Isaiah 41:15

The threshing instrument(môrâg) is a heavy sledge studded on its under surface with sharp stones or knives, drawn by oxen over the floor. See the Note in Driver's Joel and Amos, pp. 227 f. It is not a different implement from the ḥârûçof ch. Isaiah 28:27. Indeed this word ḥârûçis the one here translated "sharp"; and it may be doubted whether it has not intruded into the text as a variant to môrâg(Duhm). The instrument to which Israel is likened is "new" and "many-toothed" (lit. "possessor of mouths" i.e. edges), therefore in the highest state of efficiency.

the mountains … the hills A figure for formidable enemies; perhaps also for obstacles in general. Comp. ch. Isaiah 21:10; Micah 4:13.