Getting the right tools is essential when you get into milling work.
Milling tools are cylindrical rotary cutting tools used on milling machines to perform various milling operations. Cutting edges in a milling tool are usually on the face or periphery of the tool. For machining, these tools are selected based on the work material and type of cut that needs to be made.
This article explains milling tools by discussing various aspects like tool types, materials, surface coatings, holders, etc., and how to select the right tool for your job.
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Milling Tools – An overview
In milling, a multi-edged rotating cutting tool removes material from the workpiece.
These tools are mounted on a milling tool holder connected to the machine’s spindle.
Depending on the operation, milling tools having multiple cutters, varying lengths, specific coating, etc., are used for the job.
These tools are used in different types of machines ranging from manual to CNC milling machines.
Knowing different milling tools helps you choose the right tool for a job, as your choice can significantly affect the cut quality and job duration.
Types of Milling Tools
Milling machines use different tools to create various shapes and features.
| Milling Tool | Purpose |
|---|---|
| End Mill | For profiling, slotting, pocketing, and boring operations |
| Face Mill | To machine a smooth surface |
| Ball Cutter | To machine spherical contours or curves |
| Fly Cutter | Cheaper alternative to a face mill |
| Slab Milling Tool | To continuously remove a large amount of surface material |
| Side Milling Cutter | To cut parallel vertical slots |
| Staggered Milling Cutter | To cut parallel vertical slots with less chip interference |
| Concave & Convex Cutter | To machine convex and concave surface contours |
| Woodruff Milling Cutter | To cut keyways |
| Hobbing Cutter | To cut teeth, splines, or sprocket gears |
End Mill
End mills can cut materials axially and laterally as they have cutting teeth on the sides and end face.
They are usually flat at the bottom and have one or more flutes. They are made from HSS or cemented carbide and are commonly used in a vertical milling machine.
Face Mill
Face mills are similar to end mills but only have cutting edges at the sides. Multiple cutting teeth in the form of carbide inserts distribute the cutting load.
It is designed for facing operations and horizontal cuts up to a limited depth.
Ball Cutter
Ball cutters have hemispherical cutting ends and are used to machine spherical contours or curves at the workpiece edges. These tools are primarily used in machining centers.
Fly Cutter
Fly cutters make broad, shallow cuts on the workpiece. They generally have one or two cutters inserted in the cutting tool body and are used for face-milling operations.
Face mills perform a better job than fly cutters, but they tend to be more expensive.
Slab Milling Tool
Slab mill tools have straight or spiral cutters at their periphery.
They are used on horizontal milling machines to remove large amounts of surface material to produce flat shapes.
Side Milling Cutter
The side milling cutter utilizes cutting teeth on the sides and the periphery. They are used for straddle milling operations and for cutting slots.
Staggered Milling Cutter
Staggered mill cutters are similar to side mill cutters with teeth at the periphery and side.
The side teeth are arranged in a staggered manner which helps to prevent chip interference. It is suitable for milling slots having more depth than width.
Concave & Convex Milling Cutter
Concave and Convex milling cutters are formed cutters designed to mill convex and concave surface contours equal to a semicircle or less.
The required diameter of the circular form determines the cutter size.
Woodruff Milling Cutter
Woodruff cutters have cutting teeth on the periphery of a disc connected to a straight shank.
They have concave sides that provide clearance. These cutters are used for cutting keyways in shafts.
Hobbing Cutter
A hobbing cutter has helical cutting teeth with grooves that aid in cutting and chip removal. They are used for cutting teeth in the workpiece.
Specially designed hobs are also available for cutting splines and sprocket gears.
Materials Used to Make Milling Tools
Milling tools are made of different materials, each offering specific advantages to the cutter, thereby helping in the machining process.
The primary considerations for selecting the tool material are workpiece material, production quantity, quality, and type of machining.
Following are some of the common cutter materials used in milling tools.
| Material | Benefits |
|---|---|
| Carbon Tool Steel | Most affordable Best at low-speed jobs |
| High-Speed Steel | Hard Resistance towards wear Maintains a sharp cutting edge longer |
| Cemented Carbide | Very hard and ductile High cutting speed Good surface finish |
| Ceramic | High-speed finishing and roughing jobs Extended tool life Increased strength and toughness |
| Cubic Boron Nitride | Offers better thermal and chemical stability |
| Diamond Tool | Extremely high thermal conductivity and melting point Wear resistant Low friction |
Carbon Tool Steel
Carbon tool steel is an alloy of iron and carbon, having other elements in trace amounts to improve its properties.
It is one of the cheapest materials for making cutting tools and is suited for low-speed jobs.
These alloys often contain different trace quantities of manganese, silicon, and copper. In some cases, chromium and vanadium are added to improve hardness and grain size.
