Int J Mater Form (2010) Vol. 3 Suppl 1:971–974 DOI 10.1007/s12289-010-0931-9 © Springer-Verlag France 2010
EXPERIMENTAL AND NUMERICAL STUDY OF A MILLING MACHINEBASED DIELESS INCREMENTAL SHEET FORMING
Tisza, M.1*, Panity, I.2, Kovács, P. Z.1 1
2
University of Miskolc, Department of Materials Processing Technologies Computer and Automation Research Institute, Hungarian Academy of Sciences
ABSTRACT: In this paper, some relevant issues concerning an innovative sheet metal forming method, a milling machine-based Dieless Incremental Sheet Forming (DSF) will be analyzed. The experimental setup is based on the replacement of the commonly used dies by a second forming (Slave) tool where the movement is synchronized with the first forming (Master) tool, thus creating a flexible support. DSF can be used on both CNC machines [1] and industrial robots [2]. In this paper, the authors are only focusing on the experiments with a parallel kinematics 3-axis milling (PKM) machine with 3 additional linear actuators. The deformation state of the formed part will be analyzed using the Vialux-AutoGrid optical strain measurement system and will be compared to the results of 3D FEM modelling. KEYWORDS: die-less forming, parallel kinematics milling machine, linear actuators, optical strain measurement
1 INTRODUCTION Incremental Sheet Metal Forming (ISMF) is an innovative, flexible technology for forming complex shaped parts in small batch production without traditional dies. An original idea of ‘dieless forming’, was patented by Leszak [1]. Since then, there have been many studies to analyze various process variants [2]. A schematic view of some basic process variants systematized by Hirt [3] is shown in Figure 1.
This paper describes an investigation of the deformation mechanism of Single Point Incremental Sheet Forming. First, the new forming principle will be introduced applying a CNC milling machine with parallel kinematics and linear actuators to perform the experimental incremental sheet forming. The deformation of the sheet during the forming experiments will be analyzed using the Vialux-AutoGrid optical strain measurement system. Finally, some results of numerical simulation performed by a commercial FEM package MARC to model the process will also be presented.
2 GENERAL DESCRIPTION OF THE APPLIED EXPERIMENTAL PROCESS
Figure 1 Four basic process variants of ISF [3]
In the experimental forming, a milling machine-based Dieless Incremental Sheet Forming (DSF) was applied. The forming process is based on the replacement of the commonly used dies by a second forming (Slave) tool where the movement is synchronized with the first forming (Master) tool, thus creating a flexible support. This principle can be used on both CNC machines [4] and with industrial robots [5]. In our experiments, a new principle was used, which is similar to robotic sheet metal forming [6], however we used a CNC milling machine with so-called parallel kinematics which is integrated with linear actuators. The control system for the parallel kinematics milling machine (PKM) is an industrial FAGOR 8070 CNC controller. It can handle 28 axes (interpolated simultaneously) and 4 spindles. Four execution channels
____________________ * Corresponding author: 3515 Miskolc-Egyetemváros, Hungary, phone: +3646 565 164, fax: +3646 561 504, email address:
[email protected]
972
allow executing four different machining operations in synchronization. The disadvantages of this industrial control are that it is not adaptive and the tool path must be pre-calculated. The tool path of the Master tool is programmed using CATIA CAM software intended for surface milling. The complex slave tool path generation is solved with a C++ program. The output of this program should be in CATIA file format (for visual validation) or in NC code. The general process layout of the applied experimental equipment can be seen in Figure 2. Figure 4 The forming strategy “B”
In order to continuously support the master tool with the slave tool a predefined gap is required between the two hemispherical tools. The master and slave configuration is shown for strategy “B” in Figure 5. sheet thickness, t
master tool Tool Centre Point (TCPM) surface normal, n master tool path, TCPM (t) given
tool radius, RT
Figure 2 The process layout for DSF forming on a CNC controlled milling machine with linear actuators
Tool Centre Point (TCPS) slave tool
slave tool path, TCPS (t) sought
2.1 THE TOOL PATH CONTROL Within this project an offline slave tool path generation was developed, which is used on a commercial postprocessor’s output (Master tool path) with two forming strategies to increase part accuracy. 2.2 FORMING STRATEGIES The experiments were carried out with two forming strategies (denoted by strategy "A" and "B") with constant step depth to verify the feasibility of the forming on a truncated cone. In strategy “A”, the slave tool holds the sheet on the backside, by moving synchronously along the outer contour, constantly on the flat zone of the part (first forming level) as shown in Figure 3.
Figure 5 Master-slave tool configuration for strategy “B”
A simple formula can be derived for the loci of the slave tool centre. Using the notations given in Figure 5., the slave tool centre can be calculated for strategy “B” by the expression G n S M TCP = TCP + (2R T + t) G , (1) n where t the actual wall thickness is calculated from the initial one with the well-known sine-law, following from the volume constancy. i.e. t = t o sin (90 − α ) . o
(2)
This sine-law is obviously an approximation for the offline calculation. To compensate all the deviations, an online (real-time) calculation with adaptive control is necessary. The formula for slave tool centre for strategy “A” can be calculated by the following expression
⎡ ⎤ ⎛R⎞ TCPS = ⎢( TCPXM , TCPYM ) ⎜ ⎟ , TCPZS ⎥ (3) ⎝ r ⎠ ⎣ ⎦ S S where TCP is given by the coordinates, whilst TCPZ Figure 3 The schematic view of forming strategy “A”
In strategy “B”, the slave tool has a constant path on the outer contour and always supports the master tool right on the opposite side of the sheet as shown in Figure 4.
