A.H. Streppel

A process model for air bending

A so called `three-section? model for air bending is presented. It is assumed that a state of plane strain exists and that Bernoullis law is valid. The material behaviour is described with Swifts equation, and the change of Youngs modulus under deformation is addressed. As compared with other models, the model described in the paper is capable of generating information such as required punch displacement and the unfolded blank size, very accurately. With in-process measurement of the spring-back angle, the punch displacement can be calculated even more accurately.

The generation of bending sequences in a CAPP system for sheet-metal components

An important process-planning task in sheet-metal manufacturing is the determination of bending sequences for individual components. Computer-aided generation of these sequences, as part of a computer-aided process-planning (CAPP) system, can relieve the workload of process-planning departments, this being especially important in small batch manufacturing environments. This paper discusses the functions that have to be performed during the determination of bending sequences, focusing on accuracy aspects. The generation of bending sequences is also put into the broader perspective of an integrated CAPP system such as PART-S, which is under development presently in the authors laboratory.

Design study of the geometry of a punching/blanking tool

The cost of tooling in sheet metal industries contributes a considerable part to the overall cost of manufacturing a component. It is therefore imperative to keep down this cost by ensuring that the tool works for a long period in production without interruption. One way of achieving this objective is to reduce the stress on the tool during punching/blanking. This paper deals with the study of this problem by using the finite-element technique. 3-D finite-element models of various type of punching/blanking tools have been developed, these models enabling the analysis of the effects of variations in tool geometry on the punching/blanking force and on the deformation of the punch, a parameter highly relevant to the assessment of tool performance in terms of the accuracy of the manufactured components. The model caters also for variation in the characteristics of the tool material, in the sense that a highly wear-resistant tool is normally composed of carbide tips around its cutting profile. Computed results by FE models are checked against design standards by American Society of Manufacturing Engineers (SME). Some suggestions are offered as to how the efficiency of a punching/blanking tool can be improved.

Computer aided process planning for sheet metal based on information management

During the last few years, attention in the manufacturing cycle has shifted towards concurrent engineering (CE). With this, the integration of the different product life cycle processes has become a focus in both research and industry. However, it is obvious that the integration of all manufacturing processes, taking into account all life cycle aspects from initial functional requirements to final disposal, is hardly feasible in the traditional way. In this paper, the execution of the manufacturing cycle based on information management is explained by describing the development of a generic architecture for computer aided process planning. This architecture is elaborated upon for the field of sheet metal manufacturing in a small batch part environment.

Suitability of sheet bending modelling techniques in CAPP applications

The 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.

The Accuracy Aspect in Set-up Determination for Sheet Bending

Tolerance prescriptions, defined on a component, must be taken into account in manufacturing and in process planning. For the bending of sheet metal parts, the accuracy aspects must be addressed in sequencing and positioning procedures. In bending, the shape of a part changes not only locally, but globally as well. Therefore, sheet metal part manufacturing presents some specific problems as regards reasoning about tolerances. The paper describes the use of the so-called “tolerance tree” in sequencing procedures for bending to be used in an integrated CAPP system.

Tolerancing and sheet bending in small batch part manufacturing

Tolerances indicate geometrical limits between which a component is expected to perform its function adequately. They are used for instance for set-up selection in process planning and for inspection. Tolerances must be accounted for in sequencing and positioning procedures for bending of sheet metal parts. In bending, the shape of a part changes not only locally, but globally as well. Therefore, sheet metal part manufacturing presents some specific problems as regards reasoning about tolerances. The paper focuses on the interpretation and conversion of tolerances as part of a sequencing procedure for bending to be used in an integrated CAPP system.

Information management and design & engineering processes

In analysing design and manufacturing tasks and their mutual interactions, it appears that the underlying information of these tasks is of the utmost importance. If this information is managed in a formalized, structured way, it can serve as a basis for the control of design and manufacturing processes. The ontological description that is used for this purpose is elaborated upon. Significant in this respect is the introduction of expedient differentiations in distinct aspects of the ontology. Based on a reference model and three accompanying information structures, it is indicated how an ontological description of the information content can be applied to govern design and manufacturing processes. Examples in the field of process planning and cost estimation are used to illustrate the theory.

Determining material properties of sheet metal on a press brake

During the last decade, several attempts have been made to develop bending models for air bending. All models, whether they are sophisticated or very simple, need adequate input. Principally, the results of a bending model can never surpass the quality of the input. Whereas, e.g. the thickness of a sheet can be ascertained from straightforward measurements, the material properties are difficult to determine, and—in most cases—are not provided by the sheet metal manufacturer with adequate accuracy. In several studies it is shown that the influence of material properties on the output of bending models is substantial. For example Young’s modulus and the Ludwik–Nadai parameters are important in this respect. Based on the concepts derived from in-process control techniques, this paper describes a material test procedure that can ameliorate the input of bending models. The procedure is to be implemented on a standard press brake, without the aid of laboratory testing equipment. In order to determine the different material properties, the punch force–displacement diagram is recorded. Based on this diagram, and on fundamental knowledge of the air bending process, values for the material properties are deduced.

Tolerance reasoning and set-up planning for brakeforming

Brakeforming is widely applied in high-variety small-batch manufacturing of sheet metal components to form bends that have a straight axis and a constant radius. The process typically involves a relatively large number of set-ups. The paper describes the appropriate search technique to solve the set-up planning problem as well as some of the manufacturing rules that can be applied. Special attention is paid to the accuracy aspect in the selection of set-ups for non-90° bend angles. As an example, the set-up planning procedure is demonstrated for a sheet metal bracket.