What Is Milling? In-Depth Explanations of Machining Methods, Types, Features, and Cutting Machines

What Is Milling?

Milling is a machining method in which parts of the workpiece are cut or ground off with a milling tool that rotates at high speed. First, let's review the basics of milling.

Milling Principles

In milling, a rotating milling tool is applied to the workpiece to cut into it. Milling is also called "interrupted cutting" because the cutting tool repeatedly comes into contact then breaks contact with the workpiece.

Square, round, box-shaped, and many other shapes of workpieces can be machined, using a variety of cutting techniques, such as plane cutting and grooving. Three-dimensional products can also be made using machine tools with three-dimensional moving axes.

Difference Between Milling and Lathe Machining

In lathe machining, the workpiece is attached to the spindle and rotated while the stationary cutting tool is pressed against it to machine the workpiece into any shape. Because it features a rotating workpiece, lathe machining is suitable mainly for manufacturing cylindrical parts and machining inner diameters.

What Is the Difference between the Japanese Term "Frase" Milling and Milling?

The Japanese term "frase" milling can be translated as "milling" in English-speaking countries. Milling has its origin from the verb "to mill" or "to grind," giving rise to the terms coffee mill and pepper mill in English. Hence,milling is used synonymously with "frase" milling, and the machine tool that performs milling, a "frase" milling machine in Japanese, is called a "milling machine" in English.

What Is the Difference between Milling and Machining?

Machining is a general term for machining methods that use a machining center. A machining center is a machine tool based on an NC milling machine, and is equipped with various functions such as numerical control (NC) and automatic tool changers. Therefore, in a broad sense, machining can be considered a component of milling.

Basic Cutting Conditions for Milling

For milling, it is necessary to set the machining conditions and cutting tools appropriately based on the machining details. In this section, we will explain the cutting conditions for milling.

Cutting Speed and Rotational Speed of Tool

Cutting speed is the speed at which the milling tool comes into contact with the workpiece. In general, cutting speed is set according to various factors such as the type and hardness of the workpiece material and the material of the cutting tool. While faster cutting speeds increase machining efficiency, they also increase tool wear as well as reduce machining accuracy. Cutting speed is expressed by the following formula.

Calculation formula for cutting speed: V (m/min) = π × D × N / 1,000
π: pi
D: outer diameter of milling tool (mm)
N: tool rotational speed (min^-1)

For example, if a 10 mm diameter tool rotates 1,000 times per minute, the cutting speed can be calculated as V = π × 10 mm × 1000 rotations/min ÷ 1000 = 31.4 m/min.

Cutting Depth

Cutting depth refers to the length of the part of the milling tool that comes into contact with the workpiece in the tool's axial direction. While a larger cutting depth shortens machining time, it generates greater frictional resistance, leading to tool wear and heat.

Cutting depth is used when calculating the material removal rate (Q).

Calculation formula for chip removal rate: Q (cm³/min) = ap x ae x F / 1,000
ap: cutting depth (mm)
ae: cutting width (mm)
F: feed rate per minute (mm/min)

Table/Workpiece Feed Rate

Feed rate refers to the speed at which the tool moves over the workpiece. This speed varies depending on the type and hardness of the material being machined as well as the type of tool being used. The general formula is as follows:

Calculation formula for table feed rate: F (mm/min) = f × z × N
f: feed per tooth (mm)
z: number of teeth
N: tool rotational speed (min^-1)

For example, if a tool with 4 teeth is fed at 0.05 mm per tooth and the tool rotational speed is 1,000 rotations/min, the feed rate is 4 × 0.05 mm × 1,000 rotations/min = 200 mm/min.

Tool Rotation Direction

There are two types of tool rotation direction: upcut and downcut. The cutting conditions will differ depending on which direction is used.

Upcut is a machining method in which the rotation direction of the cutting tool and the feed direction of the table are opposite to each other. The amount cut by the tool starts from the minimum going to the maximum.
Because it cuts thoroughly, the resulting surface finish is good, but the tool wears out quickly.
Downcut is a machining method in which the rotation direction of the cutting tool and the feed direction of the table are in the same direction. The amount cut by the tool starts from the maximum going to the minimum.
Hence, frictional tool wear is relatively slow, resulting in longer tool life.
For machining centers and other machine tools, downcutting is generally recommended as it provides longer tool life and less vibration.

