- Email: [email protected]
An investigation of mandrel-free spinning
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Procedia Manufacturing 29 (2019) 145–152 Procedia Manufacturing 00 (2017) 000–000 www.elsevier.com/locate/procedia
18th International Conference on Sheet Metal, SHEMET 2019 18th International Conference on Sheet Metal, SHEMET 2019
An investigation of mandrel-free spinning An investigation of mandrel-free spinning
* Manufacturing Engineering Society International Conference 2017, MESIC 2017, 28-30 June Kishore Jawale , Evripides G. Loukaides * (Pontevedra), Spain 2017, Vigo Kishore Jawale , Evripides G. Loukaides Department of Mechanical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom Department of Mechanical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
Costing models for capacity optimization in Industry 4.0: Trade-off between used capacity and operational efficiency
Abstract Abstract Spinning is a sheet metal forming technique conventionally used to form axisymmetric shapes. Recently a flexible mandrel-free Spinningprocess is a sheet forming used technique axisymmetric shapes. Recently a flexible aconventionally a,* to bspinning spinning wasmetal successfully to form Inb,this process,mandrel-free numerically A. Santana , non-axisymmetric P. Afonsoused , geometries. A.form Zanin R.mandrel-free Wernke spinning process successfully used to form allowing non-axisymmetric geometries. In this mandrel-free spinningrequired process,for numerically controlled rollers was are used instead of a mandrel, for savings in the lead-time, cost and the material mandrel. ahas not been controlled rollersbehavior are usedofinstead of a mandrel, allowing for savings in the lead-time, cost and the material required for mandrel. The deformation this process previously analyzed numerically, although it is critical to designing toolpaths University of Minho, 4800-058 Guimarães, Portugal Theachieving deformation behaviorparts. of this hasban notFEA beenmodel previously analyzed numerically, is critical to designing toolpaths Unochapecó, 89809-000 Chapecó, SC, and successful Inprocess this work, is constructed and isBrazil used toalthough study theitstress distribution and the strain and achieving successful parts. In this work, an FEAout model is constructed andwith is used to study theexperimental stress distribution themodel strain path. Validation of the numerical model is carried by comparing forces measureable results.and The path. Validation of the numerical while modelforming is carried out by comparing forces with measureable experimental results. model shows that the stress distribution is different to that observed in the conventional spinning while the The wrinkling shows that is thesimilar stress todistribution while forming is different to that observed in the conventional spinning while the wrinkling mechanism that observed in conventional spinning. Abstract mechanism is similar to that observed in conventional spinning. © 2018 The Authors. Published by Elsevier B.V. © 2018 2019 the The Authors. Published by Elsevier Elsevier B.V. Under concept of "Industry productionlicense processes will be pushed to be increasingly interconnected, © Authors. by B.V. This is anThe open access Published article under the4.0", CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) information based on a real time basis and, necessarily, more efficient. In2019. this context, capacity optimization This is anand open access article under the CC BY-NC-ND licensemuch (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection peer-review under responsibility of the organizing committee Selection and peer-review under responsibility of the organizing committeeof ofSHEMET SHEMET2019. Selection and peer-review under responsibility the organizing contributing committee of SHEMET 2019. goes beyond the traditional aim of capacityofmaximization, also for organization’s profitability and value. Keywords: Spinning; Flexible mandrel-free spinning; Simulation; Sheet metal; Forming. suggest capacity optimization instead of Indeed, lean management and continuous improvement approaches Keywords: Spinning; Flexible spinning; Simulation; Sheet metal; Forming. maximization. The studymandrel-free of capacity optimization and costing models is an important research topic that deserves
contributions from both the practical and theoretical perspectives. This paper presents and discusses a mathematical 1. Introduction model for capacity management based on different costing models (ABC and TDABC). A generic model has been 1. Introduction developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s In commercial sheet metal spinning a circular blank is rotated at highis velocities (1000 rpm) and formed value. The trade-off capacity maximization vs operational efficiency highlighted and -it2000 is shown thatiscapacity In commercial sheet metalresults spinning a circular blank is rotated at high However, velocities (1000 - 2000 rpm) and is formed against a mandrel [1]. This in an axially symmetric component. recently a flexible mandrel-free optimization might hide operational inefficiency. against aprocess mandrel [1]. This results in prototype an axiallymachine symmetric component. recentlyoraindependently flexible mandrel-free spinning was developed. This allows rollers toHowever, move in tandem to form © 2017 The Authors. Published by Elsevier B.V. spinning process was developed. This prototype machine allows rollers to move in tandem or independently to form asymmetric components [2]. This setup (Fig. 1) has eliminated the need for component-specific mandrel and Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference asymmetricsupport components setuproller (Fig.to1)support has eliminated the need component-specific mandrel and introduced rollers [2]. and aThis blending the component beingfor formed, mimicking the conventional 2017. introduced rollers andfurther a blending rollerthat to support thecan component formed, mimicking mandrel in support spinning. It was observed the setup be used being to form components withthe theconventional help of the Keywords: Models; ABC; TDABC; Capacity Management; Idle setup Capacity; Operational mandrel Cost in spinning. It was further observed that the can be usedEfficiency to form components with the help of the 1. Introduction * *
Corresponding author. Tel.: +44 1225 385366. Corresponding author. Tel.: +44is1225 385366. E-mail address: [email protected] The cost of idle capacity a fundamental information for companies and their management of extreme importance E-mail address: [email protected]
in modern production systems. In general, it is defined as unused capacity or production potential and can be measured of the idle capacity
2351-9789 © 2018 The Authors. Published by Elsevier B.V. in several ways: tons of production, available hours of manufacturing, etc. The management 2351-9789 © 2018 Thearticle Authors. Published by Elsevier B.V. This is an open access under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) * Paulo Afonso. Tel.:article +351 253 510 253license 604 741 This is anand open access under the761; CC fax: BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection peer-review under responsibility of+351 the organizing committee of SHEMET 2019. E-mail address: [email protected] Selection and peer-review under responsibility of the organizing committee of SHEMET 2019.
