Wear is a systems problem(Ref 5, 27). The process of accurately detailing an abrasive wear failure consists of a series of defined tasks undertaken by the failure analysis specialist. Each task is designed to obtain specific information from the failed components and system. These tasks are generic to most failure analysis investigations and can be summarized in the following list (Ref 28) 1. Identify the actual materials used in the worn part, noting also the operating environment, abrasive causing the material loss, the wear debris, and any lubricant used (as needed) 2. Identify the specific wear mechanism, or combination of wear mechanisms, that caused the loss of material or change in the surface dimensions: adhesive wear. abrasive wear. corrosive wear. surface fatigue, erosive wear, and so on 3. Determine the change in the overall dimensions to the surface configuration between the worn surface and the original surface 4. Determine the relative motions involved in the tribosystem that caused the loss of material, for example, abrasive wear, including the direction and velocity of the relative motion 5. Determine the force or contact pressure between mating surfaces or between the worn surfaces, that is the counterfaces and the abrasive particles, on both the macroscopic and the microscopic level 6. Determine the wear rate by calculating the material loss over some unit of time or distance 7. Determine the coefficient of friction(if possible) 8. Identify the type of lubricant used and its effectiveness in slowing down material loss, for example, was the grease, oil, modified surface, naturally occurring oxide layer, adsorbed film, or intentional foreign material beneficial or not 9. Establish whether the observed wear is normal or abnormal for the particular application 10. Devise a solution, if required In most failures, some or even most of this information is simply not available. The more information that can be accurately determined, however, the better the chances of making a successful determination of cause and therefore, chance of remediation. These steps are used in each of the example failure analyses in this article In order to perform effective analyses of abrasive wear failures, it is not enough to have a broad-based knowledge of the mechanical situation found in the wear environment. a good background in metallurgy, ceramic science, or materials science is also necessary. In addition, access to investigative diagnostic tools is required for examination of worn sur faces and structures on both a macroscopic and microscopic level(Ref 29) This includes methods such as optical and electron microscopy, electron spectroscopy for chemical analysis, x- ay diffraction, low-energy electron diffraction, and Auger electron spectroscopy. Above all, a well documented plan of investigation is needed before any analysis is begun Once a plan of investigation is decided on, it is then necessary to develop a case history of the failure documenting as much as possible all aspects of the problem. This may not be easy to do, because in most cases detailed operating records are not kept. However, it is important to collect as much information about the operating history of the component as possible in this portion of the investigation. More importantly, it is necessary to determine at this stage whether the component and associated parts were operating properly, that is, as they were intended. If they were not, then it is easy to correct the problem by specifying the correct operating procedures and then to monitor subsequent operational situations to see if the same problem occurs However, in many cases, operation of the component and associated parts was within limits. In these instances, a thorough examination of the parts is necessary, using some or all of the analytical tools mentioned previously It is important, however, to examine the worn surfaces of the part of the failed component prior to its disassembly. Doing this provides a broad operational overview of the failure of the component, or more precisely, how all the parts fit together and operated. At this point in the analysis process, all operational questions need to be answered. Once a record is obtained, no matter how incomplete, the component can be broken down into its constituent parts, as was done in the case studies investigated in this article. Then the wear surfaces can be analyzed for manufacturing and/or material defects or any other unusual occurrences, and a reliminary determination as to the cause of the wear failure expounded. This may or may not lead to remediation procedures. If it does, these steps can be investigated on subsequent components to see if the ure was prope ly resolved(such as a change in material used for the parts or a redesign of the part to eliminate a possible design feature leading to accelerated wear ). Of course, the material or design requirements
