Novel Kinematics for Cylindrical Grinding of Brittle Materials
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy1, Y. Zhou1, C. Guo2, R. Chand3 1McMaster
University, Canada 2United Technologies Research Center, U.S.A. 3PremaTech Chand, U.S.A.
CIRP General Assembly: August 23, 2005
www.hexoloy.com
Application of brittle materials in high performance structural applications continues to be elusive despite concerted global research efforts in the last two decades
2 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Strength of brittle materials is affected by machining-induced microscopic flaws, which has an adverse influence on component performance and reliability 100 μm
ground surface
fracture surface
3 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
www.cartech.com
Material removal rates currently employed are conservative with a view to controlling surface integrity, which adds to the machining costs that are already prohibitive It is hence essential to maximize machining productivity with reference to strength degrading surface damage
4 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
A novel materialadapted cylindrical grinding process that facilitates enhanced removal rates with the least detriment to strength is presented
5 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
σT radial crack
median crack
σP
σP σT
grinding direction
Grinding of brittle materials is characterized by strength anisotropy with reference to the grinding direction, brought about by a dual population of grinding-induced microcracks Median cracks along the direction of grinding are usually larger than radial cracks that are across
σP > σT 6
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
TRANSVERSE
LONGITUDINAL
σT
σP
σP > σT
Brittle components are ground such that the grinding direction is along the application of the maximum tensile stress 7
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
[Jahanmir et al., 1998] No. of Specimens
80 Longitudinal Transverse 70 60 50 Silicon Nitride 40 30 20 10 0 200 300 400 500 600 700 800 900
Flexure Strength (MPa)
The ratio of strengths (σP /σT) depends on the material, grain size and porosity, and could be as high as 2 [Rice, 2002] ASTM C 1161 (1994): Standard test method for flexural strength of advanced ceramics at ambient temperature 8
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
TRANSVERSE
X
LONGITUDINAL
9 TRANSVERSE
www.minnesotagrinding.com
Cylindrical grinding of brittle components in conventional machine tools is bound to degrade strength, since failure in flexure is initiated at the larger median cracks 9
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
CONVENTIONAL
NOVEL
The novel process is realized through the rotation of the wheel such that the grinding lay is along the length of the component rather than across
10 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
CONVENTIONAL
NOVEL
In the novel process, the wheel-work contact area is independent of the wheel width For typical grinding parameters, the contact area would hence be lower, and so would be the forces Wheel wear in the novel process occurs along a thin circumferential band Wear can be distributed uniformly by either inclining the work, or by implementing a crossfeed 11
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
CONVENTIONAL
bs
ap
vft
vw
NOVEL
ds
vw
vft
ap
lc
Overlap ratio in the conventional process = (bs/fa), where fa is the feed/rev Overlap ratio in the novel process = (lc/fa), where lc is the geometric wheel-work contact length For the same machining time and overlap ratio, the work speed in the novel process need be higher by a factor of (bs/lc) 12
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Work material: fused silica rods (GE Type 214 Quartz) 100 mm long; diameter reduced from 7 to 6.5 mm Wet; down grinding No spark-out Identical grinding cycle time between processes
1A1 Diamond wheel, ϕ203 mm, 12.7 mm wide 140/170 grit, 75 concentration, resin bond Wheel speed Work rotational speed Work axial feed Wheel depth of cut
30 m/s 441 rpm 2.54 mm/s 10 μm/pass
TRANSVERSE
wheel
work LONGITUDINAL
wheel
work
13 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Self-aligning V-blocks
20 mm
Ground samples were tested in a four-point flexure fixture meant for round rods 15 ground samples each of novel and conventional configurations, and 15 asdrawn samples were tested in a random order 14
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Probability of Failure (%)
99.9
conventional
99 95
novel
80 60 40 20 5
as-drawn
1 40
60
80
100
120
140
Fracture Stress (MPa)
Process
Characteristic Strength (MPa)
Weibull Modulus
Novel
83.5
15.2
Conventional
64.1
8.2
The novel process corresponded to a 30% enhancement in Characteristic Strength and a higher Weibull Modulus Machining damage was the single active flaw population in ground samples
15 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
1 mm
AS-DRAWN
NOVEL
1 mm
Fracture mirrors displayed minimal mist/hackle, were incomplete and were elongated in the radial direction
CONVENTIONAL
1 mm 16 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
sample
fracture surface
FS
FS
100 μm
100 μm
GS
GS NOVEL
CONVENTIONAL
Flaws induced in the conventional process were significantly larger Fracture origins comprised distinctive V-features and were semi-elliptical The orientation of the ellipse depended on the process 17