2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under of the scientificbycommittee the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2019responsibility The Authors. Published Elsevier of B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the organizing committee of SHEMET 2019. 10.1016/j.promfg.2019.02.119
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working roller and the blending roller, without the need of the support rollers [3]. This however will change the stress distribution in the component and thus needs to be analyzed.
Support rollers Clamping
Working roller
Blending roller
(a)
Sheet (b)
Fig. 1: (a) Experimental setup; (b) Schematic side view of the setup (adapted from [1]).
Numerical simulations can be used to study the stresses in the workpiece. Previously Rentsch and Hora [4] have successfully modelled multi-pass sheet metal spinning. In this study the accuracy of numerical modelling techniques, by a direct comparison with metal spinning experiments, was investigated. The authors compared results using shell and solid elements and found that the shell elements are computationally more efficient and produce similar results when compared with solid elements. Using strain distribution analysis, the authors could identify the bending under tension (BUT) mechanism in spinning. In another study [5], a Finite Element (FE) simulation was used to study the deformation mechanics of the conventional metal spinning. The numerical results suggested that among three tool force components, the axial force is the highest while the tangential force is the lowest. The blank thickness decreases after each forward pass and there are almost no thickness changes during the backward pass. In the area between the roller contact point and the back-plate, bending effects have been observed at the beginning of the forward pass. Compressive tangential stresses are observed at the flange area near the local forming zone however they will change into tensile tangential stresses when the current contact zone of the work-piece rotates away from the roller. In the present study, the objective is to study the deformation behaviour of the mandrel-free spun component, by observing the stress distribution in the component while forming, using an FE simulation. The FE model is validated by measuring and comparing the reaction forces generated at the working roller. In the subsequent section experimental setup used for verification is introduced and then followed by a description of the FE model and a comparison of the two. Additional results made available through the numerical model concludes with the discussion. 2. Experimental setup A circular Aluminum 1050 sheet with 2 mm thickness and 385 mm diameter was spun on the flexible mandrel-free machine setup (Fig. 1) without the use of support rollers. The sheet was formed to produce a conical component with an inclined wall at a 30° angle from the initial plane of the blank. To reduce the undesirable effect of friction between the rollers and the sheet, the rollers were kept free to rotate. Additionally, grease was applied between the rollers and the sheet surface to further reduce the friction. The toolpath was generated using a custom algorithm for which the details can be found in a study done by Russo and Loukaides [3]. The working roller toolpath consisted of three forward linear passes and corresponding backward passes (see Fig. 2, Y-axis represents axial direction and X-axis represents radial direction with respect to machine’s spindle). When the working roller retracted (between forward passes) it had no contact with the workpiece, i.e. forming is performed only in forward passes. The blending roller,
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once moved to the initial position, is stationary and in contact with the component at its base for the entire duration of the forming. The spindle speed was 200 rpm and the feed-rate was 1 mm/sec. This is lower than typical spinning speeds due to limitations on the prototype machine motors. The forces on the rollers while forming are recorded using load cells. The nose radii of both the working and blending rollers are 15 mm.
Start point
End point
Fig. 2: Toolpath for working roller.