Main Machine Tools Used for Milling

Various types of machine tools are used for milling. We will introduce the major machine tools that can perform milling below.

Universal Milling Machine

A universal milling machine is a machine tool for performing machining operations manually. A big feature of these machine tools is that the skill and experience of the machine operator greatly affect machining accuracy because milling cuts are performed while manually adjusting the position of the tool. They are less expensive than computer-controlled machines and simple machining can be performed immediately, making them suitable for the production of small lots and prototypes.

NC Milling Machines

An NC milling machine is a machine tool that uses numerical control technology to perform milling. They are more expensive than universal milling machines and require programming knowledge, but are capable of handling even complex and high accuracy machining. Since the tool position is automatically controlled according to programmed commands, they are suitable for continuous production and mass production.

Machining Centers

A machining center is a machine tool that combines the function of an automatic tool changer (ATC) with an NC milling machine. In addition to milling, they can automate tasks such as drilling and tapping, enabling the machining of a variety of shapes without human intervention.

NC Gear Cutting Machine

An NC gear cutting machine is a machine tool that is specifically designed for manufacturing gears in milling. High-accuracy gears can be manufactured by programming the gear shape, size, number of teeth, and other conditions into the computer. As gears are needed in a wide variety of fields, including the automobile and machinery industries, NC gear cutting machines are widely used as machine tools that underpin the manufacturing industry in Japan.

Types of Tools Used in Milling

A variety of tools are used in milling. Face mills, end mills, among others are widely used. We will explain the types of milling tools below.

Face Mill

A face mill is a cylindrical tool with multiple teeth on the outer edges called tips, and is used to smoothly cut the surface of a workpiece. It is one of the most commonly used tools in milling machines.

End Mill

An end mill has cutting teeth at the end and sides of the tool, and is used for a wide range of applications, including groove machining, side cutting, and hole machining. It is shaped like a drill and can machine a smaller area at a time compared to a face mill, making it suitable for machining complex shapes and fine details.

Groove Mill

A groove mill is a tool for creating grooves on the surface of a workpiece. It has a disk-shaped blade at the tip. The disk has cutting teeth on its circumference, which are used to trace and cut into the workpiece to create grooves. There are several types of groove mill depending on its shape, including the T-slot cutter and dovetail cutter.

Flat Mill

A flat mill is a tool used mainly for machining flat surfaces. It has teeth on the sides of a cylindrical tool and is generally used attached to a horizontal mill. It can evenly cut a large area, making it suitable for finishing the surface of large workpieces and parts that require flatness.

Types of Milling

Milling has a wide range of applications and a variety of machining methods. In this section, we will introduce the major milling methods, such as plane cutting, side cutting, and step machining.

Plane Cutting

Plane cutting is a machining method that cuts into the plane of a material evenly to make it flat, using tools such as face mills and end mills. It is used to create a part's reference surface or to machine a material to a specified thickness.

Side Cutting

Side cutting is a method of machining the sides of a workpiece, and is used to adjust the length and width of the workpiece. An end mill is generally used for cutting, with the teeth on the periphery tracing the sides of the workpiece.

Step Machining

Step machining is a machining method to create steps on parts of the workpiece. It is used to create uneven surfaces or steps in parts, with tools that vary depending on the shape of the corner, such as corner R (arc-shaped corner) or corner C (C-shaped corner).

Groove Machining

Groove machining is used to machine grooves and slots, using tools such as end mills and groove mills. There are various types of groove machining, including T-groove machining, where a groove with a T-shaped bottom is made on the flat part of the workpiece, and dovetail machining, which creates a triangular-shaped groove.

Hole Machining

Hole machining is literally the technique for drilling holes in a workpiece, and is used to machine holes for bolts and screws. Making the inside of a hole into a shape for accommodating a screw is called tapping, making a surface smooth is called reaming, and widening the top part of a hole so that the head of a bolt can fit in is called counterboring.

