Theories of interaction of rock cutting tools


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theories of interaction of rock cutting tools in contact with the rock, different parameters, specific energy, applications, drag, point attack picks, disc cutters, and their interaction

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Theories of interaction of rock cutting tools

  1. 1. Rock Excavation Systems Mechanical ExcavationTheories of Interaction of Rock Cutting tools U.Siva Sankar Sr. Under Manager Project Planning Singareni Collieries Company Ltd E-Mail or Visit at: Breaking ProcessesThe basic principles of breaking brittle hard rock. The main considerationsin breaking rock are the forces required to induce fractures in the rock andthe energy consumed in breaking rock.Force is important because it determines the limitation on the type ofmachinery that can be used to break the rock and on the materials ofconstruction that can be used in the machinery.As the breaking mechanism of the machine changes, so would the energyrequired to break the rock since the strength of rock varies depending onthe type of stress induced on the material.Energy is important because it determines the rate at which rock breakingcan be carried out. All machines are limited in the power that can beapplied to the rock and hardness of the manufactured components of themachine. Therefore a process that demands substantial energy will resultin a slow rock breaking rate.The rock breaking process is classified into three major groups: primary,secondary, and tertiary. 1
  2. 2. Rock Breaking Processes Primary This is the application of a force by means of a hard indenter to a free rock face much larger than the indenter. This generates chips which are of a size similar to that of the indenter at the sides of the indenter and a pulverized zone immediately below the indenter.Primary breakage processes1. Impact or hammering. Dynamic forces are applied2. Percussive drilling. Application of a hard indenter to the bottom of a hole. The force is applied from one side only and the bottom of the hole is the free face. The force applied dynamically and after each application the hard indenter is moved slightly to break out more chips on the next application3. Button type cutters for raise and tunnel borers. The buttons are loaded slowly (quasi-statically) and are moved away to be re-applied elsewhere, that is, indexing occurs by rolling to the next button. Repeated applications over a large surface area maintain the flat face4. Disc type cutters for raise and tunnel borers. Hard indenter indexed by rolling. Forces at a point in the rock rise very slowly.5. Drag bit. A hard indenter forced onto the rock and indexed by dragging across the surface.6. Diamond bits. A very hard surface and very small indenter dragged across the surface. The real breaking is done by the force thrusting the diamonds against the rock. Diamonds produce very small fragments because they are small indenters. 2
  3. 3. Secondary breakage processes This is the application of forces inside a hole near to the rock face. The forces inside the hole generate tension at the sides of the hole to which produces cracks which ultimately runs to the free surface. Secondary breakage processes includes: 1. Wedging. Wedge driven into a hole which produces crack 2.Blasting. Explosive generates a pulverized zone through compression but the real breaking process is by driving tensile cracks. Tertiary breakage 1. Breaking boulders by impact or mud blasting 2. Crushing 3. Milling According to theory, the tertiary breakage process is closely related to breaking the rock in tension. From Figure can be seen that loading of a sphere by diametrically opposed forces causes a uniform tensile stress across the diametrical plane. This causes the sphere to split in tension, that is, at a stress very much lower than the uniaxial compressive stress.TertiaryThis is the application of forces from more than one side to a free surface. Tertiary breakage process Fig; Tertiary Breakage a Tensile effectIt has been found that the tertiary stress, σt, is also dependent on thesize of the rock, but not as important as the size of the indenter forprimary breakage. 3
  4. 4. Miscellaneous breakage processesSeveral other breakage processes exist, these include:1. Thermal spalling.This depends on intense heat (flame) being applied to the rock and traversed so that a high temperature gradient is produce in the rock resulting in differential expansion which produces mechanical strains and ultimately breaking of the rock. It is used in taconite and certain quarrying operations, usually in cold climates. Thermal spalling is also used for finishes on rock surfaces and where high forces must be avoided during breakage. (Commonly used ancient technique)2. Water jets.The water jets create high stagnation pressures against the surface it impinges on. Used for drilling in porous hard rock where water goes into pores and breaks grains out. Water jets are known to be wasteful on energy and are used only for special applications. Mechanical Excavation Systems Different mechanical excavation systems, like machines with; Teeth (Dozer, Shovel, Scraper, Bucket wheel excavator, Bucket chain excavator) Ripping tool (Coal Plough, ripper, rock breaker), Pick mounted rotary cutting head/drum (Roadheader, Shearer, Continuous miner, Surface miner) Disc cutters and button bits (rock drill, Mobile tunnel miner, Tunnel boring machine) Auger tool (Continuous Auger Miner, Surface Auger Miner)Application of Mechanical Systems Under Ground: Continuous Miners, Bolter miners, Auger Miners and shearers for coal or soft nonmetalics Boom type miners (road headers in soft to medium hard rocks) Rapid excavation equipment (Mobile tunnel miners,Tunner borers, raise borers, and shaft sinking rigs) for soft to medium hard and hard rocks) Surface: Rippers for very compact soil, coal, and weathered or soft rock Bucket wheel and cutting head excavators for soil or coal Augers and highwall miners for coal Mechanical dredges for placers and soil 4