3. Numerical simulation The metal spinning process can be considered a quasi-static problem which includes large membrane deformation and complex contact conditions [5]. Some authors [6] have previously studied the difference in computational time and accuracy of models solved using implicit and explicit methods. It was observed that while the implicit method resulted in slightly better accuracy the computational time was very high. Thus to study the deformation in this paper (with an acceptable computational time trade-off) the explicit method in the commercial package Abaqus was used. The material plastic property was defined using the von Mises yield criterion and isotropic hardening. To simplify the model, the rollers where modelled as analytical rigid bodies with inertia. The 4-node reduced integration shell elements (S4R in Abaqus) with approximate global size of 5mm, which generated 6450 elements, were used to model the workpiece as a deformable material. Unlike the reduced integration linear solid element, which only uses one integration point along the thickness direction, multiple integration points are used through the thickness of a shell element. Stresses and strains at each integration point of the shell element are calculated independently. 9 integration points were assigned to the elements through the thickness. Displacement boundary conditions are used in order to model the motion of the working roller. A built-in coupling condition was applied to the center of the workpiece, to represent the clamp on the spindle. A rotation velocity of 200 rpm was defined for this center point. The rollers are free to rotate about their axis. The mass scaling technique, which artificially increases the density of the material, is commonly used to speed up the analysis, but can result in unrepresentative inertial effects. A mass scaling factor of 25 was used, in accordance with successful past models for conventional spinning [5]. It was observed that the kinetic energy was less than 1% of the total internal energy, thus assuring that the mass scaling does not affect the simulation results. The coulomb’s friction criterion with a coefficient of 0.2 was used to define the friction between the rollers and sheet. This is an assumed value based on the known rollers and workpiece materials, as well as the presence of lubrication in the physical trial. The rollers are allowed to rotate freely, both in the experimental setup and in the simulation, thus reducing the influence of friction further. The numerical experiment was constructed to match the geometry of the physical experiment described above. The toolpaths were provided to Abaqus as a tabulated list of coordinates, taken from the same file provided as input to the prototype machine.
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4. Results and discussion In this section the FE simulation is validated by comparing the experimental results with the results obtained from the numerical simulation. Later the stress distribution in the formed component is compared with that observed in conventional spinning as found in earlier literature. This gives further insight into the deformation mechanism observed in flexible mandrel-free spinning. In Fig. 3, the equivalent axial forces generated at the working roller from the experimental setup and the FE simulation are compared. The forward passes are clearly separated by regions of diminishing reaction force during backward travel of the working roller. This is trivially expected, as the roller is no longer in contact with the sheet.
3rd Pass 2nd Pass 1st Pass
Fig. 3: Reaction forces generated at the working roller.
The experimental values deviate from the FE simulation with an average relative error of 23.8%. This deviation can be attributed to the following three reasons: a. The von Mises yield criteria with isotropic hardening relies on data representing equivalent plastic strain versus equivalent stress. The material data was acquired from a standard tensile test and it is only possible to measure the stress-strain curve up to a certain strain level with the proposed methods. To predict higher values, the data is simply extrapolated using mathematical models. Abaqus is capable of interpolating and extrapolating from this data, and thus giving a reasonable fit to the provided material behavior. However, more sophisticated material characterization is needed to capture behavior in the regions of higher strains. b. The von Mises yield criterion with isotropic hardening is a simple elastoplastic material law, which does not consider any changes in the yield locus. During the first tool pass the influence might be negligible, but after multiple load reversals and at higher strain levels these effects could have more significance and lead to the observed deviation. Applying more sophisticated material models, which consider the anisotropy of the material and the Bauschinger Effect (BE), might lead to an improvement of the results. c. The friction model could be more complicated than the one assumed in the FE simulation. Further to these, the modelling strategy, viz. the use of explicit method, might also contribute to the discrepancy between experiments and simulations, as stated in the beginning of the previous section. Admittedly, additional work is required to produce a more accurate simulation. However, we note that the trend of the relative error in force magnitude between FE and experiment is remarkably consistent. Hence, it is reasonable to assume that trends in other metrics remain consistent, albeit with an amount of relative error. With that in mind, the stress distribution is examined using the presented FE simulation. The development of stresses, due to contact of working roller, in the sheet at the start of the first pass can be observed in Fig. 4(a) and the same after working roller travels 5 mm can be observed in Fig. 4(b). A high stress zone
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can be seen in the sheet region which is located under the roller contact points. The sheet region between the working roller and the blending roller appears to be under low tensile stress, which is not observed in conventional spinning. The stress distribution during the initial forward pass in the case of flexible mandrel-free spinning is slightly different than the conventional spinning observed in [4, 5]. The difference is prominent at the edge of the sheet. The high tensile stress extending from the working roller to the edge of the sheet is absent in the case of this process. No significant higher tensile stresses are developed, and further they are unevenly distributed. This uneven stress distribution may cause dynamic instability and makes this process more prone to wrinkling failure. Here, the support rollers’ contact would help to minimize the effect of uneven stress distribution. (a)
Low tension zone Roller contact point
(b)
Fig. 4: Stress distribution (in MPa) at the first pass; (a) initial contact with sheet, (b) after working roller displacement of 5mm.
Fig. 5 gives a visual overview of residual stresses induced by this multi-pass spinning process. The von Mises stresses are displayed on the deformed workpiece at the end of each roller pass. The stress pattern observed at the end of all passes is similar to that observed in conventional spinning [4, 5], except for the high stresses induced in the sheet at the contact point with the blending roller. This shows that the deformation state at the end of the pass is similar in both processes. The high stress zone under the blending roller causes the required bending of the sheet at that point. Due to the substantially different tool setup, it is reasonable to assume that there might also be a difference in the amount of residual stresses induced in the component formed with different spinning processes. This needs to be investigated further.
