tools is more complex and costly than manufacturing straight flute tools, the reduction in machining vibrations and the improvement in surface finish make them worthwhile.

Solid end mills come in various common types and specifications. They can be categorized as right-hand cutting or left-hand cutting. Left-hand cutting end mills are used when it is necessary to change the direction of cutting forces to avoid exerting pressure on the workpiece. Moreover, end mills have different slot lengths, including short slots, regular slots, long slots, and extended slots. Tapered end mills can also be used, with typical tapers on each side ranging from 0.5°, 1°, 1.5°, 2°, 3°, 5°, 7°, 10°, 15°, 20°, to 25°. These tapered end mills are particularly useful in mold machining to provide draft angles for easy demolding of molded products.

Modular end mills allow cutting with both end and side edges to provide a good surface finish. The commonly used diameter range for end mills is 30-150mm, with a cutting slot depth of 13-20mm. Due to power and rigidity limitations, vertical milling machines with vertical lifting tables are limited to using smaller-sized modular end mills. Larger diameter modular end mills can be used for milling aluminum and plastic materials.

Slab mills are designed to fully utilize the required power and rigidity for high metal removal rates. However, by installing a horizontal milling attachment, a slab mill can also be used for milling operations on a vertical milling machine. Although smaller cutting parameters must be used on a vertical milling machine due to rigidity and vibration issues, this attachment is still very useful. With a horizontal milling attachment installed on a vertical milling machine, a slab mill can achieve higher metal removal rates compared to other milling cutters. In the absence of a horizontal milling machine, installing multiple milling cutters on the same arbor for simultaneous machining is an optimal approach. Slab mills are typically wider than the workpiece, allowing milling of the entire cross-section of the workpiece.

Side and face mills are Under the premise that can be tolerated, the temperature of the tool and the chips when cutting hardened steel with polycrystalline cubic boron nitride (PCBN) tools typically exceeds 1000°C.

Tool wear and tool life Tool wear typically includes the following types: ① flank wear; ② crater wear; ③ notch wear; ④ edge rounding; ⑤ edge chipping; ⑥ edge cracking; ⑦ catastrophic failure.

There is no universally accepted definition for tool life, which usually depends on different workpieces, tool materials, and cutting processes. One way to quantitatively analyze the termination point of tool life is to set an acceptable maximum flank wear limit (represented by VB or VBmax). The expected tool life can be represented by Taylor’s formula:

VcTn = C

A more commonly used form of this formula is:

VcTn × Dxfy = C

Where Vc is the cutting speed, T is the tool life, D is the cutting depth, f is the feed rate, x and y are determined experimentally, and n and C are constants determined based on experiments or published technical data, representing the characteristics of the tool material, workpiece, and feed rate.

Continuously developing optimal tool substrates, coatings, and cutting edge preparation techniques are crucial for limiting tool wear and resisting high-temperature cutting. These factors, along with the use of chip breakers and corner radii on indexable inserts, determine the applicability of each tool for different workpieces and cutting operations. The optimal combination of all these factors can extend tool life and make cutting processes more economical and reliable.

Changing the tool substrate Tool manufacturers can change the properties of cemented carbide tools by altering the particle size of tungsten carbide within the range of 1-5 μm. The particle size of the substrate material plays an important role in cutting performance and tool life. Smaller particles result in better wear resistance, while larger particles provide better toughness. Fine-grain substrates are mainly used for machining aerospace-grade materials such as titanium alloys, Inconel alloys, and other high-temperature alloys.

Additionally, increasing the cobalt content of the hard alloy tool material by 6% to 12% can achieve better toughness. Therefore, adjusting the cobalt content can meet specific requirements for cutting processes, whether it is toughness or wear resistance.

The performance of the tool substrate can also be enhanced by forming a cobalt-rich layer near the outer surface or selectively adding other alloying elements (such as titanium, tantalum, vanadium, niobium, etc.) to the cemented carbide material. A cobalt-rich layer can significantly improve the cutting edge strength, thereby enhancing the performance of roughing and interrupted cutting tools.