Wear is a systems problem (Ref 5, 27). The process of accurately detailing an abrasive wear failure consists of a series of defined tasks undertaken by the failure analysis specialist. Each task is designed to obtain specific information from the failed components and system. These tasks are generic to most failure analysis investigations and can be summarized in the following list (Ref 28): 1. Identify the actual materials used in the worn part, noting also the operating environment, abrasive causing the material loss, the wear debris, and any lubricant used (as needed). 2. Identify the specific wear mechanism, or combination of wear mechanisms, that caused the loss of material or change in the surface dimensions: adhesive wear, abrasive wear, corrosive wear, surface fatigue, erosive wear, and so on. 3. Determine the change in the overall dimensions to the surface configuration between the worn surface and the original surface. 4. Determine the relative motions involved in the tribosystem that caused the loss of material, for example, abrasive wear, including the direction and velocity of the relative motion. 5. Determine the force or contact pressure between mating surfaces or between the worn surfaces, that is, the counterfaces and the abrasive particles, on both the macroscopic and the microscopic level. 6. Determine the wear rate by calculating the material loss over some unit of time or distance. 7. Determine the coefficient of friction (if possible). 8. Identify the type of lubricant used and its effectiveness in slowing down material loss, for example, was the grease, oil, modified surface, naturally occurring oxide layer, adsorbed film, or intentional foreign material beneficial or not. 9. Establish whether the observed wear is normal or abnormal for the particular application. 10. Devise a solution, if required. In most failures, some or even most of this information is simply not available. The more information that can be accurately determined, however, the better the chances of making a successful determination of cause and therefore, chance of remediation. These steps are used in each of the example failure analyses in this article. In order to perform effective analyses of abrasive wear failures, it is not enough to have a broad-based knowledge of the mechanical situation found in the wear environment. A good background in metallurgy, ceramic science, or materials science is also necessary. In addition, access to investigative diagnostic tools is required for examination of worn surfaces and structures on both a macroscopic and microscopic level (Ref 29). This includes methods such as optical and electron microscopy, electron spectroscopy for chemical analysis, xray diffraction, low-energy electron diffraction, and Auger electron spectroscopy. Above all, a welldocumented plan of investigation is needed before any analysis is begun. Once a plan of investigation is decided on, it is then necessary to develop a case history of the failure, documenting as much as possible all aspects of the problem. This may not be easy to do, because in most cases, detailed operating records are not kept. However, it is important to collect as much information about the operating history of the component as possible in this portion of the investigation. More importantly, it is necessary to determine at this stage whether the component and associated parts were operating properly, that is, as they were intended. If they were not, then it is easy to correct the problem by specifying the correct operating procedures and then to monitor subsequent operational situations to see if the same problem occurs. However, in many cases, operation of the component and associated parts was within limits. In these instances, a thorough examination of the parts is necessary, using some or all of the analytical tools mentioned previously. It is important, however, to examine the worn surfaces of the part of the failed component prior to its disassembly. Doing this provides a broad operational overview of the failure of the component, or more precisely, how all the parts fit together and operated. At this point in the analysis process, all operational questions need to be answered. Once a record is obtained, no matter how incomplete, the component can be broken down into its constituent parts, as was done in the case studies investigated in this article. Then the wear surfaces can be analyzed for manufacturing and/or material defects or any other unusual occurrences, and a preliminary determination as to the cause of the wear failure expounded. This may or may not lead to remediation procedures. If it does, these steps can be investigated on subsequent components to see if the failure was properly resolved (such as a change in material used for the parts or a redesign of the part to eliminate a possible design feature leading to accelerated wear). Of course, the material or design requirements
