Working:
The drug to be ground is put into the cylinder of the mill and is rotated. The speed of the rotation is very different. At low speed, the mass of balls will slide or roll over each other and only a negligible amount of size reduction will occur. At a high speed, the balls will be thrown out to the walls by centrifugal force and no grinding will occur. But at about 2/3rd of the speed, the centrifugal force just occurs, the balls are carried almost to the top of the mill and cascading occurs. By this way, the maximum size reduction is effected by the impact of particles between the balls and by attrition between the balls. After a suitable time, the material is taken out and passed through a sieve to get powder of the required size.
Diagram :
Advantages:
i)It can produce very fine powder
ii)It is capable of grinding wide variety of materials of differing character & of different degree of hardness.
iii)It can be used in a completely enclosed form , which makes it especially suitable for use with toxic materials.
iv)Can be sterilized & usedfor parenteral& ophthalmic preparations.
v)Can be used for batch as well as continuous grinding
vi)can be used for dry as well as wet grinding.