Suitability of sheet bending modelling techniques in CAPP applications

Abstract

Industrial SummaryThe use of CNC machine tools, together with decreasing lot sizes and stricter tolerance prescriptions, has led to changes in sheet-metal part manufacturing. In this paper, problems introduced by the difference between the actual material behaviour and the results obtained from analytical models and FEM simulations are discussed against the background of the required predictable accuracy in small-batch part manufacturing and FMS environments. The topics are limited to those relevant to bending along straight lines. Applications of analytical and empirical models in the area of CAPP and adaptive control are discussed. Process planning for sheet bending, however, should not be treated as an individual task but must be related to other process-planning tasks. An integrated CAPP system for sheet metal, PART-S, is introduced briefly.

Simplified springback prediction in Al/PP/Al sandwich air bending

“…It was reported that wrapping of the sheet around the punch depends on the parameters such as material properties, punch stroke, and die opening [34]. In the case of air bending, using small tool-radius in combination with the large die opening, no wrap around zone was observed [35]. Material modeling affects prediction of wrap around zone [36,37].…”

Sheet-metal bend sequence planning subjected to process and material variations

“…Accurate blank dimension calculation is mandatory to achieve 3D-formed part accuracy. Analytical models of sheet-metal bending are based on the assumptions devised by Ludwik in 1905 [38]. In the actual industrial environment, these assumptions are almost impossible to meet due to various material and process variations.…”

“…Empirical equations utilize correction factors to overcome the effects of thickness and material properties variation. Various international standards provide empirical blank development equations for a variety of sheet-metal grades [38]. According to DIN 1541 permissible thickness variation for cold-rolled steel is 15-20 % [39].…”

An Application of Iso-Gum in the Method for Estimating the Dimensional Errors of Bent Parts

“…According to studies reported in (de Vin et al, 1996), (Hagenah, 2003), (Nguyen et al, 2005), (Singh et al, 2004) and (Streppel et al, 1993), the dimensions achieved from bending operations suffer from various sources of errors. As instructed by (ISO, 1997), once a quantity Q is expressed by a function of n normally distributed variables in such form as in (1), then the deviation Q ∆ of the quantity Q from the nominal value 0 Q can be expressed through a combination of the influencing errors, as expressed in (2) ( ) 0 ,.., ,..,…”

“…Geometric approximations and material behaviour assumptions are applied in both modelling and calculation. Due to the discrepancies between the model and reality, there exists an error in the calculation of the unfolding (Streppel et al, 1993). The subsequent cutting step may also provide various dimensional precision, depending on the selected cutting process.…”