whetstone, tool grinder
碬 centers on a stone used for sharpening or grinding: a whetstone or tool grinder. Both meanings describe the same abrasive stone tool.
碬 combines 石 (stone) with 叚, which likely contributes the sound. The stone radical points to a stone tool, consistent with the whetstone meaning.
The stone 石 on the left is the material, and the right side 叚 looks like a grinding surface with a handle. Picture a stone block used to sharpen blades.
For カ, imagine a car mechanic using a whetstone to sharpen a tool: car -> カ, and the stone grinds the metal.