What Is Milling?
Milling is a type of cutting process in which a tool is rotated to cut an object.
A cylindrical milling tool with multiple cutting edges is rotated at high speed to cut a workpiece (metal, plastic, rubber, etc.) fixed to a table, also known as a milling machine. The word “milling” is derived from German, French, and Dutch, and is also called a milling cutter in English.
The tool and the workpiece to be milled are placed on the milling machine, and the tool and table are moved up, down, left, right, back, and forth to perform the process.
There are various types of milling machines and milling tools, and by using different types depending on the shape to be machined. Various types of machining can be performed, such as machining of flat surfaces, sides, and grooves of objects, and drilling.
Uses of Milling
Because it is possible to cut shapes with high precision according to drawings, milling is used for gears, molds, and precision machine parts that require high dimensional accuracy.
Types of Milling Processes
The following five types of milling are the main types of milling.
1. Plane Milling
This is a machining process to cut a flat surface of a fixed workpiece. Front-face milling machines, flat milling machines, and end mills are mainly used. The front-face milling machine is used for machining large areas with high accuracy, the flat milling machine is used for rough machining and semi-finishing where accuracy is not required, and the end milling machine is used for machining small areas and complex machining.
2. Side Milling
This is a machining process to cut the side of a workpiece fixed to a table. Front milling machines, side milling machines, and end mills are mainly used. Horizontal milling machines use front and side milling machines, while vertical milling machines use end mills.
3. Step Machining
This is a machining process to cut one step down from the flat surface of a fixed workpiece. Front milling machines, side milling machines, and end mills are mainly used. When the area of the step is large or the step is low, front milling is mainly used. When the area is narrow or the step is high, side milling or end milling is mainly used.
4. Groove Machining
Grooving is the process of cutting a groove on the flat surface of a workpiece. The groove is cut to a specified depth and then the table is moved back and forth, left and right. There are not only grooves but also the following types of groove shapes.
- T-groove: A groove that looks like a reversed T of the alphabet.
- Dovetail Groove: Trapezoidal groove that widens toward the bottom
- Pocket: An indentation on a flat surface.
- Slit: A narrow, deep groove.
- Keyway: groove for fitting machine parts
Milling tools include end mills, T-slot milling machines, dovetail milling machines, metal saws, and split milling machines. T-slot milling machines are used in conjunction with end mills for cutting t-slots, dovetail milling machines are used for cutting dovetail grooves, and metal saws and split milling machines are used for slits.
5. Hole Milling
Hole drilling involves drilling holes using a drilling tool and boring to widen the holes created by drilling. Milling machines and end mills are used.
Drills and end mills are generally used for drilling, while end mills are used for boring and drilling that requires precision.
Types of Tools Used for Milling
Milling tools used in milling operations are mainly made of high-speed steel or cemented carbide and are broadly classified into the following two types, depending on how they are mounted on the milling machine.
- Bore type: A type with a hole in the center of the tool
- Shank type: Rod type
Milling tools are classified into the following 7 types according to the “shape to be cut” and “application”.
1. Front Face Milling Cutter
A facemill is a bore-type milling tool with a circular cutting edge. Facemills are characterized by their ability to machine large areas with high precision and efficiency. On the other hand, they are not suitable for machining small areas.
Front-face milling tools are often used mainly for cutting flat surfaces on vertical milling machines. Another feature of front-face milling tools is that they can be used many times by changing the tip of the cutting tool.
2. Flat Milling Machine
Flat milling is a bore-type milling tool with the cutting tool attached to the periphery of a cylinder at the end of the tool. Flat milling is mainly used on horizontal milling machines.
It is characterized by its high efficiency in frontal cutting and faster working speed than frontal milling. On the other hand, they are less precise than frontal milling machines, so they are often used for rough machining and semi-finishing where precision is not required.
3. End Mill
An end mill is an elongated, drill-like, shank-type milling tool. The end mill is the only milling tool that is versatile enough to handle all milling operations. It is often used when the cutting area is small or when performing complex machining operations due to its low cutting efficiency.
There are various types depending on the number of cutting edges and the shape of the cutting tool, as shown below.
