Fundamental Studies On Sheet Metal Incremental Forming Based On Parallel Machine Tools

Over the past several decades, an innovative flexible prototyping technology– sheet metal incremental forming has been an extensive focus of research for sheet metal prototyping and small scale productions. In this forming process, a Numeric-Controlled tool path is pre-determined based on a part's CAD model. The sheet blank is clamped along its periphery, and is incrementally deformed into a desired shape by one or more stylus tools traveling along the prescribed path. Due to advantages of high flexibility, low-cost and short lead-time, the technology has high potential for producing low to middle volume components in aeronautic and astronautic, automotive and shipbuilding industries. This paper focuses on the fundamental studies of incremental forming based on serial machine tool and parallel machine tool. Experimental, numerical and theoretical methods are employed. Major undertakings and conclusions are summarized as follows:(1) The current state-of-art of research efforts on incremental forming is reviewed in the Introduction. Conventional incremental forming processes based on serial machine tools can be categorized into two types, i.e., Negative Incremental Forming (NIF) and Positive Incremental Forming (PIF). The different characteristics of the two forming methods are noted. In NIF, only simple part shapes such as cones and pyramids can be formed. On the other hand, complex parts can be successfully produced in PIF, although a die is needed to support the sheet blank. The applications of NIF to hole-flanging and irregular parts flanging processes are studied. The forming process of PIF for a complex part is also presented. In this process, a supporting die is first milled on the same incremental forming machine which doubles as a milling machine. The bump phenomenon in NIF when the forming angle is small is studied in detail, the same part is also formed with PIF method, and there is no evidence that the bump phenomenon exists. Invetigations are also conducted for sheet metal embossed character using both NIF and PIF methods. It is demonstrated that PIF has a better performance than NIF, indicating that PIF should have wider applications in practice.(2) A Freeform Incremental Forming (FIF) based on parallel machine tools is developed at the Scientific Research Laboratory of Ford Motor Company where two stylus tools are synchronized in motion and deform a sheet metal blank from opposite sides as they are traveling to form a product shape. The two stylus tools are driven by two robots which are mounted reversely on the machine base. The new technology provides significant advantages for sheet metal fabrication process in terms of cost and flexibility since forming dies are completely eliminated. A brief review of the control system and related software are also presented. To improve the accuracy of the kinematics of the parallel machine tools, a laser-tracking measuring instrument is deployed to gauge the positioning errors of the upper and lower robots. The measured data is then used to compensate the tool path over the entire working space before experiments are to be conducteed. This method is proved to be effective for improving the dimensional accuracy of the machine system. Conventional NIF and PIF methods which are originally based on serial machine tools can also be realized using the new parallel machine tool. Four new incremental forming methods are implemented. They are: (a) The upper tool and lower tool form different features separately. This could be interpreted as the combination of NIF from opposite sides. A relative complex part can be formed by this method; (b) One stylus tool forms the product shape while the other tool supports the sheet blank at the surroundings. This method can avoid sagging of the sheet blank at the top of the part; (c) The two stylus tools are synchronized in motion to form the part. In theory any part geometry can be formed by this method. No supporting dies are needed; and (d). The two stylus tools are also synchronized in motion. But the forming sequence is from inside of the part to the outside. There are no vertical travels for either of the two tools except in the first forming level during this forming process.(3) The positioning relationships of two stylus tools are derived for the synchronized FIF process based on the geometry information for cones. A user subroutine based on EXCEL VBA is developed to generate tool paths. The native program used in robot control software can be also directly exported from this subroutine. Since the quality of formed part can be improved by spiral tool paths, other treatment methods for complex part shapes such as the transformation from contour paths to spiral paths are also developed.