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Factors affecting wheel collapse in grinding
CIRP Annals - Manufacturing Technology 58 (2009) 307–310
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er. com/ci rp/ def a ult . asp
Factors affecting wheel collapse in grinding J. Badger The Grinding Doc Consulting, Austin, TX, USA Submitted by J. Webster (1), Cool-Grind Technologies
A R T I C L E I N F O
A B S T R A C T
Keywords: Grinding Machinability Roundness
An investigation was made into the phenomenon of wheel collapse, where power increases steadily with attritious wear until a critical value is reached, upon which power drops rapidly to a steady-state value and the wheel loses its roundness. Causes and mechanisms were investigated, along with the relationships between amount of material removed at collapse, power at collapse, post-collapse steadystate power, cooling, wheel self-sharpening, and G-ratio. Examples from laboratory tests and production tests are used. Practical recommendations are given for delaying or eliminating collapse. ß 2009 CIRP.
1. Introduction Wear of grinding wheels is typically divided into three regimes: attritious wear, grit fracture, and bond fracture [1]. Attritious wear is associated with larger normal and tangential forces and, consequently, higher power. The degree of grit fracture and bond fracture determines the amount of self-sharpening of the wheel. A typical wheel-wear profile [2] shows a high degree of initial wheel wear as the wheel ‘cleans up’ loosely held grits, followed by a stable regime of steady wheel wear, and finally a regime of rapid wear, often accompanied by ‘grinding burn’ [1] and/or a transition from nucleate boiling to film boiling [3,4]. This is sometimes referred to as a ‘surge’ in power. Much research has been done on this phenomenon, which has been summarized previously [3,4]. There is another form of wheel wear, where power, beginning at some initial value P0, increases gradually with attritious wear until a critical value, PC, is reached. At this point wheel wear is rapid and power drops to a steady-state value, PSS. Aside from the gradual increase and decrease in power, there is no visible surge accompanying this phenomenon. A typical power profile is given in Fig. 1 [5]. Little has been written about the subject. One grinding-wheel manufacturer referred to this phenomenon as wheel ‘collapse’ in a technical brochure [6], stating that it is caused by groups of grits breaking off at once, resulting in an out-of-round wheel. Other authors have experienced in their tests, but have not elaborated on it. Malkin and Murray [7] found normal and tangential forces, Fn and Ft, increased until a maximum value was reached, then decreased to a steady-state value of around 65–90% of the maximum, where it remained for the remainder of the test. They also found that less-aggressive dressing conditions increased not only the initial forces, but also the forces at collapse, meaning collapse could occur over a wide range of forces. Different dressing conditions did not appear to affect significantly the amount of material removed before 0007-8506/$ – see front matter ß 2009 CIRP. doi:10.1016/j.cirp.2009.03.048
the onset of collapse. Sultana [8] found that dressing affects the power and forces at the onset of collapse, that dressing did not affect the amount of work material removed upon collapse, and that post-collapse power was 70–90% the maximum power. Metzger [9] found unpredictable decreases in grinding power when surface grinding tungsten-carbide with a resin-bonded diamond wheel, with power after collapse being 65–80% of the power at collapse. G-ratios in experiments that experienced collapse were on average 50% lower than experiments where collapse did not occur. Metzger explained this phenomenon as grits from one section along the circumference of the wheel being pulled out due to wear-flat development, which in turn puts a larger load on grits along another section of the circumference, which initiates pull-out there, with this cycle leading to overall rapid macroscopic wheel wear. Unfortunately, aside from Metzger’s short paper, very little has been written about this phenomenon. This paper describes an investigation into the phenomenon of collapse, using data from laboratory tests and actual production operations. It explores the mechanisms behind collapse, its relationship to grindability and G-ratio, forces and power, material removed upon collapse, attritious wear, and post-collapse power. It also examines the effect of cooling and grinding aggressiveness on collapse. Finally, it gives practical recommendations on delaying or eliminating collapse to increase productions rates and reduce the risk of burn and chatter. 2. Experimental Experiments were done both in the laboratory and on the shop floor in a production environment. Laboratory tests, described previously [5], were done on a 3.5 kW Chevalier surface grinder with a 100%-white-Al203, 46-grit, H-grade, vitrified-bonded grinding wheel on various grades of powder-metallurgy (PM) and conventionally produced (CONV) high-speed steel using 6%
J. Badger / CIRP Annals - Manufacturing Technology 58 (2009) 307–310
308
Fig. 1. Power profile.
