On forces, formability and geometrical error in metal incremental sheet forming. A. Fiorentino*, A. Attanasio, R. Marzi and E.

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1 Int. J. Materials and Product Technology, Vol. 40, Nos. 3/4, On forces, formability and geometrical error in metal incremental sheet forming A. Fiorentino*, A. Attanasio, R. Marzi and E. Ceretti Department of Mechanical and Industrial Engineering, University of Brescia, V. Branze 38, Brescia, Italy ing.unibs.it ing.unibs.it *Corresponding author C. Giardini Department of Design and Technologies, University of Bergamo, V. Salvecchio 19, Bergamo, Italy Abstract: The present paper deals with incremental sheet forming (ISF), a metal forming process developed in the last 20 years. Main advantages characterising this technology are its high flexibility and the possibility of reducing development times and costs. ISF consists of a simple hemispherical tool, moved by a CNC machine or a robot, which locally deforms a metal sheet moving along a defined path. The desired part profile can be obtained using three ISF techniques: single point incremental forming (SPIF) and two points incremental forming (TPIF) with either negative or positive die. In this work, a part, whose geometry was chosen to be representative for sheet formability through ISF, was formed considering the different ISF techniques. Moreover, the influence on the produced part of the adopted tool path step depth increment was investigated. Experimental tests were conducted and the results were analysed in terms of forming forces, material formability, process accuracy and final part thickness. Keywords: incremental sheet forming; ISF; force measurement; formability; FLD; thickness; geometrical error. Reference to this paper should be made as follows: Fiorentino, A., Attanasio, A., Marzi, R., Ceretti, E. and Giardini, C. (2011) On forces, formability and geometrical error in metal incremental sheet forming, Int. J. Materials and Product Technology, Vol. 40, Nos. 3/4, pp Biographical notes: Antonio Fiorentino is an Assistant Professor at the Mechanical and Industrial Engineering Department at Engineering Faculty of Brescia University. He obtained his PhD in Technologies and Energetic Systems for Mechanical Industry from the University of Brescia. His research areas of interests are in metal plastic deformation: sheet incremental forming, Copyright 2011 Inderscience Enterprises Ltd.

2 278 A. Fiorentino et al. sheet and tube hydroforming; necking. forming forces, processes parameters influence and optimisation; machining. micromachining in biomedical devices production; friction. materials, surface finishing and process condition influence on friction; material characterisation; finite element model; and development of FEM model and simulation of studied cases. Aldo Attanasio is an Assistant Professor at the Mechanical and Industrial Engineering Department at Engineering Faculty of Brescia University. His research activities cover the scientific field namely: technology and manufacturing systems. In particular, his researches are focused on experimental and simulative analysis of cutting operations. In the field of plastic deformations, he studied several traditional and innovative processes, such as the Mannesmann process, the sheet incremental forming process by CNC machine, the tube and sheet hydroforming processes. All these researches were based on both experimental and simulative approaches in order to increase the accuracy of the process analysis and to design and to validate simulative models able to forecast the process parameters influence on the final product. Roberto Marzi is a PhD student in Technologies and Energetic Systems at Engineering Faculty of Brescia University. His PhD topics are innovative sheet forming techniques: hydroforming and the incremental forming. He had experience in development of forces acquisition system for turning operations and of FEM model. Elisabetta Ceretti is an Associate Professor at the Mechanical and Industrial Engineering Department at Engineering Faculty of Brescia University. Her research areas of interest are: plastic deformation processes of metal: deep drawing, stamping, extrusion, forging, ring test, tube rolling and drawing, incremental forming and hydroforming; FEM simulation of metal cutting operations; optimisation of cutting operations, tool reliability, optimal cutting speed determination, tool path optimisation (applied both to macro and micro operations); analysis of tool breakage; online (vibration and force gauges) and offline (neural network) control systems; optimisation in the use of flexible working cells and system; and application of quality systems in bettering the working management and in continuous improvement of the processes (FMEA and FMECA). Claudio Giardini is a Full Professor at the Design and Technology Department at Engineering Faculty of Bergamo University. His research fields of interest are: metal plastic deformation: deep drawing, stamping, extrusion forging, extrusion, ring test, tube rolling and drawing; innovative sheet and tube metal processes (incremental forming and hydroforming); development of optimisation methods for plastic deformation processes; study of limit formability, damage and welding criteria; development of analytical and numerical models; optimisation (in stochastic environment) of cutting operations; tool wear mechanism and growth; lubrication and process parameters influence on workpiece (residual stress, surface finishing); analysis of tool breakage; online (vibration and force gauges) and offline (neural network) control systems; optimisation in flexible working cells and systems; and quality tools, working management control systems. He carried out many applied researches with several industrial companies, with national and international research centres in the USA and in Germany.

