1. A drill bit assembly for use in combination with a drill string including a plurality of longitudinally aligned hollow drill pipes, comprising: an elongated tubular keying head member removably fixed with and longitudinally aligned with the lowermost drill pipe in the drill string; drill bit housing means movable longitudinally upwardly and downwardly of the drill string, and including a tubular cutting head section longitudinally slidably arranged around said keying head member and non-rotatable with respect thereto, and a tubular back-up section disposed around a drill pipe above said cutting head section; rotary drilling bit means fixed to the lower end surface of said cutting head section; holding means on said cutting head and back-up sections for axially securing said sections to each other while permitting said sections to rotate with respect to each other; bearing means operatively arranged between said sections for allowing said cutting head to rotate with the rotating drill string while said back-up section remains stationary; tubular secondary back-up members disposed around a drill pipe above said back-up section of said drill bit housing means; and attachment means operatively connected to said drill bit assembly for lifting and lowering said drill bit assembly out of and into a well.

6. A drill bit assembly for use in combination with a drill string including a plurality of longitudinally aligned hollow drill pipes, comprising: an elongated tubular keying head member having a generally square cross-section removably fixed with and longitudinally aligned with the lowermost drill pipe in the drill string; drill bit housing means moveable longitudinally upwardly and downwardly of the drill string, and including a tubular generally cylindrically shaped cutting head section having its interior bore formed at least in part in a generally square cross-sectional shape conforming substantially to the shape of said keying head member and longitudinally slidably arranged on said keying head member, and a tubular back-up section disposed around a drill pipe above said cutting head section and including a pair of semi-cylindrical body members articulated to each other for opening and closing around the drill string; locking means for locking said body members around the drill string; said back-up head having a radially inwardly extending annular lip, one half of which is on each of said body members, on the lower end portion thereof, said cutting head section having a circumferential groove formed in the external surface thereof, said lip fitting loosely into said groove such that said sections are axially secured to each other while said sections can rotate with respect to each other; thrust bearing means operatively arranged between said cutting head and back-up sections for allowing said cutting head section to rotate with the rotating drill string while said back-up section remains stationary; rotary drilling bit means fixed to the lower end surface cutting head section and arranged for cutting a well hole while permitting said cutting head section to be moved upwardly and downwardly of the drill string; and power driven lifting means fixed with said back-up section for lifting and lowering said cutting head and back-up sections together into and out of a well.

7. A drill collar assembly for use in combination with a drill string including a plurality of longitudinally aligned hollow drill pipes and a drill bit assembly having a stationary upper back-up section, comprising: an elongated tubular body shaped to fit longitudinally slidably around drill pipes and having external dimensions slightly less than the diameter of the well hole being drilled such that lateral shifting of the drill will be prevented, said body including a plurality of longitudinal sections articulated to each other for opening and closing around a drill string; locking means for locking said sections around the drill string; and stabilizer means for positioning said body centrally of the well hole supported by said body and including a plurality of roller members extending radially outwardly of said body, and biasing means supported by said body and connected with said roller members outwardly of said body.

10. A drill bit assembly for use in combination with a rotatable drill string including a plurality of longitudinally aligned hollow drill pipes and adapted to be raised and lowered into and out of a well hole without disturbing said drill string, comprising: drill bit housing means movable longitudinally upwardly and downwardly of said drill string, and including: i. a tubular cutting head section connected to the bottom portion of the lowermost section of said drill string in a manner to permit relative longitudinal sliding motion but to prevent relative rotational motion therebetween, and ii. a tubular back-up section disposed around a drill pipe above said cutting head section; rotary drilling bit means connected to the lower portion of said cutting head section; holding means on said cutting head and back-up sections for axially securing said sections to each other while permitting said sections to rotate with respect to each other; and bearing means operatively arranged between said cutting head section and said back-up section for allowing said cutting head to rotate with the rotating drill string while said back-up section remains stationary.

11. A drill bit assembly with a combination of a drill string including a plurality of a longitudinally aligned hollow drill pipes, comprising: a power means removably fixed longitudinally aligned with the lowermost drill pipe and non-rotatable with respect thereto; an elongated tubular keying head member removably fixed with and longitudinally aligned with said power means and rotatable by said power means; drill bit housing means movable longitudinally upwardly and downwardly of the drill string, and including a tubular cutting head section longitudinally slidably arranged around said keying head member and non-rotatable with respect thereto, and a tubular back-up section disposed around a drill pipe above said cutting head section; rotary drilling bit means fixed to the lower end surface of said cutting head section; holding means on said cutting head and back-up sections for axially securing said sections to each other while permitting said sections to rotate with respect to each other; bearing means operatively arranged between said sections for allowing said cutting head to rotate with the rotating drill string while said back-up section remains stationary; tubular secondary back-up members disposed around a drill pipe above said back-up section of said drill bit housing means; and attachment means on said back-up section of said drill bit housing and on said secondary back-up members for lifting said cutting head, back-up section and secondary back-up members together into and out of a well.

12. A drill collar assembly adapted for use in combination with a drill string, said drill string comprising a plurality of longitudinally aligned hollow drill pipes extending from the surface of the ground to the bottom of a well and a drill bit assembly at the lower end of the drill pipes, said drill bit assembly having a larger diameter than the drill pipes and being vertically keyed to the lower end of the drill pipes for vertical movement of the drill bit assembly relative to the drill pipes, said drill collar assembly comprising an elongated hollow tube-like member having an outside diameter smaller than the diameter of the well and an inside diameter larger than the drill pipes and the joints thereof but smaller than the outside diameter drill bit assembly, whereby said collar may be lifted to the top oF the well without lifting the drill pipes and may be lowered to rest upon the drill bit assembly, said drill collar assembly comprising stabilizing members supported by the tube-like member, movable radially, and biased outwardly to continuously engage the wall of the well, whereby said drill collar assembly is radially positioned in the well during positioning of the drill bit assembly and during drilling.