木 depicts a tree with branches and roots. As radical it tells you 械 was originally a wooden implement — most ancient tools and weapons were carved from wood, so the wood-radical is logical even when modern tools are metal.
戒 supplies the sound (jiè shifted to xiè). 戒 itself shows hands holding a halberd — a guard at the ready. The combined image is a wooden weapon-on-watch, the source of meanings tool, weapon, instrument.
