In one embodiment, the present invention recites forming a number of first openings in a first substrate. Disposed rearwardly of a rearwardmost end of the tip, on a tapered side surface of a head of the bit body is an annular ledge which extends laterally outwardly sufficiently far to deflect cuttings laterally away from the bit. By using the invention, the combination of external mapping under the two-level sub-carrier mapping manner in the partial frequency multiplexing with the localized mapping and the distributed mapping can be realized. The withdrawable rack, provided with the inventive locking device for the electric switch displaceable in the withdrawable rack via the actuating shaft, is also disclosed. The overlapped digital video signal is then transmitted to one of the sets of manipulation and display device through the client interface. The trays according to the present invention have an improved rigidity and improved resistance to deformation and bending, improvements that have been achieved without increasing the amount of plastic material used for the manufacture of the tray but relying only on a specific geometry thereof.