CNC MILLING MACHINE

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. 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

Milling Machine Parts

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

∘

∘ 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

The cutting action 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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

A horizontal milling machine is a machining tool with its spindle parallel to the ground. It's suitable for heavy-duty milling tasks and offers better chip evacuation. The workpiece is positioned on the bed, and the cutting tool, mounted on a horizontal arbor, removes material as it rotates. This configuration is advantageous for producing grooves, slots, and complex shapes with stability and efficiency.

A vertical milling machine features a spindle that is perpendicular to the worktable, allowing it to perform tasks like drilling, plunging, and face milling. This design is versatile and well-suited for various applications, including precision cutting and shaping of metal, wood, and plastics. Vertical mills often have a quill for precise depth control, making them ideal for tasks requiring accurate vertical movements.

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.

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

It has a high rate of flank wear
High crater wear
Using a poor material cutter or machine with a loose arbour, vibration is generated. This can cause poor work on the workpiece.
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 summarizes all your learning about the applications.

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

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