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On the selection and assessment of input variables for the characteristic diagram based correction of thermo-elastic deformations in machine tools

Ihlenfeldt S., Naumann C., Putz M.

Journal of Machine Engineering

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2018

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Vol. 18, No. 4

25--38

EN

It is a well-known problem of milling machines, that waste heat from motors, friction effects on guides and most importantly the milling process itself greatly affect positioning accuracy and thus production quality. An economic and energy-efficient method of correcting this thermo-elastic positioning error is to gather sensor data from the machine tool and the process and to use that information to predict and correct the resulting tool center point displacement using high dimensional characteristic diagrams. On the one hand, the selection of which and how many input variables to use in the characteristic diagrams is critical to their performance. On the other hand, however, there are often a great number of possible variable combinations available and testing them all is practically impossible. This paper will discuss the suitability of many different input variable types and present a new method of input variable selection which will be compared to existing methods and demonstrated on measurements performed on a machine tool.

2

Efficient quantification of free and forced convection via the decoupling of thermo-mechanical and thermo-fluidic simulations of machine tools

Glaenzel J., Ihlenfeldt S., Naumann C., Putz M.

Journal of Machine Engineering

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2018

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Vol. 18, No. 2

41--53

EN

Thermo-elastic deformations represent one of the main reasons for positioning errors in machine tools. Investigations of the thermo-mechanical behaviour of machine tools, especially during the design phase, rely mainly on thermo-elastic simulations. These require the knowledge of heat sources and sinks and assumptions on the heat dissipation via convection, conduction and radiation. Forced convection such as that caused by moving assemblies has both a large influence on the heat dissipation to the surrounding air. The most accurate way of taking convection into account is via computational fluid dynamics (CFD) simulations. These simulations compute heat transfer coefficients for every finite element on the machine tool surface, which can then be used as boundary conditions for accurate thermo-mechanical simulations. Transient thermo-mechanical simulations with moving assemblies thus require a CFD simulation during each time step, which is very time-consuming. This paper presents an alternative by using characteristic diagrams to interpolate the CFD simulations. The new method uses precomputed thermal coefficients of a small number of load cases as support points to estimate the convection of all relevant load cases (i.e. ambient conditions). It will be explained and demonstrated on a machine tool column.

3

Optimized grid structures for the characteristic diagram based estimation of thermo-elastic tool center point displacements in machine tools

Putz M., Ihlenfeldt S., Naumann C., Glänzel J.

Journal of Machine Engineering

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2017

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Vol. 17, No. 3

36--50

EN

It is a well-known problem of milling machines, that waste heat from motors, friction effects on guides and also the milling process itself greatly affect the positioning accuracy and thus the production quality. An economic and energy-efficient method of correcting this thermo-elastic positioning error is to gather sensor data from the machine tool and the process and to use that information to predict and correct the resulting tool center point displacement using high dimensional characteristic diagrams. The size of these characteristic diagrams depends on the number of input variables (sensors) and the fineness of the discretization of the grid. While the number of sensors can usually not be reduced without affecting the quality of the prediction, it is often possible to minimize the size of characteristic diagrams through the use of adaptive grid refinement. This ensures that the finest grid sections correspond with the load cases that have the largest local gradients. Through such adaptive refinement, it is possible to reduce storage capacity and computation time without significant loss of precision. The aim of this article is to examine, test and compare different methods of adaptive grid refinement. For this, simulation data from a machine tool is used.

4

Flexible coupling of drive and guide elements for parallel-driven feed axes to increase dynamics and accuracy of motion

Peukert C., Merx M., Müller J., Ihlenfeldt S.

Journal of Machine Engineering

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2017

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Vol. 17, No. 2

77--89

EN

Productivity enhancement is achievable by increasing the dynamics of machines. This can be accomplished by decreasing the moving mass or by increasing the driving forces. One possibility for increasing the drive forces lies in the use of multiple parallel-acting actuators. Because of the mechanical coupling between the drives, undesirable interference of the actuators occurs. This limits the control bandwidth of the feed axis. Feed axes of machine tools are mostly equipped with linear guides fitted with rolling elements. Since they dispose of just one degree of freedom, small errors in their alignment or thermally induced errors cause constraining loads. In this paper compliant mechanisms are used to mechanically decouple the slide from the guide and drive elements. The application of flexible joints in gantry axes with linear motors and ball screws is investigated. The dynamic behaviour of the feed axis components are modelled using the Finite Element Method. By applying the modal reduction technique, the dynamic behaviour is included in an elastic multibody simulation. With the mechanical decoupling of the drive and guide elements it is possible to increase the control bandwidth of the system. Deviations in motion may also be corrected with parallel-driven feed axis by applying compliant mechanisms.

5

Implementation and demonstration of characteristic diagram as well as structure model based correction of thermo-elastic tool center point displacements

Putz M., Ihlenfeldt S., Kauschinger B., Naumann C., Thiem X., Riedel M.

Journal of Machine Engineering

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2016

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Vol. 16, No. 3

88--101

EN

In milling machines, waste heat from motors, friction effects on guides and most importantly the milling process itself greatly affect positioning accuracy and thus production quality. Therefore, active cooling and lead time are used to reach thermal stability. A cheaper and more energy-efficient approach is to gather sensor data from the machine tool to predict and correct the resulting tool center point displacement. Two such approaches are the characteristic diagram based and the structure model based correction algorithms which are briefly introduced in this paper. Both principles have never been directly compared on the same demonstration machine, under the equal environmental conditions and with the same measurement setup. The paper accomplishes this comparison ona hexapod kinematics examined in a thermal chamber,where the effectiveness of both approaches is measured and the strengths and weaknesses of both are pointed out.

6

Processing with parallel kinematics - an experimental analysis

Neugebauer R., Wabner M., Ihlenfeldt S.

Journal of Machine Engineering

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2008

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Vol. 8, No. 1

54--62

EN

This article summarizes experimental analyses for the determination of process parameters that can be achieved in an industrial machine tool with parallel kinematics. Focus is on the examination of the machining results' dependency on position and direction. Therefore milling experiments are conducted, testing different positions and directions within the workspace. Surface quality, process forces, acceleration as well as vibration characteristics of the working platform are assessed. Based on measured frequency responses the stability limits are determined at different locations within the workspace. By these examinations it can be confirmed that the dependency of static and dynamic machine properties on position and direction has an influence on machining results. Best results are achieved in central positions of the machine used for most of respective operations. Furthermore it's represented that the machining results are comparable to conventional serial machines though parallel kinematics show a stronger dependency on direction.