Distortion control is one of the most important concerns of the metal processing industry, which also includes the fine-blanking technology. Fine-blanking of sheet metal involves metal flow and shearing, which result in complex deformation. Distortion occurs during the removal of parts from the tool due to the relaxation of internal stresses. Internal stresses that are generated during fine-blanking have an important effect on the shape and size of parts after fine-blanking. The distortion of ring type products during fine-blanking was analyzed by using dimension measurements, microscopic observation and numerical simulation.Kontrola distorzije je jedan od najvažnijih poslova u metaloprerađivačkoj industriji, što također vrijedi i za tehnologiju preciznog utiskivanja. Precizno utiskivanje trake uključuje tok i rezanje materijala, što se odražava na složenost deformacije. Distorzija nastaje tijekom uklanjanja dijelova iz alata zbog oslobađanja unutarnjih naprezanja. Promjena oblika tijekom preciznog utiskivanja prstenova analizirana je mjerenjima dimenzija, mikroskopskim zapažanjima i numeričkim simulacijama

B. Kosec

D. Česnik

M. Bizjak

V Bratuš

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157METALURGIJA 51 (2012) 2, 157-160D. ČESNIK, V. BRATUŠ, B. KOSEC, M. BIZJAKDISTORTION OF RING TYPE PARTS DURING FINE-BLANKINGReceived – Prispjelo: 2011-05-27Accepted – Prihvaćeno: 2011-07-21Original Scientifi c Paper – Izvorni znanstveni radISSN 0543-5846METABK 51(2) 157-160 (2012)UDC – UDK 621.73.042:669.1=111 Distortion control is one of the most important concerns of the metal processing industry, which also includes the fi ne-blanking technology. Fine-blanking of sheet metal involves metal fl ow and shearing, which result in complex deformation. Distortion occurs during the removal of parts from the tool due to the relaxation of internal stresses. Internal stresses that are generated during fi ne-blanking have an important eff ect on the shape and size of parts after fi ne-blanking. The distortion of ring type products during fi ne-blanking was analysed by using dimension measurements, microscopic observation and numerical simulation. Key words: Fine-blanking, distortion, internal stressesDistorzija dijelova prstena tijekom preciznog utiskivanja. Kontrola distorzije je jedan od najvažnijih poslova u metaloprerađivačkoj industriji, što također vrijedi i za tehnologiju preciznog utiskivanja. Precizno utiskivanje trake uključuje tok i rezanje materijala, što se odražava na složenost deformacije. Distorzija nastaje tijekom uklanjanja dijelova iz alata zbog oslobađanja unutarnjih naprezanja. Promjena oblika tijekom preciznog utiskivanja prstenova analizirana je mjerenjima dimenzija, mikroskopskim zapažanjima i numeričkim simulacijama. Ključne riječi: precizno utiskivanje, distorzija, unutarnja naprezanjaINTRODUCTIONThe aim of the investigation was to analyze the dis-tortion of ring type products during ﬁ ne-blanking. Dis-tortion control during the production cycle is an impor-tant need within the metals processing industry, there-fore numerous investigations were performed [1-10]. It is possible that the shape and size of a product change when removing it from the tool during cold form-ing because of the relaxation of stresses. Internal stresses occur as a consequence of different strain levels at differ-ent locations at the same time. These differences are caused by the different strength of coexistent phases in the material, the different actual strain as a consequence of the tool shape and temperature gradients [11]. The inﬂ uence of the above described process on the ﬁ ne-blanked parts was analyzed. The second important dimension criterion is the de-viation from perpendicularity from the cutting edge to the face, the so called taper. Fine-blanked parts are al-ways slightly tapered, therefore the inner shapes have a larger dimension on the die roll side (Figure 1). The amount of taper to be expected is about 0,0026 mm per millimetre of edge width. Taper is inﬂ uenced by many factors, thus should be examined separately in each in-dividual case [12]. D. Česnik, M. Bizjak, B. Kosec, Faculty of Natural Science and Engi-neering, University of Ljubljana, SloveniaV. Bratuš, Hidria Institute for Materials and Technologies, Sp. Idrija, Slovenia EXPERIMENTAL SETUP Distortion was analyzed during the ﬁ ne-blanking of ring type products from a cold rolled steel sheet C15E (1.1141) with the chemical composition, given in Table 1. The mechanical properties of the used steel sheet are given in Table 2.Table 1 Chemical composition of used steel sheet C15EElement C Si Mn S P/mas.