3 On forces, formability and geometrical error Introduction Incremental sheet forming (ISF) (Jeswiet, 2001; Jeswiet et al., 2005) is a metal forming process which found a large development in the last two decades. Main characteristics of this technique are its high flexibility, short development time and cost reduction. In fact, with the use of a machine, which is generally a CNC (Kopac and Kampus, 2005) or either a robot (Lamminen et al., 2004) or a self-designed device (Jeswiet et al., 2005), a simple hemispherical tool is moved along a predefined path so to locally deform a metal sheet. Robots are characterised by a wide working range and make ISF process more flexible, but their lower sustainable working loads and stiffness make robots less precise than CNC machines. Different tools were developed and tested (Hirt et al., 2003a), starting from the one-piece configuration (rod with fixed hemispherical end) to the rotating hydrodynamically sustained spherical head (Iseki and Naganawa, 2002). The head can be made of different materials with different wear resistance: plastic, mild steel, cemented steel or ceramic (CBN) that can be carbide coated so to reduce friction and to avoid chemical reaction with the formed sheet. To be competitive, a technology must be economically suitable. Due to its long manufacturing time, incremental forming results competitive when small production batches are considered (prototyping, pre-series) (Ambrogio et al., 2003), while its main application markets (Allwood et al., 2005) are characterised by higher product unit cost, longer lead time, poor geometrical features and low tolerances. Customised products, such as medical prostheses (Ambrogio et al., 2005; Tanaka et al., 2005), or specific applications, like metallic foam filled sandwich panels (Jackson et al., 2008), represent an interesting market for ISF. From the ecological point of view, incremental forming showed many advantages in terms of energy request, die material, noise emission and chemical waste (Hirt et al., 2003b). The process can be realised without bottom die [single point incremental forming (SPIF), the simplest configuration], or with the presence of a full or partial bottom die [two point incremental forming (TPIF)] which can be positive or negative (Park and Kim, 2003). If the die represents an additional constraint for the sheet during forming operation, the process flexibility is reduced and the equipment costs increase (Ambrogio et al., 2006a). Therefore, low cost material such as resin, wood or low carbon steel can be used. In both cases (SPIF and TPIF) the sheet edges are clamped by a blankholder frame which is still in SPIF or moves in TPIF when positive die is considered. Being a local deforming process, the formability is limited by the maximum allowed local thinning which mainly depends on the sheet material. Due to the likeness with spinning forming (Figure 1), the sheet final thickness (t f ) in ISF was thought to be ruled by the so called sine law (1) which relates t f with the initial sheet thickness (t 0 ) and the part wall slope (θ). The sine law imposes that no vertical wall (θ = 90 ) can be formed because a null sheet thickness is expected. Indeed, further studies showed that the use of a proper number of intermediate tool passes, called multistage tool paths (Hirt et al., 2003a; Cerro et al., 2006; Verbert et al., 2008), allows to overcome the sine law limit (Iseki and Naganawa, 2002) and, moreover, to control the final part thickness (Kim and Yang, 2000).

