A milling machine is a versatile and essential machine tool used in the manufacturing industry for shaping solid materials, especially metals. It operates by rotating a cutting tool against a workpiece to remove material and create the desired shape. The milling process is highly precise and can produce complex components with intricate contours and surfaces. Milling machine come in various types, including horizontal, vertical, universal, and CNC (Computer Numerical Control) milling machines. The choice depends on the specific application, type of material, and production requirements. In a typical setup, the workpiece is securely clamped on the machine's table, and the cutting tool is mounted on a spindle that rotates at high speed.

• The milling machine is one of the interesting machines from the past. From gear cutting & slotting to machining flat surfaces, it does everything. Sometimes, these machines are called ‘MTM’ – Multi-Tasking Machines.

In this article, we shall discuss the working principle of Milling machine with parts. We will also learn the types of milling machines, their advantages, disadvantages, and applications. The topic is so well-known that it has been a favourite theory subject in exams like SSC JE ME and GATE ME. Along with this, take the help of this article to brace yourself for the upcoming RRB JE Mechanical examinations.

What is a Milling Machine?

The process of removing material from a workpiece by advancing rotary cutters cutter into it is called milling. The process is carried out by adjusting pressure, speed of cutter head, and direction of feed.  A milling machine is a piece of equipment that removes a layer of material from the surface by using a multi-point cutting tool. With the aid of numerous cutting edges, the milling cutter revolves at high speed while rapidly removing metal. Cutters can be mounted simultaneously in groups. This machine is well-known for its high level of accuracy, even to the measure of microns. A glance at the history - In 1818, Eli Whitney created the milling machine. Here is an image of a typical milling machine in a workshop.

Fig 1: Typical Milling Machine

There are various parts and components in the Milling machine. Applicants appearing in the engineering jobs recruitment exams are advised to go through all the parts of milling machine thoroughly. Each part in a Milling machine has an identical function. It is important to understand the function of each component of milling machine. Now, the first reaction we get while looking at any machine is to identify the parts. This section helps you learn the important parts that are responsible for the functioning of these machines.

Fig 2: Milling Machine Parts

Let us refer to the milling machine diagram as we study further.

• Base: It supports every component of the machine and can dampen vibrations imparted by milling operations. Some machines have hollow bases that act as cutting fluid reservoirs. The base is generally constructed out of cast iron for rigidity.
• Column: The major supporting part, positioned vertically on the base, is the column. It is box-shaped. It contains the table feed drive mechanism and spindle. A dovetail guideway is provided at the front of its vertical face to support the knee.
• Knee: The knee, grey iron casting, slides along vertical guideways to modify the gap between the cutter and the workpiece on the table. This vertical motion is done by utilizing elevating screw provided below the knee.
• Elevating Screw: The elevating screw has threads that can provide upward and downward movement to the knee and table by rotating it using a handwheel.
• Saddle: The saddle is mounted on the knee at an angle of 90 \(^{\circ}\) to the face of the column. It can be moved transversely along the guideways on the knee.
• Table: The table, mostly made of cast iron, rests on guideways in the saddle and provides support for the workpiece. And, this workpiece placed on the worktable is moved in either of the three directions:

  • Transverse motion generated by the saddle's movement about the knee.

  • Vertical motion produced by the vertical motion of the knee. 

  • A hand wheel affixed to the side of the elevating screw provides longitudinal motion.

• Overarm: Another name for this is ‘overhanging arm’. It is installed on the top of the column and functions as a support for the opposite end of the arbour and spindle, which extends beyond the column face.
• Arbor Support: The arbour is a machined shaft that supports and drives the milling cutters. It is attached to the overhanging arm clamping in any position. We can also view it as an extension of the spindle where milling cutters are placed and firmly secured.

Working Principle of Milling Machine

Candidates can check the working of milling machine here. After understanding the working of various parts of milling machine, it is quite easy to understand the working principle. The cutting action in Milling machine is accomplished by feeding the workpiece against a rotating cutter. The workpiece is firmly secured to the table while a multitooth cutter positioned on the arbour rotates. The cutter rotates at a rapid rate, while the material is fed. 

Working Principle of Milling Machine

The workpiece can be fed vertically, longitudinally, or diagonally. The spindle speed, table feed, depth of cut, and direction of cutter rotation are the most important process factors.

Types of Milling Operations - Overview

Milling operations are generally classified into four categories based on direction of cut, operation type, cutter orientation and machining purpose. Aspirants preparing for the competitive exams are advised to refer to the classification of milling operations. It helps to understand which operation should be used as per the requirement. For more details, candidates can refer to the table provided below.