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(a)
(b)
(c)
Fig. 5: Mises stress distribution (in MPa) at the end of, (a) 1st pass; (b) 2nd pass; c) 3rd pass.
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An additional forward pass was introduced to the model to understand the deformation mechanism of wrinkling in flexible mandrel-free spinning. The wrinkling, as presented in Fig. 6, shows toothed stress pattern similar to that observed in conventional sheet metal spinning [5]. Wrinkling in conventional spinning typically occurs in regions not
Fig. 6: Stress (in MPa) distribution in flexible mandrel free spinning at wrinkling.
in contact with the mandrel, hence that similarity is perhaps unsurprising. Three elements from different positions were selected to investigate the plastic deformation at difference locations on the workpiece. Both the position of elements and the respective in-plane principal strains are presented in Fig. 7. Position A indicates an element under the blending roller, position B is in the middle of the cone and position C is at
B
C
A
(a)
(b)
Fig. 7: (a) Positions of selected elements; (b) Strain paths for corresponding position.
the edge of the formed cone. The black cross mark intersecting strain paths in Fig. 7a represents the end of pass 1, green cross mark represent end of pass 2 and red cross mark represent end of pass 3. The element under the blending roller follows a plane strain path, the element positioned at the middle of the cone follows approximately uniaxial strain path and the element positioned at the edge of the formed cone follow shear
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strain path. Shear deformation is usually desired in this process to ensure thickness uniformity. However, due to the complex mechanics of the process, a range of deformation from plane strain to shear is observed. In general, in incremental forming, maximum fracture strain is achieved when the material follows a uniaxial strain path [7]. Thus, this strain path is desirable to achieve higher deformation. 4. Conclusion An FEA simulation was created for flexible mandrel-free spinning. The numerical simulation was validated using the experimental axial forces generated at the working roller and the stress distribution was examined in the sheet at different stages of forming. The stress distribution in the case of flexible mandrel-free spinning, at the end of the passes, is similar to that observed in the conventional spinning. However, there is a difference in stress distribution during forming. There is a development of low tensile stress zone in-between the blending and working rollers. The stresses generated at the edge are also different than in conventional spinning. The uneven stresses generated at the edge of the sheet can cause early wrinkling failure in flexible mandrel-free spinning without support rollers, compared to conventional spinning. The wrinkling failure mechanism for this process is similar to that observed in the conventional spinning. The strain path followed by the elements at different positions range from plane strain to shear. This preliminary work demonstrates the possibility to use an FE model to further understand this process. In particular, the absence of a mandrel introduces distinct deformation mechanics, which need to be understood better to optimize both the hardware configuration and the toolpath design on the machine. Another avenue of study is for nonaxisymmetric workpieces, which have only been studied experimentally on this process. As shown here, the FE model can reveal pertinent metrics and trends, not readily available in an experimental setting. Acknowledgements The authors would like to acknowledge funding for this study through an Innovate UK project (FELDSPAR) and earlier funding from Nissan Motor Co., Ltd., UK. The authors also appreciate the contribution of Iacopo M. Russo through data and discussions. References [1] O. Music, J. M. Allwood and K. Kawai, A review of the mechanics of metal spinning, Journal of Material Processing Tech., 210 (2010), 3-23. [2] O. Music and J. M. Allwood, Flexible asymmetric spinning, CIRP Ann., 60 (2011), 319–322. [3] I. M. Russo and E. G. Loukaides, Toolpath generation for asymmetric mandrel-free spinning, Procedia Eng., 207 (2017), 1707–1712. [4] B. Rentsch, N. Manopulo, and P. Hora, Numerical modelling, validation and analysis of multi-pass sheet metal spinning processes, Int. J. Material Forming., 10 (2017), 641–651. [5] L. Wang and H. Long, Investigation of material deformation in multi-pass conventional metal spinning, Material Design, 32 (2011), 2891– 2899. [6] M. E. Tamer, O. Music, I. Ozdemir, B. Baranoglu, A. Sakin, and I. Durgun, Simulation for Incremental Sheet Forming Process : a Comparison of Implicit and Explicit Finite Element Analysis with Experimental Data, in 7th International Conference and Exhibition on Design and Production of Machines and Dies/Molds, Antalya, Turkey, 2013. [7] K. Jawale, J. F. Duarte, A. Reis, and M. B. Silva, Characterizing fracture forming limit and shear fracture forming limit for sheet metals, Journal of Material Processing Technology, 255 (2018), 886–897.
