This paper comprises an experimental study for a complex geometry part obtained by incremental forming. Due to the process complexity (the presence of forces on three directions—a vertical one and two in the blank's plane), a three axes CNC milling machine, capable of describing the complex paths covered by the punch for obtaining the truncated cone-shaped parts, has been chosen. To obtain a truncated cone, three different trajectories were selected: in first and second variants after each vertical press having a constant step, the punch covers a circular path. The differences show that the following circular trajectory can start at the same point or can be shifted at an angle of 90° from the previous press point. In the last variant, the punch performs a spatial spiral trajectory. The main objective of our study was to determine the optimal forming strategy, by shifting the press position of the punch and the path it follows to obtain a truncated cone through single point incremental forming. Thus, the strain distribution can be homogeneous, and the thickness reduction and the process forces are minimal.
The incremental sheet metal forming process is a modern method of material forming processes, with huge potential regarding the degree of flexibility and customization of the parts made through this process. Within the analysed process, the strain is created by means of a numerically controlled punch. The punch follows a trajectory defined by the CNC program of a milling machine on which strains are produced. The material is strained locally only in the area that comes into contact with the punch, hence the name incremental forming, the material being formed gradually.
With regard to single point incremental forming, Pohlak et al. [
Another approach to the single point incremental forming process has been identified in Silva’s analytical studies
Multistage deformation path strategies for single point incremental forming (SPIF) are revised with the purpose of improving sheet thickness distribution and forming a vertical wall surface for cylindrical cups by Liu et al. [
Yamashita et al. [
Li et al. [
A deformation analysis of incremental forming process is presented by Cui et al. [
Bambach et al. [
Silva and Martins [
In the single point incremental forming study, experimental research methods were used to highlight the mechanics of the forming process, the forces required in the forming process, the accuracy of the surface obtained, the formability of various materials, the optimum path of the punch, and so forth.
The mechanics of the incremental forming process was examined by comparing single point and two points incremental forming from Jackson’s experimental perspective [
Experimental research on single point incremental forming was mainly directed at determining the forces involved in the process. Minutolo et al. [
The accuracy of the parts obtained through single point incremental forming was studied by Ambrogio et al. [
Hussain and Gao [
Attanasio et al. [
Fu et al. [
This paper aims at studying the influence of the forming strategy (by choosing the area of punch press in the first step and also its subsequent path) on the main strains, thickness reduction, and forces to obtain truncated cone-shaped parts.
Experimental investigations were carried out on a three axes CNC milling machine, DMG Veco, capable of describing the complex paths covered by the punch in order to obtain truncated cone-shaped parts. The milling machine table was fixed with an elastic element enabling the measurement of forces, and the elastic element was fitted with the die. The punch was fitted on the crunch of the CNC milling machine.
The experimental layout is presented in Figure
The experimental layout.
The second horizontal component has the same variation as the first but out of phase with an angle of 90 degrees, for which it was not taken into account in this study. The signal acquired from the analogue digital board KPCI 3108 can be processed, filtered, and saved via a virtual instrument created using the Matlab program. The actual forming force values of the components were calculated by means of the interdependence ratio between the electrical signals received and force magnitude, determined using calibration curves.
The die used in the incremental forming consists of a support plate, a port die plate, a die, and a blankholder. The die is circular, with a diameter of
To measure the strains “offline,” an Argus optical measurement system produced by Gom was used (Figure
The Argus optical measurement system used for strain and thickness reduction evaluation.
The strains were measured with a camera having a lens’s focal length of 12 mm. The pieces were placed on a turntable. Four coded bars and 12 markers enabling the calibration of the optical system were stacked on the pieces. Strain accuracy for the Argus optical measurement system is 0.1%. The program installed on the optical analyser can measure the strains in three ways: the engineering strains, the logarithmic strains, and the Green strains. In this paper, we determined the logarithmic strains.
The main objective of the present study was to determine the optimal forming strategy by shifting the penetrating position of the punch and the path it follows to obtain a truncated cone through single point incremental forming, so that the strain distribution can be homogeneous and thickness reduction minimal. To carry out the tests, a DC04 steel sheet (SR EN 10130-2000) with a thickness of 0.7 mm was used. DC04 is commercial deep-drawing steel commonly used in automotive industry. The material elasticity modulus is
Because the Argus measurement system allows precisely the determination of the main strains and thickness reduction on a calibrated area, limited from dimensional point of view, it was chosen for this study to achieve some small pieces, so that the accuracy of the results is as high as possible. The diameter of the large base of the truncated cone was chosen such that
The three different trajectories used for the produced parts (a) V1, (b) V2, and (c) V3.
The experimental researches were repeated five times for each type of the chosen trajectories both in terms of the determination of main strains and thickness reduction as well as forces in two directions so that any errors will be eliminated.
Table
Variant |
|
|
Thickness reduction [mm/mm] |
---|---|---|---|
V1 | 0,5928 | 0,163 | 0,679 |
V2 | 0,6119 | 0,1129 | 0,673 |
V3 | 0,502 | 0,113 | 0,5986 |
It can be observed that, for all three path types, the major strain (Figure
The major strain variation for the specified three different trajectories.