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
σ∝Φ
a
π 2
NOVEL
100 μm
Φ( a b ) = ∫
cos 2 θ + ( a b )2 sin 2 θ dθ
0
is the stress intensity shape factor
σ n σ c = ( ac an ) ( Φ n Φc )
100 μm
CONVENTIONAL
Process
Average Strength (MPa)
Average flaw size a (μm)
Average flaw aspect ratio (a/b)
Ф(a/b)
Novel (n)
80.6
32
0.61
1.286
Conventional (c)
60.7
72
0.54
1.239
{
σn σc =
1.33 (measured) 1.56 (analysis)
18 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Relative Frequency
0.6 conventional novel
0.5
0.4 0.3 0.2 0.1 0.0 0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Surface Roughness Ra (μm) NOVEL
CONVENTIONAL
The mean roughness obtained in the novel process (1.31±0.04 μm Ra) is fairly higher than that in the conventional process (1.00±0.04 μm Ra) This is due to the overlap ratio in the novel process being an order of magnitude lower, which pertains to a relatively insignificant spark-out 19
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Flexural Strength (MPa)
Extent of spark-out can be enhanced in the novel process without incurring any increase in the grinding cycle time by increasing the work speed Increase in the work speed would increase the grit depth of cut In the novel process this can be expected to not have any adverse effect on strength
LONGITUDINAL
1000 800 600 400
TRANSVERSE Silicon Nitride
200 0
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
Grit Depth of Cut (microns) [Strakna et al., 1996]
800
Char. Strength (MPa)
[Mayer and Fang, 1994]
1200
700
LONGITUDINAL
600 500 400
TRANSVERSE
300 200 Silicon Nitride
100 0 0
500
1000
1500
2000
3
Vol. Removal Rate (mm/min) Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
20
55th CIRP General Assembly August 2005, Antalya, Turkey
The novel process can also be accomplished using a cup-wheel The grinding lay will depend on the position of the work with respect to the wheel center 21
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Conclusions
The scientific basis and proof-of-concept for a kinematic configuration especially suited for cylindrical grinding of brittle materials is presented For the same cycle time, the proposed novel configuration related to reduced strength variability and a 30% increase in Characteristic Strength, in the grinding of quartz samples Implications of this technology are significant in that several components made of brittle materials comprise cylindrical features that require grinding 22
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
For more information please see: P. Koshy, Y. Zhou, C. Guo, R. Chand, Novel kinematics for cylindrical grinding of brittle materials, Annals of CIRP 54 (2005) 289-292.
23 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
University, Canada 2United Technologies Research Center, U.S.A. 3PremaTech Chand, U.S.A.
CIRP General Assembly: August 23, 2005
www.hexoloy.com
Application of brittle materials in high performance structural applications continues to be elusive despite concerted global research efforts in the last two decades
2 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Strength of brittle materials is affected by machining-induced microscopic flaws, which has an adverse influence on component performance and reliability 100 μm
ground surface
fracture surface
3 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
www.cartech.com
Material removal rates currently employed are conservative with a view to controlling surface integrity, which adds to the machining costs that are already prohibitive It is hence essential to maximize machining productivity with reference to strength degrading surface damage
4 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
A novel materialadapted cylindrical grinding process that facilitates enhanced removal rates with the least detriment to strength is presented
5 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
σT radial crack
median crack
σP
σP σT
grinding direction
Grinding of brittle materials is characterized by strength anisotropy with reference to the grinding direction, brought about by a dual population of grinding-induced microcracks Median cracks along the direction of grinding are usually larger than radial cracks that are across
σP > σT 6
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
TRANSVERSE
LONGITUDINAL
σT
σP
σP > σT
Brittle components are ground such that the grinding direction is along the application of the maximum tensile stress 7
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
[Jahanmir et al., 1998] No. of Specimens
80 Longitudinal Transverse 70 60 50 Silicon Nitride 40 30 20 10 0 200 300 400 500 600 700 800 900
Flexure Strength (MPa)
The ratio of strengths (σP /σT) depends on the material, grain size and porosity, and could be as high as 2 [Rice, 2002] ASTM C 1161 (1994): Standard test method for flexural strength of advanced ceramics at ambient temperature 8
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
TRANSVERSE
X
LONGITUDINAL
9 TRANSVERSE
www.minnesotagrinding.com
Cylindrical grinding of brittle components in conventional machine tools is bound to degrade strength, since failure in flexure is initiated at the larger median cracks 9
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
CONVENTIONAL
NOVEL
The novel process is realized through the rotation of the wheel such that the grinding lay is along the length of the component rather than across