Milling Machine Operations

Based on the working principle, several operations are performed on milling machines in various industries where they are employed. Below are a few discussed in brief.

Up-Milling Operation

The up-milling is also known as conventional milling. In this, a cutter revolving in the opposite direction of the workpiece removes the metal in the form of tiny chips as shown in the figure below. The chip thickness varies from minimum at the beginning and maximum at the end as the cutter advances. The primary drawbacks of the up-milling technique are the tendency of the cutting force to lift the work from the fixtures and the poor surface finish.

Fig 3: Up-milling or Conventional Milling

Down Milling Operation

A down-milling operation is also known as ‘climb milling’. In this operation, the cutter rotates in the same direction as the feed. The chip thickness varies from maximum at the beginning to minimum at the end. There is always less friction in climb milling which produces less heat. Thin slots, lengthy cuts, and sharpening of the pieces can be easily achieved in this operation. 

Fig 4: Down Milling Operation

Plain Milling Operation

The most commonly used milling machine operation is plain milling. It is also referred to as slab milling (Hence, the labelling in the image below). The workpiece is firmly mounted on the machine before this operation. After choosing the proper speed and feed, the machine is then turned on. This operation creates a smooth and horizontal surface that is parallel to the axis of rotation of the cutter as shown in the figure below.

Fig 5: Plain or Slab Milling

Face Milling Operation

The most simple operation on a milling machine is face milling. This procedure is carried out with a face milling cutter that is rotated about an axis perpendicular to the work surface as shown in the figure below. By turning the crossfeed screw of the table, the depth of the cut can be changed. This operation creates a flat surface with the cutter positioned on a stub arbour. 

Fig 6: Face Milling

Gang Milling Operation

The operation of simultaneously milling multiple surfaces of a workpiece by feeding the table against numerous cutters with the same or various diameters mounted to the arbour is called gang milling operation.The speed of this group of cutters is determined by the cutter with the greatest diameter. The approach reduces machining time significantly and is commonly utilised for repeated tasks. 

Fig 7: Gang Milling Operation

Straddle Milling Operation

The milling operation in which two surfaces are milled simultaneously is a straddle milling operation. With two side milling cutters mounted to the same arbour, straddle milling creates flat and vertical surfaces on both sides of the workpiece. Using adequate spacing collars, the distance between the two cutters is regulated. T-slot milling is an exceptional illustration of straddle milling. The operation is used to create hexagonal or square surfaces.

Fig 8: Straddle Milling Operation

Angular Milling Operation

The process of generating an angular surface on a workpiece which is not at right angles to the spindle axis of the milling machine is known as angular milling. The angular groove may have a single or double included angle which depends on the type and geometry of the angular cutter employed. V-block manufacturing is an example of angular milling.

Fig 9: Angular Milling

Form Milling Operation

The process of creating an uneven shape like convex, concave, or any other form employing form cutters is called form milling operation. The selection of form cutters depends on the shape needed. This operation has a 20% - 30% slower cutting rate than plain milling.

Fig 10: Form Milling

Side Milling Operation

The process of side milling involves using a side milling cutter to create a flat, vertical surface on the side of a workpiece. The depth of cut is provided by adjusting the vertical feed with the help of the screw on the table.

Fig 11: Side Milling

Keyway Milling Operation

The operation of making keyways, grooves, and slots of various shapes and sizes is called keyway milling. It can be done with an end mill, a side milling cutter, a plain milling cutter, or a metal slitting saw. Keyways generally have minute dimensions of width or depth. Hence, a special tool is necessary for keyway milling like standard helical or staggered teeth cutters.

Fig 12: Keyway Milling

Profile Milling Operation

The process of replicating an intricate shape of a master die on a workpiece is known as profile milling. For milling profiles, various cutters like helical plain cutters are utilised. One of the milling cutters that is frequently used in profile milling is the end mill cutter.

Fig 13: Profile Milling

Thread Milling Operation

The precise threads are produced in small or big quantities using single or multiple thread milling cutters. The thread is finished by feeding the rotating cutter longitudinally over a distance equal to the pitch length of the thread. Thread milling operations are carried out on specialised thread milling machines. 

Fig 14: Thread Milling 

Gear Cutting Operation

A form-relieved cutter on a milling machine executes the gear-cutting operation. The cutter type may be either cylindrical or end mill. The cutter profile is made to precisely match the gear tooth spacing. A universal diving head is used to hold the workpiece while a process called indexing creates evenly spaced gear teeth on a gear blank.