Tools made from these materials resist abrasion and maintain a sharp cutting edge.
Carbon tool steel cutters are used to machine soft metals such as aluminum, copper, magnesium, etc.
They are limited to working at temperatures under 250℃. If the tool heats beyond this threshold, it’ll lose its hardness, which impacts the milling job.
High-Speed Steel (HSS)
High-speed steel is a high-carbon steel alloy that combines steel with molybdenum or tungsten and small amounts of chromium and vanadium.
The alloying elements considerably improve their properties, increasing hardness, wear resistance, and efficiency at high operating temperatures.
Heat treatment of HSS is essential for enhancing its properties as heating changes the steel’s internal structure, resulting in increased hardness.
It can retain the tool’s hardness up to 650℃, but manufacturers recommend using a coolant for increased tool life.
HSS tools can maintain their sharp cutting edge even after a long job cycle. You can even re-sharpen them multiple times, providing a long tool life.
Several grades of HSS milling tools are available, each having varying properties.
Cemented Carbide
Cemented carbide has a high hardness and strength making it optimal for cutting tools.
It is made by mixing carbide particles with a binding material such as cobalt.
The metal binder provides ductility to the tool while carbide contributes to the hardness resulting in long-lasting resilient tools.
Carbide tools have a high cutting speed and can retain hardness at temperatures up to 1000°C.
Machining operations using these tools result in a better surface finish. It is used in high-volume machining and for cutting rigid materials such as stainless steel.
Due to high material cost, carbide tools are usually produced as inserts, while the shank is made of carbon tool steel to reduce tool cost.
This results in cost savings without compromising cutting performance.
Ceramic Tool
Ceramic tools usually are made of aluminum oxide or silicon nitride.
Tools made of aluminum oxide are used for high-speed finishing operations, while silicon nitride tools are used for rough machining jobs.
Many other additives are added to improve ceramic tools’ strength and toughness, thereby increasing their tool life and productivity.
Ceramics tools provide excellent high-temperature performance by retaining their hardness and chemical inertness. They are also highly resistant to corrosion and wear.
They are remarkably faster than HSS and suitable for dry machining as coolant is not required. This is due to their lower friction coefficient at the cutting interface and low thermal conductivity.
Cubic Boron Nitride
Cubic boron nitride (CBN) is an inorganic compound of boron and nitrogen which exhibits varying properties in different forms.
The cubic crystalline form is similar to diamond and slightly softer but possesses better thermal and chemical stability.
It does not exist naturally but is produced in laboratories and widely used in abrasive components and cutting tools.
Tools made of boron nitride can be used for precision grinding and cutting hard materials due to their lower wear rates and ability to maintain tolerances.
It stays thermally and chemically stable at temperatures up to 1300°C. It also forms a layer of boron oxide at the surface, which prevents further oxidation at high temperatures.
CBN tools should use oil-based coolants as the oxide layer dissolves in water, increasing the wear rate.
Diamond Tool
Diamond is a hard material with extremely high thermal conductivity and melting point.
Its high strength, wear resistance, and low friction coefficient makes it suitable to use as an abrasive and in cutting tools.
The diamond grains are bonded at the cutting edge using sintered metal alloys, resin, ceramic, or other bonding materials.
Diamond-coated milling tools provide a good surface finish and result in a highly precise machining operation with close tolerances.
It is used to machine tough materials such as carbide alloys, ceramics, and non-ferrous metals such as copper and its alloys.
These tools are not suitable for use with steel as diamond does not remain chemically inert at high temperatures and may react with iron and other metals.
Tool Coating
Most cutting tools come with some form of coating to improve their surface properties like hardness, resistance towards wear, surface oxidation, fatigue, and thermal shock.
Improved performance and longer tool life are also achieved due to the surface coatings applied to the milling tools.
The thermal insulation effect of coatings improves hot hardness. Coatings also aid in lubrication by providing a smooth cutter surface that minimizes friction and improves chip removal.
| Material | Benefits |
|---|---|
| Titanium Nitride | Economical Improves tool life Resistance to wear and abrasion |
| Aluminum Titanium Nitride | Allows for high-speed machining of hard metals Resistance to thermal shock and oxidation High hardness |
| Titanium Aluminum Nitride Nano | Superior tool life Increased run time on ferrous metals Hard and tough |
| Zirconium Nitride | Improved lubricity (less friction between the tool and stock) Hard and resistant to abrasion |
| Titanium Diboride | High strength and hardness Resistance to erosion Good adhesion to the cutter Best to work on aluminum and magnesium alloys |
| Diamond Coatings | To work on non-ferrous materials like graphites, ceramics, composites, carbides, etc. |
Coatings Suitable for Cutting Ferrous and Exotic Materials
The following coating materials provide good cutting performance to the tool when cutting ferrous and exotic materials like ceramics, hard plastic, etc.
