An Analysis of Strain in Chip Breaking Using Slipline Filed Theory with Adhesion Friction at Chip/Tool Interface

By: Chawla, B SContributor(s): Das, N S [Supervisor] | Biswas, C K [Supervisor] | Department of Mechanical EngineeringMaterial type: TextTextLanguage: English Publisher: 2005Description: 222 pSubject(s): Engineering and Technology | Mechanical Engineering | MechatronicsOnline resources: Click here to access online Dissertation note: Thesis (Ph.D)- National Institute of Technology, Rourkela 2005 Summary: Despite rapid growth in the applications of metal machining in manufacturing, a comprehensive analysis of the problem of chip control has always been a difficult task. This is because of the complex mechanism of the chip formation process and a lack of knowledge of the factors that influence chip form/chip breakability under a given set of input machining conditions such as work material properties, tool geometry, chip breakers and cutting conditions. Consequently, the solution to the problem has been approached empirically with a limited degree of success. In the present investigation, an attempt has been made to examine chip breaking by a step-type chip breaker using the rigid-plastic slip-line field theory. Orthogonal machining is assumed and the deformation mode is analysed using the solutions proposed earlier by Kudo and Dewhurst. The rake face friction is represented by the adhesion friction law suggested by Maekawa et al. The fields are constructed and analysed by the matrix operational procedure developed by Dewhurst and Collins. Limit of validity of the fields has been determined from the consideration of overstressing of the rigid vertices at the chip and the workpiece and also from the consideration that friction angle along the tool face nowhere becomes negative. The extent of ‘material damage’ is assessed by computing the cumulative shear strain suffered by the material in passing through the primary shear line and secondary deformation zones, by a method due to Atkins et al. Variation of total strain, breaking strain and the chip curl radius as a function of the chip breaker height and its distance from the cutting edge is studied. The variation of strain across the chip thickness is estimated. The accuracy of prediction of the degree of chip breaking by some of the breakability criterion is examined in the light of rigid-perfectly plastic slip-line field theory. It is found that as the chip breaker moves away from the cutting edge the radius of chip curvature (Rchip/t0), tool-chip contact length (ln/t0), specific cutting energy (Fc/t0), cutting ratio ζ and total strain ²t in the chip increase while the breaking strain and the secondary strain decrease. This observation is found to be influenced both by uncut chip thickness t0 and tool rake angle γ. The cutting force increases as WTR increases and rake angle γ decreases, however, the reverse trend is exhibited by chip breaker force Fb. The amount of shear strain in the secondary deformation zone is found to be about 10 to 15 % of total strain. The trend of variation of total strain ²t , specific cutting energy (Fc/t0) and the breaking strain ²b with chip breaker position supports the view that chip breaking is governed mainly by the breaking strain and not by “material damage” or by specific cutting energy consumed during machining. Experimental investigation has been carried out to validate the theoretical observations. Orthogonal machining tests were carried out on mild steel tubes using HSS tools with 10 % cobalt. Chip breaking was accomplished using a step-type chip breaker. Chip thickness and chip curl radius were measured using an image analyser. For the chips, the shift in the position of the neutral axis from the centre was calculated using the theory of bending of curved beams. The chip curl radius before breaking was determined taking into account the elastic recovery of the chips. Breaking strain was calculated from a simplified formula, ²b = tchip/(2 Rchip) and this was correlated with the degree of chip breaking. A procedure for chip breaker design to achieve effective breaking is also suggested. It is seen that chip breakability criteria based on t0, tchip and Rchip predict the effectiveness of chip breaking more accurately than those based on specific cutting energy and material damage.
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Thesis (Ph.D)- National Institute of Technology, Rourkela 2005

Despite rapid growth in the applications of metal machining in manufacturing, a comprehensive analysis of the problem of chip control has always been a difficult task. This is because of the complex mechanism of the chip formation process and a lack of
knowledge of the factors that influence chip form/chip breakability under a given set
of input machining conditions such as work material properties, tool geometry, chip
breakers and cutting conditions. Consequently, the solution to the problem has been
approached empirically with a limited degree of success.
In the present investigation, an attempt has been made to examine chip breaking
by a step-type chip breaker using the rigid-plastic slip-line field theory. Orthogonal
machining is assumed and the deformation mode is analysed using the solutions proposed
earlier by Kudo and Dewhurst. The rake face friction is represented by the
adhesion friction law suggested by Maekawa et al. The fields are constructed and
analysed by the matrix operational procedure developed by Dewhurst and Collins.
Limit of validity of the fields has been determined from the consideration of overstressing
of the rigid vertices at the chip and the workpiece and also from the consideration
that friction angle along the tool face nowhere becomes negative. The extent of ‘material
damage’ is assessed by computing the cumulative shear strain suffered by the
material in passing through the primary shear line and secondary deformation zones,
by a method due to Atkins et al. Variation of total strain, breaking strain and the
chip curl radius as a function of the chip breaker height and its distance from the
cutting edge is studied. The variation of strain across the chip thickness is estimated.
The accuracy of prediction of the degree of chip breaking by some of the breakability
criterion is examined in the light of rigid-perfectly plastic slip-line field theory.
It is found that as the chip breaker moves away from the cutting edge the radius
of chip curvature (Rchip/t0), tool-chip contact length (ln/t0), specific cutting energy
(Fc/t0), cutting ratio ζ and total strain ²t
in the chip increase while the breaking
strain and the secondary strain decrease. This observation is found to be influenced
both by uncut chip thickness t0 and tool rake angle γ. The cutting force increases as
WTR increases and rake angle γ decreases, however, the reverse trend is exhibited
by chip breaker force Fb. The amount of shear strain in the secondary deformation
zone is found to be about 10 to 15 % of total strain. The trend of variation of total
strain ²t
, specific cutting energy (Fc/t0) and the breaking strain ²b with chip breaker
position supports the view that chip breaking is governed mainly by the breaking
strain and not by “material damage” or by specific cutting energy consumed during
machining.
Experimental investigation has been carried out to validate the theoretical observations.
Orthogonal machining tests were carried out on mild steel tubes using
HSS tools with 10 % cobalt. Chip breaking was accomplished using a step-type chip
breaker. Chip thickness and chip curl radius were measured using an image analyser.
For the chips, the shift in the position of the neutral axis from the centre was calculated
using the theory of bending of curved beams. The chip curl radius before
breaking was determined taking into account the elastic recovery of the chips. Breaking strain was calculated from a simplified formula, ²b = tchip/(2 Rchip) and this was correlated with the degree of chip breaking. A procedure for chip breaker design to
achieve effective breaking is also suggested.
It is seen that chip breakability criteria based on t0, tchip and Rchip predict the
effectiveness of chip breaking more accurately than those based on specific cutting energy and material damage.

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