Fig 15: Gear Cutting or Milling Operation

As we just discussed, this operation involves a process called Indexing. Indexing is the process of splitting the circumference of the workpiece into an arbitrary number of equal sections. The perimeter of the gear blank is divided into 'n' equal sections, and each tooth is milled individually.  Almost all gear teeth divisions can be covered by index plates through crank rotation. The index crank is always next to the spindle making it easier to index the divisions to fractions of a turn. This is done to accurately cut the gear teeth spacing. 

Fig 16: Indexing Plates 

The movement of the index crank is calculated by a formula which is given below,

Index Crank Movement = \(\frac{40}{N}\), where N \(\to\) is the required number of divisions.

Classification of Milling Machine - Overview

Milling operations are mainly classified into six types. There are various factors that determine the category of a milling machine. It is important to choose the right milling operation as per the operation to be performed. Candidates can refer to the table below for more information. 

Types of Milling Machines

As we concluded our learning about the various operations performed on the machines, it is natural to be curious about the types. Let us study all the classification of milling machines.

Horizontal Milling Machine

These machines can hold and rotate the spindle horizontally to remove material from the surface of a fixed workpiece. Cutting tools for these machines are thicker and shorter.

Vertical Milling Machine

This is a particular type of milling machine that can hold the spindle in a vertical configuration while rotating it to remove material from the surface of a stationary workpiece. Cutting tools for vertical milling machines are longer and thinner.

Column and Knee Type Milling Machine

This milling machine has a vertical column attached to the bed which has gear drives that can move the saddle and knee. On top of the knee is a saddle that can move in transverse directions, allowing the worktable to move along it.

Fixed Bed Milling Machine

These devices are rigidly constructed. The table is fixed to the guideways of the bed. There are no options for transverse or vertical adjustment and the table can only move at a right angle to the spindle axis.

Simplex Milling Machine

The spindle head can only move in one direction with this machine. Surfaces cut using these machines are flat and have a good surface finish.

Duplex Milling Machine

This machine is a combination of two machines that are arranged across from one another. They operate independently. Two spindles are used simultaneously to machine the workpiece on this machine while it functions as one unit.

Triplex Milling Machine

The spindle of these machines travels in three directions: x, y, and z axes. These machines are extremely accurate.

Universal Milling Machine

It can be modified to fit a variety of milling tasks. The table is turned to any angle between 0 \(^{\circ}\) and 45 \(^{\circ}\) on either side. In addition to the three motions previously stated, the table may have a fourth movement if it is fed at an angle towards the milling cutter. This machine is capable of producing milling cutters, spur, bevel, spiral, and twist drills.

There are other special types of milling machines apart from these which are listed below.

• Rotary table type
• Drum type
• Profile milling machine
• Planetary type
• Tracer controlled type
• Pantograph type
• CNC milling machine

Specification of Milling Machine

While working in an industry, the types of these machines are considered. But there are terms that the machine shop operators discuss before shipping or considering investing in these machines. They are called a specification of milling machines.

1. Movement range of the worktable with its size measurements
2. Maximum longitudinal movement allowed
3. Maximum speed of the spindle in rpm
4. Power capacity of the motor
5. Total floor space required
6. Overall weight of the machine

These are some of the factors considered for each machine.

Advantages of Milling Machine

Every machine should have considerable advantages to be used for a longer period. Here are some of the advantages of these machines.

1. The cutter having numerous cutting edges rotates at a high speed which results in a high rate of metal removal.
2. It also follows that these machines produce better surface finishes
3. Considering the CNC machines, the software is used to manage the milling process. Hence, it has improved accuracy and finish, increasing production overall.
4. Milling machines are known for their higher accuracy compared to other machines.

Disadvantages of Milling Machines

The machines have been noted with some disadvantages that hinder their use for some of the machining applications.

1. It has a high rate of flank wear
2. High crater wear
3. Using a poor material cutter or machine with a loose arbour, vibration is generated. This can cause poor work on the workpiece.
4. The chips formed during the machining can get clogged in the teeth of the cutter.

Application of Milling Machine

Now, the applications of these machines are almost known. Yet, this section summarises all your learning about the applications.

1. Milling machines are highly adaptable
2. Typically, they are used to manufacture flat surfaces
3. They are also capable of producing uneven surfaces
4. They can be used to bore, cut gears, create slots, and drill
5. Milling can be utilised to machine curved surfaces.

Now that we have understood important concepts about milling machines. Test your knowledge on this topic by taking any test from the SSC JE ME Test series or the GATE ME Test series. Our super teachers can help you crack the GATE ME exam. You can multiply your efforts to crack the AE/JE Mechanical exam.

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