Titanium Nitride
Titanium nitride is a general-purpose coating having a golden color and is one of the most common materials used to coat cutting tools.
It improves the tool’s life, wear resistance, abrasion resistance, and cutting performance.
Aluminum Titanium Nitride
Aluminum titanium nitride is a composite coating. It is primarily employed on tools used for high-speed machining of hard metals in harsh conditions.
It provides high hardness along with resistance to thermal shock and oxidation. Adding aluminum also results in the retention of hot hardness.
The high hot hardness allows for dry operation with high feed rates and improved tool life.
Since the aluminum oxide layer is produced at high temperatures, it has reduced thermal conductivity.
Titanium Aluminum Nitride Nano
Titanium Aluminum Nitride Nano (TiAlN Nano) is a premium blue-colored coating that provides superior tool life and cycle times on ferrous metals than other coatings.
When titanium aluminum nitride is mixed with silicon, it produces a nanocomposite coating, further improving the tool’s hardness and toughness.
TiAlN Nano coating is suitable for machining more rigid materials such as hardened steels, tool steels, etc. It is not recommended for machining aluminum.
Coatings Suitable for Non-Ferrous and Non-Metallic Materials
The following coatings are best for milling tools used to machine non-ferrous (aluminum, copper, titanium, etc.) and non-metallic materials.
Zirconium Nitride
Zirconium nitride coatings improve the tool’s hardness, abrasion resistance, and lubricity.
The coating forms a hard ceramic layer on the tool’s surface, about 2 – 5 microns thick.
It improves the cutting performance on non-ferrous materials and is widely used to coat cutters, bits, etc.
Suitable materials include non-ferrous alloys such as brass, copper, bronze, and aluminum. It improves tool life by up to 5 times over non-coated tools.
Titanium Diboride
Titanium diboride is a ceramic with high strength and hardness. It provides exceptional resistance to erosion during machining and has good adhesion to the substrate.
The coating minimizes material build-up at the cutting edge resulting in increased tool life. It is recommended when working on aluminum and magnesium alloys.
Most other coatings react with aluminum during cutting, but titanium diboride has a low affinity towards aluminum.
Diamond Coatings
Diamond coatings come in various forms and structures. Due to its low operating temperature range, it is considerably higher in cost and suited for specialized applications with non-ferrous material.
It works on graphites, ceramics, composites, carbides, and other non-ferrous metals and alloys such as aluminum, copper, brass, etc.
Structure of a Standard Milling Tool Holder
The tool holder is the machine part that connects the tool to the milling machine.
It firmly holds the tool to effectively transfer the cutting force to the workpiece with utmost accuracy and accounts for runout and balance of the milling operation.
There are three major components on a tool holder:
Taper
Tapper is a cone-shaped part on the tool holder that is connected to the spindle.
The tool holder is selected based on the spindle design, as a machine only accept tool holder having a specific type of tapper.
Many tappers are available based on the mounting style, such as Morse taper, NMTB taper, etc.
Flange
A flange is a gripping component of the tool holder.
Robotic components on machining centers, like the automatic tool changer (ATC), use the flange to grab and move the tool from the spindle.
Collet Pocket
A collet is a segmented band or sleeve used to tighten a shaft. The collet is inserted in the pocket and tightened using various collet nuts.
Types of Milling Tool Holders
| Tool Holder | Benefits |
|---|---|
| Collet Chuck | Supports tools of multiple sizes Can be customized Best for high-accuracy finishing operations |
| End Mill Holder | Can hold large and heavy tools used in heavy machining jobs Higher grip force than collet chucks |
| Hydraulic Tool Holder | Offers a high degree of precision Easy to work with Ideal for tools like drills, reamers, end mills, etc. |
| Milling Chucks | Its symmetrical design provides high accuracy and good balance. Can hold many types of tools Has minimal runout and axial movement |
| Shrink Fit Holders | Uniformly grabs the tool shank because of the thermal effect High gripping force Only requires minimal accessories |
Collet Chuck
Collet chucks are versatile tool holders designed for using multiple types and sizes of cutting tools. It uses a slotted collar to hold them firmly.
They are available in different sizes and types and can be custom-made for specific applications. These tool holders are preferred for high-accuracy finishing operations.
Single-angle and double-angle are the two types of collet chucks available.
ER collets are an example of a single-angle system having high concentricity and balance. They are suitable for drilling and light milling operations at high speeds.
Double-angle collet systems are used where sufficient clearance is not available. It is a simple chuck design but lacks the concentricity and grip required for high-speed precision operations.
