Charles Ellicott Commentary Isaiah 41:15

"Behold, I have made thee [to be] a new sharp threshing instrument having teeth; thou shalt thresh the mountains, and beat them small, and shalt make the hills as chaff." — Isaiah 41:15 (ASV)

A new sharp threshing instrument. —The instrument described is a kind of revolving sledge armed with two-edged blades, still used in Syria, and, as elsewhere (Micah 4:13), is the symbol of a crushing victory. The next verse continues the image, as in Jeremiah 15:7; Jeremiah 51:2.