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Available Availableonline onlineatatwww.sciencedirect.com www.sciencedirect.com Procedia Manufacturing 00 (2018) 000–000 Procedia Manufacturing 00 (2018) 000–000
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www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Manufacturing 29 (2019) 145–152 Procedia Manufacturing 00 (2017) 000–000 www.elsevier.com/locate/procedia
18th International Conference on Sheet Metal, SHEMET 2019 18th International Conference on Sheet Metal, SHEMET 2019
An investigation of mandrel-free spinning An investigation of mandrel-free spinning
* Manufacturing Engineering Society International Conference 2017, MESIC 2017, 28-30 June Kishore Jawale , Evripides G. Loukaides * (Pontevedra), Spain 2017, Vigo Kishore Jawale , Evripides G. Loukaides Department of Mechanical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom Department of Mechanical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
Costing models for capacity optimization in Industry 4.0: Trade-off between used capacity and operational efficiency
Abstract Abstract Spinning is a sheet metal forming technique conventionally used to form axisymmetric shapes. Recently a flexible mandrel-free Spinningprocess is a sheet forming used technique axisymmetric shapes. Recently a flexible aconventionally a,* to bspinning spinning wasmetal successfully to form Inb,this process,mandrel-free numerically A. Santana , non-axisymmetric P. Afonsoused , geometries. A.form Zanin R.mandrel-free Wernke spinning process successfully used to form allowing non-axisymmetric geometries. In this mandrel-free spinningrequired process,for numerically controlled rollers was are used instead of a mandrel, for savings in the lead-time, cost and the material mandrel. ahas not been controlled rollersbehavior are usedofinstead of a mandrel, allowing for savings in the lead-time, cost and the material required for mandrel. The deformation this process previously analyzed numerically, although it is critical to designing toolpaths University of Minho, 4800-058 Guimarães, Portugal Theachieving deformation behaviorparts. of this hasban notFEA beenmodel previously analyzed numerically, is critical to designing toolpaths Unochapecó, 89809-000 Chapecó, SC, and successful Inprocess this work, is constructed and isBrazil used toalthough study theitstress distribution and the strain and achieving successful parts. In this work, an FEAout model is constructed andwith is used to study theexperimental stress distribution themodel strain path. Validation of the numerical model is carried by comparing forces measureable results.and The path. Validation of the numerical while modelforming is carried out by comparing forces with measureable experimental results. model shows that the stress distribution is different to that observed in the conventional spinning while the The wrinkling shows that is thesimilar stress todistribution while forming is different to that observed in the conventional spinning while the wrinkling mechanism that observed in conventional spinning. Abstract mechanism is similar to that observed in conventional spinning. © 2018 The Authors. Published by Elsevier B.V. © 2018 2019 the The Authors. Published by Elsevier Elsevier B.V. Under concept of "Industry productionlicense processes will be pushed to be increasingly interconnected, © Authors. by B.V. This is anThe open access Published article under the4.0", CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) information based on a real time basis and, necessarily, more efficient. In2019. this context, capacity optimization This is anand open access article under the CC BY-NC-ND licensemuch (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection peer-review under responsibility of the organizing committee Selection and peer-review under responsibility of the organizing committeeof ofSHEMET SHEMET2019. Selection and peer-review under responsibility the organizing contributing committee of SHEMET 2019. goes beyond the traditional aim of capacityofmaximization, also for organization’s profitability and value. Keywords: Spinning; Flexible mandrel-free spinning; Simulation; Sheet metal; Forming. suggest capacity optimization instead of Indeed, lean management and continuous improvement approaches Keywords: Spinning; Flexible spinning; Simulation; Sheet metal; Forming. maximization. The studymandrel-free of capacity optimization and costing models is an important research topic that deserves
contributions from both the practical and theoretical perspectives. This paper presents and discusses a mathematical 1. Introduction model for capacity management based on different costing models (ABC and TDABC). A generic model has been 1. Introduction developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s In commercial sheet metal spinning a circular blank is rotated at highis velocities (1000 rpm) and formed value. The trade-off capacity maximization vs operational efficiency highlighted and -it2000 is shown thatiscapacity In commercial sheet metalresults spinning a circular blank is rotated at high However, velocities (1000 - 2000 rpm) and is formed against a mandrel [1]. This in an axially symmetric component. recently a flexible mandrel-free optimization might hide operational inefficiency. against aprocess mandrel [1]. This results in prototype an axiallymachine symmetric component. recentlyoraindependently flexible mandrel-free spinning was developed. This allows rollers toHowever, move in tandem to form © 2017 The Authors. Published by Elsevier B.V. spinning process was developed. This prototype machine allows rollers to move in tandem or independently to form asymmetric components [2]. This setup (Fig. 1) has eliminated the need for component-specific mandrel and Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference asymmetricsupport components setuproller (Fig.to1)support has eliminated the need component-specific mandrel and introduced rollers [2]. and aThis blending the component beingfor formed, mimicking the conventional 2017. introduced rollers andfurther a blending rollerthat to support thecan component formed, mimicking mandrel in support spinning. It was observed the setup be used being to form components withthe theconventional help of the Keywords: Models; ABC; TDABC; Capacity Management; Idle setup Capacity; Operational mandrel Cost in spinning. It was further observed that the can be usedEfficiency to form components with the help of the 1. Introduction * *
Corresponding author. Tel.: +44 1225 385366. Corresponding author. Tel.: +44is1225 385366. E-mail address: [email protected] The cost of idle capacity a fundamental information for companies and their management of extreme importance E-mail address: [email protected]
in modern production systems. In general, it is defined as unused capacity or production potential and can be measured of the idle capacity
2351-9789 © 2018 The Authors. Published by Elsevier B.V. in several ways: tons of production, available hours of manufacturing, etc. The management 2351-9789 © 2018 Thearticle Authors. Published by Elsevier B.V. This is an open access under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) * Paulo Afonso. Tel.:article +351 253 510 253license 604 741 This is anand open access under the761; CC fax: BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection peer-review under responsibility of+351 the organizing committee of SHEMET 2019. E-mail address: [email protected] Selection and peer-review under responsibility of the organizing committee of SHEMET 2019.