(4) Experiments are conducted and sheet metal parts are formed using the new methods of incremental forming with parallel machine tools. A squeeze factor method which controls the clearance of two forming tools is developed to improve the part's dimension accuracy. A series of experiments have demonstrated that forming angle has significant impact on the squeeze factor. The squeeze factor should be adjusted when the forming angle changes. However, it is observed that it is difficult to get the dimensions accurate and without springback when the forming angle is bigger than 45o, even a very small squeeze factor is used. Therefore the combination of squeeze factor and multistage method is used instead in this case. This new strategy is shown to be an effective method for improving dimensional accuracies.(5) Numerical simulation processes based on LS-DYNA software are carried out for incremental forming. It includes model setup (the pre-process), the analysis, and data interpretation (post-process). A cone-shaped part is used as an example to compare the differences in using shell element vs. solid element for incremental forming simulations. Strains of solid element models and the middle surface strains of shell element models match very well. Since the simulation with shell element models requires significantly less computing time and the accuracy is acceptable, it is used for all subsequent simulations. Firstly, the hole-flanging process is simulated. Forming forces and thicknesses of the flange under different part sizes and tool diameters are measured, with the goal to obtain the optimized tool diameter. Secondly, the effect of different forming parameters such as forming angles, tool diameters and step sizes on the thinning band phenomenon is investigated. It is concluded that the forming angle is the biggest effect, followed by is the tool diameter. The thinning band gradually develops when the forming angle is increasing. It suggests that the formability can be increased with a smaller tool. This conclustion agrees well with experimental observations found in available literatures. A transitional arc with a smaller forming angle can be added on the top of the part to avoid the excessive thinning. Lastly, the multistage forming method is simulated. It is concluded that careful considerations should be taken for the increment of wall angle at each stage when designing multistage forming in order to minimizing excessive thinning and "Fold Over" while able to achieve the final shape in reasonable number of steps. The forming process where two stylus tools are used is also simulated, with experiments conducted to verify simulation results. The result is that, bending at the top just occurs when the forming is at beginning. After certain time the bending at top doesn't have any changes. Based on this result which is obtained from simulation process, a complex part is formed by FIF process using parallel machine tools. In this part, a pre-formed feature is added at the top of the part. During the forming process of the added feature, bending phenomenon happens. Then the main body of the part is formed after this feature. The experimental result shows that, there will be no bending at the formed part. Also the part dimension accuracy can be improved.(6) Wall thickness distribution plays an important role in sheet metal forming process. Strains are often used as an indication of deformation severity. In incremental forming, the Cosine law t = t0cosφwhere t is the wall thickness, t 0 is the initial sheet thickness andφis the forming angle is generally used to estimate the thickness after forming. However this equation is difficult to be used for complex or irregular parts such as a simple skew cone. Based on the experimental and numerical experiences, a new general mathematical model is proposed. This model is based on part geometry and tool paths. After some algebra, the equation can be expressed asεss = ?ln(cosφ)=?ln(Nr ?nr), where Nr is part's forming surface normal and nr is the normal direction of the tool path plane. Several classic part shapes such as a simple symmetric cone, a hyperbolic cone, a spherical cone, a skew cone and an elliptical cone are studied. Equations for strain distributions are derived. For the regular-shaped symmetric cone, hyperbolic cone and spherical cone, it is the same as the traditional Cosine law. For the skew cone and elliptical cone, the equation is more complex. In this analytical derivation, the formula for strains depends only on the part geometry parameters and tool path directions. Results from numerical simulations of the forming process using LS-DYNA and the corresponding experimental processes are compared with the analytical models for hyperbolic cone, skew cone and elliptical cone. Excellent correlations are found. It is demonstrated that the analytical model developed in this thesis is reliable and efficient in the prediction of strain distributions for incremental forming process. This mathematical model also can be used in any part shapes to forecast strain distributions. The application process on a complex part is illustrated based on the combination of EXCEL VBA and finite element software to show the accuracy and advantage of the new analytical model. It is demonstrated that the process can be used for forming a wide range of complex parts and suitable for industrial applications.