“Two working rolls of small diameter and four or more back up rolls of large diameter” are 

Cluster Mill

• Here each of the working rolls is backed up by two or more large backup rolls.
• This rolling mill is 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 length.

Two-high Rolling Mill

• It has two high rollers.
• Both the roller revolves at the same speed but in opposite direction.
• The space between the rollers can be adjusted by raising or lowering the upper roll which is adjustable.

Three-high Rolling Mill

• It consists of three rolls arranged one above other.
• 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 is used for rolling of continuous passes in a rolling sequence without reversing the drives.

• This results in a higher rate of production than the two-high rolling mill.

Four-high Rolling Mill

• Here two rolls are in direct contact with the work-piece and the other two rolls are used as backup rolls.
• The two rolls which are in direct contact with the work-piece 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 thickening of rolled plates at the centre.

Tandem Milling

• It consists of numbers of non- reversing two-high rolling mills arrange one after other.
• 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-up will be required and they will consume a lot of labour and work.