A system and method to measure the clock skew between transmitting and receiving devices operating with independent clock sources over a packet network is described. The array substrate comprises a substrate, a gate metal layer, a gate insulation layer, a semiconductor layer, a patterned metal layer, a flat layer, and a pixel electrode. The grindstone is disposed within the movable range of the cutter blade driven by the robot and is placed in a position at which the cutter blade can pressure contact the cutter blade.