  5. 5. Mechanical Excavation Systems Classifications of cutting tool based on Cutting Action •Type of •Mode of •Specific tool •Machines using tool tool action types •Drag tool •Applies a force •Diamond drill bit •Rotary drilling roughly parallel machine to the rock •Pick •Roadheader surface (point attack and •Continuous miner wedge) •Shearer •Indenter •Applies a force •Brazed drill bit •Rotary percussive normal to the •Button drill bit drilling machine rock surface •Tricone Roller •Rotary drilling drill bit machine •Raise borer •Disc cutter •Tunnel boring machine (TBM) •Mobile miner •Raise borerMechanical Excavation Systems The main difference between indenters and drag bits is that an indenter breaks rock by applying a force that is predominantly in a direction normal to the rock surface. Comparatively, a sharp drag bit applies the main force in a direction predominantly parallel to the rock surface. The breaking mechanism for both is actually a tensile fracture. Because the drag tool initiates tensile fractures in a more direct manner, with less crushing, it is more efficient than an indenter. However, indenters are by far the most widely used type of tool; why is this? The reason lies in the strength of the tool itself. The materials used for the cutting edge must be hard but, because of this property, they are also brittle. The mode of action of a drag tool induces bending, or tensile stresses in the tool cutting edge and makes catastrophic failure of the tool more likely. An indenter, on the other hand, is loaded mainly by a compressive force along its main axis and the material of which it is made is inherently strong in compression. (Hood and Roxborough 1992.) 5
  6. 6. Mechanical Excavation Systems These basic cutting methods, defined in terms of tool type, are and include: 1.Drag bit cutting. Drag type 2. Point-attack bit cutting. 3. Disk cutting. 4. Roller cutting. Indenters 5. Button cutting. Fig: Rock Cutting TechniquesMechanical Excavation Systems Drag bit cutting and Point-attack bit cutting. The application of both drag bits and point-attack bits is similar. The tools are inserted in tool holders (or boxes), which are integral parts of the cutting head, and may be held in place by a circlip or spring. Point-attack bits are commonly free to rotate in their holders. It has been claimed that this feature promotes more even tool wear (self sharpening) and better overall tool life. During cutting, the bits are pushed into the rock, developing cutting forces parallel to the direction of head rotation and normal forces parallel to the direction of head thrust. As these forces build up to critical values, a macroscopic failure surface develops ahead of the bit, and a piece of rock spalls away. Road headers, Continuous Miners (Bolter Miners & Surface Miners) and Shearers use drag and point-attack bits almost exclusively. These tools also find application on tunnel boring machine (TBM) cutter heads, but in this role they are generally limited to machines operating in weaker formations. 6
  7. 7. Mechanical Excavation Systems Disk cutters generally consist of solid steel alloy discs with a tapered cutting edge. The disk is mounted in a bearing and is free to roll in response to applied forces acting parallel to the rock surface. These rolling forces are analogous to the cutting forces applied in drag bit cutting. Thrust and drag forces are applied to the disk through the bearing and act normal and parallel respectively to the rock surface. Thrust forces acting on the cutting head push the cutter into the rock building up stresses which cause local rock failure. Disks used in practice may be of the simple type, or may consist of multi-edge varieties, including types with successively smaller disk diameters giving a tapered or conical arrangement. Frequently these multi-row disks employ carbide inserts with chisel points imbedded nearly flush with the circumference. Simple disk cutters are used primarily on full face TBMs, and multi-row disks on raise boring machines (RBMs). Breaking Process Under a Disc Cutter Fig: Model for disk cutting (Roxborough and Phillips, 1975a). 7
  8. 8. Mechanical Excavation Systems Roller or mill-tooth cutting is similar to disk cutting except that instead of a tapered disc edge, the tool is equipped with circumferential teeth. As the cutter moves in response to rolling forces, each tooth in turn is pushed into the rock, acting like a wedge, and causing local failure. Button cutters consist of cylindrical or conical tool bodies inset with tungsten carbide buttons. The tool is mounted in a bearing in the same way as disk cutters or roller cutters and is free to roll in response to applied forces acting parallel to the rock surface. Thrust forces cause high stress concentrations beneath each button as they roll across the rock surface, resulting in local failure and pulverization of the rock. The area of influence of each button is small and results in a fine-grained product. Button cutting is used in applications in which high rock strength and