Basics of drilling 4


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Basics of drilling 4

  1. 1. BASICS OF DRILLING-4 I. Satyanarayana, M.Tech,MBA,MCADy. Manager, Project Planning, SCCL,, visit ISN
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  12. 12. Roller Cone Bits —These bits usually consist of three cone elementsmounted on rolling bearings. The rock is broken by indentation as thecutting elements that project from the surfaces of the cones are pressed, bya thrust force applied to the bit, into the rock surface on the hole bottom.A torque also is applied to the bit; this causes the cones to roll on theirbearings and brings other cutting elements into contact with the rock face.The bits illustrated in Fig. 9.1.37 show both hardened steel teeth andcemented tungsten carbide buttons as the cutting elements. Both of thesetypes of tool are used commonly today.Also shown in this figure are nozzles directed at the rock face. A fluid,either compressed air or a drilling mud, is conducted down the center ofthe drill pipe and directed at high velocity onto the face through thesenozzles. This fluid serves at least three purposes. One is to cool thebearings. Another is to remove the rock fragments from the face as theyare produced by the action of the cutting elements. The third is totransport this rock debris to the surface. The debris-laden fluid returns tothe surface up the annulus between the drill pipe and the hole wall. ISN
  13. 13. Roller cone bits with a) milled steel teeth, and b) cemented tungstencarbide inserts ISN
  15. 15. Down-the-Hole Drills —The previous discussion makes itclear that in a conventional percussive drill the mechanismthat imparts energy to the rock remains outside the hole andthis energy is transmitted in the hole through the drill rod(s)and the bit. As the name, down-hole (DTH) drill implies,with this percussive unit the mechanism for impartingenergy to the rock is located in the hole. The piston in thesedrills is powered by compressed air. Holes 4 to 8 in. (100 to200 mm) in diameter up to 500 ft (150 m) long can be drilledwith a DTH drill (Anderson, 1982).This system eliminates energy transmission losses in the drillrods; thus a major advantage of a DTH drill is a constantpenetration rate that is independent of hole depth. ISN
  16. 16. Down-the-hole drill.• check valve;• air metering plug;• air distributor;• piston;• cylinder;• piston-stem bearing;• washer;• bit retaining ring;• splined chuck ISN
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  18. 18. Rotary Drills: Small-diameter rotary drills are used for drilling weakerrock types. These drills usually employ a rotary hydraulic motor toprovide the drill torque, and a hydraulic cylinder to provide the thrust.They are mounted, in a similar manner to percussive drills, on drilljumbos for face drilling and for roof drilling.Large-diameter holes in both underground and, more frequently, insurface mines typically are drilled with roller cone bits. The essentialfeatures of a rotary drill rig for a surface mining operation are shown inFig. 9.1.50. These features include a motor as the rotary drive, a chainpull-down arrangement operated by hydraulic rams to provide thrust, arod handling device, a compressor for flushing the rock debris from thehole bottom, hydraulic leveling jacks, a dust collection system, and a mainmotor, either diesel or electric. ISN
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  20. 20. Reverse circulation flushing (left) and dual string ISNcirculation (right)
  21. 21. Conclusion• The principal drilling methods used in mines today are mechanical ones in which a drill drives cutting tools into rock by means of static (simple rotation) or dynamic force (impact).• Alternative methods, employing heat, flame, high-pressure water, or high voltage electric discharges, for example, are used only in particular situations or in laboratory studies.• Percussion rock drills are the most commonly used equipment for drilling in small-scale surface or underground mining situations, whereas rotary crushing drills or down the hole drills (DTHs) are generally employed for mining in large- scale surface mines. ISN
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