2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under of the scientificbycommittee the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2019responsibility The Authors. Published Elsevier of B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the organizing committee of SHEMET 2019. 10.1016/j.promfg.2019.02.119
Kishore Jawale et al. / Procedia Manufacturing 29 (2019) 145–152 Kishore Jawale/ Procedia Manufacturing 00 (2018) 000–000
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working roller and the blending roller, without the need of the support rollers [3]. This however will change the stress distribution in the component and thus needs to be analyzed.
Support rollers Clamping
Working roller
Blending roller
(a)
Sheet (b)
Fig. 1: (a) Experimental setup; (b) Schematic side view of the setup (adapted from [1]).
Numerical simulations can be used to study the stresses in the workpiece. Previously Rentsch and Hora [4] have successfully modelled multi-pass sheet metal spinning. In this study the accuracy of numerical modelling techniques, by a direct comparison with metal spinning experiments, was investigated. The authors compared results using shell and solid elements and found that the shell elements are computationally more efficient and produce similar results when compared with solid elements. Using strain distribution analysis, the authors could identify the bending under tension (BUT) mechanism in spinning. In another study [5], a Finite Element (FE) simulation was used to study the deformation mechanics of the conventional metal spinning. The numerical results suggested that among three tool force components, the axial force is the highest while the tangential force is the lowest. The blank thickness decreases after each forward pass and there are almost no thickness changes during the backward pass. In the area between the roller contact point and the back-plate, bending effects have been observed at the beginning of the forward pass. Compressive tangential stresses are observed at the flange area near the local forming zone however they will change into tensile tangential stresses when the current contact zone of the work-piece rotates away from the roller. In the present study, the objective is to study the deformation behaviour of the mandrel-free spun component, by observing the stress distribution in the component while forming, using an FE simulation. The FE model is validated by measuring and comparing the reaction forces generated at the working roller. In the subsequent section experimental setup used for verification is introduced and then followed by a description of the FE model and a comparison of the two. Additional results made available through the numerical model concludes with the discussion. 2. Experimental setup A circular Aluminum 1050 sheet with 2 mm thickness and 385 mm diameter was spun on the flexible mandrel-free machine setup (Fig. 1) without the use of support rollers. The sheet was formed to produce a conical component with an inclined wall at a 30° angle from the initial plane of the blank. To reduce the undesirable effect of friction between the rollers and the sheet, the rollers were kept free to rotate. Additionally, grease was applied between the rollers and the sheet surface to further reduce the friction. The toolpath was generated using a custom algorithm for which the details can be found in a study done by Russo and Loukaides [3]. The working roller toolpath consisted of three forward linear passes and corresponding backward passes (see Fig. 2, Y-axis represents axial direction and X-axis represents radial direction with respect to machine’s spindle). When the working roller retracted (between forward passes) it had no contact with the workpiece, i.e. forming is performed only in forward passes. The blending roller,
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once moved to the initial position, is stationary and in contact with the component at its base for the entire duration of the forming. The spindle speed was 200 rpm and the feed-rate was 1 mm/sec. This is lower than typical spinning speeds due to limitations on the prototype machine motors. The forces on the rollers while forming are recorded using load cells. The nose radii of both the working and blending rollers are 15 mm.
Start point
End point
Fig. 2: Toolpath for working roller.
