Does anyone know what are the principle of working of a coffee grinder?

Baout what kind of a grinder you want to know? They are different and the principles may be different as well.

well, what are the difference - can you explain?

That's easy. Find here the topcis about blade and burr grinders and read them.

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Life is what happens when you are making other plans. (John Lennon)

OK, OK! I will tell you something to save your time. \r\n\r\nThere are two main types of grinders - burr and blade.\r\n\r\nBurr grinders are considered the best type of coffee grinder ensuring the best coffee flavor. These grinders are more expensive than the blade ones. In a burr grinder coffee beans are crushed by burrs, which are serrated steel parts with conical or flat surfaces that fit together. They crush beans into a uniform consistency that can be controlled by using differemt grinding settings. \r\n\r\nBurr mills can be hand-cranked or electric. The hand-cranked coffee grinder is favored by people who like to be a little more involved in the process, but can also be handy when camping or traveling.\r\n\r\nBlade coffee grinders consist of a simple motorized fan-like blade that spins in a hopper, or enclosed plastic casing into which whole beans have been placed. The grind they can produce is more suitable for automatic drip coffee maker, but not recommended for espresso,because of its inconsistency. You can only control the process of grinding by allowing more time for finer ground coffee, and less time for a course grind. Even so the end result is a combination of granulates varying in size.

__________________
Life is what happens when you are making other plans. (John Lennon)

Dear Trinity, thank you very much!