abrasivity preclude the use of other methods. These cutters also find application as reaming cutters used for final profiling on RBMs and TBMs. Mechanical Excavation Systems Pick The picks consist of a steel body containing a recess into which a cemented carbide tip is brazed. The cemented carbide tip is the cutting portion of the pick, and consists of two materials, tungsten carbide and cobalt, sintered together to form a matrix of car bide grains within a cement of fused carbon. The most important physical properties of the cemented carbide are hardness and toughness. The value of both these properties can be varied by the amount of cobalt present, as shown in Fig. I. If the carbide is too hard, premature fracturing will occur, and, if it is too soft, the material will wear away too quickly. Thus, for optimum cutting performance, a balance between the two properties is necessary, dependent upon the quality of the coal being cut. Fig: variation of Toughness and hardness of pick with % of Cobalt 8
  9. 9. Mechanical Excavation Systems Drag Pick Types Radial picks Forward attack Picks, and Point attack Picks Radial and Forward attack Picks (Chisel or Wedge Picks) Conical or Point Attack PicksRadial picks – Chisel or Wedge pick Mechanical Excavation Systems These tools are designed such that the axis of pick shank is normally parallel to the radial line of cutting head/drum. They are generally suitable for cutting soft and medium-hard rocks and coal. Radial picks generate lower forces than those of point attack tools, when pristine. The normal force is of low magnitude compared to cutting force.Forward attack Picks - Chisel or Wedge pick These picks are also termed tangential picks, together with point attack picks, due to the orientation of their tool axis. The design and the geometry of tool tip is similar to that found on radial picks Chisel or wedge pick may be having either flat bottom surface or round bottomed surface.Point attack picks Conical tip and cylindrical shank. Shank axis is inclined relative to the rock surface. The tool is designed to rotate by the action of cutting to produce even wear and is therefore favoured in abrasive rocks. Often, however, dirt clogs the tool, so that it cannot rotate. 9
  10. 10. Mechanical Excavation SystemsCutting parameters Cutting Geometry of Drag Pick Schematic Drawing of Forces acting on a Conical BitCutting parameters Mechanical Excavation SystemsA simple Drag pick with the forces acting on it is illustrated in Figure. The resultant force Pa may be resolved into three mutually perpendicular components: Cutting force (Fc), acting in the direction of cutting; Normal force (FN) perpendicular to the direction of Fc; and Sideways force (Fs) normal to the plane on which Fc and FN lie.Clearance Angle: Clearance angle, which is between the lower surface of pick and a plane parallel to the cutting direction, also has pronounced affects on the pick forces. Investigations have shown that tool forces drop sharply after a value of around 5°and stay sensibly constant. To meet the kinematic needs, the clearance angle is generally designed to be around 10 degrees. 10
  11. 11. Mechanical Excavation SystemsRake Angle: Cutting and normal forces decrease monotonically with increasing rake angle as seen in Figure. Most of the benefit to pick forces has been achieved at a rake angle of 20° , beyond which further marginal improvement is at an increasing penalty to pick strength and its potential to survive. Rake angle can be either +ve or -ve. Rake angles between +20 and +30 degrees can be chosen for weak rocks and coal cutting. High rake angles may not be beneficial since picks with these angles are more susceptible to gross failure. Mechanical Excavation SystemsAttack Angle The angle of attack which is the angle between the tool axis and the tangent of the cutting path, is another parameter affecting the performance of point attack picks. This angle provides a good contact between the pick and rock and failure to position the pick at its correct angle of attack will significantly alter the effective tool geometry. In order to offset the value of clearance angle, the angle of attack is to be larger, e.g. at 90 degrees cone angle, the angle of attack should be at least 55 degrees. It is also reported that at high rotational speed this angle should not exceed 48° .Tilt angle: It is the angle between cutter axis to the vertical line normal to direction of cutting. Tilt angles of 65 to 70 degrees offered the lower specific energy and relative freedom from vibration problems. 11
  12. 12. Breakout Angle: Fig. Effect of breakout. Fig. Breakout between neighbouring Picks When a pick cuts its way through a material, some of it breaks away at each side of the pick; this is referred to as side splay or breakout Usually the sides of the groove are irregular, but over the total cut length the average slope of the sides, termed the breakout angle, can be considered constant for a particular material Efficient cutting is achieved through the maximum use of breakout, and pick lacing patterns should be designed so as to continually repeat the cutting sequence that produces it.where s = spacing between the tools, d= depth of cut, and θ = breakout angle. If the breakout angle for a particular material has been determined then s/d can be calculated. Mechanical Excavation SystemsWear Angle: The wearflat is almost parallel to thecutting direction; however, it generallytends to incline in the opposite directionand forms a wear angle. This angle is around few degrees andbecomes smaller for the hardest andstrongest materials. Occurrence of wearflat changes the tooltip geometry and, consequently, results inthe generation of higher tool forces. The normal force is the most affectedcomponent by the wear, e.g. a wearflataround 1mm can drastically increaseFn/Fc ratio. Wear Development It is also reported that a large clearance of Drag Pickangle relieves the wear effect andprovides better overall efficiency even if,as a consequence, a small or slightlynegative rake angle is introduced. 12
