【正文】 y defect product can be reduced greatly and thus product quality and reliability is improved. To achieve this, an online tool condition monitoring system using a single chip microputer for detecting tool breakage during cutting process is discussed in this paper. Conventionally, PCbased monitoring systems are used in most research works. The major shorting of PCbased monitoring systems is the incurred cost. To reduce costs, the tool condition monitoring system was built with an Intel 8051 singlechip microprocessor and the design is described in this paper. The 8051 tool monitoring system uses a strain gauge for measuring cutting force。 according to the force feature, the tool monitoring system can easily recognize the breakage of the cutting tool with its tool breakage algorithm. The experimental results show that the lowcost 8051 tool monitoring board can detect tool breakage in three successive products successfully.Keywords: Tool condition monitoring。 Tool breakage。 Quality and reliability。