operation procedure of the present invention are explained hereinafter when biasing the fence unit 70 relative to the coupled locating plate 16 the rip fence 71 is moved above the angle scale 12 and the supplementary frame bar 14 to turn the suspension arm 40 about the screw rod 45 at this time the magnetic sensor 50 detects a change of the value of magnetic induction relative to the magnetic strip 25 of the magnetic plate assembly 20 and sends the detection result to the circuit module of the display unit 60 enabling the circuit module of the display unit 60 to calculate the amount of angular displacement and display the calculated value on display module 61 further after installation of the fence unit 70 in one of the two locating plates 16 of the carrier 10 the angular position 90 degrees angular position of the fence unit 70 relative to the angle scale 12 and the angular position 90 degrees angular position of the magnetic sensor 50 relative to the magnetic plate assembly 20 can be calibrated based on the reference of the calibration bolt 15 further the fence unit 70 can be biased inwards within the range of 90 degrees to 45 degrees to adjust the cutting angle of the workpiece still further when wishing to change the position of the fence unit 70 on the carrier 10 loosen the lock bolt 30 and then move the magnetic plate assembly 20 and the lock bolt 30 away from the corresponding locating plate 16 and then couple the magnetic plate assembly 20 to the other locating plate 16 and then fasten tight the lock bolt 30 to.