3. Numerical simulation The metal spinning process can be considered a quasi-static problem which includes large membrane deformation and complex contact conditions [5]. Some authors [6] have previously studied the difference in computational time and accuracy of models solved using implicit and explicit methods. It was observed that while the implicit method resulted in slightly better accuracy the computational time was very high. Thus to study the deformation in this paper (with an acceptable computational time trade-off) the explicit method in the commercial package Abaqus was used. The material plastic property was defined using the von Mises yield criterion and isotropic hardening. To simplify the model, the rollers where modelled as analytical rigid bodies with inertia. The 4-node reduced integration shell elements (S4R in Abaqus) with approximate global size of 5mm, which generated 6450 elements, were used to model the workpiece as a deformable material. Unlike the reduced integration linear solid element, which only uses one integration point along the thickness direction, multiple integration points are used through the thickness of a shell element. Stresses and strains at each integration point of the shell element are calculated independently. 9 integration points were assigned to the elements through the thickness. Displacement boundary conditions are used in order to model the motion of the working roller. A built-in coupling condition was applied to the center of the workpiece, to represent the clamp on the spindle. A rotation velocity of 200 rpm was defined for this center point. The rollers are free to rotate about their axis. The mass scaling technique, which artificially increases the density of the material, is commonly used to speed up the analysis, but can result in unrepresentative inertial effects. A mass scaling factor of 25 was used, in accordance with successful past models for conventional spinning [5]. It was observed that the kinetic energy was less than 1% of the total internal energy, thus assuring that the mass scaling does not affect the simulation results. The coulomb’s friction criterion with a coefficient of 0.2 was used to define the friction between the rollers and sheet. This is an assumed value based on the known rollers and workpiece materials, as well as the presence of lubrication in the physical trial. The rollers are allowed to rotate freely, both in the experimental setup and in the simulation, thus reducing the influence of friction further. The numerical experiment was constructed to match the geometry of the physical experiment described above. The toolpaths were provided to Abaqus as a tabulated list of coordinates, taken from the same file provided as input to the prototype machine.
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4. Results and discussion In this section the FE simulation is validated by comparing the experimental results with the results obtained from the numerical simulation. Later the stress distribution in the formed component is compared with that observed in conventional spinning as found in earlier literature. This gives further insight into the deformation mechanism observed in flexible mandrel-free spinning. In Fig. 3, the equivalent axial forces generated at the working roller from the experimental setup and the FE simulation are compared. The forward passes are clearly separated by regions of diminishing reaction force during backward travel of the working roller. This is trivially expected, as the roller is no longer in contact with the sheet.
3rd Pass 2nd Pass 1st Pass
Fig. 3: Reaction forces generated at the working roller.
The experimental values deviate from the FE simulation with an average relative error of 23.8%. This deviation can be attributed to the following three reasons: a. The von Mises yield criteria with isotropic hardening relies on data representing equivalent plastic strain versus equivalent stress. The material data was acquired from a standard tensile test and it is only possible to measure the stress-strain curve up to a certain strain level with the proposed methods. To predict higher values, the data is simply extrapolated using mathematical models. Abaqus is capable of interpolating and extrapolating from this data, and thus giving a reasonable fit to the provided material behavior. However, more sophisticated material characterization is needed to capture behavior in the regions of higher strains. b. The von Mises yield criterion with isotropic hardening is a simple elastoplastic material law, which does not consider any changes in the yield locus. During the first tool pass the influence might be negligible, but after multiple load reversals and at higher strain levels these effects could have more significance and lead to the observed deviation. Applying more sophisticated material models, which consider the anisotropy of the material and the Bauschinger Effect (BE), might lead to an improvement of the results. c. The friction model could be more complicated than the one assumed in the FE simulation. Further to these, the modelling strategy, viz. the use of explicit method, might also contribute to the discrepancy between experiments and simulations, as stated in the beginning of the previous section. Admittedly, additional work is required to produce a more accurate simulation. However, we note that the trend of the relative error in force magnitude between FE and experiment is remarkably consistent. Hence, it is reasonable to assume that trends in other metrics remain consistent, albeit with an amount of relative error. With that in mind, the stress distribution is examined using the presented FE simulation. The development of stresses, due to contact of working roller, in the sheet at the start of the first pass can be observed in Fig. 4(a) and the same after working roller travels 5 mm can be observed in Fig. 4(b). A high stress zone
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can be seen in the sheet region which is located under the roller contact points. The sheet region between the working roller and the blending roller appears to be under low tensile stress, which is not observed in conventional spinning. The stress distribution during the initial forward pass in the case of flexible mandrel-free spinning is slightly different than the conventional spinning observed in [4, 5]. The difference is prominent at the edge of the sheet. The high tensile stress extending from the working roller to the edge of the sheet is absent in the case of this process. No significant higher tensile stresses are developed, and further they are unevenly distributed. This uneven stress distribution may cause dynamic instability and makes this process more prone to wrinkling failure. Here, the support rollers’ contact would help to minimize the effect of uneven stress distribution. (a)
Low tension zone Roller contact point
(b)
Fig. 4: Stress distribution (in MPa) at the first pass; (a) initial contact with sheet, (b) after working roller displacement of 5mm.