3D Machining

3D machining is an advanced technology for performing machining by simultaneously controlling three axes X/Y/Z. It is suitable for parts with three-dimensional shapes and curved surfaces, and is generally used in combination with a CAD/CAM system.

Milling Examples

How is milling actually performed in manufacturing? In this section, we will look at some examples of milling.

Multi-face machining

Multi-face machining refers to the machining of multiple faces of a workpiece. Typical examples are 6-face, 5-face, and 4-face machining. When performing multi-face machining with a universal milling machine, the workpiece is re-mounted each time a surface is finished. In contrast, machining centers and multi-tasking machines can perform multi-face machining without the need to load and unload the workpiece.

Hexagonal Machining

Hexagonal machining is a method for machining the sides of a workpiece or part of the sides into a hexagonal shape. It is used for fastening parts such as bolts and nuts.

Helical Machining

Helical machining is a machining technique in which the tool is moved to cut in a circular or elliptical path on the X/Y-axes and in the positive or negative direction on the Z-axis. The term helical in helical machining can be translated into the Japanese word for spiral-shaped. Literally, helical machining is characterized by gradually cutting into the workpiece along a spiral path.

Grooving

Grooving is a machining technique to provide some recess in a corner when a pin angle (sharp angle) cannot be machined. When grinding inner surfaces, curved sections may remain at corners if there is no recess. Grooving is carried out at corners to prevent these curved sections.

Features and Advantages of Milling

Milling is known for its ability to machine with high accuracy across a wide range of uses, among other features. In this section, we will introduce the advantages of milling.

Capable of High Accuracy Machining

A major feature of milling is that it is highly accurate. In particular, NC milling machines and machining centers can perform machining under computer control, making it possible to maintain high productivity while minimizing product variations.

Capable of Machining a Variety of Shapes

As we have seen, there are numerous machining techniques in milling. Any shape can be machined flexibly using milling tools. High flexibility and freedom are the defining features of milling. Machine tools that are capable of 5-axis machining can also perform complex three-dimensional cutting.

Reduces Machining Steps and Increases Work Efficiency

With NC milling machines and machining centers, the machining steps are directed by a program. Therefore, machining work can be automated once a program is created. Machining centers can also switch cutting tools automatically, making it possible to significantly reduce the task of changing tools.

What are the Disadvantages and Precautions When Milling?

While milling has many advantages, it also has some precautions. Let's check the disadvantages characteristic to milling and precautions when machining.

Some Shapes Cannot Be Machined

Milling is versatile and capable of machining a wide array of shapes, but it is not all-purpose. For example, milling cannot machine the inside of a corner into a pin angle. Because the tool is rotating, it inevitably creates a corner radius (corner on a circular arc).

Tools and Fixtures Appropriate for the Material Are Essential

When milling, the tools and fixtures for use must be selected correctly to suit the material to be machined. Incorrect tool selection or fixing method can lead to machine malfunction or the workpiece being dislodged, or in the worst case, an accident. For these reasons, it is vital to perform machining safely and efficiently when milling, taking into account the work process, compatibility of workpiece and tools, and other factors.

Toward More Efficient Manufacturing for Small and Medium-Sized Businesses

Due to labor shortages, the manufacturing industry needs to transform, in order to reduce manpower and boost productivity. How can manufacturing businesses address this issue?

Dealing With the Difficulty of Acquiring Skilled Workers

The skills and experience of skilled workers form the core of product manufacturing. Losing these will have a direct impact on quality. Because of this, an increasing number of companies are addressing the urgent issue of manpower shortage by extending the employment period of skilled workers or re-employing them, among other measures.

Promoting Automation

With the progress of technological developments in machine tools, not a few companies are solving their manpower shortage through capital investment. As we have already seen, labor-intensive processes can be simplified and productivity improved by adopting machining centers and multi-tasking machines. Moreover, automation can help reduce employee workload, allowing resources to be allocated to more creative and high-value work. Such aggressive capital investment will lead to sustainable growth for the company.