4 280 A. Fiorentino et al. Figure 1 Sine law The single path itself influences the material formability as shown in Kopac and Kampus (2005), where different tool trajectories were tested on a simple geometry. The results showed that it is possible to double the reached depth choosing the appropriate tool path. In Giardini et al. (2004), different tool path strategies were considered showing how the part feasibility is significantly influenced by the tool trajectory. The possibility to improve the formed part accuracy optimising the adopted tool path was shown in Attanasio et al. (2006) where an automotive component was formed maximising the dimensional accuracy and the surface quality, minimising the sheet thinning. Compared to other sheet forming techniques, ISF allows higher formability: in fact, when comparing the forming limit curves (FLCs) (Shim and Park, 2001) it can be observed that ISF points are higher, up to three times (Jeswiet et al., 2005) than conventional drawing ones. On the other hand, dimensional and geometrical accuracy still represents a limit for ISF. Different strategies are under investigation to improve the accuracy such as the local heating of the workpiece, so to reduce the springback effects (Duflou et al., 2008). The accuracy can be enhanced also with the implementation of tool path optimisation algorithms which can determine a trajectory that compensates the profiles errors (Hirt et al., 2003c; Ambrogio et al., 2004; Ambrogio and Filice, 2005; Rauch et al., 2008). Forces knowledge is essential when considering sheet failure prevention (Ambrogio et al., 2006b; Filice et al., 2006) or verification of maximum force required in IF manufacturing processes. In fact, for a certain blank material the maximum force represents an overall limit to the maximum workable thickness especially when using forming equipment characterised by limited stiffness such as robots. Moreover, forces are usually used to validate analytical and FEM models (Ceretti et al., 2004; Hirt and Bambach, 2005; Quin et al., 2005; Giardini et al., 2005; Oleksik et al., 2008; Decultot et al., 2008). This scenario results in a lot of attention paid on ISF as testified by the large amount of papers that were published on it, but many cloudy areas are still present and it is there that this work wants to investigate. In fact, the actual adopted investigation approach is an experience and trial-and-error-based method which evaluates different process configuration (TPIF, SPIF, tool paths, ) so to obtain the part features as close as possible to the desired ones. As consequence, there is not a set of global rules that relate the desired results with a proper certain process configuration. This paper objective is to make some clearness on these aspects, adopting a methodical approach to investigate the effects of different technological solutions (TPIF or SPIF) and tool path parameters on the process and on the formed part. In particular, being the final part surface quality and the process time related to the distance between two tool passes, different step depths were tested. To investigate on how these aspects

5 On forces, formability and geometrical error 281 affects the process and the final part features, a simple and representative geometry for the process was chosen (Attanasio et al., 2009). In previous works, the authors studied the effects of the process parameters on the process in terms of surface finishing or part feasibility, while in this research the tested solutions were compared in terms of working forces using a self-designed equipment for force measurement (Attanasio et al., 2008a, 2009), in terms of final part geometry accuracy (geometrical and dimensional errors) and of part formability (maximum drawing depth and wall slope thickness and membrane deformation). Figure 2 Experimental equipment, (a) strain gages equipped tool (punch) (b) load cells equipped table (table) (see online version for colours) (a) (b)

