Novel cylindrical grinding kinematic for 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.
Submitted by Professor Malkin
CIRP General Assembly: August 23, 2005

P. Koshy, Y. Zhou, C. Guo, R. Chand
55th CIRP General Assembly
August 2005, Antalya, Turkey
 Application of brittle
materials in high
performance structural
applications continues
to be elusive despite
concerted global
research efforts in the
last two decades
www.hexoloy.com
2

 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

 Material removal rates
currently employed are
conservative with a view to
controlling surface integrity,
which adds to the
machining costs that are
already prohibitive
www.cartech.com

It is hence essential to maximize machining productivity with reference to strength degrading surface damage
4

 A novel material-adapted
cylindrical
grinding process that
facilitates enhanced
removal rates with the
least detriment to
strength is presented
5

σ
median crack
P
σ
radial crack
grinding direction
T
σ
P
T
σ
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
σ
P
σ
T
>

Median cracks along the direction of grinding are usually larger than radial cracks that are across
6

P
σ
TRANSVERSE
T
σ
LONGITUDINAL
Brittle components are ground such that the grinding direction is along the application of the maximum tensile stress
σ
P
σ
T
>
7

 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

[Jahanmir et al., 1998]
Flexure Strength (MPa)
200
300
400
500
600
700
800
900
0
10
20
30
40
50
60
70
80
Transverse
No. of Specimens
Longitudinal
Silicon Nitride
8

LONGITUDINAL

TRANSVERSE
X
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
TRANSVERSE
www.minnesotagrinding.com
9

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

 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 cross-feed
CONVENTIONAL
NOVEL
11

b
s
v
ft
a
p
v
w
CONVENTIONAL
Overlap ratio in the conventional process = (bs/fa), where fais the feed/rev
l
c
d
s
v
w
v
ft
a
p
NOVEL

Overlap ratio in the novel process = (lc/fa), where lcis 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

 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 speed30 m/s
Work rotational speed441 rpm
Work axial feed2.54 mm/s
Wheel depth of cut10 μm/pass
TRANSVERSE
LONGITUDINAL
work
work
wheel
wheel
13

 Ground samples were tested
in a four-point flexure
fixture meant for round rods
20 mm
Self-aligning
V-blocks

15 ground samples each of novel and conventional configurations, and 15 as- drawn samples were tested in a random order
14

140
Probability of Failure (%)
40
60
80
100
120
1
5
20
40
60
80
95
99
99.9
Fracture Stress (MPa)
novel
conventional
as-drawn
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

1 mm
Fracture mirrors displayed minimal mist/hackle, were incomplete and were elongated in the radial direction
1 mm
NOVEL
CONVENTIONAL
1 mm
AS-DRAWN
16

 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
FS
GS
100 μm
FS
GS
100 μm
NOVEL
CONVENTIONAL
fracture surface
sample
17

)()aa(cnnccnΦΦ=σσθθθπ dsin)ba(cos)ba(∫+=Φ20222
is the stress intensity shape factor
a
σ
Φ
∝
100 μm
NOVEL
CONVENTIONAL
100 μm
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
{
=cnσσ
1.33 (measured)
1.56 (analysis)
18

Relative
Frequency
2.0
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Surface Roughness Ra(μm)
conventional
novel
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
NOVEL
CONVENTIONAL
19

 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
[Strakna et al., 1996]
0
500
1000
1500
2000
0
100
200
300
400
500
600
700
800
Char. Strength (MPa)
3
Vol. Removal Rate (mm
/min)
LONGITUDINAL
TRANSVERSE
Silicon Nitride
[Mayer and Fang, 1994]
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0
200
400
600
800
1000
1200
Flexural Strength (MPa)
Grit Depth of Cut (microns)
LONGITUDINAL
TRANSVERSE
Silicon Nitride

In the novel process this can be expected to not have any adverse effect on strength

Increase in the work speed would increase the grit depth of cut
20

 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

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 cylindrical grinding kinematic for brittle materials

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