For both the path with successive presses in the same area (V1) and the one with successive presses shifted at an angle of 90° from the previous press (V2), we notice that the value of the major strain does not have a homogenous variation on the circular path (Figures
Unlike the other two variants considered, in a spiral trajectory (V3), the major strain values
The minor strain also reaches a maximum value in the initial point of punch press along a vertical direction for V1 and for the spiral trajectory, while for variant V2 there are four areas of maximum local values corresponding to the press points (Figures
The minor strain variation for the specified three different trajectories.
The thickness reduction variation for the specified three different trajectories.
This is due to the material fold that is formed before the punch press in these two types of paths. In the spiral path (V3), the process is continuous, with no repeated punch presses along the vertical and circular directions, leading to the smoothing of the material. The material folding in the area before the punch press is a major disadvantage of the two types of paths (V1 and V2), resulting in reduced accuracy of the formed parts and increased forces involved in the process. The minor strain maximum values are 0.163 for V1, 0.1129 for V2, and 0.113 for V3.
The thickness reduction has a variation that is similar to the major strain for all three types of paths. The peak value is reached at the points of initial punch press on a vertical direction. Higher values similar to those of the major strain are reached and decrease as the diameter of the circular path reduces (Figure
The best variant for manufacturing truncated cone-shaped parts in terms of the distribution of strains is the one involving the use of a spiral path, as the distribution of major strains and relative thickness is more homogeneous, and relative thickness has lower values.
Table
Variant |
|
|
---|---|---|
V1 | 414 | 1004 |
V2 | 361 | 928 |
V3 | 391 | 811 |
As regard force values
The horizontal force (
In the case of successive punch presses in the same area (V1) and the spiral path (V3), the variation of force
As to the variation of component
Figure
The vertical force (
In the variant with successive presses in the same area (V1), vertical force variation reaches a local maximum after each vertical punch press, followed by a sudden drop and then by a smooth decrease until a circular path is completed, when there is another local minimum. This variation is repeated at each work cycle until the peak force value along the vertical direction is reached. This peak is reached near the area where the last vertical steps are performed. Here, it is noticed that the differences between the local highs and lows of each circular path stay approximately constant throughout the entire path.
In the variant with successive presses offset at an angle of 90° from the previous entry (V2), the variation of the vertical component of the force is similar to that of variant V1 for the areas where full circles are completed. The only difference is the emergence of areas with minimum values
In the variant where the geometry of the part is described by a spiral path (V3), the variation of the vertical component of the force is smooth with no local maximum or minimum. The vertical component of the force increases until reaching a maximum value (close to the area where the last steps are performed) and then remains relatively constant until just before the end of part processing. As in the case of the horizontal component, it is noticed that the best processing variant is the spiral path, which has smaller peaks and a smooth variation.
Although SPIF is a very flexible process and requires very simple equipment, it has a major disadvantage, namely, precision of the obtained parts. To determine which of the three paths is better in terms of accuracy of the resulting piece, the same optical measuring system Argus was used. With this device, 3D geometries of measured parts were performed after deformation. These geometries were exported in STL format and compared to the model that was supposed to be achieved. To highlight which of the three paths is better to obtain a more accurate part, the pieces were sectioned through the area with the largest deviations from the profile (Figure
The position of the section plan utilised to measure profiles and comparison of resulting profiles and the ideal shape of the part.
From this point of view, the most unfavorable case is the one of the forming in variant V2 (trajectory shifted at an angle of 90° from the previous press point), where this convexity can reach values of up to 0.91 mm. Also a big convexity (0.68 mm) is reached in variant V1 (trajectory which start in the same spot). As can be seen from Figure
This disadvantage can be removed by using a backing plate (TPIF) or minimizing the distance between the forming area and the blankholder. Another disadvantage is the springback. When the punch is removed, the elastic deformations caused by it are removed, and the material tends to recover, and the final form of the piece is slightly smaller than the original. The pillow effect is another disadvantage of SPIF and is present on the small base of the truncated cone parts, causing a slight curvature of the undeformed material in that area.
The analysis of the strain and force variation of truncated cone-shaped parts indicates that the optimum strain strategy is the one in which the punch follows a spiral path. A reduction of about 18% in the maximum major strain value occurs in the case of the spiral path compared to the other two strain strategies, and for the minor strain approximately the same maximum value is reached compared to the deflected press strategy; in the case of thickness reduction, a decrease of approximately 13.5% is obtained compared to the other two strain strategies. Moreover, strains and thickness reduction are evenly distributed for the spiral path compared to the other two strategies.
In the case of force variation, it was noticed that there is not only a decrease in the maximum value for the spiral path strategy of 23% compared to the press strategy in the same area and of 14% compared to the strategy of successive presses offset at an angle of 90° from the previous entry but also a smooth impact-free variation that could lead to the reduction in the dimensional deviations of the parts produced.
In terms of precision of parts obtained through SPIF, the best path is V3, and the resulting profile is the closest to the desired one than in the other two cases.
A future research direction may comprise the study of the strain strategy for parts with different geometries such as truncated pyramid parts, where the strain reaches maximum values due to the geometric structure of the part.