6 282 A. Fiorentino et al. 2 Experimental equipment The experimental equipment used for the forming forces measurement (Figure 2) is composed by a double acquisition system: a strain gages equipped tool with changeable spherical head [Figure 2(a)] and a load cells equipped table [Figure 2(b)]. The punch is mounted on a CNC milling machine mandrel while the table is mounted on the machine working table (Attanasio et al., 2008a, 2009). In previous works, the equipments showed a very good repeatability in terms of low forming forces data scattering and a good overlapping between the developed measuring devices was shown too. Due to the higher dynamic effects that were shown to affect the punch signals, table device was chosen for forces measuring and punch for verification. The table was designed in order to allow the use of a bottom die that can be positioned, when necessary, on the die frame while a modular blankholder clamps the sheet during the process. The blankholder can move along Z axis by means of a CNC controlled brushless motor; in this way both negative and positive dies can be tested. 3 Experimental tests Part geometries usually adopted for studying ISF technique are simple (pyramid, conical, ) or specific ones (common commercial part) that hardly can be correlated to each other. The geometry of the part to be formed in the present research was the one chosen in Attanasio et al. (2009) which is simple and representative of the process. In particular, it showed to be able to highlight ISF process effects and limits. The part profile (Figure 3) starts from a horizontal plane and gradually reaches a vertical inclination. With respect to traditional testing cone or pyramid shaped geometries (Taleb-Araghi et al, 2009; Emmens et al., 2009), the proposed one allows to evaluate the sheet wall maximum reachable inclination (θ max ) using one single geometry. In this way it is possible, for example, to study how the forming forces change as the wall slope changes. Moreover, the adopted geometry allows investigating two different types of deformation depending on which workpiece feature is considered: the punch in fact moves along straight lines (X or Y axes) or along a circular trajectory in the plane (four arcs in XY plane). Moreover, it does not put any limits on testable blank materials and thicknesses, process configurations (SPIF or TPIF), tool path and other process parameters. For those reasons, the adopted geometry can be successfully used to represent the different and various part geometries which characterise actual commercial parts. The different combination of ISF technological solutions and tool path that were tested are (Table 1): the absence or the presence of a die (SPIF or TPIF), the part geometry (positive or negative) and the tool path step depth. SPIF positive case was not considered since evidently a positive geometry cannot be realised dieless because of the missing sheet constraints. Geometrical error, formability, surface finishing of the formed part change as the adopted path strategy changes. In particular, working with constant Δθ increment allows controlling the scallop effect which greatly influences the sheet surface finishing quality (Attanasio et al., 2006, 2008b). The step depth increment ΔZ between two tool passes and the corresponding angle Δθ are related by the following expression (2). ΔZ( θ ) = R [ cos( θ Δθ) cos( θ )] (2)

7 On forces, formability and geometrical error 283 As a consequence, when the wall profile inclination θ is low, constant Δθ increment tool paths allow to use closer tool passes (low ΔZ values). On the other hand, this strategy shows its limits when high θ value profiles are formed. In fact high ΔZ are imposed when it is more difficult to form the profile and low step depth increment should be required. On the contrary, working with constant ΔZ easily allows to control the process when high inclination profiles are formed, but does not allow to control the surface finishing, especially for low θ values, since too far tool passes in XY plane are obtained (Attanasio et al., 2006). Figure 3 Tool path, part and die geometries, Δθ and ΔZ parameters

8 284 A. Fiorentino et al. Table 1 Design of experiments and process parameters Tool path Die Geometry Test name A [ ] No Negative SPIF Neg A9 Yes Negative TPIF Neg A9 Yes Positive TPIF Pos A9 Sheet Lubrication Tool Repetitions FeP04 steel 0.8 mm thick Grease (Agip GR MU EP 2) Material Feed rate Rotation Rs C m/min None 9 mm Starting from the above considerations and reminding that low Δθ and ΔZ increments give better results in terms of scallop (and surface finishing) but longer process times, optimised tool paths can be obtained combining the two strategies. For example, when realising low inclination profiles, upper Δθ limited constant ΔZ tool path can be used and vice versa when high inclination profiles are considered. In this work the attention is focused on constant Δθ tool path, leaving to the future a constant ΔZ tool paths investigation. The tool paths are generated by means of a customised self-designed CAM once the process parameters (i.e., Δθ, sheet thickness, tool and die dimensions, feed rate) are defined. Experiments (Table 1) were conducted on FeP04 deep drawing steel sheets 0.8 mm thick using a punch with a head radius RS = 9 mm. Before the test, on each blank surface, a reference grid for deformation measurements was applied. The previous study (Attanasio et al., 2008a) showed a good tests repeatability, therefore two repetitions were performed for each test. 2 4 Results and discussion The experimental results were compared in terms of forming forces, maximum reachable depth (Z MAX ), wall inclination before rupture (θ MAX ), formed part geometrical error, sheet deformation and thinning. The depth of the last tool pass before rupture was considered as the maximum reachable depth because the sheet rupture always occurred during Z depth increment. 4.1 Detected forces The force profiles detected by the table force measuring device in the analysed cases are reported in Figures 5, 6 and 7 top-left corner. Some likeness are evident, in fact it is possible to observe that forces in XY plane (T X and T Y ) have a periodic profile whose max amplitude increases as the process proceeds (that is as θ increases). Also the force along Z axis (T Z ) does not have a constant profile. Its amplitude increases at each tool pass presenting four local maxima inside each pass (Fig. 4), corresponding to the four corner of the formed part (Fig. 5 top-left). To perform a numerical comparison between

