# Milling Machine Definition, Types, Cutting Tools, Industrial Applications, Processes & Operations

## Milling Definition

Milling machine is one of the most versatile conventional machine tools with a wide range of metal cutting capability. Many complicated operations such as indexing, gang milling, and straddle milling etc. can be carried out on a milling machine.

This training module is intended to give you a good appreciation on the type of milling machines and the various types of milling processes. Emphasis is placed on its industrial applications, operations, and the selection of appropriate cutting tools.

On completion of this module, you will acquire some of these techniques from the training exercises as illustrated in figure 1. However, to gain maximum benefit, you are strongly advised to make yourself familiar with the following notes before undertaking the training activities, and to have a good interaction between yourself and the staff in charge of your training.

Assessment of your training will be based on a combination of your skill and attitude in getting the work done.

## Cutting Tools

### Cutting Tools for Horizontal Milling

1. Slab Mills: For heavy cutting of large and flat surfaces.

2. Side and Face Cutters

3. Slitting Saws: For cutting deep slots or for parting off.

### Cutting tools for Vertical Milling

1. End Mills: Commonly used for facing, slotting and profile milling.

2. Rough Cut End Mills: For rapid metal removal.

3. Slot Drills: For producing pockets without drilling a hole before hand.

4. Face Milling Cutters: For heavy cutting.

### Involute gear cutter

The image shows a Number 4 cutter from an involute gear cutting set. There are 7 cutters (excluding the rare half sizes) that will cut gears from 12 teeth through to a rack (infinite diameter). The cutter shown has markings that show it is a

• 10 DP (diametrical pitch) cutter
• That it is No. 4 in the set
• that it cuts gears from 26 through to 34 teeth
• It has a 14.5 degree pressure angle

### Hobbing cutter

These cutters are a type of form tool and are used in hobbing machines to generate gears. A cross section of the cutters tooth will generate the required shape on the workpiece, once set to the appropriate conditions (blank size). A hobbing machine is a specialised milling machine.

## Industrial Applications

Milling machines are widely used in the tool and die making industry and are commonly used in the manufacturing industry for the production of a wide range of components as shown in figure 11. Typical examples are the milling of flat surface, indexing, gear cutting, as well as the cutting of slots and key-ways.

## Milling Processes

Milling is a metal removal process by means of using a rotating cutter having one or more cutting teeth as illustrated in figure below.

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.

### Spindle Speed

Spindle speed in revolution per minute (R.P.M.) for the cutter can be calculated from the equation:

[latexpage]

$N = \frac{CS \times 1000}{\pid}$

where —

N = R.P.M. of the cutter

CS = Linear Cutting Speed of the material in m/min. ( see table 1 )

d = Diameter of cutter in mm

### Feed Rate

Feed rate (F) is defined as the rate of travel of the workpiece in mm/min. But most tool suppliers recommend it as the movement per tooth of the cutter (f). Thus,

F = f . u . N

where —

F = table feed in mm/min

f = movement per tooth of cutter in mm ( see table 1 )

u = number of teeth of cutter

N = R.P.M. of the cutter

where

C.S. and feed rate for some common material:

Table: 1

 Tool Material High Speed Steel Carbide Material Cutting Speed Feed (f) Cutting Speed Feed (f) Mild Steel 25 0.08 100 0.15 Aluminium 100 0.15 500 0.3 Hardened Steel — — 50 0.1

### Depth of Cut

Depth of cut is directly related to the efficiency of the cutting process. The deeper the cut the faster will be the production rate. Yet, it still depends on the strength of the cutter and the material to be cut.

For a certain type of cutter, a typical range of cut will be recommended by the supplier. Nevertheless, it should be noted that a finer cut is usually associated with a better surface finish as well as a long tool life.

### Direction of Cutter Rotation

1. Up Cut Milling

In up cut milling, the cutter rotates in a direction opposite to the table feed as illustrated in figure below. It is conventionally used in most milling operations because the backlash between the leadscrew and the nut of the machine table can be eliminated.

2. Down Cut Milling

In down cut milling, the cutter rotates in the same direction as the table feed as illustrated in figure below. This method is also known as Climb Milling and can only be used on machines equipped with a backlash eliminator or on a CNC milling machine. This method, when properly treated, will require less power in feeding the table and give a better surface finish on the workpiece.

## Typical Milling Operations

### 1. Plain Milling

Plain milling is the milling of a flat surface with the axis of the cutter parallel to the machining surface. It can be carried out either on a horizontal machine or a vertical machine as shown in figure below.

### 2. End Milling

End Milling is the milling of a flat surface with the axis of the cutter perpendicular to the machining surface as shown in figure below.

### 3. Gang Milling

Gang milling is a horizontal milling operation that utilises three or more milling cutters grouped together for the milling of a complex surface in one pass. As illustrated in figure below, different type and size of cutters should be selected for achieving the desire profile on the workpiece.