Fig. 5 gives a visual overview of residual stresses induced by this multi-pass spinning process. The von Mises stresses are displayed on the deformed workpiece at the end of each roller pass. The stress pattern observed at the end of all passes is similar to that observed in conventional spinning [4, 5], except for the high stresses induced in the sheet at the contact point with the blending roller. This shows that the deformation state at the end of the pass is similar in both processes. The high stress zone under the blending roller causes the required bending of the sheet at that point. Due to the substantially different tool setup, it is reasonable to assume that there might also be a difference in the amount of residual stresses induced in the component formed with different spinning processes. This needs to be investigated further.
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(a)
(b)
(c)
Fig. 5: Mises stress distribution (in MPa) at the end of, (a) 1st pass; (b) 2nd pass; c) 3rd pass.
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An additional forward pass was introduced to the model to understand the deformation mechanism of wrinkling in flexible mandrel-free spinning. The wrinkling, as presented in Fig. 6, shows toothed stress pattern similar to that observed in conventional sheet metal spinning [5]. Wrinkling in conventional spinning typically occurs in regions not
Fig. 6: Stress (in MPa) distribution in flexible mandrel free spinning at wrinkling.
in contact with the mandrel, hence that similarity is perhaps unsurprising. Three elements from different positions were selected to investigate the plastic deformation at difference locations on the workpiece. Both the position of elements and the respective in-plane principal strains are presented in Fig. 7. Position A indicates an element under the blending roller, position B is in the middle of the cone and position C is at
B
C
A
(a)
(b)
Fig. 7: (a) Positions of selected elements; (b) Strain paths for corresponding position.
the edge of the formed cone. The black cross mark intersecting strain paths in Fig. 7a represents the end of pass 1, green cross mark represent end of pass 2 and red cross mark represent end of pass 3. The element under the blending roller follows a plane strain path, the element positioned at the middle of the cone follows approximately uniaxial strain path and the element positioned at the edge of the formed cone follow shear
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strain path. Shear deformation is usually desired in this process to ensure thickness uniformity. However, due to the complex mechanics of the process, a range of deformation from plane strain to shear is observed. In general, in incremental forming, maximum fracture strain is achieved when the material follows a uniaxial strain path [7]. Thus, this strain path is desirable to achieve higher deformation. 4. Conclusion An FEA simulation was created for flexible mandrel-free spinning. The numerical simulation was validated using the experimental axial forces generated at the working roller and the stress distribution was examined in the sheet at different stages of forming. The stress distribution in the case of flexible mandrel-free spinning, at the end of the passes, is similar to that observed in the conventional spinning. However, there is a difference in stress distribution during forming. There is a development of low tensile stress zone in-between the blending and working rollers. The stresses generated at the edge are also different than in conventional spinning. The uneven stresses generated at the edge of the sheet can cause early wrinkling failure in flexible mandrel-free spinning without support rollers, compared to conventional spinning. The wrinkling failure mechanism for this process is similar to that observed in the conventional spinning. The strain path followed by the elements at different positions range from plane strain to shear. This preliminary work demonstrates the possibility to use an FE model to further understand this process. In particular, the absence of a mandrel introduces distinct deformation mechanics, which need to be understood better to optimize both the hardware configuration and the toolpath design on the machine. Another avenue of study is for nonaxisymmetric workpieces, which have only been studied experimentally on this process. As shown here, the FE model can reveal pertinent metrics and trends, not readily available in an experimental setting. Acknowledgements The authors would like to acknowledge funding for this study through an Innovate UK project (FELDSPAR) and earlier funding from Nissan Motor Co., Ltd., UK. The authors also appreciate the contribution of Iacopo M. Russo through data and discussions. References [1] O. Music, J. M. Allwood and K. Kawai, A review of the mechanics of metal spinning, Journal of Material Processing Tech., 210 (2010), 3-23. [2] O. Music and J. M. Allwood, Flexible asymmetric spinning, CIRP Ann., 60 (2011), 319–322. [3] I. M. Russo and E. G. Loukaides, Toolpath generation for asymmetric mandrel-free spinning, Procedia Eng., 207 (2017), 1707–1712. [4] B. Rentsch, N. Manopulo, and P. Hora, Numerical modelling, validation and analysis of multi-pass sheet metal spinning processes, Int. J. Material Forming., 10 (2017), 641–651. [5] L. Wang and H. Long, Investigation of material deformation in multi-pass conventional metal spinning, Material Design, 32 (2011), 2891– 2899. [6] M. E. Tamer, O. Music, I. Ozdemir, B. Baranoglu, A. Sakin, and I. Durgun, Simulation for Incremental Sheet Forming Process : a Comparison of Implicit and Explicit Finite Element Analysis with Experimental Data, in 7th International Conference and Exhibition on Design and Production of Machines and Dies/Molds, Antalya, Turkey, 2013. [7] K. Jawale, J. F. Duarte, A. Reis, and M. B. Silva, Characterizing fracture forming limit and shear fracture forming limit for sheet metals, Journal of Material Processing Technology, 255 (2018), 886–897.