% 0,14 0,18 0,45 0,004 0,007Table 2 Mechanical properties of used steel sheetRp0,2 /MPa Rm /MPa Rp0,2/Rm /- n / -285 405 0,70 0,205C /MPa E /GPa A /%675 170 40A two row tool was used, which allowed us to deter-mine the inﬂ uence of the different positions of parts in Figure 1 Taper-W of fi ne-blanked edges 158  METALURGIJA 51 (2012) 2, 157-160D. ČESNIK et al.: DISTORTION OF RING TYPE PARTS DURING FINE-BLANKINGthe steel sheet on the distortion. The position of both rows is presented in Figure 2. The ring type samples had a nominal inner diameter (ID) of 46,07 mm and a nominal outer diameter (OD) of 62,07 mm. The thickness of the steel sheet was 6 mm. The research work was focused on the changes of the inner hole size and shape. The ID was measured with a MarVision MS 442 3D measuring device with a resolu-tion of 2,9 + L/100 μm. It was measured at three differ-ent locations according to the die roll and burr side; 0,8 mm, 3 mm and 5,3 mm from the die roll side of the product (Figure 3). Die roll indicates the starting point of cutting and burr the end point of cutting. Ninety sam-ples were measured from row 1 and ninety samples from row 2. The results were complemented by microscopic ob-servations, residual stress measurement and numerical simulation of ﬁ ne-blanking.RESULT AND DISCUSSIONThe shape of the inner hole is presented in Figure 4 and Figure 5. The angle 0° represents the direction of rolling. On both rows the effect of the ears and the edge of the steel sheet is visible – the radius is larger at those points. Row 1 and row 2 are symmetrically positioned (Figure 2) on the steel sheet, therefore the shape of the inner hole is also symmetrical (Figure 4 and Figure 5).It was expected that taper between 0,8 mm and 5,3 mm from the die roll side would be 0,0117 mm. The measured taper (Figure 6) was 0,0125 mm, which is in good agreement with the expected theoretical taper. Thus the radius at 0,8 mm should be 0,0125 mm larger than at 5,3 mm. But this is not true, considering the re-sults presented in Figure 4 and Figure 5. Quite the op-posite, the radius is larger at most points at 5,3 mm. This could be explained by the uneven deformation during ﬁ ne-blanking, which resulted in the generation of inter-nal residual stresses that are released during the remov-al of parts from the tool. In Figure 7 uneven deformation during ﬁ ne-blank-ing is presented with the help of microscopic observa-Figure 2 Position of both rows on steel sheet Figure 3  Schematically presented ring type sample and measured levelsFigure 4 Shape of inner hole of parts from row 1Figure 5 Shape of inner hole of parts from row 2Figure 6 Taper measured under stereomicroscope159METALURGIJA 51 (2012) 2, 157-160D. ČESNIK et al.: DISTORTION OF RING TYPE PARTS DURING FINE-BLANKINGtion and numerical simulation. The ﬁ ne-blanking proc-ess was interrupted at punch penetration of 4 mm into the steel sheet. Similar deformation was observed in both cases. On one hand this conﬁ rms the model for numerical simulation and on the other it provides an in-sight into uneven deformation that occurs during the ﬁ ne-blanking process. The material on the inner side of the ring part bends towards the ring and the material on the outer side away from it, therefore the ring type part is bended slightly towards the outside cutting edge. Such uneven deformation results in the distortion of parts as a consequence of the relaxation of internal stresses after they are removed from the tool. The result of distortion after cutting and removal of parts from the tool is presented in Figure 8. It was notice-able that parts were distorted in such a way that the radius at 5,3 mm should have been larger than at 0,3 mm, which is in good agreement with the results presented in Figure 4 and Figure 5. The orientation of this distortion is op-posite to the orientation of distortion due to taper. When analyzing the overall distortion we must consider the dis-tortion that occurs as a consequence of taper and uneven deformation. Thus on the internal shapes the larger di-mension is not always on the die roll side.Different parameters affect the above described dis-tortion, thus the radius and difference at 5,3 and 0,8 mm are not the same all over the trajectory. An important thing is the amount of the surrounding material, there-fore the difference at 5,3 and 0,8 mm is the maximum at the edge of the steel sheet; angles from 225° to 315° for row 1 and from 45 ° to 135° for parts from row 2. With parts from row 2 the difference is greater at angles from 270° to 45°, where there is less surrounding material than at angles from 135° to 270°. When comparing the radius at angles from 0° to 180° for parts from row 1 and the radius at angles 180° to 360° for parts from row 