carpenter's file, file smooth
銼 centers on a metal tool used for smoothing or shaping by abrasion: a carpenter's file, or the action of filing something smooth.
銼 combines 金 (metal) with 坐, which likely contributes a phonetic element. The character refers to a metal file, fitting the metal radical.
The metal radical 金 on the left suggests a metal tool. The right side 坐 looks like two people sitting on the ground, but here imagine them sitting and filing a piece of metal smooth with a carpenter's file.
For サ, picture a carpenter filing a metal saw (サ) smooth: the sound of the saw cues サ while the file does its work.