"Per" is used to describe the tool that you use, "kun" is more for accompaniment.
So you might say "Mi iras al la lernejo kun mia amiko" (I am going to the school with my friend) because your friend is just going along with you, but "Mi iras al la lernejo per buso" (I am going to school by bus) because the bus is the tool you use to go there - it is not merely something that happens to be going there with you.
Similarly, "Mi tranĉas la panon kun amiko" (I cut the bread with a friend) indicates accompaniment through "kun", "Mi tranĉas la panon per tranĉilo" (I cut the bread with a knife) indicates the tool you use through "per".
tondilo = tondas + il (tool) so does tondas have the same meaning with trancxilas?
"tondi" is specifically to cut with scissors or shears; "tranĉi" is more generally to cut with a sharp blade.