soluble-oil coolant. The grinding power was measured with a Hall-effect power meter (resolution 0.04 kW) sampling at 20 Hz. The net grinding power was calculated by subtracting the idle power and the power to accelerate the coolant (idle power + coolant-acceleration power = 1.6 kW). Additional tests were done on a single PM grade (ASP2055: C: 1.65%; Cr: 4.0%; Mo: 4.6%; W: 6.3%; Co: 9%; V: 3.2%; Nb: 2.1%) at three different depths of cut (ae) and on a single CONV grade (M2: C: 0.87%; Cr: 4.2%; Mo: 5%; W: 6.4%; V: 1.9%) with poor cooling (coolant velocity = vc = 5 m/s, vc/ vs = 0.20) and improved cooling (vc = 23 m/s, vc/vs = 0.92). Production tests were done on a 25 kW Gefra flute-grinding machine, creep-feed grinding 3.125 mm-diameter M2 HSS drills using a 30%-ceramic-grit 100-mesh resin-bonded wheel with neatoil coolant, normally dressing 30 mm every 11 parts with a singlepoint diamond. Grinding was done at three different feedrates: vw = 600, 780 and 960 mm/min, giving aggressive values [10] of 11.2, 14.6, 18.0 and material-removal rates of 12.9, 16.8 and 20.7 mm3/mm/s, respectively (vs = 58 m/s; ae = 1.292 mm; wheel diameter = 305 mm).
Fig. 2. G-ratio and power at onset of collapse.
3. Results from laboratory tests 3.1. Onset of collapse: relationship to G-ratio and power The relationships between G-ratio, the power at the onset of collapse and the amount of material removed at the time of collapse are shown in Fig. 2. Since G-ratio and power are both parameter-dependant, values for G-ratio are given relative to the standard grade M2 (G-ratio = 2.15) and values of power are given relative to the original power, P0, which was the same for all grades [5] and corresponded to a specific energy, ec, of 130 J/mm3. The figure shows a consistent trend, with grades with low Gratios collapsing much earlier than grades with high G-ratios. Grades that collapsed earlier did so with a lower value of PC. Whether the grade was PM or CONV had no apparent bearing on the results.
Fig. 3. G-ratio and power at onset of collapse.
3.2. Power at collapse and post-collapse power The relationship between the power at collapse, PC, and the steady-state post-collapse power, PSS, is given in Fig. 3. Regardless of the pre-collapse power, post-collapse power was always on the order of 80% of the pre-collapse power. This is similar to what was found by previous researchers [7–9]. Again, the ratio appears to be independent of whether it is a PM grade or a CONV grade. 3.3. Effect of aggressiveness Additional test were done on a single, high-alloy PM grade at three different depths of cut to determine the effect of aggres-
Fig. 4. Effect of aggressiveness on onset of collapse.
J. Badger / CIRP Annals - Manufacturing Technology 58 (2009) 307–310
309
Fig. 5. Effect of cooling on collapse.
siveness on collapse. Aggressiveness, Aggr, is defined as [10] Aggr ¼ 1; 000; 000
yw ys
rffiffiffiffiffi ae ds
(1)
where vw is the table speed, vs is the wheel speed, ae is the depth of cut, ds is the wheel diameter Aggr values were 24.8, 30.4 and 35.1. The results (Fig. 4) showed that more aggressive grinding conditions delayed the onset of collapse or even eliminated it throughout the life of the test. Higher aggressiveness values also decreased the rate of power increase, P/P0, presumably through the size effect and better wheel self-sharpening. 3.4. Effect of cooling Tests with poor cooling (low-velocity coolant, vc/vs = 0.20) and good cooling (high-velocity coolant, vc/vs = 0.92) showed that good cooling drastically reduces the increase in power, delays collapse and decreases the power at collapse (Fig. 5).
