3 February 2016

What is milling | Operations | Applications | Advantages

Milling is a type of machining during which the rotary motion is always performed by a tool, and feed motion is performed by a tool or by a workpiece. The machines that are used for milling are called milling machines

Milling is the most common form of machining, a process of material removal, which by cutting off the unwanted material can create a variety of features on a part. The workpiece is a piece of pre-shaped material that is secured to the fixture that is attached within the milling machine to a platform itself. The cutting toil with sharp teeth is also secured in the milling machine and rotates at high speeds. By feeding the workpiece into the rotating cutter, the material is cut away from this workpiece in the form of small chips to create the desired shape. 

The milling machine can also hold more than one cutters at a time this is why a milling machine finds wide application in production work. 

Milling machine used to produce non-axially symmetrical parts with many features, such as holes, slots, pockets, and even three-dimensional contours of the surface.

Milling machinery can be operated manually using a device called a computer numerical control or CNC milling machine. There are often one or two additional axes in addition to the traditional X, Y, and Z axes found in a manual machine and a CNC friction machine. These additional axes can provide greater flexibility and accuracy. 

CNC machines eliminate the need for a machine operator which can prevent possible accidents as well as save on labour costs. The main advantage of the milling machine is that it can be used with a high degree of accuracy to literally perform any operation.

According to the relative moment between tool and workpiece milling is classified below:

1. Down Milling :

The down milling is also called climb milling.

It is the process of removing metal by a cutter that is rotated in the same direction of travel of the workpiece. 

2. Up Milling :

The up milling is called conventional milling.

It is the process of removing metal by a cutter which is rotated against the direction of travel of the workpiece.

There is another major milling process classify below :

1. Face Milling :

In this milling process cutting action occurs primarily at the end corners of the milling cutter. Face milling is used to cut flat surfaces into the workpiece or to cut flat-bottomed cavities.

2. Peripheral Milling :

In this milling process cutting action occurs primarily along the cutter's circumference, so that the cutter's shape finishes with the cross-section of the milled surface. In this case, cutter blades can be viewed as scooping out the workpiece material. Peripheral friction is suitable for cutting deep slots, threads, and gear teeth. 

3. Chamfer Milling : 

A chamfered end mill makes a peripheral cut along a workpiece edge or a feature in this milling process to create an angled surface known as a chamfer. This chamfer can typically be machined on either the outside or inside of a part with an angle of 45 degrees and can either follow a straight or curved path. 

4. End Milling :

To machine a specified feature such as a profile slot, pocket, or even a complex surface contour, an end mill makes either peripheral or slot cuts determined by the step-over distance across the workpiece. The depth of the feature can be machined in a single pass, or it can be reached through machining at a lower axial cutting depth and multiple passes. 

Milling operation capabilities : 
  • Shapes - Solid cubic and complex, Flat, Thin-walled cylindrical, cubic and complex, and solid cylindrical. 
  • Part size - Length : 0.04 to 72 in Width : 0.04 to 72 in 
  • Materials - Metals such as Alloy Steel, Carbon Steel, Cast Iron, Stainless Steel, Aluminum, Copper, Magnesium, Zinc
  • Material - Ceramics such as Composites, Lead, Nickel, Tin, Titanium, Elastomer, Thermoplastics, Thermosets
  • Surface finish - Typical: 32 to 125 µin Feasible: 8 to 500 µin
  • Tolerance - Typical: + or - 0.001 in Feasible: + or - 0.0005 in
Milling Machine Operation :

1. Plain Milling
2. Face Milling
3. Side Milling
4. Straddle Milling
5. Angular Milling
6. Gang Milling
7. Form Milling
8. Profile Milling
9. End Milling
10. Saw Milling
11. Milling keyways, grooves, and slots.
12. Gear cutting
13. Helical Milling
14. Cam Milling
15. Thread Milling

For detailed information for all of the above operation: Milling machine operation

Types of Milling Machines :

There are many different types of milling machine available. Milling machines are categorized by their orientation to their workpiece and their degree of motion.

Knee-Type :

Knee-type milling machines use a knee-supported vertical workspace, which is a vertical casting adjustable. The knee supports a saddle and a customizable workspace can be adjusted.

Plain Vertical and Horizontal :

Milling machines can be oriented either vertically or horizontally with a standard work surface. Typically, the tool assembly is attached to a turret and swivel, typically parallel to the workspace. To enforce tight tolerances, the turret and swivel allow the tool to move freely around the workpiece. 

Universal Horizontal Milling Machine :

A universal horizontal milling machine differs from the horizontal plain type because it has a table swivel housing that allows the table to move 45 degrees from the horizontal standard position. This movement of the workpiece facilitates angular or helical milling operations.

Ram-Type and Universal Ram-Type Milling Machines :

In relation to the workpiece, a ram-type machine is used to allow the tooling to position itself on a larger range of space. On a movable housing, the ram-type machine has a spindle that can move in a set horizontal plane. The universal ram-type milling machine includes a swivel housing that increases the range of movement of cutting. 

Swivel Cutter Head Ram-Type Milling Machine :

A milling machine can rotate from a fully vertical position to a completely horizontal position with a swivel cutter. The work table also moves, providing a very liberal movement and orientation degree for the user. Many swivel cutters include settings that are both automatic or manually driven, increasing the options for operation. 

Advantages of the milling process :
  • All material was compatible. 
  • Very good tolerances.  
  • Short lead times. 
Disadvantages of the milling process :
  • Limited shape complexity. 
  • Part may require several operations and machines. 
  • High equipment costs. 
  • Significant tool wear. 
  • A large amount of scrap. 
Applications of the milling process :