WORKING PRINCIPLE AND TYPES OF MILLING MACHINE

We have discussed in our previous post CNC Cylindrical Grinding machine about the fundamentals and objective of CNC cylindrical grinding machine, we have also seen various parameters which must be measure on cylindrical ground and various sensors used for smooth operation of CGM and their importance in CGM operation. 

Today we will focus on milling operation, where we will see here the basic of milling process, Principle of milling operation, up milling versus down milling and finally we will see the classification of milling machines.  

So let us start to discuss first the basic of milling operation

Milling is basically known as most common type of machining process of removing the unwanted material from workpiece. Workpiece will be machined by inserting against one rotating cutter and rotating cutter will have a number of cutting sharp edges. Unwanted material will be cut away in small-small chips to secure the desired shape. Workpiece might be curved, flat or with irregular surface. Milling process is basically prefer to use in order to produce parts those are axially unsymmetrical and with various features e.g. holes, pockets, slots or might be with 3D surface contours.

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Milling machine will have one spindle and one reciprocating adjustable working table. Spindle will be driven by electrical motor and it will mount and rotate the milling cutter. Working table will mount the workpice and insert against rotating cutter.

Working Principle of Milling machining operation

Adjustable working table of milling machine will hold the workpiece and movement of adjustable working table will govern the feeding of workpiece against rotating milling cutter. As we have discussed above, rotating cutter will be mounted with spindle which will be driven by electric motor and hence rotating cutter will rotates with high rpm. Rotating cutter will have only one motion i.e. rotating motion and apart from rotating motion, there will not be any movement in rotating cutter. When workpiece will be inserted against rotating cutter, rotating cutter teeth will remove the unwanted material from the workpiece to secure the desired shape.

Figure shows the principle of milling process. We have shown here up milling and down milling on the basis of direction of feeding of workpiece with respect to direction of rotating cutter.

Let us see the comparison between up milling and down milling

S. No.	Milling Operation : Up milling	Milling Operation : Down milling
1	Direction of feeding of workpiece and direction of rotating cutter will be opposite	Direction of feeding of workpiece and direction of rotating cutter will be same
2	Up milling process used for machining hard materials	Down milling process used for machining soft materials
3	Unwanted material will be removed in form of thicker chips	Unwanted material will be removed in form of thinner chips
4	Larger clamping force will be required because cutting force will be in direction of upwards and will tend to lift the workpiece	Larger clamping force will not be required because cutting force will be in direction of downwards and will tend to press the workpiece against the working table.
5	Chips will be accumulated at cutting area	Chips will be thrown away during cutting process
6	Surface finishing will be poor	Surface finishing will be good

Classification of milling machines

Milling machines are classified basically in two types as mentioned here

1. Vertical milling machine
2. Horizontal milling machine

Image courtesy: Google

There are few more classifications of milling machines.

1. Knee type milling machine
2. Ram type milling machine
3. Bed type milling machine
4. Planner type milling machine
5. Special purpose milling machine

Special purpose milling machine

1. Thread milling machine
2. CNC milling machine
3. Tracer controlled milling machine

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