4. Results from production tests 4.1. Standard feedrate For the flute-grinding production tests, thirty parts were ground without dressing and measurements were taken of the change in final part dimension. This distance is a combination of (i) wheel wear and (ii) wheel/workpiece deflection, also referred to as ‘push off’, due to the normal force. Each part was examined for visible oxidation burn in the bottom of the flute. The test was done at three different feedrates, vw. The wheel was dressed before each of the three tests. The results are given in Fig. 6. For vw = 600 mm/min (a), parts with significant oxidation burn are indicated in red, those with slight oxidation burn in brown and those with no oxidation burn in black. If we look first at vw = 600 mm/min, after dressing the part dimension starts to increase. At part five oxidation burn appears. By part 10 size has increased by 35 mm. Between part 10 and part 11 the part dimension decreases by about 14 mm and oxidation burn disappears. Since the wheel self-sharpens – it does not ‘grow’ – this must be caused by a decrease in normal force and ‘push off’. Then, part dimension starts to increase again, burn becomes moderate and then severe, and then, at part 22, part dimension falls again by about the same amount, accompanied by the disappearance of oxidation burn. As before this drop occurred after around 10 parts. The cycle then starts to repeat itself before the test was stopped. 4.2. Increased feedrate Next, the wheel was dressed and the feedrate was increased by 30%, to vw = 780 mm/min (b), and the test repeated. Parts 1–5
Fig. 6. Part dimension from production tests.
exhibited no oxidation burn and parts 6–30 exhibited severe oxidation burn. At the higher feedrate, part dimension increased at about the same average rate as before. However, the increase was far more steady, without any apparent collapse of the wheel until part 23, where it appears to be more gradual. Finally, the wheel was dressed again and the feedrate was increased by 60%, to vw = 960 mm/min (c). Parts 1–5 exhibited no visual oxidation burn, parts 6, 11 and 12 slight oxidation burn, and the remainder of the parts severe oxidation burn. This time part dimension increased at a slower rate than both previous tests, with no apparent collapse throughout. 5. Discussion 5.1. Mechanisms Several mechanism can be postulated for the cause of collapse: (i) thermal, with high temperatures causing a phase change in the material and/or a transition from nucleate to film boiling; (ii) loading, with increased accumulation of material leading to high
J. Badger / CIRP Annals - Manufacturing Technology 58 (2009) 307–310
310
collapse occurred had chatter marks (presumably due to the out-of-true wheel), burn, and tolerance issues. Collapse appears to go hand-in-hand with high normal forces. Although collapse occurs earlier in difficult-to-grind grades, Figs. 4 and 6 show that collapse can be delayed by grinding more aggressively. This is true for both resin- and vitrified-bonded wheels. Also, collapse can be delayed and the rate of power increase can be decreased dramatically by higher coolant velocities. 6. Summary
Fig. 7. SEM pictograph of wheel after grinding.
forces and eventual striping of the loading, taking whole grits with it; or (iii) mechanical, with large forces simply causing either grit fracture or bond fracture. It is unlikely that the cause is thermal, as no power surge was evident, as was seen by previous researchers [3,4]. Also, for a given set of grinding conditions, collapse was seen at a wide range of value for power. It is unlikely that the cause is loading, as no evidence of loading was seen in SEM photographs taken after grinding (Fig. 7), either at the grain tips or within the porosity of the wheel. Collapse was very dependent, however, on wheel dulling, as seen in all examples, with grades that dulled more rapidly collapsing much earlier. However, for a given set of grinding conditions, collapse occurred over a wide range of power values (and, hence, values for resultant force). Grades with higher G-ratios collapsed at much higher power. This could be explained by the change in wheel topography with wheel dulling and wheel wear. As the wheel dulls, forces acting on individual cutting points increase, and either grit fracture or bond fracture occurs. If this self-sharpening of the wheel occurs at a slow rate (as in the less-aggressive grinding conditions or in high-grindability steel grades), there will not be a high-enough degree of wheel self-sharpening, and normal forces will be extremely high, resulting in early collapse. 5.2. Clustered wear In Fig. 3, we see that post-collapse power is directly related to power at collapse. This means that at least some of the pre-collapse topography of the wheel still exists after collapse. The decrease in power is caused by a higher maximum chip thickness after collapse due to an effective greater grit spacing from the missing grit clusters, which results in a lower specific energy. Also, no drastic changes in part dimension were seen in Fig. 6, indicating that, if wheel wear is extreme, it is only extreme in clusters around the circumference. 5.3. Practical implications Wheel collapse is a phenomenon that should be avoided because of chatter. All specimens in laboratory tests where
b Collapse is rapid wheel wear occurring in localized clusters around the wheel circumference. b Post-collapse power is consistently around 80% of power at collapse. b Collapse is followed by lower grinding power, less grinding burn, and lower normal forces. b Steel grades with low G-ratios collapse earlier than grades with high G-ratios. b Steel grades with low G-ratios collapse at lower values of power than grades with high G-ratios. b More aggressive grinding conditions delay the onset of collapse, most likely due to better self-sharpening conditions. b Better cooling delays the onset of collapse. b Better cooling greatly reduces the power at collapse. b Collapse is most likely caused by wheel dulling and high grinding forces prior to collapse, but this has not been proven. b More consistent wheel hardness may affect the onset of collapse.
