what is the disadvantages of up milling?

Answer 1

I assume you mean conventional milling (mill turn causes teeth to cut with travel, tooth moves "up" through material), as opposed to climb milling (mill turn causes teeth to cut against travel, mill tooth cuts "down" through the work).

With conventional milling, the chip starts infinitely thin and gradually gets thicker. At the point the mill cutter exits the work, the chip is at its thickest. The disadvantage of this is that for very slick or high-powered cutters (high feedrate), the cutter will "grind" temporarily before the chip becomes thick enough to force the cutter to "bite" and actually start making a chip.

Climb milling is just the opposite. The chip starts thick and gradually gets thinner.

Use climb milling for roughing and high speed / high removal rate cutting. Use conventional milling for light finish cuts with coolant and high-speed steel tools.

Answer 2

the main disadvantage being, very low material removal rates. its basically used for better surface finish.