is only calculated once, i.e. at the first level of forming. The experimental parameters applied in the two forming strategies are summarised in Table 1. During the experiments deep-drawing oil for high pressure application was used on both sides of the sheet. Due to
973
some technical limitations, in our experiments only the master tool was rotating. Table 1: Experimental parameters
Forming speed: Pitch: Material: Sheet thickness: Forming Strategy: Master Tool Diameter: Slave Tool Diameter:
3000 mm/min 0.5 mm Al 1050 0.6 mm strategy “A” or “B” 10 mm 6 mm
3.2 EXPERIMENTAL RESULTS WITH OPTICAL STRAIN MEASUREMENT
During the forming experiments various parameters were investigated. Here we present only a limited set of experimental results mainly focusing on the deformation process particularly on wall thickness changes. For the investigation of the deformation state of the formed parts the AutoGrid optical strain measurement system was applied. Before the experiments a square grid was prepared on the blank. The deformed grid can be seen on the formed truncated cone as shown in Figure 8.
3 EXPERIMENTAL RESULTS Two sets of experiments were performed with the two different forming strategies (“A” and “B”) which were described in section 2.2. A pre-defined final depth of 100 mm was set with the outer diameter D = 380 mm, and the wall angle α = 45o. 3.1 RESULTS WITH THE FORMING STRATEGIES “A” AND “B”
Applying the forming strategy “A” the bottom of the truncated cone was broken before reaching the predefined final depth of 100 mm, as shown in Figure 6. This crack is characteristic for this type of forming [7].
Figure 6 Forming failure with the forming strategy “A”
Figure 8 The formed part with the deformed grid
In Figure 9 the 3D image of the deformed grid can be seen.
Figure 9 The image of 3D deformed grid
Successful forming resulted good quality part applying the forming strategy “B”, however, the slave tool lost the contact after 42 cycles as shown in Figure 7.
Figure 7 Forming result with and without slave tool contact
Figure 10 The variation of the true major strain along the formed part
974
The distribution of the principal strain components (ϕ1, ϕ2, ϕ3) along the section illustrated by the section plane (see Figure 10.) is shown in Figure 11. True strain components
0,6 0,4
φ1
0,2
φ2
0,0
φ3
- 0,2 - 0,4 - 0,6
0
20
40
60
80
100
Distance, mm
Figure 11 Distribution of the true strain components along the section plain shown in Figure 10
Similar figures can be obtained for the sheet thickness changes as shown in Figure 12. In Figure 13, the sheet thickness variation along the section plane shown in Figure 12. can be seen.
4 FEM MODELLING A 3D FEM model was created to study the deformation state of the process performed experimentally. The same geometric and tool parameters were applied as described in the experiments. The stress-strain relationship for the material Al 1050 was used with the well known power law (σ=111ε0.14). To reduce the lengthy solution time rigid tool materials were considered. The simulation results showed a good agreement with the experimentally measured strain components and thickness variation shown in Figure 10. to Figure 13.
5 CONCLUSIONS In this paper a new concept of a truly dieless incremental forming process was introduced. This is based on the replacement of the conventional dies by a programmable movement of a slave tool synchronized with the master tool, thus creating a truly flexible tooling system. The experiments were evaluated by optical strain measurement focusing mainly on the deformation state and on the thickness variation.
ACKNOWLEDGEMENT The works presented in this paper were partly financed by the EU 6th Framework Program with the acronym “SCULPTOR” (NMP2-CT-2005-014026) and the EUREKA project (EUREKA-HU-ISMFP-08). Both financial supports are gratefully acknowledged.
REFERENCES
Figure 12 Sheet thickness variation on the formed part
Since, the original blank sheet thickness to = 0.6 mm, it can be seen from Figure 13. that significant wall thickness reduction occurred along the conical surface. The lowest measured thickness was about 0.329 mm. Sheet thickness variation
0,7 0,6 0,5 0,4 0,3
0
20
40
60
80
100
Distance, mm
Figure 13 Sheet thickness variation along the section plane shown in Figure 12
[1] Leszak, E.: Apparatus and process for incremental dieless forming, US Patent, US 3342051A1, published 1967.09.19. [2] Jeswiet, J. et al: Asymmetric single point incremental forming of sheet metal, Annals of CIRP. v. 54. pages 623-650. 2005. [3] Hirt, G., Ames, J., Bambach, M., Kopp, R.: Forming strategies and process modelling for CNC incremental sheet forming, in Proc. 7th Int. Conf. on Comp. Plasticity, Barcelona, pages 1-8. 2003. [4] Maidagan E., Zettler J., Bambach M., Rodríguez P. P., Hirt G.: A new incremental sheet forming process based on flexible supporting die system. Key Engineering Materials, 344:607-614, 2007. [5] Meier H.., Dewald O., Zhang J.: A New RobotBased Sheet Metal Forming Process, In: Proc. Shemet 2005, pages 465-470, 2005. [6] Panity, I.: Designing Tool Path Generating Strategies for Novel Incremental Sheet Forming. In: OWD 2008, X. International PhD Workshop, pages 142-146, 2008. [7] Micari F, Ambrogio G. and Filice L.: Shape and dimensional accuracy in single point incremental forming: state of the art and future trends. In: AMPT’ 2006. Las Vegas, pages 1-4, 2006. [8] Tisza, M.: New innovations in sheet metal forming, in Proc. of microCAD’09 Int. Conf., Miskolc, pages 67-72. 2009.
