Left metal radical (the side-form of 金) - the indexing radical, marking 镰 as a metal tool. Same radical builds 锄 hoe, 锹 spade, 锯 saw. Whenever a Chinese word names an iron implement of farming or craft, expect 钅 anchoring the left.
Right 廉 supplies the sound lián, an exact match with no drift. The 'upright, frugal' sense plays no part in the meaning; pure sound peg. Same phonetic appears in 帘 curtain (also lián) and 濂 the river name. 镰刀 names the curved sickle paired with the hammer on Chinese state heraldry.
