Rolling mills are classified based on the number and arrangement of the rolls in it. Rolling is a metal forming process in which metal stock is passed through one or more pair of rolls to reduce its thickness and make the thickness uniform.
These machines are available in different shapes and sizes and having different number of rolls.These machines have minimum 2 rolls and can have maximum number of rolls is not limited. Some rolls are in direct contact with the workpiece and others are used as backup rolls and their main function is to prevent the deflection of the rolls which are in direct contact with workpiece.
Now the main types of rolling machines are :-
i) Two high rolling mills .
ii) Three high rolling mills .
iii) Four high rolling mills.
iv) Tandem rolling mills.
v) Cluster rolling mills.
i) Two high rolling mills :-
This type of mill has two rollers arranged as shown in figure below. Both the roller revolves at same speed but in opposite direction. The space between the rollers can be adjusted by raising or lowering the upper roll which is adjustable. To reduce the thickness of workpiece, it can be feeded from one direction only. However, there is another kind of two high rolling mills in which workpiece can be feeded frim both direction.
Based on the direction of feed available, two high rolling mills are further divided into types :-
a) Non -Reversilng two high rolling mill
In this type of rolling mill, the rolls can rotate in one direction only and hence the workpiece can be feeeded from one direction only.
b) Reversilng two high rolling mill
In this type of rolling mills, the rolls can rotate in both direction forward and reverse and hence the workpiece can be feeded from both the direction.
ii) Three High Rolling Mills :-
This type of rolling mills consists of three rolls arranged one above other as shown in figure. The direction of rotation of upper and lower rolls are same but the middle roll rotates in the opposite direction. This type of rolling mills are used for rolling of continuous passes in a rolling sequence without reversing the drives. This results in a higher rate of production then the two-high rolling mill.
iii) Four High Rolling Mill :-
In this type of rolling machine, two rolls are in direct contact with the workpiece and the other two rolls are used as backup rolls. The two rolls which are in direct contact with the workpiece are smaller than backup rolls and are called working rolls. Backup rolls are used to prevent the deflection of the smaller rolls, which otherwise would result in thickenning of rolled plates at the centre.
iv) Tendem or Continuous rolling Mill :-
It consists of a number of non reversing two-high rolling mills arrange one after other. So that the material can be passed through all of them in sequence.It is suitable for mass production work only, because for smaller quantities quick changes of set upwill be required and they will consume lot of labor and work.
iv) Cluster Rolling Mill :-
In this type of rolling mill, each of working roll is backed up by two or more larger backup rolls. These rolls are arranged as shown in figure. This rolling mill are used in rolling hard thin materials. For rolling hard thin materials, it may be necessary to employ work rolls of very small diameter but of considerable lenght . In such cases adequate support of the working rolls can be obtained by using a cluster-mill.
v) Planetary Rolling Mill :-
In this type of rolling mill, a large backup roller is surrounded by many planetary working rolls. Each planetary rolls gives constant reduction. It is used to reduce large thickness of single pass of steel strip. Its rolling capacity is more than cluster rolling mill but less than rolling mill.