Two-Dimensional Study of Rock Breakage in Drag-Bit Drilling at Atmospheric Pressure

Abstract

This paper presents some preliminary results of twodimensional cutting tests of dry limestone samples at atmospheric pressure. Cutting tips having rake angles of + 30°, + 15°, 0°, -15° and -30° were used to make cuts on Leuders limestone samples at six depths of cut ranging from .005 to .060 in. at cutting speeds of 15, 50, 109 and 150 ft/min. The vertical and horizontal force components on the cutting tips were recorded with an oscilloscope equipped with a polaroid camera. Motion pictures of the cutting process at camera speeds of 5,000 to 8,000 frames/sec were taken at strategic points in the variable ranges.The movies provide considerable insight into the brittle failure mechanism in rocks. It appears that chipgenerating cracks usually have an initial orientation which is related to the resultant of the externally applied forces. The latter part of the crack curves upward toward the free surface being cut, this part being governed by some type of cantilever bending or prying. The linear and angular motion of the loosened chips also indicate the tensile nature of brittle failure. Analyses of the forces on the cutting tips indicate that: (1) relatively small increases in vertical loading result in large cut-depth increases for sharp tips (rake angles > 0°); (2) tool forces increase at an increasing rate as t~ rake angle decreases, particularly for rake angles < 0°; and (3), for the range of this study, rate of loading had little effect on the maximum forces. Both the movies and visual inspection of the cuttings indicated that the volume of rock removed by chipping was much larger than that by any grinding mechanism, even for tips having negative rake angles. Cutting size increases with increased cut depth and rake angles, and decreases slightly at high cutting speeds, the depth of cut having by far the most influence. The amount of contact between the rock and the cutting tip was always less than the depth of cut and rarely exceeded 0.010 in. even for cuts of 0.060 in.

Numerical Studies on Chip Formation in Drag-Pick Cutting of Rocks

“…These assumptions are contrary to the tensile failure theories proposed by Evans (1962) in which he has postulated a curvilinear failure plane. The experimental studies conducted by Evans (1962) and Gray et al (1962), and numerical simulations by Ingraffea (1987) also reveals a curvilinear path of failure. Svensson and Freshwater (1963) from their photoelastic studies on the chip formations concluded that the chip formation was more due to the development of tensile stresses normal to the crack path.…”

“…Unlike metal cutting, the broken pieces of rocks do not stick to the uncut rock domain. Gray et al (1962) studied rock breakage under drag-bit drilling using high speed photography and found that fully formed chips are thrown out at high velocities and at a linear acceleration of 150-300 times the acceleration due to gravity. This makes the crushing process difficult to simulate using Finite Element Method, which uses a continuum approach to solve displacements and resulting stresses.…”

A study of rock cutting with point attack picks

“…The frictional heat generated is also responsible for the temperature rise at the pick-rock interface, which has a significant effect on the wear rate of cutting tool [90,93]. A number of researchers realised that severe reduction in drag bit performance and significant thermal stress in the cutting tool as well as the rock might be caused by the high temperatures encountered at the pick-rock interface [94][95][96][97][98][99][100]. Thus temperature measurement at the pick-rock interface is extremely important in determining the performance and economy of drag bits as well as providing a good indicator of the thermal energy generated in the rock cutting process which can be used to assess the risk of frictional ignition during pick cutting.…”

“…Meanwhile, the occurrence of elevated interface temperature could arise from the considerable frictional heat or thermal energy generated between cutter picks and rock, which in turn, has a significant effect on the determination of the wear rate of the cutting tool [90,93]. It is pointed out by many researchers that severe reduction in pick performance and significant thermal stress in rock as well as the tool could ultimately be caused by the elevated temperatures encountered in pick-rock interface [94][95][96][97][98][99][100]. Thus the determination of pick cutting temperature is extremely necessary as it will assist in assessing the performance and economics of drag bits and quantifying the risk of frictional ignition.…”

Stiffness and Strength of Rock Cutting and Drilling Tools – Drag Bit and Roller Disc Cutters

“…Pioneering studies on drag picks were conducted in the early 1950s for cutting coal, shale and soft sandstone. Starting in the late 1960s, effects of cutter geometry, cutting speed and rock properties on cutting forces were comprehensively investigated on coal seam and weak rocks [69,93,188,215]. In the 1970s, to assess the technical feasibility of ongoing tunnelling and drilling machines, other aspects of the cutting problem such as wear mechanism and specific energy were further identified and studied from both theoretical and experimental points of view [168,192].…”