Chip Formation in Micro-cutting

Transcription

Faculté Polytechnique
Chip Formation in Micro-cutting
F. Ducobu, E. Rivière-Lorphèvre, E. Filippi
Francois.Ducobu@umons.ac.be
Machine Design and Production Engineering
Department
Introduction
 Miniaturisation  increasing demand for micro-components
 development of micro-manufacturing techniques
 Micro-milling = one of them
 Micro-milling = the fastest and flexible micro-machining process
 to produce complex 3D micro-forms
 with sharp edges
 and good surface quality
 in many materials (metal alloys, polymers and ceramics)
 Uses a micro-mill rotating at high speed
 Applications quite varied: micro-injection moulds, watch
components,…
 Chae, J., Park, S., Freiheit, T., 2006, Investigation of micro-cutting operations, Int. J. Machine Tools and Manufacture, 45: 313-332.
Université de Mons
François Ducobu | FPMs – Machine Design and Production Engineering Department | NCTAM 2012
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Plan
A. Chip formation specificities in micro-cutting
B. Model presentation
C. Results in macro-cutting
D. Influence of the depth of cut
E. Conclusions
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CHIP FORMATION SPECIFICITIES IN MICRO-CUTTING | MODEL PRESENTATION | RESULTS IN MACRO-CUTTING | INFLUENCE OF THE DEPTH OF CUT | CONCLUSIONS
A. Chip formation specificities
in micro-cutting
 Micro- and macro-milling concepts are similar
 Scaling-down of the process
 changes in the process
 micro-cutting phenomenon cannot be considered as a
simple scaling of micro-cutting
 Lead to several chip formation specificities in micro-cutting
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1. Minimum chip thickness
 Depth of cut and feed per tooth very small  no chip is formed
below a critical value called “minimum chip thickness”
 Estimation of its value = one of the present challenges in micromilling
 Moreover machined material and tool geometry greatly affect its
value, complicating its estimation
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2. Size effect
 Size effect, at small depth of cut
= non-linear increase in the specific cutting energy when the
depth of cut decreases
3. Influence of the machined material
 At the microscopic scale, the microstructure of the machined
material takes importance
 Its granular structure must be taken into account
 The material can no longer be considered as
homogeneous and isotropic ≠ macro-cutting
 Filiz, S., Conley, C., Wasserman, M., Ozdoganlar, O., 2007, An experimental investigation of micro-machinability of copper 101
using tungsten carbide micro-endmills, Int. J. Machine Tools and Manufacture, 47: 1088-1100.
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B. Model presentation
 Lagrangian Finite Element Method (FEM) model to study the depth
of cut influence on chip formation in orthogonal cutting
 Numerical simulations performed with ABAQUS/Explicit v6.8
 Important characteristic of the model = its validity in micro-cutting
but also in macro-cutting
 Allows to study changes in the cutting mechanism from macroto micro-cutting with one single model
 Ability to form saw-toothed chips in macro-cutting = one of the
requirements and difficulties introduced by the multi-scale aspect
of the model
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 2D plane strain model
 Take into account the chip formation area
 Explicit Lagrangian formulation because:
 Interest focused on
 the transient phase of the chip formation
 the absence of chip formation
 Production of saw-toothed chips morphologically close to experimental ones,
which cannot be achieved with an ALE formulation, contrary to Lagrangian
formulation
 Ducobu, F., Filippi, E., Rivière-Lorphèvre, E., 2009, Chip Formation and Minimum Chip Thickness in Micro-milling, Proceedings of the
12th CIRP Conference on Modeling of Machining Operations, 339-346.
 Ducobu, F., Filippi, E., Rivière-Lorphèvre, E., 2009, Investigations on Chip Formation in Micro-milling, Proceedings of the 9th
International Conference on Laser Metrology, CMM and Machine Tool Performance, 327-336.
 Ducobu, F., Rivière-Lorphèvre, E., Filippi, E., 2010, An ALE Model to Study the Depth of Cut Influence on Chip Formation in
Orthogonal Cutting, Proceedings of the Eighth International Conference on High Speed Machining, 202-207.
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 Tool:
 Rake angle: 15°
 Clearance angle: 2°
 Edge radius: 20 µm
 Cutting speed: 75 m/min
 Initial workpiece shape
= rectangular box
 Friction at the tool – chip interface implemented using Coulomb’s
friction
 All the friction energy is converted into heat
 Initial temperature set to 25°C
 Only conduction is considered and all the faces are adiabatic
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 Workpiece material: Titanium alloy Ti6Al4V:
 Homogeneous  simplification of its actual granular structure
 Behaviour described by the Hyperbolic TANgent (TANH) law [11] = JohnsonCook law taking account of the strain softening effect
 Strain softening could explain the formation of saw-toothed Ti6Al4V chips
 taking it into account  more realistic chip
 Tool material: tungsten carbide described by a linear elastic law
 Calamaz, M., Coupard, D., Girot, F., 2008, A new material model for 2D numerical simulation of serrated chip formation when
machining titanium alloy Ti-6Al-4V, Int. J. Machine Tools and Manufacture, 48: 275-288.
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 Lagrangian formulation  chip separation criterion needed
 Chip formation possible thanks to an “eroding element” method
 Criterion based on the temperature dependent tensile failure of
Ti6Al4V
 Tensile failure value reached in an element
 deleted from the visualisation
and all its stress components are put to zero
 Suppression of a finite element
 introduction of a crack in the workpiece
 making it possible for the chip to come off
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C. Results in macro-cutting
 Validation of the model: comparison of the modelled saw-toothed
macro-chip (h = 280 µm) and cutting forces to experimental cutting
