The force sensor is essentially required in controlling robot manipulator in such applications as precise assembly of mechanical parts, deburring and polishing and various kinds of 6-axis force sensors are developed for these application. This paper presents the algorithm of horizontal assembly of circular cross-sectional workpiece using 3-axis force sensor and procedure to develop the sensor. The sensor is calibrated and tested using AID converter and 16 bit micro computer. The result is $pm$0.03% FS of zero stability, 0.1%FS of linearity and $pm$0.05% FS of resolution. The sensor will be used in the research of robot application such as assembly and deburring interfaced with micro computer based robot controller which is under development at the robotics lab.