to solve an abrasive wear failure, or any failure for that matter, may prove to be too expensive, leading to other solution approaches, such as the complete redesign of the component and the associated parts that failed References cited in this section 1. E. Rabinowicz, Friction and Wear of Materials, 2nd ed, John Wiley Sons, Inc., 1995, p 5-7 2. V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures, 2nd ed, John Wiley Sons, Inc 1987 ,p1-10 5. H. Czichos, Systems Approach to Wear Problems, Wear Control Handbook, M B. Peterson and wo Winer, Ed, American Society of Mechanical Engineers, 1980, p 17-34 27. H Czichos, Tribology, Elsevier Science Publishing Co, 1978, p 14-23 28. R.C. Tucker, Wear Failures, Failure Analysis and Prevention, Vol 1l, Metals Handbook, 9th ed American Society for Metals, 1986, p 145-162 29. K.C. Ludema, Friction, Wear, Lubrication: A Textbook in Tribology, CRC Press, 1996, p 205-21 Abrasive wear Failures Jeffrey A. Hawk and Rick D. Wilson, Albany Research Center; Daniel R. Danks, Danks Tribological Services; Matthew T. Kiser, Caterpillar Inc Examples of Abrasive Wear Example 1: Jaw-Type Rock Crusher Wear. Although rock crushing and mineral comminution components are expected to lose surface material during their operating lifetime, this loss of material still results in reduced and/or eliminated component function and leads directly to failure. In this type of system, mineral ore flows down a feed chute into the upper portion of the crushing zone, which consists of two plates, one stationary and one moving. The chunks of rock enter at the top and are reduced in size each time the jaws cycle toward each other. The mineral then moves through the crushing zone until it reaches the desired size at the bottom, where the crushed pieces exit through the gap at the bottom of the plate assembly each time the plates separate to accept new rock Figure 14 shows the jaw crusher wear plates after processing a quantity of mineral ore In this case, the crusher plates are of the size and type outlined in ASTMG 89. The plates were quenched-and-tempered, low-alloy steel -0.30 wt%C at 514 HB hardness), with an elliptical motion of the movable plate relative to the fixed plate (Ref 30). In this case, the entering rock is 50 to 75 mm(2 to 3 in. ) in size. On exiting the jaw crusher plate assembly, the mineral is approximately less than 6 mm(0.2 in in size. Notice that for this jaw crusher configuration, the stationary plate absorbs the most severe gouging-abrasive wear(right side of the plate ). Table 4 summarizes the damage characteristics on both the stationary and moveable plates Thefileisdownloadedfromwww.bzfxw.com
to solve an abrasive wear failure, or any failure for that matter, may prove to be too expensive, leading to other solution approaches, such as the complete redesign of the component and the associated parts that failed. References cited in this section 1. E. Rabinowicz, Friction and Wear of Materials, 2nd ed., John Wiley & Sons, Inc., 1995, p 5–7 2. V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures, 2nd ed., John Wiley & Sons, Inc., 1987, p 1–10 5. H. Czichos, Systems Approach to Wear Problems, Wear Control Handbook, M.B. Peterson and W.O. Winer, Ed., American Society of Mechanical Engineers, 1980, p 17–34 27. H. Czichos, Tribology, Elsevier Science Publishing Co., 1978, p 14–23 28. R.C. Tucker, Wear Failures, Failure Analysis and Prevention, Vol 11, Metals Handbook, 9th ed., American Society for Metals, 1986, p 145–162 29. K.C. Ludema, Friction, Wear, Lubrication: A Textbook in Tribology, CRC Press, 1996, p 205–211 Abrasive Wear Failures Jeffrey A. Hawk and Rick D. Wilson, Albany Research Center; Daniel R. Danks, Danks Tribological Services; Matthew T. Kiser, Caterpillar Inc. Examples of Abrasive Wear Example 1: Jaw-Type Rock Crusher Wear. Although rock crushing and mineral comminution components are expected to lose surface material during their operating lifetime, this loss of material still results in reduced and/or eliminated component function and leads directly to failure. In