Milling Process – Definition , Milling Manufacturing Processes

Definition Of Milling :

Milling is a process performed with a machine in which the cutters rotate to remove the material from the work piece present in the direction of the angle with the tool axis. With the help of the milling machines one can perform many operations and functions starting from small objects to large ones.

Milling machining is one of the very common manufacturing processes used in machinery shops and industries to manufacture high precision products and parts in different shapes and sizes.

Introduction to Milling :

Milling machine is one of the important machining operations. In this operation the workpiece is fed against a rotating cylindrical tool. The rotating tool consists of multiple cutting edges (multipoint cutting tool). Normally axis of rotation of feed given to the workpiece. Milling operation is distinguished from other machining operations on the basis of orientation between the tool axis and the feed direction, however, in other operations like drilling, turning, etc. the tool is fed in the direction parallel to axis of rotation.

The cutting tool used in milling operation is called milling cutter, which consists of multiple edges called teeth. The machine tool that performs the milling operations by producing required relative motion between workpiece and tool is called milling machine. It provides the required relative motion under very controlled conditions. These conditions will be discussed later in this unit as milling speed, feed rate and depth of cut.

Normally, the milling operation creates plane surfaces. Other geometries can also be created by milling machine. Milling operation is considered an interrupted cutting operation teeth of milling cutter enter and exit the work during each revolution. This interrupted cutting action subjects the teeth to a cycle of impact force and thermal shock on every rotation. The tool material and cutter geometry must be designed to bear the above stated conditions. Depending upon the positioning of the tool and workpiece the milling operation can be classified into different types.

Read More : Working Principle Of Milling | Classification Of Milling Machine

Milling Process

The milling machine involves the following processes or phases of cutting:

Milling Cutters
There are a lot of cutting tools used in the milling process. The milling cutters named end mills have special cutting surfaces on their end surfaces so that they can be placed onto the work piece by drilling. These also have extended cutting surfaces on each side for the purpose of peripheral milling. The milling cutters have small cutters at the end corners. The cutters are made from highly resistant materials that are durable and produce less friction.

Surface Finish
Any material put through the cutting area of the milling machine gets regular intervals. The side cutters have got regular ridges on them. The distance between the ridges depends on the feed rate, the diameter of the cutter and the quantity of cutting surfaces. These can be the significant variations in the height of the surfaces.

Gang Milling
This means that more than two milling cutters are involved in a setup like the horizontal milling. All the cutters perform a uniform operation or it may also be possible that the cutter may perform distinct operations. This is an important operation for producing duplicate parts.

Milling is a metal removal process by means of using a rotating cutter having one or more cutting teeth as illustrated in figure 
Cutting action is carried out by feeding the workpiece against the rotating cutter. Thus, the spindle speed, the table feed, the depth of cut, and the rotating direction of the cutter become the main parameters of the process. Good results can only be achieved with a well balanced settings of these parameters.

milling process
milling process

Milling Process cutting Parameter : 

There are three major cutting parameters to be controlled in any milling operation. These three parameters are cutting, speed, feed rate and depth of cut. These parameters are described below.

Cutting Speed
Cutting speed of a milling cutter is its peripheral linear speed resulting from operation. It is expressed in meters per minute. The cutting speed can be derived
from the above formula.

V = πDN /1000

where,

d = Diameter of milling cutter in mm,
V = Cutting speed (linear) in meter per minute, and
n = Cutter speed in revolution per minute.

Spindle speed of a milling machine is selected to give the desired peripheral speed of cutter.

Feed Rate
It is the rate with which the workpiece under process advances under the revolving milling cutter. It is known that revolving cutter remains stationary and feed is
given to the workpiece through worktable. Generally feed is expressed in three ways.

Feed per Tooth
It is the distance traveled by the workpiece (its advance) between engagement by the two successive teeth. It is expressed as mm/tooth (ft).

Feed per Revolution
Travel of workpiece during one revolution of milling cutter. It is expressed as mm/rev. and denoted by f(rev).

Feed per Unit of Time
Feed can also be expressed as feed/minute or feed/sec. It is the distance advances by the workpiece in unit time (fm).

Above described three feed rates are mutually convertible.
(fm). = n x  f(rev).
where n = rpm of cutter.

It can be extended further as
(fm). = n x  f(rev). = z x n x f(rev)

where z = Number of teeth in milling cutter.

Read More About Milling Cutter : Types Of Milling Cutters used For Various Milling Operation

Depth of Cut
Depth of cut in milling operation is the measure of penetration of cutter into the workpiece. It is thickness of the material removed in one pairs of cutter under
process. One pairs of cutter means when cutter completes the milling operation from one end of the workpiece to another end. In other words, it is the
perpendicular distance measured between the original and final surface of workpiece. It is measured in mm.

Milling Operations

Owing to the variety of shapes possible and its high production rates, milling is one of the most versatile and widely used machining operations. The geometric form created by milling fall into three major groups:

  • Plane surfaces: the surface is linear in all three dimensions. The simplest and most convenient type of surface;
  • Two-dimensional surfaces: the shape of the surface changes in the direction of two of the axes and is linear along the third axis. Examples include cams;
  • Three-dimensional surfaces: the shape of the surface changes in all three directions Examples include die cavities, gas turbine blades, propellers, casting patterns, etc.
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