A screw which can be more quickly screwed into a work piece and has better chip-exhausting effect. The screw has a rod body and higher thread and lower thread formed on the rod body. On a conic section of the screw, a cutting thread with another guide angle larger than the guide angle of the lower thread directly extends from the lower thread. Accordingly, the lower thread of the screw has better cuffing, drilling and chip-exhausting function. Alternatively, the screw can have single thread or double threads with equal height. On a predetermined section of the rod body of the screw, a cutting thread with changed guide angle is directly formed as a part of the thread to achieve better cutting, drilling and chip-exhausting function.
1. A screw comprising:
a rod body having a conic section; the conic section having a tip end;
a higher thread formed on the rod body and the conic section; the higher thread having a distal end located on said conic section; and
a lower thread formed on the rod body and the conic section; the lower thread being interposed between the higher thread from a lower end of the rod body to the tip end of the conic section; the lower thread having a smaller diameter than the higher thread and having a distal end located on said conic section; wherein said distal end of the lower thread sharply increases in pitch in a direction towards said tip end to form a general straight cutting tread which terminates at a location spaced from the tip end and spaced from the higher thread; and
wherein the distal end of the higher thread extends closer to the tip end of the conic section than the distal end of the lower thread.