this type of system, mineral ore flows down a feed chute into the upper portion of the crushing zone, which consists of two plates, one stationary and one moving. The chunks of rock enter at the top and are reduced in size each time the jaws cycle toward each other. The mineral then moves through the crushing zone until it reaches the desired size at the bottom, where the crushed pieces exit through the gap at the bottom of the plate assembly each time the plates separate to accept new rock. Figure 14 shows the jaw crusher wear plates after processing a quantity of mineral ore. In this case, the crusher plates are of the size and type outlined in ASTM G 89. The plates were quenched-and-tempered, low-alloy steel (~0.30 wt% C at 514 HB hardness), with an elliptical motion of the movable plate relative to the fixed plate (Ref 30). In this case, the entering rock is 50 to 75 mm (2 to 3 in.) in size. On exiting the jaw crusher plate assembly, the mineral is approximately less than 6 mm (0.2 in.) in size. Notice that for this jaw crusher configuration, the stationary plate absorbs the most severe gouging-abrasive wear (right side of the plate). Table 4 summarizes the damage characteristics on both the stationary and moveable plates. The file is downloaded from www.bzfxw.com
Scale, inches Fig 14 Photograph of the movable(top) and stationary(bottom) wear plates from the jaw crusher. The feed moves across the plates from left to right, so the most severe wear occurs at the exit area toward the rig ght. Table 4 Jaw crusher plate damage summary Plate Location Damage type Typical event Relative Displacement (from top) sIze material loss direction mm In Moveable Zone 0-50 0-2 Blunt with flow 120.5None Zone 50-100 2-4 Mixed rm 25 1.mInor Up Zone 100- 4-7 Sharp and deep, 251.0 Moderate Zone 175- Small, direct 6 0.25 Minor p 210 8.3 enetration Stationary Zone 0-2 Blunt, direct 120.5 None Zone 50-100 24 Sharp, direct 180.75Moderate Zone 100- sHarp,rm 120.5 Majority p 3 21083 rm, relative movement direction; that is, direction the surface metal is flowing Example 2: Electronic Circuit Board Drill Wear. Very small drill bits are used for drilling holes in electronic printed circuit boards(PCBs). To be economical, the drilling process must be completed quickly, because of the large number of holes in each board. This high-speed drilling operation thus requires automatic drilling machines capable of identifying hole location, starting and stopping quickly, and changing worn drill bits as needed. The maximum drilling rate for the system is the rate of maximum drill bit breakage. Generally, the optimal drilling conditions are determined by pushing the drilling conditions to their limit, that is, until drill bit failure. The failure of the drill bits is then analyzed Commercial drill bits vary in size, but popular sizes for drilling holes in PCBs are on the order of 0. 343 to 0.457 mm(0.0135 to 0.0180 in. )in diameter by 6.35 mm(0.25 in. )in length. In this case, the drill bits are made of obalt with approximately 50 vol% of uniformly distributed 0. 1 um sintered WC particles in the cobalt matrix Vickers hardness of the drill bits was measured at 1589 HV. The PCBs are composite materials, with layers of fiberglass epoxy resin sandwiched between copper layers. A PCB contains as many as 14 layers of copper and fiberglass-resin layers. The glass fibers in the fiberglass have a hardness of 500 Hv, while the copper has a hardness of 40 HV
Fig. 14 Photograph of the movable (top) and stationary (bottom) wear plates from the jaw crusher. The feed moves across the plates from left to right, so the most severe wear occurs at the exit area toward the right. Table 4 Jaw crusher plate damage summary Location (from top) Typical event size Plate mm in. Damage type mm in. Relative material loss Displacement direction Zone 1 0–50 0–2 Blunt with flow 12 0.5 None Up Zone 2 50–100 2–4 Mixed rm 25 1.0 Minor Up Zone 3 100– 175 4–7 Sharp and deep, rm 25 1.0 Moderate Up Moveable Zone 4 175– 210 7– 8.3 Small, direct penetration 6 0.25 Minor Up Zone 1 0–50 0–2 Blunt, direct penetration 12 0.5 None None Zone 2 50–100 2–4 Sharp, direct penetration 18 0.75 Moderate Up Stationary Zone 3 100– 210 4– 8.3 Sharp, rm 12 0.5 Majority Up rm, relative movement direction; that is, direction the surface metal is flowing Example 2: Electronic Circuit Board Drill Wear. Very small drill bits are used for drilling holes in electronic printed circuit boards (PCBs). To be economical, the drilling process must be completed quickly, because of the large number of holes in each board. This high-speed drilling operation thus requires automatic drilling