An electronically activated tool (50) comprising a tool body, cutter blocks and sensors with means for attachment to a drilling support and rotation so as to permit the simultaneous underreaming and measurement of the diameter of a wellbore (especially in oil and gas drilling) characterized by a means of activation using an electronic signal sent by mud-pulse, fiber-optics, wireless transmission or other means which may also communicate commands and receive data from the underreamer during drilling, at least one radially extendable cutter block (62) incorporating positional sensors adapted to measure the relative position of the cutter block to the tool, at least one calliper means (76) to measure wellbore diameter, all of which are inter-linked by a means of communication using receivers, sensors and microprocessors and a system that optimizes underreaming or expansion (22) by simultaneously comparing and correlating wellbore diameter data (22) and underreaming parameters in real-time or in memory.