Furthermore, when selecting a tool substrate that matches the workpiece material and machining method, five additional substrate characteristics should be considered: fracture toughness, transverse rupture strength, compressive strength, hardness, and thermal shock resistance. For example, if the hard alloy tool experiences edge chipping along the cutting edge, a substrate material with higher fracture toughness should be selected. In the case of direct failure or damage to the cutting edge, a solution may be to choose a substrate material with higher transverse rupture strength or compressive strength. For machining operations with higher cutting temperatures, such as dry cutting, tool materials with higher hardness are typically preferred.

Under the premise that the maximum temperature can be tolerated, when using polycrystalline cubic boron nitride (PCBN) tools for cutting hardened steel, the temperature of the tool and chips usually exceeds 1000°C.

Tool wear and tool life typically include the following types: ① flank wear; ② crater wear; ③ notch wear; ④ dulling of cutting edge; ⑤ chipping of cutting edge; ⑥ cracking of cutting edge; ⑦ catastrophic failure.

There is no universally accepted definition of tool life, which typically depends on different workpieces, tool materials, and cutting processes. One way to quantitatively analyze the termination point of tool life is to set an acceptable maximum flank wear limit (represented by VB or VBmax). The Taylor equation can be used to express the expected tool life, given by VcTn = C. A more commonly used form of this equation is VcTn × Dxfy = C, where Vc is the cutting speed, T is the tool life, D is the cutting depth, f is the feed rate, x and y are determined experimentally, and n and C are constants determined based on experiments or published technical data, representing the characteristics of the tool material, workpiece, and feed rate.

The optimal combination of evolving tool substrates, coatings, and cutting edge preparation techniques is crucial for limiting tool wear and resisting high-temperature cutting. These factors, along with the use of chip breakers and corner radii on indexable inserts, determine the suitability of each tool for different workpieces and cutting processes. The optimal combination of these factors can extend tool life, making cutting processes more economical and reliable.

Changing the tool substrate involves modifying the grain size of tungsten carbide within the range of 1-5 μm. Tool manufacturers can alter the properties of hard alloy tools by adjusting the grain size of the substrate material. Smaller grain sizes result in better wear resistance, while larger grain sizes provide better toughness. Fine-grained substrates are mainly used for machining aerospace-grade materials such as titanium alloys, Inconel alloys, and other high-temperature alloys.

Furthermore, increasing the cobalt content of hard alloy tool materials by 6%-12% can improve their toughness. Therefore, the cobalt content can be adjusted to meet the requirements of specific cutting processes, whether it’s for toughness or wear resistance.

The performance of the tool substrate can also be enhanced by forming a cobalt-rich layer near the outer surface or selectively adding other alloying elements (such as titanium, tantalum, vanadium, niobium, etc.) to the hard alloy material. A cobalt-rich layer can significantly improve the cutting edge strength, thereby enhancing the performance of roughing and interrupted cutting tools.

Additionally, when selecting a tool substrate that matches the workpiece material and machining conditions, five other substrate characteristics should be considered: fracture toughness, transverse rupture strength, compressive strength, hardness, and thermal shock resistance. For example, if the hard alloy tool exhibits chipping along the cutting edge, a substrate material with higher fracture toughness should be chosen. In cases where the tool experiences direct failure or breakage of the cutting edge, a solution may involve using a substrate material with higher transverse rupture strength or compressive strength. For machining applications with high cutting temperatures, such as dry cutting, it is generally advisable to choose a tool substrate with higher hardness.

Choosing the right coating also contributes to improving the cutting performance of tools. Current coating technologies include:

1. Titanium nitride (TiN) coating: This is a universal PVD and CVD coating that enhances the hardness and oxidation temperature of the tool.
2. Titanium carbonitride (TiCN) coating: By adding carbon to TiN, the hardness and surface finish of the coating are improved.
3. Titanium aluminum nitride (TiAlN) and aluminum titanium nitride (AlTiN) coatings: The combination of aluminum oxide (Al2O3) layer and these coatings can improve the tool life for high-temperature cutting processes. Aluminum oxide coatings are particularly suitable for dry cutting and near-dry cutting. AlTiN coatings, with a higher aluminum content, exhibit higher surface hardness compared to TiAlN coatings with higher titanium content. AlTiN coatings are typically used for high-speed cutting operations.
4. Chromium nitride (CrN) coating: This coating offers good anti-adhesion properties and is a preferred solution against built-up edge formation.
5. Diamond coating: Diamond coatings significantly enhance the cutting performance of tools when machining non-ferrous materials. They are particularly suitable for processing graphite, metal matrix composites, high-silicon aluminum alloys, and other highly abrasive materials. However, diamond coatings are not suitable for cutting steel, as the chemical reaction between diamond and steel can degrade the adhesion between the coating and the substrate.

