Makaralı bükme işleminde ince sacların kenar dalgalanma kusurunun deneysel incelenmesi

Experimental investigation of edge waviness during cold roll forming of thin sheet metal

Özet

Makaralı bükme işlemi arka arkaya yerleştirilmiş makara setlerinin hareket ettirilmesi ile sac malzemenin makaraların arasından geçerken kenarının bükülerek istenilen profilin elde edilmesi olarak tarif edilebilir. Çok çeşitli kesitler bu yöntemle imal edilmekte ve sanayi üretiminde makaralı bükme işleminin payı hızla artmaktadır. Yaklaşık yüzyıllık bir işlem olmasına rağmen, tezgah ve makara tasarımı, günümüz teknolojisinde halen deneysel olarak elde edilen bilgilere ve çalışanların pratik deneyimlerine bağlıdır. Yüksek maliyet ve harcamalardan ötürü sistematik olarak deneysel çalışmalar çok az yapılabilmektedir. Makaralı bükme tezgahı tasarımı istenilen profilin en az makara seti kullanılarak elde edilmesi hedefi ile yapılmaktadır. Makaralı bükme işleminde görülen kusurlar içinde kenar yüzeyde dalgalanma en sık görüleni olmakla birlikte, sac yüzeyinde/taban alanında cep oluşumu, dikey ve yatay yönde eğilme, burkulma ve burulma sayılabilir. Bu çalışmada ince sacların makaralı bükme işleminde kenar dalgalanma kusuru deneysel olarak incelenmiştir. Kenar dalgalanmasının davranışının daha iyi anlaşılması ve bu kusurun kontrol altında tutulması makara setleri ve tezgah tasarımının daha düşük maliyetle ve daha hızlı yapılabilmesi açısından önem taşımaktadır. Deneysel çalışmada Taguçi yaklaşımı ile üç değişkenli ve iki düzeyli L8 tam eşleştirmeli deney yöntemi benimsenmiş ve elde edilen sonuçlar aynı yöntemle incelenmiştir. Sac malzemenin kenar yüksekliği değişimi en büyük etki olarak gözlenmiş, ayrıca yüzey kalitesine etkisi olan kenar yüksekliği ile malzeme cinsi ve makara dizilişi ile malzeme cinsi arasında etkileşimler tespit edilmiştir.

Abstract

Cold roll forming (CRF) is the process of forming the sheet metal by passing it through a series of rotating rolls arranged in tandem that bend and shape the sheet metal. CRF is gaining more significance between the other metal forming in today’s manufacturing environment. The producers request tighter tolerances, improved surface quality and increased variety of cross sections that require the parameters affecting the process to be revisited in order to understand the effects and interactions among them. The design of the process is still based on empirically gained data and practical experience of employees. Systematic experimental researches have been done with limited forms because of high costs. The CAD tools in this field require to be improved in order to simulate and analyze the process in a better way. In this regard the latest design software packages are being sold with finite element analysis interfaces, as the design software packages are limited with the empirical methods & analogy with the previous similar shapes. On the producers’ side it is very important to deliver the CRF machine as early as possible once an order is received. However, only after production, the CRF machines can be tuned to prevent any undesired shape defects that require additional time to fix some of which may need a rework on the machine. On the customer side the time loss is the biggest risk as it is a mass production machine and the tuning and design change costs which are always accompanied with time and profit loss. Due to complex deformations unlike bending or deep drawing, CRF requires careful modeling of the process. The most frequent defect observed during CRF process of symmetrical u-shape profiles is the edge waviness. The web-buckling, twist, bend in the horizontal and vertical plane are the other defects, which can be observed during CRF. The introduced empirical equations i.e. forming angle method, forming rate method, Bhattacharyya method, Ona & Jimma method are the ones used by CRF machine & flower pattern designers. However these equations generally offer roll set numbers or flower patterns within a large safety margin. Therefore, for complex shapes the designer’s experience is still the most reliable tool for deciding the design parameters. In this study Taguchi method as used in order to investigate edge waviness defect during CRF process of thin sheet metal applying full factorial L8 design method. During the experimental study the effects of the parameters of roll setup (the angle difference at the roll sets 40 and 55 degrees), flange length (12 mm or 17 mm) and material mechanical properties (mild steel or aluminum) on edge waviness and interrelations between them were investigated. The study shows that the flange length has the biggest effect on the edge waviness formation. The roll setup and material type has equal effects and are observed to have equal magnitudes after flange length. In addition to these effects, the interactions between the flange length, material type and roll setup are observed and detailed in this study. The interaction between roll setup and edge length shows that as the angle increment between two roll sets decrease, the edge waviness decreases for both of the edge length values. In this regard if there is a need to work with changing edge length, the angle increments between the roll sets should be reduced. This interaction result is especially important for the state of the art CRF machine design to manufacture parts with variable cross section. The second interaction between edge length and material type shows that as the edge length decreases the surface waviness decreases for both of the material types of aluminum and mild steel. The importance of this interaction is important in the classical CRF technology to control the material non-homogeneity. In other words using a larger edge length the CRF machine becomes sensitive to material mechanical properties changes. Another defect observed during the experimental study was the (longitudinal) bow. Due to the large angle difference between the last two roll stands a weak correlation is observed between the edge waviness and the bow formation in this study. The experimental results, within the limits of this study, can be summarized as follows: 1) The edge length is the most important parameter to control the edge waviness. 2) The mechanical properties of the materials are equally important with the roll angle increment. 3) There is an interaction between roll angle increment and edge length. 4) There is another interaction between edge length and material properties. Shorter edge length controls the material mechanical properties non-homogeneity.