Acknowledgments The author would like to thank Stephen Malkin; Mike Hitchiner; Tony Hudson at Sutton Tools, Australia; and Erasteel Kloster in So¨derfors, Sweden. References [1] Malkin S, Cook NH (1971) The Wear of Grinding Wheels: Part 1—Attritious Wear. Transactions of ASME Journal of Engineering for Industry 933:1120– 1128. [2] Krabacher EJ (1959) Factors Influencing the Performance of Grinding Wheels. Transactions of ASME Journal of Engineering for Industry 81:187– 200. [3] Malkin S, Guo C (2008) Grinding Technology: Theory and Applications of Machining with Abrasives. second ed. . pp. 189–192. [4] Andrew C, Howes T, Pearce T (1985) Creep Feed Grinding. Holt, Rinehart and Winston, Eastbourne, East Sussex, UK. pp. 76–85. [5] Badger J (2007) Grindability of Conventionally Produced and Powder-Metallurgy High-Speed Steel. Annals of CIRP 56/1:353–356. [6] Slip Naxos. Va¨stervik Sweden. (1995) Handbok i precisionsslipning. . p. 17. [7] Malkin S, Murray T (1977) Comparison of Single Point and Rotary Dressing of Grinding Wheels. Fifth North American Metalworking Research Conference, May 23–25, 278–283. [8] Sultana S (2004) Effect of Dressing Parameters on Grinding Wheel Wear, Master’s Thesis, Dalhousie University, Halifax, Novia Scotia, pps. 44–46, 54. [9] Metzger J (1989) Wheel Performance in Superabrasive Grinding. Industrial Diamond Review 3/89:116–117. [10] Badger J (2008) Practical Application of Aggressiveness and Chip Thickness in Grinding. Annals of the CIRP 3rd International Conference High Performance Cutting (HPC), Dublin, Ireland, 599–606.
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er. com/ci rp/ def a ult . asp
Factors affecting wheel collapse in grinding J. Badger The Grinding Doc Consulting, Austin, TX, USA Submitted by J. Webster (1), Cool-Grind Technologies
A R T I C L E I N F O
A B S T R A C T
Keywords: Grinding Machinability Roundness
An investigation was made into the phenomenon of wheel collapse, where power increases steadily with attritious wear until a critical value is reached, upon which power drops rapidly to a steady-state value and the wheel loses its roundness. Causes and mechanisms were investigated, along with the relationships between amount of material removed at collapse, power at collapse, post-collapse steadystate power, cooling, wheel self-sharpening, and G-ratio. Examples from laboratory tests and production tests are used. Practical recommendations are given for delaying or eliminating collapse. ß 2009 CIRP.
1. Introduction Wear of grinding wheels is typically divided into three regimes: attritious wear, grit fracture, and bond fracture [1]. Attritious wear is associated with larger normal and tangential forces and, consequently, higher power. The degree of grit fracture and bond fracture determines the amount of self-sharpening of the wheel. A typical wheel-wear profile [2] shows a high degree of initial wheel wear as the wheel ‘cleans up’ loosely held grits, followed by a stable regime of steady wheel wear, and finally a regime of rapid wear, often accompanied by ‘grinding burn’ [1] and/or a transition from nucleate boiling to film boiling [3,4]. This is sometimes referred to as a ‘surge’ in power. Much research has been done on this phenomenon, which has been summarized previously [3,4]. There is another form of wheel wear, where power, beginning at some initial value P0, increases gradually with attritious wear until a critical value, PC, is reached. At this point wheel wear is rapid and power drops to a steady-state value, PSS. Aside from the gradual increase and decrease in power, there is no visible surge accompanying this phenomenon. A typical power profile is given in Fig. 1 [5]. Little has been written about the subject. One grinding-wheel manufacturer referred to this phenomenon as wheel ‘collapse’ in a technical brochure [6], stating that it is caused by groups of grits breaking off at once, resulting in an out-of-round wheel. Other authors have experienced in their tests, but have not elaborated on it. Malkin and Murray [7] found normal and tangential forces, Fn and Ft, increased until a maximum value was reached, then decreased to a steady-state value of around 65–90% of the maximum, where it remained for the remainder of the test. They also found that less-aggressive dressing conditions increased not only the initial forces, but also the forces at collapse, meaning collapse could occur over a wide range of forces. Different dressing conditions did not appear to affect significantly the amount of material removed before 0007-8506/$ – see front matter ß 2009 CIRP. doi:10.1016/j.cirp.2009.03.048
