Simulation aided design of fixture systems and machine tools

The design of machine tools or complex fixtures for workpieces requires profound knowledge to evaluate design variants or new concepts. Finite Element Analysis (FEA) is an appropriate tool to support the designer but usually process values like realistic cutting forces are not taken into account. This is especially problematic when dealing with machining dynamics. Consequently, the designer often has to rely on his assumptions of the influence of process specific variables on the design’s dynamic behavior.

Within the projects iNTEFIX (European Union's Seventh Framework Programme) and ZIM Prima (Program to encourage the Industrial Collective Research (Industrielle Gemeinschaftsforschung, IGF) by Federal Ministry for Economic Affairs and Energy) fixtures for the milling of impellers and of structural parts for airplanes as well as midscale machine tools were designed and tested. The cooperation partners, the Institute of Manufacturing Technology and Quality Management (IFQ) and several companies contributed their specific expertise.

The iNTEFIX project comprises the simulation of actively induced specific excitations having a stabilizing effect on the process dynamics. The simulation is used to optimize the excitation strategy and the equipment like the vibratory fixture systems and piezo-electric actuators, which were realized by the project partner IFQ. In the iNTEFIX project another application of the process simulation system was to efficiently design a fixture system, enabling the compensation of workpiece deformations caused by residual stresses in milling. In addition to the calculation of stability limits, the simulation system is used to predict the changing engagement conditions if residual stresses occur.

Using the FEA-calculated dynamic properties of a machine tool structure in different variants (IFQ), the process simulation (ISF) is applied in a topology optimization process of machine tool components due to process dynamics and lightweight design. The major goals of the Prima project were the achievement of stable process conditions and the minimization of the effect of thermal loads on the machine tool properties. The figure describes simulated stability limits for three variants of a z-slide structure for a Fooke ENDURA 700 linear milling machine.

Fig.: iNTEFIX: Fixture systems to reduce chatter and residual stress induced deformation. ZIM Prima: Process simulations to evaluate different structural variants