machines capable of identifying hole location, starting and stopping quickly, and changing worn drill bits as needed. The maximum drilling rate for the system is the rate of maximum drill bit breakage. Generally, the optimal drilling conditions are determined by pushing the drilling conditions to their limit, that is, until drill bit failure. The failure of the drill bits is then analyzed. Commercial drill bits vary in size, but popular sizes for drilling holes in PCBs are on the order of 0.343 to 0.457 mm (0.0135 to 0.0180 in.) in diameter by 6.35 mm (0.25 in.) in length. In this case, the drill bits are made of cobalt with approximately 50 vol% of uniformly distributed 0.1 μm sintered WC particles in the cobalt matrix. Vickers hardness of the drill bits was measured at 1589 HV. The PCBs are composite materials, with layers of fiberglass epoxy resin sandwiched between copper layers. A PCB contains as many as 14 layers of copper and fiberglass-resin layers. The glass fibers in the fiberglass have a hardness of 500 HV, while the copper has a hardness of 40 HV
The drilling process is computer operated and numerically controlled. Typical operating conditions are as follows: the feed for drilling PCB holes is approximately 2000 mm(80 in )/min. The speed corresponds to the number of drill bit rotations per minute, and for this application, it was between 80,000 and 100,000 rpm. The hit rate or the number of holes drilled in a particular time interval, was 80 holes/min. After 1500 hits, the drill bits were resharpened. The material removal rate(MRR) is 4.8 mm/s and is given by the following equation (Ref 31) d MRR=n/, 4000 where d is the diameter in millimeters, n is the rotational speed in revolutions per second, and fr is the feed per revolution in millimeters In this study, a sharp, new 0.343 mm(0.0135 in )diameter drill bit was loaded into a tool holder and operated at the appropriate drilling conditions to drill a hole in a PCB. Figure 15 shows the results of this operation: a hole 0.3404 mm(0.0134 in in diameter. Notice that the hole has sharp edges and round, straight internal dimensions. The drill bit made chips of fiberglass and copper foil while cutting through the PCB. The glass fibers break by brittle fracture on impact with the cutting edge of the drill bit. Intense stress develops under the surface of the glass fiber where it makes contact with the bit, generating a plastic zone and a median vent crack in the plane of the stress field. Lateral cracks also develop and curve back to the fiber surface, liberating wear debris of glass fibers and epoxy. When the drill bit cuts through the layers of copper, ordinary ductile metal chips form by shearing. The chips tend to flow up the rake face of the drill bit and deposit in the flute, causing copper buildup on the edge Fig. 15 A scanning electron micrograph of the details of a hole drilled with a new, sharp drill bit. Note the clean hole with only a minor amount of damage to the hole periphery One way a drill bit fails is by catastrophic breakage from, for example, too high a load applied to the bit. a dull drill bit is susceptible to the buildup of wear debris on the flute. As wear debris accumulates on the bit, the friction force between the bit and hole increases. The increased friction generates additional heat, exacerbating change in bit diameter due to thermal expansion. The buildup of wear debris also rounds the cutting edge. h 9 wear debris buildup on the bit cutting edge. This reduces bit strength and increases hole size, a direct result in order to maintain cutting efficiency, the cutting force must be increased, which leads to increased shear stress on the drill bit shank. When the applied shear stress exceeds that of the bit material, it fractures, usually in the region between the spiral flutes and the shank Another way that drill bits ultimately fail is by accelerated wear. Drill bits typically have a break-in period where sharp burs on the drill bit are worn away. This break-in period is followed by steady-state wear when the majority of the holes are drilled with uniformly acceptable characteristics(Fig. 15). The majority of the tool lifetime is spent in the steady-state wear regime. Taylor's tool lifetime equation can be used to predict this time (Ref 32) Thefileisdownloadedfromwww.bzfxw.com
