Online monitoring of machining processes is revealed as a critical tool for detecting tool wear, influencing the determination of the remaining useful lifetime of cutting tools. Embracing the ethos of Industry 4.0, the study emphasizes the automatic monitoring of cutting forces, surface roughness, power consumption, tool wear, and tool life, citing their indispensable role in mitigating unfavourable machining conditions such as chatter vibrations, tool breakage, and compromised dimensional accuracy. The paper underscores the pivotal role played by advanced sensor systems in achieving enhanced machining characteristics, characterized by reduced human effort, minimized errors, and streamlined production times. The exploration extends to a comprehensive overview of online detection systems, encompassing sensors and signal processing software tailored for mechanical machining operations. Commencing with an up-to-date literature introduction, the paper systematically navigates through the types of sensors employed in machining, online detection methods, and addresses pertinent challenges while offering insightful suggestions. In a nutshell, the paper summarized its findings and provided future insights, particularly centred on the industry 4.0 theme. It is important to note that this review offers significant assistance to researchers and academics in the industrial sectors.