10 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
CONVENTIONAL
NOVEL
In the novel process, the wheel-work contact area is independent of the wheel width For typical grinding parameters, the contact area would hence be lower, and so would be the forces Wheel wear in the novel process occurs along a thin circumferential band Wear can be distributed uniformly by either inclining the work, or by implementing a crossfeed 11
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
CONVENTIONAL
bs
ap
vft
vw
NOVEL
ds
vw
vft
ap
lc
Overlap ratio in the conventional process = (bs/fa), where fa is the feed/rev Overlap ratio in the novel process = (lc/fa), where lc is the geometric wheel-work contact length For the same machining time and overlap ratio, the work speed in the novel process need be higher by a factor of (bs/lc) 12
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Work material: fused silica rods (GE Type 214 Quartz) 100 mm long; diameter reduced from 7 to 6.5 mm Wet; down grinding No spark-out Identical grinding cycle time between processes
1A1 Diamond wheel, ϕ203 mm, 12.7 mm wide 140/170 grit, 75 concentration, resin bond Wheel speed Work rotational speed Work axial feed Wheel depth of cut
30 m/s 441 rpm 2.54 mm/s 10 μm/pass
TRANSVERSE
wheel
work LONGITUDINAL
wheel
work
13 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Self-aligning V-blocks
20 mm
Ground samples were tested in a four-point flexure fixture meant for round rods 15 ground samples each of novel and conventional configurations, and 15 asdrawn samples were tested in a random order 14
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Probability of Failure (%)
99.9
conventional
99 95
novel
80 60 40 20 5
as-drawn
1 40
60
80
100
120
140
Fracture Stress (MPa)
Process
Characteristic Strength (MPa)
Weibull Modulus
Novel
83.5
15.2
Conventional
64.1
8.2
The novel process corresponded to a 30% enhancement in Characteristic Strength and a higher Weibull Modulus Machining damage was the single active flaw population in ground samples
15 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
1 mm
AS-DRAWN
NOVEL
1 mm
Fracture mirrors displayed minimal mist/hackle, were incomplete and were elongated in the radial direction
CONVENTIONAL
1 mm 16 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
sample
fracture surface
FS
FS
100 μm
100 μm
GS
GS NOVEL
CONVENTIONAL
Flaws induced in the conventional process were significantly larger Fracture origins comprised distinctive V-features and were semi-elliptical The orientation of the ellipse depended on the process 17
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
σ∝Φ
a
π 2
NOVEL
100 μm
Φ( a b ) = ∫
cos 2 θ + ( a b )2 sin 2 θ dθ
0
is the stress intensity shape factor
σ n σ c = ( ac an ) ( Φ n Φc )
100 μm
CONVENTIONAL
Process
Average Strength (MPa)
Average flaw size a (μm)
Average flaw aspect ratio (a/b)
Ф(a/b)
Novel (n)
80.6
32
0.61
1.286
Conventional (c)
60.7
72
0.54
1.239
{
σn σc =
1.33 (measured) 1.56 (analysis)
18 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Relative Frequency
0.6 conventional novel
0.5
0.4 0.3 0.2 0.1 0.0 0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Surface Roughness Ra (μm) NOVEL
CONVENTIONAL
The mean roughness obtained in the novel process (1.31±0.04 μm Ra) is fairly higher than that in the conventional process (1.00±0.04 μm Ra) This is due to the overlap ratio in the novel process being an order of magnitude lower, which pertains to a relatively insignificant spark-out 19
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Flexural Strength (MPa)
Extent of spark-out can be enhanced in the novel process without incurring any increase in the grinding cycle time by increasing the work speed Increase in the work speed would increase the grit depth of cut In the novel process this can be expected to not have any adverse effect on strength
LONGITUDINAL
1000 800 600 400
TRANSVERSE Silicon Nitride
200 0
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
Grit Depth of Cut (microns) [Strakna et al., 1996]
800
Char. Strength (MPa)
[Mayer and Fang, 1994]
1200
700
LONGITUDINAL
600 500 400
TRANSVERSE
300 200 Silicon Nitride
100 0 0
500
1000
1500
2000
3
Vol. Removal Rate (mm/min) Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
20
55th CIRP General Assembly August 2005, Antalya, Turkey
The novel process can also be accomplished using a cup-wheel The grinding lay will depend on the position of the work with respect to the wheel center 21
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
Conclusions
The scientific basis and proof-of-concept for a kinematic configuration especially suited for cylindrical grinding of brittle materials is presented For the same cycle time, the proposed novel configuration related to reduced strength variability and a 30% increase in Characteristic Strength, in the grinding of quartz samples Implications of this technology are significant in that several components made of brittle materials comprise cylindrical features that require grinding 22
Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
For more information please see: P. Koshy, Y. Zhou, C. Guo, R. Chand, Novel kinematics for cylindrical grinding of brittle materials, Annals of CIRP 54 (2005) 289-292.
23 Novel Kinematics for Cylindrical Grinding of Brittle Materials P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly August 2005, Antalya, Turkey
