plowshare, barb, lancet
鎞 centers on sharp metal tools: a plowshare for cutting earth, a barb for piercing, or a lancet for surgery. The common thread is a pointed metal blade used to cut or pierce.
鎞 is a phono-semantic compound: 金 (metal) suggests a metal tool, while the right side 比 over 口 with a dot likely contributes the sound. The exact historical development of the right-side component is uncertain.
The left side 金 (metal) tells you this is a metal object. The right side looks like a comparison 比 of mouths 口 with a dot above, like a sharp point. Imagine a metal blade comparing different cutting edges, each as sharp as a lancet or plowshare.
For ヘイ, picture a metal hay cutter: hay -> ヘイ, and the plowshare slices through hay like a sharp lancet.