pestle
碓 centers on a stone tool used for pounding or grinding: a pestle. The meaning is concrete and specific, without broad abstract extensions.
碓 combines 石 (stone) with 隹, which likely contributes the sound. The character refers to a stone pestle or mortar, fitting the stone component.
A stone (石) on the left, and a bird-like shape (隹) on the right. Imagine a heavy stone pestle shaped like a bird, used for pounding grains.
For カク, picture a stone pestle cracking a nut with a sharp sound: crack -> カク. The stone tool cracks the shell.