  13. 13. Mechanical Excavation Systems Important measures of cutting performance•Yield (Q) •The volume f rock produced by cutting - depends on penetration depth (d), breakout angle (θ) and distance•Specific Energy cut work done by the cutting force (FC) to excavate unit •The(SE) volume of yield. •Dependent •Rock strength and toughness on: •Degree of fracturing •Machine type and method of operation •Tool type and condition •Available tool forces (machine size and power) •Penetration depthSpecific energy is one of the most important factors in determining theefficiency of cutting systems and defined as the work to excavate a unitvolume of rock. Hughes and Mellor demonstrated that specific energy mightbe formulated as in the following:Where, SE is specific energy, E is secant elasticity modulus from zero to loadto failure and Sc is compressive strength of rock. Detailed rock cutting tests, however, showed that specific energy was not only a function of rock properties but it was also closely related to operational parameters such as rotational speed, cutting power of excavation machines and tool geometry. Roxborough reported that specific energy decreased dramatically to a certain level with increasing depth of cut and decreasing tool angle.The effect of the spacing between cutsand depth of cut (or penetration) oncutting efficiency is explained in Figure.If the line spacing is too close , thecutting is not efficient because the rockis over-crushed; in this region, toolwear is also high due to the high frictionbetween tool and rock. Fig: General effect of cutter spacing on specific energy. 13
  14. 14. Mechanical Excavation SystemsPointed Attack Vs Chisel or Wedge or Radial Picks In terms of pick shape, when operating at the same rake and clearance angles and depth of cut, the pointed pick requires the least cutting and normal force. The chisel pick requires the greatest forces. Due to the increased penetrating capability of the pointed pick, for a given available normal force, pointed picks operate more efficiently than the chisel bit. Pointed picks can but deeper for a given level of force, whereas chisel bit cut more material for a given depth of penetration. Mechanical Excavation SystemsTheoretical and experimental studies on cutting forces A number of scientists have formulated mathematical models to improve the design of the excavation machines and find the best configuration of the cutting tools for more efficient cutting process. Evans, Evans and Pomeroy extended theoretical works of Evans were used to establish the basic principles of the cutting process and these have been widely used in the efficient design of excavation machines such as shearers, continuous miners and road headers. Evans demonstrated theoretically that tensile strength and compressive strength were dominant rock properties in rock cutting with chisel picks and point attack tools. He also formulated optimum spacing for chisel picks as three to four times the pick width.Where FC is cutting force, d is depth of cut, w is tool width, α is rake angle,σt is tensile strength, σC is compressive strength and φ is tip angle. 14
  15. 15. Mechanical Excavation Systems Theoretical and experimental studies on cutting forcesGoktan suggested a modification on Evans’ cutting theory for point attacktools as indicated in Equation below and concluded that the force valuesobtained with this equation were close to previously published experimentalvalues and could be of practical value;Where ψ is friction coefficient between cutting tool and rockGoktan used Evans’ theories to compare the cutting efficiency of pointattack tools and wedge–shaped picks and concluded that the ratio oftensile to compressive strength was the main parameter governing therelative efficiency. Performance of Disc Cutters Fig: Model for disk cutting (Roxborough and Phillips, 1975a). Fig. Interplay between pick width and spacing. Fig: General effect of cutter spacing on specific energy. 15
  16. 16. Performance of Disc, Button and Pineapple cuttersA ranking of cutting efficiency of tool types, in terms of specific energy,places the steel disk cutter as the most efficient, the disk-button cutter next,and the pineapple cutter as least efficient.However, the wear resistance, and therefore the capability of cutting strongabrasive formations, is the reverse of this efficiency ranking.Consequently, steel disks tend to be used for cutting weaker, less abrasiverocks, and pineapple cutters are used for machining the most abrasive andtoughest formations.First, in contrast with drag bits, the efficiency of the rock breakage processdoes not decrease when disk cutters are used in a groove deepening mode.Second, the value of this optimum spacing depends on the depth of cuttaken and on the rock type and with drag bits an optimum s/d value of 2 to 3and with disk cutters this value is in the range 5 to 10.Third, the efficiency of the rock breakage process is independent of whetherthe grooves are cut simultaneously, with multiple disks on a single hub,orequentially, with independent disks. 16