9 On forces, formability and geometrical error 285 the tests, the maximum absolute values of the forces in the XY plane were chosen for each pass (F X,MAX and F Y,MAX ). The F Z,MEAN values, calculated as the mean value of T Z in the considered step interval excluding the force peaks ( A interval in Figure 4), were chosen as representative for the force normal to the XY plane. Figure 4 Reference forces value used in experimental results comparison Note: F Z,MEAN refers to T Z and P Z average values in the A interval Figure 5 Typical forming force profiles (top-left) and elaborated forces values in SPIF Neg tests (see online version for colours)

10 286 A. Fiorentino et al. Figure 6 Typical forming force profiles (top-left) and elaborated forces values in TPIF Neg tests Figure 7 Typical forming force profiles (top-left) and elaborated forces values in TPIF Pos tests

11 On forces, formability and geometrical error 287 The obtained F X,MAX, F Y,MAX and F Z,MEAN values calculated for the tested cases as θ increases are plotted in Figures 5, 6 and 7. A common forces behaviour can be observed. In particular, they are all affected by both θ and Δθ (Δθ influence is clearly visible when θ becomes greater than 20 ). In fact all forces values grow as θ or Δθ raise. In both cases, the forces increase is due to the increasing amount of material which is deformed by the punch since the amount of material in contact with the tool increases as the wall slope θ and the step depth Δθ increase. Differences in F X,MAX and F Y,MAX values (that should be equal in the same test due to the process symmetry) are due to the sheet blank anisotropy. The highest force of F X,MAX and F Y,MAX (called F X-Y,MAX ) was used for studying the process parameters effects Negative geometry SPIF vs. TPIF Figure 8 compares the effects of the die presence on forming forces when a negative geometry is concerned. TPIF requires higher forming forces than SPIF because, in the first case, the die represents an obstacle for the sheet deformation so requiring higher deformation forces. Figure 8 Forces comparison between SPIF Neg and TPIF Neg processes TPIF negative vs. positive geometries Figure 9 compares the force values obtained in TPIF Pos and TPIF Neg tests. It is evident how a positive tool path requires lower forces with respect to a negative path for high θ values. When θ is low, the working forces are lower when using a negative tool path even if, in this case, the differences are much smaller. The comparison shows different force curve profiles; in fact using a positive geometry, the force curves have a predominant negative curvature while the ones with a negative geometry have not. An explanation can be given considering the movable blankholder used in positive profile forming (Figure 3) which stretches and pulls the sheet as it moves along Z axis. In negative profile forming, the blankholder is still and therefore all the pushing action along Z is exerted by the punch. Moreover, the blankholder effect is more relevant as the wall slope θ is higher being ΔZ higher according to formula (2). In fact in this case the stretching action exerted by the blankholder is higher and forces increase slowly.