The drilling process is computer operated and numerically controlled. Typical operating conditions are as follows: the feed for drilling PCB holes is approximately 2000 mm (80 in.)/min. The speed corresponds to the number of drill bit rotations per minute, and for this application, it was between 80,000 and 100,000 rpm. The hit rate, or the number of holes drilled in a particular time interval, was 80 holes/min. After 1500 hits, the drill bits were resharpened. The material removal rate (MRR) is 4.8 mm3 /s and is given by the following equation (Ref 31): ² 4000 r d MRR nf p = where d is the diameter in millimeters, n is the rotational speed in revolutions per second, and fr is the feed per revolution in millimeters. In this study, a sharp, new 0.343 mm (0.0135 in.) diameter drill bit was loaded into a tool holder and operated at the appropriate drilling conditions to drill a hole in a PCB. Figure 15 shows the results of this operation: a hole 0.3404 mm (0.0134 in.) in diameter. Notice that the hole has sharp edges and round, straight internal dimensions. The drill bit made chips of fiberglass and copper foil while cutting through the PCB. The glass fibers break by brittle fracture on impact with the cutting edge of the drill bit. Intense stress develops under the surface of the glass fiber where it makes contact with the bit, generating a plastic zone and a median vent crack in the plane of the stress field. Lateral cracks also develop and curve back to the fiber surface, liberating wear debris of glass fibers and epoxy. When the drill bit cuts through the layers of copper, ordinary ductile metal chips form by shearing. The chips tend to flow up the rake face of the drill bit and deposit in the flute, causing copper buildup on the edge. Fig. 15 A scanning electron micrograph of the details of a hole drilled with a new, sharp drill bit. Note the clean hole with only a minor amount of damage to the hole periphery. One way a drill bit fails is by catastrophic breakage from, for example, too high a load applied to the bit. A dull drill bit is susceptible to the buildup of wear debris on the flute. As wear debris accumulates on the bit, the friction force between the bit and hole increases. The increased friction generates additional heat, exacerbating wear debris buildup on the bit cutting edge. This reduces bit strength and increases hole size, a direct result of a change in bit diameter due to thermal expansion. The buildup of wear debris also rounds the cutting edge. Thus, in order to maintain cutting efficiency, the cutting force must be increased, which leads to increased shear stress on the drill bit shank. When the applied shear stress exceeds that of the bit material, it fractures, usually in the region between the spiral flutes and the shank. Another way that drill bits ultimately fail is by accelerated wear. Drill bits typically have a break-in period, where sharp burs on the drill bit are worn away. This break-in period is followed by steady-state wear when the majority of the holes are drilled with uniformly acceptable characteristics (Fig. 15). The majority of the tool lifetime is spent in the steady-state wear regime. Taylor's tool lifetime equation can be used to predict this time (Ref 32): The file is downloaded from www.bzfxw.com
/T=C where V is the cutting speed, T is the tool life, C is a material-dependent constant, and n is a constant that depends on tool material and cutting conditions. Eventually, the sharpness of the bit degrades to a point where edge wear-debris accumulation becomes a problem. At this point, the drill bit has effectively failed, because the characteristics of the hole have significantly changed(Fig. 16). Examining Fig. 16, it is clear that the worn drill bit has slid away from the intended hole location by 0.3810 mm(0.015 in. ) approximately one drill bit diameter. The margins of the hole are also rounded, and it is now oversized by 12.6% 200um ig. 16 A scanning electron micrograph of a hole drilled with a worn drill bit. Note that the final position of the hole is not where the drilling started (i.e the drill wandered across the surface before"biting") and the ragged nature of the periphery of the hole. The microstructure of these drill bits has been optimized, so that further improvements in hardness and wear resistance may not be possible. However, some things can be done to increase the productivity of the drilling operation by properly determining drill bit life and changing them at the beginning of the accelerated wear phase of the tool. One way to do this is by measuring the drilling force on the chuck, using a three-axis force dynamometer. When the cutting force deviates from the steady-state constant force condition, the bit must be resharpened or replaced. Feeds and speeds of the drilling operation in combination with changing the cutting angle of the drill bits may also lower the number of fractured bits Example 3: Grinding Plate Wear Failure Analysis. In this example, a 230 mm(9 in )diameter disk attrition mill was scheduled to grind 6.35 mm(0. 