results
 Experiments performed on a
lathe
 Workpiece = shaft comporting
flanges in the form of
successive slices of equal
thickness
 Tool width larger than disks
 Cutting process: plunge
condition ≈ orthogonal cutting
 Fixation of the tool  high
rigidity
 Use of a tailstock to avoid
workpiece displacements and
vibrations
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 Morphology of the modelled chip very close to the experimental
one
 For each tooth a slipping band is formed in the primary shear
zone, as expected
 It vanishes as the tool moves forward, initiating the tooth
formation
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 Cyclic evolution of the cutting force = typical of saw-toothed chip
formation: a drop in the force = formation of a tooth
 Link between force evolution and teeth formation, 7 teeth
 Simulated force of the same order but smaller than experiments 
choice of TANH parameters?
 Same observations for FF
 Simulated force smaller
than experiments 
influence of the friction,
difficult to measure
and model
 The model is able to model qualitatively the chip formation of
Ti6Al4V in orthogonal cutting
 Suitable for the study of the depth of cut influence on chip formation
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D. Influence of the depth of cut
 For a determined material, minimum chip thickness depends on
 depth of cut (h)
 tool edge radius (r)
 Study of the influence of the depth of cut on chip formation with
8 decreasing values of the depth of cut for a constant tool edge
radius (20 µm)
h (µm) 280 100 40 20 10
h/r
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5
2
1
5
2.5
1
0.5 0.25 0.125 0.05
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1. Chip morphology
 From saw-toothed chip to the cutting refuse including segmented
chip  chip morphology evolving away from macro-cutting
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 From h/r = 0.25: material seems to be pushed, deformed,
not sheared anymore
h/r = 14
h/r = 2
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Pa
h/r = 5
h/r = 0.5
h/r = 0.25
h/r = 0.125
h/r = 0.05
h/r = 1
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 For h/r values under 0.125 no chip is formed and a small amount
of material accumulates in front of the tool
 This small amount grows when the tool moves forward until it
reaches a thickness greater than the minimum chip thickness
 It is then removed from the workpiece
 Critical h/r concerning the change in the mechanism of chip
formation: between 0.125 (2.5 µm) and 0.25 (5 µm)
h/r = 0.5
h/r = 0.25
103 Pa
h/r = 0.125
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h/r = 0.05
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2. Cutting forces
 h/r decreases  teeth are less deep then disappear
 Same observation for the cyclic evolutions of the forces
Experiments
 Forces ratio = FF/CF
 h/r decreases  forces ratio increases
 When forces ratio > 1: change in the cutting phenomenon: FF > CF
 If critical ratio value = 2  minimum chip thickness value between 5 µm and 10 µm
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3. Specific cutting energy
 Specific cutting energy = cutting force on the area of the chip section
 Mean normalized = mean simulated for each case divided by experiments
 Size effect highlighted: non-linear increase happens when the depth of cut
decreases
 Critical h/r value: between 0.25 (5 µm) and 0.5 (10 µm)
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4. Elastic recovery
 Elastic recovery (or elastic spring back ) of the workpiece after the
tool tip passage.
 Increase of its value when the depth of cut decreases:
from 0.45% for h = 280 µm to 25% for h = 1 µm
 Significant importance for small depths of cut
 Large value relatively to the small depths of cut
 Contributes to increase:
 Feed force
 Slipping force
 Specific cutting energy
 hm < 10 µm (exponential evolution)
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5. Minimum chip thickness prediction
 It is obvious that the minimum chip thickness is less a precise and
single value than a range of values with unclear limits
 According to the model results, for Ti6Al4V with the geometry and
the cutting conditions considered:
 The elastic recovery sets the upper limit of the values range under 10 µm
 The lower limit is set around 2.5 µm by the morphological aspect
 The 2 others criterions lead to a value between 5 µm and 10 µm
 Minimum chip thickness resulting value in these conditions
= of the order of 25% of the cutting edge radius of the tool with a
lower limit around 12.5% and an upper limit inferior to 50%
 This order of magnitude is confirmed in literature
 Filiz, S., Conley, C., Wasserman, M., Ozdoganlar, O., 2007, An experimental investigation of micro-machinability of copper 101 using
tungsten carbide micro-endmills, Int. J. Machine Tools and Manufacture, 47: 1088-1100.
 Vogler, M.P., DeVor, R.E., Kapoor, S.G., 2004, On the modeling and analysis of machining performance in micro endmilling, Part I:
surface generation, J. Manufacturing Science and Engineering, 126:685-694.
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E. Conclusions
 Transition from macro- to micro-cutting  changes in the cutting
phenomenon
 Study of the influence of the depth of cut on chip formation with a
2D Lagrangian finite element model
 Chip formation evolves away from macro-cutting when the depth
of cut decreases
 Specific micro-cutting features reported in literature like:
 Minimum chip thickness
 Negative effective rake angle
 Increase of the importance of the feed force
 Size effect
are highlighted in the results
 Importance and role of the elastic recovery of the workpiece is
highlighted and added to the micro-cutting features list
 A minimum chip thickness prediction has been performed
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Thank you for your attention
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Lagrangian
ALE
Experiments
103 Pa
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Chip Formation in Micro-cutting

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