the onset of collapse. Sultana [8] found that dressing affects the power and forces at the onset of collapse, that dressing did not affect the amount of work material removed upon collapse, and that post-collapse power was 70–90% the maximum power. Metzger [9] found unpredictable decreases in grinding power when surface grinding tungsten-carbide with a resin-bonded diamond wheel, with power after collapse being 65–80% of the power at collapse. G-ratios in experiments that experienced collapse were on average 50% lower than experiments where collapse did not occur. Metzger explained this phenomenon as grits from one section along the circumference of the wheel being pulled out due to wear-flat development, which in turn puts a larger load on grits along another section of the circumference, which initiates pull-out there, with this cycle leading to overall rapid macroscopic wheel wear. Unfortunately, aside from Metzger’s short paper, very little has been written about this phenomenon. This paper describes an investigation into the phenomenon of collapse, using data from laboratory tests and actual production operations. It explores the mechanisms behind collapse, its relationship to grindability and G-ratio, forces and power, material removed upon collapse, attritious wear, and post-collapse power. It also examines the effect of cooling and grinding aggressiveness on collapse. Finally, it gives practical recommendations on delaying or eliminating collapse to increase productions rates and reduce the risk of burn and chatter. 2. Experimental Experiments were done both in the laboratory and on the shop floor in a production environment. Laboratory tests, described previously [5], were done on a 3.5 kW Chevalier surface grinder with a 100%-white-Al203, 46-grit, H-grade, vitrified-bonded grinding wheel on various grades of powder-metallurgy (PM) and conventionally produced (CONV) high-speed steel using 6%
J. Badger / CIRP Annals - Manufacturing Technology 58 (2009) 307–310
308
Fig. 1. Power profile.
soluble-oil coolant. The grinding power was measured with a Hall-effect power meter (resolution 0.04 kW) sampling at 20 Hz. The net grinding power was calculated by subtracting the idle power and the power to accelerate the coolant (idle power + coolant-acceleration power = 1.6 kW). Additional tests were done on a single PM grade (ASP2055: C: 1.65%; Cr: 4.0%; Mo: 4.6%; W: 6.3%; Co: 9%; V: 3.2%; Nb: 2.1%) at three different depths of cut (ae) and on a single CONV grade (M2: C: 0.87%; Cr: 4.2%; Mo: 5%; W: 6.4%; V: 1.9%) with poor cooling (coolant velocity = vc = 5 m/s, vc/ vs = 0.20) and improved cooling (vc = 23 m/s, vc/vs = 0.92). Production tests were done on a 25 kW Gefra flute-grinding machine, creep-feed grinding 3.125 mm-diameter M2 HSS drills using a 30%-ceramic-grit 100-mesh resin-bonded wheel with neatoil coolant, normally dressing 30 mm every 11 parts with a singlepoint diamond. Grinding was done at three different feedrates: vw = 600, 780 and 960 mm/min, giving aggressive values [10] of 11.2, 14.6, 18.0 and material-removal rates of 12.9, 16.8 and 20.7 mm3/mm/s, respectively (vs = 58 m/s; ae = 1.292 mm; wheel diameter = 305 mm).
Fig. 2. G-ratio and power at onset of collapse.
3. Results from laboratory tests 3.1. Onset of collapse: relationship to G-ratio and power The relationships between G-ratio, the power at the onset of collapse and the amount of material removed at the time of collapse are shown in Fig. 2. Since G-ratio and power are both parameter-dependant, values for G-ratio are given relative to the standard grade M2 (G-ratio = 2.15) and values of power are given relative to the original power, P0, which was the same for all grades [5] and corresponded to a specific energy, ec, of 130 J/mm3. The figure shows a consistent trend, with grades with low Gratios collapsing much earlier than grades with high G-ratios. Grades that collapsed earlier did so with a lower value of PC. Whether the grade was PM or CONV had no apparent bearing on the results.