2 it is visible that the difference is greater with samples from row 2 that have less surrounding material. What is more, the radius for row 1 at 0,8 mm was bigger than at 5,3 mm, which means that distortion due to taper is more signiﬁ cant than the uneven deformation that oc-curs because of the surrounding material. It is believed that an important role is also played by the mechanical properties of the steel sheet and the condition of the tool, especial the clearance between active elements that becomes larger during the production cycle as a consequence of wear. Greater clearances allow greater horizontal movement of active elements during cutting, which can affect the direction and magnitude of distor-tion. Additional research work is necessary.The hole-drilling strain-gauge method was used to determine the residual stresses after ﬁ ne-blanking on the die roll and burr side. Residual stresses were meas-ured on the ear of the ring type parts. The results are presented in Figure 9 and Figure 10. Measurements conﬁ rm the uneven deformation and uneven generation of internal stresses. Part of the internal stresses is re-a)b)Figure 7  Deformation during fi ne-blanking; a) stereomicroscope, b) Contour from numerical simulation. Displacement increases 10 timesFigure 8  Distortion after fi ne blanking and removal of parts from tool. Contour from numerical simulation. Displacement increases 40 times-200-160-120-80-40040801201602000 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 1,1Distance from surface / mmResidualstresses/N/mm2σmin σmaxFigure 9 Residual stresses on die roll side of parts-200-160-120-80-40040801201602000 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 1,1Distance from surface / mmResidualstresses/N/mm2σmin σmaxFigure 10 Residual stresses on burr side of parts160  METALURGIJA 51 (2012) 2, 157-160D. ČESNIK et al.: DISTORTION OF RING TYPE PARTS DURING FINE-BLANKINGlived by distortion during the removal of parts from the tool, while the other part stays locked in. Stresses are higher and tensile on the die roll side, whilst lower and compressive on the burr side. CONCLUSIONDistortion during ﬁ ne-blanking is a complex phe-nomenon and can be affected by several different pa-rameters. The ﬁ rst important thing is characteristic for ﬁ ne-blanking – deviation from perpendicularity from the cutting edge to the face, the so called taper. The magni-tude and particular orientation of taper are well known.The second thing is distortion due to the relaxation of internal stresses during the removal of parts from the tool, which is more complex and is inﬂ uenced by many different factors, such as: the shape and position of parts in the steel sheet, the mechanical properties of the steel sheet and the condition of tools.Both of the above described processes affect the dis-tortion of parts during ﬁ ne-blanking. They can counter-balance or amplify each other, depending on the distor-tion caused by internal stresses. In our example they counterbalance. In order to have control over the distortion of ﬁ ne-blanked parts one has to control the amount of taper and above all distortion due to the relaxation of residual stresses. First of all, a lot of attention needs to be fo-cused on choosing the right position of parts on the steel sheet, the right steel sheet properties and the right tool design. Secondly, good control is needed over steel sheet properties and the condition of tools. ACKNOWLEDGEMENTThe operation was partly ﬁ nanced by the European Union, the European Social Fund. It was implemented in the framework of the Operational Programme for Human Resources Development for the time period from 2007-2013, Priority axis 1: Promoting entrepre-neurship and adaptability, Main type of activity 1.1.: Experts and researchers for competitive enterprises.We also wish to express our gratitude to the Fine-blanking division of the company Hidria Rotomatika Ltd., Spodnja Idrija and C3M – Centre for Computa-tional Continuum Mechanic, Ljubljana.REFERENCES[1] E. Silveira, A. M. Irisarri, Study on the distortion of steel worm shafts, Engineering Failure Analysis, 16 (2009) 3, 1090 – 1096.[2]  A. K. Nallathambi, Y. Kaymak, E. Specht, A. Bertram, Sensitivity of material properties on distortion and residual stresses during metal quenching processes, Journal of Ma-terials Processing Technology, 210 (2010), 204-2110.[3]  E. Brinkmeister, J. Solter, Prediction of shape deviations in machining, CIRP Annals - Manufacturing Technology, 58 (2009) 5, 507 - 510.[4]  J. R. Cho, W. J. 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Distorzija dijelova prstena tijekom preciznog utiskivanja

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