- 2-flute end mill
- 3-flute end mill
- Multi-flute end mill
- Taper end mills
- Ball end mills
- Square end mill
4. Side Milling Cutter
A side milling cutter is a bore-type milling tool with cutting edges on the periphery and sides of the disk at the tooltip. There are different types of cutting edges, such as “normal cutting edge,” “rough cutting edge,” and “staggered cutting edge,” and they are used according to the intended use. It is often used mainly for grooving and side cutting.
5. Square Milling Cutter
Angle milling is a bore-type milling tool with an angled cutting edge around the circumference of the disk at the tooltip. It is often used for V-grooving on horizontal milling machines and dovetail milling on vertical milling machines.
6. Drill
A drill is a shank-type milling tool with a cutting tool attached to the end of the tool. It is similar in shape to an end mill, but unlike an end mill, it does not have a cutting edge on the side of the drill and cannot machined on the side. It is characterized by its specialization in hole drilling.
7. Groove Milling Cutter
The groove milling cutter is a bore-type milling tool with a cutting edge on the periphery of the tooltip. It is specialized for groove milling. Various types of tools are used depending on the shape of the groove, as shown below.
- Groove, pocket: end mill, side milling tool
- T-slot machining: T-slot milling
- Dovetail milling: Dovetail milling
- Helical milling: End mills
- Keyway milling: end mills, slot milling
- Half-moon keyway milling: Semi-circular keyway milling
- Slitting: Metal saws, mortise, and tenon milling
Types of Milling Machines Used for Milling
There are six types of milling machines used in milling operations. The differences are in the direction in which the tool is mounted, the direction of travel, and the control method.
1. Vertical Milling Machine
This is a type of milling machine in which the tool is attached perpendicularly to the ground. It is often used for flat-surface machining and R-face machining. Depending on how the tool and table move, they are classified into the following three types.
- Bed type: Both the table and the tool move
- Knee type: Only the table moves
- Ram Type: Bed-type milling machine with tool moving back and forth
The knee type is characterized by good visibility that allows machining at eye level and is suitable for machining light materials or when machining and measuring are performed multiple times. On the other hand, the bed type is less prone to dimensional changes caused by repeated use and is suitable for producing things in large quantities.
2. Horizontal Milling Machine
This is a type of milling machine in which the tool is attached horizontally to the ground. Chips from the milling process tend to fall directly to the bottom of the machine, resulting in high cutting efficiency. It is often used for grooving, deep drilling, side milling of plate materials, and cutting.
3. Universal Milling Machine
This type of milling machine has a table that can rotate horizontally and is equipped with both vertical and horizontal functions. In addition to being able to perform both vertical and horizontal machining with a single machine, it is also capable of performing complex machining, such as gear cutting and drilling.
4. Tabletop Milling Machine
This is a small tabletop milling machine. They are often used at home for DIY, etc. Most models are vertical type.
5. NC Milling Machine / CNC Milling Machine
This is a milling machine that can perform machining automatically by numerical control.
There is also a CNC milling machine, which is a computer-controlled milling machine that uses a computer and a program to control the machining process. The advantage is that the operator does not need to watch the machine at all times because machining can be done automatically, but the disadvantage is that it takes time to learn how to operate the machine and develop the program.
NC milling machines and CNC milling machines are available in the following types:
- Vertical NC milling machine
- Horizontal NC milling machine
- Desktop NC milling machine
6. Machining Center
Machining centers are NC/CNC milling machines with the added function of automatically changing tools. Some machining centers are capable of position control not only vertically, horizontally, and vertically but also in five axes, including rotation, making it possible to cut more complex shapes.
While machining centers can be programmed for automatic machining without any operator intervention, their high cost and the time required to learn how to operate them and develop programs are their disadvantages.
The following types of machining centers are available.
- Vertical machining centers
- Horizontal machining centers
- Double-column machining center
- 5-axis machining center
The difference between Milling and Turning
In cutting operations, lathe turning is often confused with milling. Both are cutting processes that use rotation to cut the workpiece, but the difference is in what is rotated and the intermittency of the process.
In lathe turning, the workpiece is rotated and shaved by hitting it against a stationary tool, whereas in milling, the tool is rotated and shaved by hitting it against a stationary workpiece. In lathe turning, the workpiece is always in contact with the stationary tool (continuous cutting), whereas in milling, the stationary workpiece and the tool are in repeated contact and non-contact (intermittent cutting).