Abstract: A hinge assembly includes a first rotating module and a second rotating module. The first rotating module includes a frame and a pivot mechanism positioned on the frame. The second rotating module includes a supporting body, a shaft, a rotating washer, a stationary washer, a resilient ring, and a nut. A first end of the shaft is fixed on the frame, and the supporting body, the rotating washer, the stationary washer, the resilient ring, and the nut are orderly sleeved on a second end, opposite to that of the first end, of the shaft. The nut fixes the supporting body, the rotating washer, the stationary washer, and the resilient rings to the shaft. The rotating washer and the stationary washer forms at least one protrusion and at least depression for receiving the at least one protrusion respectively. The pivotal shaft is configured to drive the stationary washer to rotate.

Abstract: An exemplary surface flatness testing device (10) includes a light emitting unit (11), a light receiving unit (12), a platform (13), an adjusting unit (14) and a processing unit (15). The light receiving unit faces the light emitting unit. The platform is located between the light emitting unit and the light receiving unit, which is configured for holding a workpiece (18) having a testing surface (181). The adjusting unit is disposed between the light emitting unit and the light receiving unit. The adjusting unit and the testing surface of the workpiece are controlling light from the light emitting unit to the light receiving unit. The processing unit is configured for processing signals from the light receiving unit. The present invention also provides a surface flatness testing method.