In recent years, the market share of PVD-coated tools has expanded, and their prices are now comparable to CVD-coated tools. CVD coatings typically have a thickness of 5-15 μm, while PVD coatings have a thickness of approximately 2-6 μm. When applied to the tool substrate, CVD coatings generate unwanted tensile stress, while PVD coatings contribute to beneficial compressive stress on the substrate. Thicker CVD coatings usually significantly reduce the cutting edge strength of the tool. Therefore, CVD coatings are not suitable for tools that require a very sharp cutting edge.

The use of new alloying elements in the coating process can improve the adhesion and performance of the coatings. For example, Iscar’s 3P Sumo Tec treatment technology enhances the toughness, smoothness, and resistance to chipping of both PVD and CVD coatings. The Sumo Tec coating technology also reduces friction, thereby lowering energy consumption during machining, while improving resistance to built-up edge formation.

The Sumo Tec coating process reduces the occurrence of microcracks on the surface of the tool caused by differential shrinkage after CVD coating. Similarly, the process eliminates the formation of detrimental droplets on the surface of the coating during PVD coating.

In order to make the coating surface smoother, reduce the cutting temperature and prolong the tool life, it is possible to adopt higher cutting speeds. Another example is Iscar’s Do-Tec coating technology. This technique deposits a layer of TiAlN PVD coating on a medium-temperature CVD Al2O3 coating. This composite coating has excellent wear resistance and anti-chipping properties, making it highly suitable for various grades of cutting inserts used in high-speed cutting of cast iron. The expected cutting speeds can reach 650-1200 sfm (depending on the material type and machining conditions).

The preparation of cutting edges is crucial in many cases and can determine the success or failure of machining. The parameters for edge preparation, or edge honing, should be determined based on specific machining requirements. For instance, the requirements for edge honing differ between cutting inserts used for high-speed finishing of steel and those used for rough machining. Edge honing can be applied to cutting inserts for almost any type of carbon steel or alloy steel, while its application on inserts for stainless steel and special alloy materials may be limited. The honing amount can range from as small as 0.007mm to as large as 0.05mm. In order to enhance the cutting edge performance in harsh machining conditions, it is also possible to create micro T-shaped land by edge honing.

Generally, cutting inserts used for continuous turning and milling of most steels and cast irons require a significant level of edge honing. The honing amount depends on the grade of the hard alloy and the type of coating (CVD or PCD coating). For heavy interrupted cutting inserts, heavy honing or creating T-shaped land is a prerequisite. Depending on the coating type, the honing amount can be close to 0.05mm.

On the contrary, when machining stainless steel and high-temperature alloys, it is necessary to maintain a sharp cutting edge as these materials tend to form built-up edges. Only slight honing can be applied, as low as 0.01mm, and even smaller honing amounts can be customized. Similarly, cutting inserts for aluminum alloys require sharp cutting edges.

For example, Iscar produces various types of cutting inserts with spiral cutting edges. These cutting edges are formed by uniformly moving around a cylindrical surface along the axis. One of the advantages of this spiral edge design, which approximates a helical line, is smoother cutting motion. Unlike the cutting motion of straight edges, the spiral cutting edge simulates the motion of a helical groove end mill. The cutting edge gradually enters the cut during the “helical” motion, instead of entering all at once, which reduces chatter and improves surface finish.

Furthermore, spiral cutting edges can withstand higher cutting loads, achieve higher metal removal rates, and reduce stress. Due to the reduced cutting pressure and heat on the tool, another advantage of spiral cutting edges is the extension of tool life.

Understanding the mechanisms of tool wear and adopting new technologies to counteract them can improve tool life and machining efficiency. In today’s market, companies not only need to compete locally but also participate in global competition. Therefore, fully leveraging all competitive advantages is crucial.