For that reason, I think {magh chIpwI'} is better for "lawn mower". (There's another, completely subjective reason. I grew up watching my uncle cut turf, which involves not trimming the grass, but cutting out entire sections of a mat of grass, with sod still attached. The machine that does that, I would immediately describe as a {magh pe'wI'}, and describing a lawn mower with the same term feels just incorrect.)

QeS 'utlh