Fig. 3. G-ratio and power at onset of collapse.
3.2. Power at collapse and post-collapse power The relationship between the power at collapse, PC, and the steady-state post-collapse power, PSS, is given in Fig. 3. Regardless of the pre-collapse power, post-collapse power was always on the order of 80% of the pre-collapse power. This is similar to what was found by previous researchers [7–9]. Again, the ratio appears to be independent of whether it is a PM grade or a CONV grade. 3.3. Effect of aggressiveness Additional test were done on a single, high-alloy PM grade at three different depths of cut to determine the effect of aggres-
Fig. 4. Effect of aggressiveness on onset of collapse.
J. Badger / CIRP Annals - Manufacturing Technology 58 (2009) 307–310
309
Fig. 5. Effect of cooling on collapse.
siveness on collapse. Aggressiveness, Aggr, is defined as [10] Aggr ¼ 1; 000; 000
yw ys
rffiffiffiffiffi ae ds
(1)
where vw is the table speed, vs is the wheel speed, ae is the depth of cut, ds is the wheel diameter Aggr values were 24.8, 30.4 and 35.1. The results (Fig. 4) showed that more aggressive grinding conditions delayed the onset of collapse or even eliminated it throughout the life of the test. Higher aggressiveness values also decreased the rate of power increase, P/P0, presumably through the size effect and better wheel self-sharpening. 3.4. Effect of cooling Tests with poor cooling (low-velocity coolant, vc/vs = 0.20) and good cooling (high-velocity coolant, vc/vs = 0.92) showed that good cooling drastically reduces the increase in power, delays collapse and decreases the power at collapse (Fig. 5).
4. Results from production tests 4.1. Standard feedrate For the flute-grinding production tests, thirty parts were ground without dressing and measurements were taken of the change in final part dimension. This distance is a combination of (i) wheel wear and (ii) wheel/workpiece deflection, also referred to as ‘push off’, due to the normal force. Each part was examined for visible oxidation burn in the bottom of the flute. The test was done at three different feedrates, vw. The wheel was dressed before each of the three tests. The results are given in Fig. 6. For vw = 600 mm/min (a), parts with significant oxidation burn are indicated in red, those with slight oxidation burn in brown and those with no oxidation burn in black. If we look first at vw = 600 mm/min, after dressing the part dimension starts to increase. At part five oxidation burn appears. By part 10 size has increased by 35 mm. Between part 10 and part 11 the part dimension decreases by about 14 mm and oxidation burn disappears. Since the wheel self-sharpens – it does not ‘grow’ – this must be caused by a decrease in normal force and ‘push off’. Then, part dimension starts to increase again, burn becomes moderate and then severe, and then, at part 22, part dimension falls again by about the same amount, accompanied by the disappearance of oxidation burn. As before this drop occurred after around 10 parts. The cycle then starts to repeat itself before the test was stopped. 4.2. Increased feedrate Next, the wheel was dressed and the feedrate was increased by 30%, to vw = 780 mm/min (b), and the test repeated. Parts 1–5
Fig. 6. Part dimension from production tests.
exhibited no oxidation burn and parts 6–30 exhibited severe oxidation burn. At the higher feedrate, part dimension increased at about the same average rate as before. However, the increase was far more steady, without any apparent collapse of the wheel until part 23, where it appears to be more gradual. Finally, the wheel was dressed again and the feedrate was increased by 60%, to vw = 960 mm/min (c). Parts 1–5 exhibited no visual oxidation burn, parts 6, 11 and 12 slight oxidation burn, and the remainder of the parts severe oxidation burn. This time part dimension increased at a slower rate than both previous tests, with no apparent collapse throughout. 5. Discussion 5.1. Mechanisms Several mechanism can be postulated for the cause of collapse: (i) thermal, with high temperatures causing a phase change in the material and/or a transition from nucleate to film boiling; (ii) loading, with increased accumulation of material leading to high
J. Badger / CIRP Annals - Manufacturing Technology 58 (2009) 307–310
310
collapse occurred had chatter marks (presumably due to the out-of-true wheel), burn, and tolerance issues. Collapse appears to go hand-in-hand with high normal forces. Although collapse occurs earlier in difficult-to-grind grades, Figs. 4 and 6 show that collapse can be delayed by grinding more aggressively. This is true for both resin- and vitrified-bonded wheels. Also, collapse can be delayed and the rate of power increase can be decreased dramatically by higher coolant velocities. 6. Summary
Fig. 7. SEM pictograph of wheel after grinding.
