When metal parts are being manufactured, the workpieces go through heating and cooling phases, in which they are exposed to considerable fluctuations in temperature. During the manufacture of raw material blanks (semi-finished products) using high-temperature methods such as casting or forging, internal compression and tensile stresses, also known as residual stresses occur at various points in the semi-finished products.
Parts of the material are removed during the subsequent material removal process, often milling or grinding in which the workpiece takes on the required end geometry. The forces arising from the internal stresses in the remaining material lose their equilibrium as a result of the material removal. As a consequence, there is a change in the residual stress within the part resulting in variations in both the structure of the material and in the volume of the part. This frequently leads to change in the shape of the part, which is referred to as distortion. Residual stress distortion is particularly prevalent in the period following unclamping when the clamping system ceases to act as an effective counterforce.
Serious residual stress distortion, which is an important factor in lightweight engineering design, occurs particularly in the case of large-scale parts and parts with thin walls. Semi-finished goods therefore undergo "low stress annealing" in which they are kept at a high temperature thereby allowing the residual stresses in the workpiece to diminish significantly, in order to reduce distortion. A further option is to straighten the distorted parts following a mechanical machining operation. This entails bending the workpiece into the required shape. However, straightening is a very complex process and not always possible, particularly in the case of complex geometries. Reducing or eliminating distortion is always time-consuming and costly; it is not uncommon for the workpiece to have to be scrapped in the end, even after considerable effort has been made to salvage it.
Part distortion is a relevant cost factor, not a marginal inconvenience: In a study conducted by Boeing, it emerged that re-working and waste costs resulting from part distortion amount to more than 290 million dollars annually. According to the German Engineering Federation (VDMA), the cost to the manufacturers of drive technology of eliminating part distortion total some 850 million Euro in Germany alone. If the problem of part distortion could be reduced, profits reported by the mechanical engineering sector could be markedly increased.
The aim of the "VoKoES" research project was to predict and compensate for part distortion caused by residual stresses during 5-axis milling operations and to reflect this in the CAM programming.
The research project was divided into four sub-tasks in order to achieve the aims of the project:
Finally, the researchers implemented the compensation methods they had developed in a CAM module.
"VoKoES" enables metal parts to be produced more economically by paving the way for resource-efficient, flexible manufacture particularly of thin-walled integral components.