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, Canada2United Technologies Research Center, U.S.A.

3PremaTech Chand, U.S.A.

Submitted by Professor Malkin

CIRP General Assembly: August 23, 2005

Novel Kinematics for Cylindrical Grinding of Brittle Materials 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

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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 prohibitivew

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It is hence essential to maximize machining productivity with reference to strength degrading surface damage

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

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σ

median crack

Pσ

radial crack

grinding directionTσ

P

Tσ

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

σPσ T> Median cracks along the direction of grinding are usually larger than radial cracks that are across

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Pσ

TRANSVERSE

Tσ

LONGITUDINAL

Brittle components are ground such that the grinding direction is along the application of the maximum tensile stress

σPσ T>

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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

01020304050607080 Transverse

No.

of S

peci

men

s Longitudinal

Silicon Nitride

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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

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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

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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

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bs

vft

ap

vw

CONVENTIONAL

Overlap ratio in the conventional process = (bs/fa), where fa is the feed/rev

lc

ds

vwvft

ap

NOVEL

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)

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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 wide140/170 grit, 75 concentration, resin bond

Wheel speed 30 m/sWork rotational speed 441 rpmWork axial feed 2.54 mm/sWheel depth of cut 10 µm/pass

TRANSVERSE

LONGITUDINAL

work

work

wheel

wheel

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Ground samples were tested in a four-point flexure fixture meant for round rods

20 mm

Self-aligningV-blocks

15 ground samples each of novel and conventional configurations, and 15 as-drawn samples were tested in a random order

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140

Pro

babi

lity

of F

ailu

re (%

)

40 60 80 100 12015

20406080

9599

99.9

Fracture Stress (MPa)

novelconventional

as-drawn

Process Characteristic Strength (MPa)

WeibullModulus

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

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1 mm

Fracture mirrors displayed minimal mist/hackle, were incomplete and were elongated in the radial direction

1 mmNOVEL

CONVENTIONAL1 mm

AS-DRAWN

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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

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)()aa( cnnccn ΦΦ=σσ

θθθπ

dsin)ba(cos)ba( ∫ +=Φ2

0

222

is the stress intensity shape factor

aσ 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 1.286Conventional (c) 60.7 72 0.54 1.239

{=cn σσ 1.33 (measured)1.56 (analysis)

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Rel

ativ

e Fr

eque

ncy

2.00.6 0.8 1.0 1.2 1.4 1.6 1.80.0

0.1

0.2

0.3

0.4

0.5

0.6

Surface Roughness Ra (µm)

conventionalnovel 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

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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 20000

100

200

300

400

500

600

700

800

Cha

r. S

treng

th (M

Pa)

3Vol. 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 0.350

200

400

600

800

1000

1200

Flex

ural

Stre

ngth

(MP

a)

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

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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

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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

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novel cylindrical grinding kinematic for brittle materials

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