12 288 A. Fiorentino et al. Figure 9 Forces comparison between TPIF Neg and TPIF Pos processes This aspect is important when robot IF is considered since its low stiffness, with respect to CNC machines, does not permit high part geometrical accuracy unless the forming forces are limited. Another important aspect is that lower forces acting on the tool and on the die allow to contain the die and tool wear and the friction acting between punch and sheet. Figure 10 Formed parts geometry detected by the CMM (see online version for colours) 4.2 Part formability and geometrical error In order to be able to choose the most appropriate process variables, it is very important to understand how the different parameter combinations affect the material formability in terms of workpiece geometry and deformation the material can undergo before rupture. The part formability was estimated in terms of maximum reachable depth (Z MAX ) and wall slope (θ MAX ). Due to sheet springback and tool deformation, the actual part geometry is different from the theoretical one. In order to estimate the effect of these phenomena, two values where calculated according to (3) and (4). They represent the differences in terms of E Z and E θ (Figure 11), between the part ideal geometry and its actual one which was measured using a coordinate measuring machine (CMM) (Figure 10).

13 On forces, formability and geometrical error 289 EZ = ZMAX = ZIdeal (3) E θ =θ θ (4) MAX Ideal Geometrical accuracy is one of the weak aspects of ISF processes, therefore the geometrical accuracy of the process was evaluated by comparing theoretical and actual sheet geometries by means of a self-designed software. After aligning the two geometry using the part flange and its symmetry, this software calculates the local distance between the two geometries furnishing its maximum value. An example is showed in Figure 11 in the case of SPIF with negative geometry. This distance between the two profiles was taken as geometrical error. Figure 11 Geometrical error estimation method (referring to SPIF Neg 6 ) (see online version for colours) Figure 12 compares the results for all the considered cases. The highest formability is obtained in dieless forming (SPIF), but the very low geometrical accuracy does not make it widely suitable. In fact, the highest values of Z MAX and θ MAX obtained when SPIF is adopted, are due to the overall deformation of the sheet (visible in Figure 13) as the absence of the die results in a lack of constraints for the formed part. This leads to a deeper final part whose geometry is much more different from the ideal one. When TPIF is concerned, it is possible to observe that the part formability is significantly improved as Δθ decreases. In fact, for lower Δθ values the amount of deformed material at the same time is lower, so a more gradual deformation is obtained and rupture occurs at higher depths. Moreover, the adoption of a positive tool path slightly improves the part formability. The geometrical error results show that, within TPIF cases, the geometrical accuracy is not significantly influenced by the adopted tool path strategy.

14 290 A. Fiorentino et al. Figure 12 Formability and Geometrical Accuracy comparison between TPIF Pos, TPIF Neg and SPIF Neg. Figure 13 Comparison of final geometries: SPIF and TPIF for Δθ = 2 (see online version for colours) When E Z and E θ values are considered, it is noticeable that an improved part accuracy can be reached adopting a TPIF Neg process. Moreover, a solution based on over-forming technique can be used to compensate springback and equipment deformation for SPIF Neg and TPIF Pos strategies. 4.3 Part limit deformation In order to evaluate the sheet membrane deformations in terms of maximum and minimum elongation (Figure 14) according to formula (5), a circular reference grid (diameter 5 mm) was applied on the blank surfaces before the tests. Due to the impossibility of estimating the deformation in the rupture zones, the measures were taken in the most deformed safe areas located in the deepest part zones. The membrane deformation comparison (Figure 15) shows that the IF technique seems to not affect the material formability which is, instead, affected by Δθ. In particular, the lower the Δθ, the higher the formability. Combining these results with the ones reported in Figure 12 it is possible to state that lower Δθ values allow to reach higher depths, slopes and material deformations. The use of SPIF technique allows reaching higher depths and slopes than TPIF, due to the die absence, but the actual sheet membrane deformation is the same in both cases even if, as already said, the geometrical error is higher than the one obtained with TPIF.