End Mill Holder
End mill holders are used to hold milling tools for heavy machining operations. They have a higher grip force than collet chucks, and it rigidly holds the tool in place using set screws.
They are available in various lengths and have a tapered design with a small nose diameter resulting in improved rigidity and reduced vibrations.
Hydraulic Tool Holder
Hydraulic tool holders are used for a process where a high degree of precision is required. Hydraulic fluid forces help center the tool with uniform pressure, resulting in concentric and rigid tool holding.
The tool is placed in the holder, and the screw is tightened, which causes the hydraulic pressure to rise. This pressure increase causes the sleeve to expand and hold the tool shank.
Hydraulic tool holders are very efficient as they allow for high removal rates and are ideal for tools requiring high accuracy, such as drills, reamers, end mills, etc.
Milling Chucks
Milling chucks are rigid and accurate tool holders with a high gripping force. They have a symmetrical design which provides high accuracy and good balance.
They are designed for a specific tool size, and reducing collets are used for lower-diameter tools. They are highly versatile in terms of the types of tools they can use.
It has a straight collet that provides uniform clamping force, increasing rigidity and having minimal runout and axial movement.
They also have a simple locking system that locks the tool in place with high force. These properties make it suitable for heavy milling applications at high speeds.
Better tool-holding properties also contribute to longer tool life and better surface finish. These holders are preferred for large-diameter milling tools.
Shrink Fit Holders
Shrink fit holders have an undersized hole for the tool. The hole’s size is thermally expanded to fit the tool in place.
Once the holder cools down, it uniformly grabs the tool shank to provide an even clamping force on the tool.
Uniform force distribution results in high concentricity, and material contraction provides high grip force. It has a considerably higher grip force than hydraulic holders with similar levels of runout.
Induction heaters are used to heat the holder leading to reduced tool changing time.
The tool holder only requires minimal accessories, but you have to invest in the tool holder’s heating and cooling equipment for efficient operation.
Hence it requires a high initial investment, but the benefits outweigh the cost as it can significantly improve production rate, tool life, and quality.
How to Select The Right Milling Tool for Your Job – Tool Selection Guide
Selecting the right tool can contribute a lot towards achieving the project goals.
There are a variety of factors and considerations which play a significant role in choosing the right tool.
A lot depends upon the material to be worked upon and the shape and form of the workpiece required.
To determine the right cutting tool, you must also consider the average cutting speed, direction (plunge or feed), and the required finish.
Investing in a more robust cutting tool contributes to the project’s overall quality while preventing breakage and slippage during the process. Also, they minimize wear and result in accurate machining.
Milling tools with a higher chip clearance make rougher cuts, while tools with multiple flutes remove less material and are best at finishing operations.
For a balanced setup, use rough tools at the start to remove material and then run a final round using a finishing tool.
Note that setting the correct speed and feed rate depending on the material and cutter in hand is essential for accurate milling.
Generally, the feed rate (IPM) is determined by multiplying speed in RPM, chip load in inches per tooth (IPT), and the number of flutes.
IPM = RPM × IPT x No. of flutes
IPT, also known as chip load, is the measurement of the amount of material that a cutter removes per revolution of the cutting tool.
Selecting the right tool holder can positively influence the results of machining.
It should provide the necessary gripping force at the required speed apart from having an optimal runout and balance for the operation.
Milling tools can cost anywhere between $10 and $1,000, or even more if you are looking for some fancy options.
Generally, adjustable tools like a face mill with replaceable inserts cost more than tools having inserts welded into them.
Final Thoughts
Milling is a major cutting operation in various manufacturing processes.
The advancements in tool design, material, coatings, and machines have greatly improved cutting performance at a reduced cost.
Cutting tools having better operational capabilities are usually costlier, but they compensate for the cost through higher cutting speed, quality, accuracy, longer tool life, etc.
Investing in the right tool having higher quality and grade gives dividends in the long run by significantly improving the quality of the finished goods.
It is crucial to understand and have knowledge of new developments in tooling to perform an efficient and economical operation.
Frequently Asked Questions
What are the types of milling machines?
There are mainly four types of milling machines – knee type, column type, fixed bed type, and planar. The knee and column-type machine move the workpieces in the vertical direction, and the saddle moves in the transverse direction. The fixed bed-type milling machine has a moving spindle that can move in one or all directions. Lastly, planar-type milling machines provide vertical movement to the tool, and the table moves to give feed.
How to select a cutting tool based on the milling machine?
The right cutting tool for a milling machine is decided based on the machine’s power, rigidity, operational capabilities, tool holder, etc.
What are the limitations of a milling machine?
The limitations of a milling machine depend on its work area, the number of axes, tool clamping range, spindle speed, power of the motor, machine’s overall construction, weight, etc.