forces and eventual striping of the loading, taking whole grits with it; or (iii) mechanical, with large forces simply causing either grit fracture or bond fracture. It is unlikely that the cause is thermal, as no power surge was evident, as was seen by previous researchers [3,4]. Also, for a given set of grinding conditions, collapse was seen at a wide range of value for power. It is unlikely that the cause is loading, as no evidence of loading was seen in SEM photographs taken after grinding (Fig. 7), either at the grain tips or within the porosity of the wheel. Collapse was very dependent, however, on wheel dulling, as seen in all examples, with grades that dulled more rapidly collapsing much earlier. However, for a given set of grinding conditions, collapse occurred over a wide range of power values (and, hence, values for resultant force). Grades with higher G-ratios collapsed at much higher power. This could be explained by the change in wheel topography with wheel dulling and wheel wear. As the wheel dulls, forces acting on individual cutting points increase, and either grit fracture or bond fracture occurs. If this self-sharpening of the wheel occurs at a slow rate (as in the less-aggressive grinding conditions or in high-grindability steel grades), there will not be a high-enough degree of wheel self-sharpening, and normal forces will be extremely high, resulting in early collapse. 5.2. Clustered wear In Fig. 3, we see that post-collapse power is directly related to power at collapse. This means that at least some of the pre-collapse topography of the wheel still exists after collapse. The decrease in power is caused by a higher maximum chip thickness after collapse due to an effective greater grit spacing from the missing grit clusters, which results in a lower specific energy. Also, no drastic changes in part dimension were seen in Fig. 6, indicating that, if wheel wear is extreme, it is only extreme in clusters around the circumference. 5.3. Practical implications Wheel collapse is a phenomenon that should be avoided because of chatter. All specimens in laboratory tests where
b Collapse is rapid wheel wear occurring in localized clusters around the wheel circumference. b Post-collapse power is consistently around 80% of power at collapse. b Collapse is followed by lower grinding power, less grinding burn, and lower normal forces. b Steel grades with low G-ratios collapse earlier than grades with high G-ratios. b Steel grades with low G-ratios collapse at lower values of power than grades with high G-ratios. b More aggressive grinding conditions delay the onset of collapse, most likely due to better self-sharpening conditions. b Better cooling delays the onset of collapse. b Better cooling greatly reduces the power at collapse. b Collapse is most likely caused by wheel dulling and high grinding forces prior to collapse, but this has not been proven. b More consistent wheel hardness may affect the onset of collapse.
Acknowledgments The author would like to thank Stephen Malkin; Mike Hitchiner; Tony Hudson at Sutton Tools, Australia; and Erasteel Kloster in So¨derfors, Sweden. References [1] Malkin S, Cook NH (1971) The Wear of Grinding Wheels: Part 1—Attritious Wear. Transactions of ASME Journal of Engineering for Industry 933:1120– 1128. [2] Krabacher EJ (1959) Factors Influencing the Performance of Grinding Wheels. Transactions of ASME Journal of Engineering for Industry 81:187– 200. [3] Malkin S, Guo C (2008) Grinding Technology: Theory and Applications of Machining with Abrasives. second ed. . pp. 189–192. [4] Andrew C, Howes T, Pearce T (1985) Creep Feed Grinding. Holt, Rinehart and Winston, Eastbourne, East Sussex, UK. pp. 76–85. [5] Badger J (2007) Grindability of Conventionally Produced and Powder-Metallurgy High-Speed Steel. Annals of CIRP 56/1:353–356. [6] Slip Naxos. Va¨stervik Sweden. (1995) Handbok i precisionsslipning. . p. 17. [7] Malkin S, Murray T (1977) Comparison of Single Point and Rotary Dressing of Grinding Wheels. Fifth North American Metalworking Research Conference, May 23–25, 278–283. [8] Sultana S (2004) Effect of Dressing Parameters on Grinding Wheel Wear, Master’s Thesis, Dalhousie University, Halifax, Novia Scotia, pps. 44–46, 54. [9] Metzger J (1989) Wheel Performance in Superabrasive Grinding. Industrial Diamond Review 3/89:116–117. [10] Badger J (2008) Practical Application of Aggressiveness and Chip Thickness in Grinding. Annals of the CIRP 3rd International Conference High Performance Cutting (HPC), Dublin, Ireland, 599–606.