A tool for cutting the isthmuses in slots of a plate comprising PCBs includes: a base plate, a spindle having a shank and a cutting tip, a spindle support fixed to the base plate and revolvingly supporting the spindle, a head fixed to the base plate and provided with a through hole wherein the shank of the spindle is disposed, a guide mounted on the head and provided with a through hole wherein the cutting tip of the spindle is disposed, in such manner that a portion of the cutting tip of the spindle protrudes from the guide, the guide having a guide rib disposed in alignment with the cutting tip of the spindle and suitable for being inserted into the holes of the plate comprising the PCBs to guide a relative movement of the plate comprising the PCBs with respect to the tool.