25 in. )diameter quartz particles to a 0.075 mm(0.003 in. )diameter powder Due to severe wear on the grinding plates, however, the unit was unable to complete the task of grinding the rock. The unit was disassembled and an analysis performed to determine the cause of plate failure. Several steps were used to analyze the disk attrition mill failure, which included both an analysis of the grinding plates and the rock to be comminuted. Historical information was collected about the disk attrition mill and used to compare the present operating conditions with previous conditions where acceptable product was produced Several wear mechanisms were identified and used to make recommendations for future improvements Grinding Plates. The disk attrition mill is shown in Fig. 17. It consists of a heavy gray cast iron frame, a gravity feeder port, a runner, and a heavy-duty motor. The frame and gravity feeder weighed over 200 kg(440 lb)and in some areas, was over 25 mm(I in )thick. gray cast iron is considered ideal for the machine frame and base because it absorbs vibration and promotes smooth, quiet operation during grinding. To obtain the operating speed of 200 rpm, a gear system is used to transmit the torque from the 2 hp motor. The runner consisted of a 50 mm(2 in. )diameter shaft and two gray cast iron grinding plates (i.e., the parts used to grind the mineral)
VTn = C where V is the cutting speed, T is the tool life, C is a material-dependent constant, and n is a constant that depends on tool material and cutting conditions. Eventually, the sharpness of the bit degrades to a point where edge wear-debris accumulation becomes a problem. At this point, the drill bit has effectively failed, because the characteristics of the hole have significantly changed (Fig. 16). Examining Fig. 16, it is clear that the worn drill bit has slid away from the intended hole location by 0.3810 mm (0.015 in.), approximately one drill bit diameter. The margins of the hole are also rounded, and it is now oversized by 12.6%. Fig. 16 A scanning electron micrograph of a hole drilled with a worn drill bit. Note that the final position of the hole is not where the drilling started (i.e., the drill wandered across the surface before “biting”) and the ragged nature of the periphery of the hole. The microstructure of these drill bits has been optimized, so that further improvements in hardness and wear resistance may not be possible. However, some things can be done to increase the productivity of the drilling operation by properly determining drill bit life and changing them at the beginning of the accelerated wear phase of the tool. One way to do this is by measuring the drilling force on the chuck, using a three-axis force dynamometer. When the cutting force deviates from the steady-state constant force condition, the bit must be resharpened or replaced. Feeds and speeds of the drilling operation in combination with changing the cutting angle of the drill bits may also lower the number of fractured bits. Example 3: Grinding Plate Wear Failure Analysis. In this example, a 230 mm (9 in.) diameter disk attrition mill was scheduled to grind 6.35 mm (0.25 in.) diameter quartz particles to a 0.075 mm (0.003 in.) diameter powder. Due to severe wear on the grinding plates, however, the unit was unable to complete the task of grinding the rock. The unit was disassembled and an analysis performed to determine the cause of plate failure. Several steps were used to analyze the disk attrition mill failure, which included both an analysis of the grinding plates and the rock to be comminuted. Historical information was collected about the disk attrition mill and used to compare the present operating conditions with previous conditions where acceptable product was produced. Several wear mechanisms were identified and used to make recommendations for future improvements. Grinding Plates. The disk attrition mill is shown in Fig. 17. It consists of a heavy gray cast iron frame, a gravity feeder port, a runner, and a heavy-duty motor. The frame and gravity feeder weighed over 200 kg (440 lb) and, in some areas, was over 25 mm (1 in.) thick. Gray cast iron is considered ideal for the machine frame and base, because it absorbs vibration and promotes smooth, quiet operation during grinding. To obtain the operating speed of 200 rpm, a gear system is used to transmit the torque from the 2 hp motor. The runner consisted of a 50 mm (2 in.) diameter shaft and two gray cast iron grinding plates (i.e., the parts used to grind the mineral)