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Machine tool with more accurate, stable and higher precision for the machining components cannot be developed without the precise control of the temperature rise by appropriate cooling. The machine tool coolers are the best managers of oil (or water) temperature in avoiding the deviation of spindle centerline for machine tools. However, the machine tool coolers are facing the control hunting of cooling temperature and the dramatic variation of heat load during high speed machining. In this study, the experimental investigation using hot-gas by-pass scheme were conducted to evaluate the temperature control accuracy of process cooling water. Effects of delay time control use pulse width modulation (PWM) techniques for by-pass valve under variation of process cooling load and different throttling devices been have investigated. The accuracy of temperature control as well as the frequency of ON-OFF times under different loading and different delay time has been compared comprehensively. The experimental results revealed that control accuracy within 0.2°C (0.36 °F) of cooling water temperature can be achieved with 5 seconds delay time controlled by adjusting hot-gas by-pass valve. The proposed hot-gas by-pass cooling system with PWM controlled time delay scheme is not only more cost-effective than inverter-driven system, but also can provide higher durability for by-pass solenoid valve and perform precise temperature control specific for high-accuracy process cooling application.