15 On forces, formability and geometrical error 291 Figure 14 Reference grid and its deformation Figure 15 SPIF-TPIF formability comparison in negative geometry forming in terms of material membrane deformation In order to compare the IF formability with respect to standard forming process, the theoretical FLC, derived from the material properties for FeP04 steel 0.8 mm thick using the Keeler Law, is reported in Figure 15. It is evident that IF allows greater deformations with respect to standard forming process. 4.4 Part thickness The final part thickness was measured each 5 mm along the part symmetry plane. In Figure 16 the thickness plots are reported. It is evident how the part thickness decreases where the wall slope θ increases in that direction. Moreover, being IF a local deformation process, the sheet thickness in the not worked zone (central and outer zones) remains unchanged. As far as the process parameters are concerned, it is evident that Δθ influences the final part thickness while the presence of the die does not. In particular, the lower the Δθ, the lower the minimum thickness. This confirms the membrane deformation observation, i.e., the sheet can undergo a more severe thinning before failure, since its formability is improved.

16 292 A. Fiorentino et al. Figure 16 Final part thickness (see online version for colours)

17 On forces, formability and geometrical error Conclusions and further studies In the present paper, an experimental tests campaign on ISF was presented where different process techniques and parameters (presence of the die, positive-negative geometry and tool path) and their effects on forming forces, material formability and final part geometry were considered. The collected data on forming forces allowed to conclude that when significant force reduction and geometrical accuracy improvement are the main objectives, TPIF with positive geometry technique using low Δθ tool path should be adopted. Low Δθ increments increase the material formability in terms of membrane deformation. In particular, the material formability improvement was verified measuring the part final thickness and showing the increasing of the maximum thinning the material can undergo before rupture. As a consequence, maximum depth and wall slope before rupture increase. Higher depth can be reached using SPIF instead of TPIF techniques, but with a significant loss in geometry accuracy. Moreover in this case, the membrane deformation measures showed that the reachable higher depths and wall slopes are the effects of the sheet bending under the tool action when die is absent and do not derive from an increasing of the material formability. It is evident how the choice among the different process parameters greatly influences the process costs since TPIF requires the presence of a die and a stiffer machine due to the higher required forming forces. Δθ parameter influences the operation costs too since lower Δθ values mean higher working times. In all the tested cases, the developed geometry was useful in studying the process main aspects and characteristics. Next studies will be oriented on testing the effects of the different tool path strategies using a constant ΔZ step depth increment. References Allwood, J.M., King, G.P.F. and Duflou, J. (2005) A structured search for applications of the incremental sheet-forming process by product segmentation, IMechE Part B. J. of Eng. Manufacture, Vol. B2, pp Ambrogio, G. and Filice, L. (2005) A simple strategy for improving geometry precision in single point incremental forming, paper presented at the 8th ICTP, Cluj-Napoca, Romania, pp , in proceedings. Ambrogio, G., Costantino, I., De Napoli, L., Filice, L., Fratini, L. and Muzzupappa, M. (2004) Influence of some relevant process parameters on the dimensional accuracy in incremental forming: a numerical and experimental investigation, J. of Mat. Proc. Tech., Vol , pp Ambrogio, G., De Napoli, L., Filice, L., Gagliardi, F. and Muzzuppa, M. (2005) Application of incremental forming process for high customized medical product manufacturing, J. of Mat. Proc. Tech., Vol , pp Ambrogio, G., De Napoli, L., Filice, L., Micari, F. and Muzzupappa, M. (2006a) Some considerations on the precision of incrementally formed double-curvature sheet components, paper presented at the 9th ESAFORM 2006, Glasgow, UK, pp , in proceedings. Ambrogio, G., Di Lorenzo, R. and Micari, F. (2003) Analysis of the economical effectiveness of incremental forming process: an industrial case study, paper presented at the 6th AITeM, Gaeta, Italy, pp.37 38, in proceedings.

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