1,721,010 research outputs found
Enhancing position accuracy of CNC machine tools by means of direct measurement of deformation
No full textThe positioning accuracy of computer numer- ical control (CNC) machine tools is mainly limited by the manufacturing accuracy of their linear and circular motion axes and by the long-term dimensional stability of their structures. Maximizing this accuracy can prove to be a particularly challenging task, especially for large-sized systems. In fact, heat-induced deformations, long-period deformation of foundations and the manu- facturing process itself, these all cause time-dependent structural deformations of the machine body, which are difficult to model and to predict. The usual approach is a model-based prediction of structural deformations, which is followed by a compensation of positioning er- rors at CNC level. This approach is often limited by the complexity of the problem from both geometrical (sys- tem geometry can be very complex and it can vary in time) and physical (it is difficult to model and consider any possible load type and loading condition) point of view. As a consequence, only limited success has been achieved in active error compensation based on the modelling of the relationship between the generalized dynamic loads and the structural deformation field. This paper illustrates a different approach in active error compensation, which exploits a new measurement system able to provide real-time measurement of the displacement field of a given structural component,
P. Bosetti (B) · S. Bruschi Department of Mechanical and Structural Engineering, University of Trento, 38123 Trento, Italy e-mail: [email protected]
without any model about its dynamic/thermal structural behavior
Some remarks on formability and microstructural features of incrementally formed sheets as a function of geometrical parameters.
No full textSeveral investigations have been carried out in recent years to study the fundamental aspects of incremental sheet forming operations, with particular attention to determination of material formability and evaluation of the formed component geometry. Some of these investigations –based on both experimental and numerical studies – tried to give a comprehensive explanation of deformation mechanisms that arise during incremental sheet forming, and which possibly affect the material formability. However, none of the proposed theories are today fully accepted by the scientific community.
The objective of the paper is to study the effect that geometrical parameters have on the component formability and on its microstructural characteristics. Axi-symmetric parts characterized by a varying slope with depth and by different initial slopes were incrementally formed until the first crack appeared. The formed components were measured with a Coordinate Measuring Machine and their geometry and thickness profiles evaluated. It is shown that the final thickness of the component is strongly dependent on geometrical parameters: in particular, the presence of one or two minima in the thickness is outlined, as a consequence of the different predominant deforming mechanisms that lead to sheet failure.
In order to study the relevant deformation mechanisms, microstructural characterization was conducted on the failure zone of the formed parts, by means of optical microscopy to evaluate the part microstructure as compared to the not deformed blank. The observed micro- structural features were then compared to the equivalent ones of tensile sheet specimens deformed until fracture, in order to evidence similarities and differences, and utilized to support or deny theories about incremental forming deformation mechanisms available in litera- ture
In-line monitoring of cut surfaces in interrupted cutting operations.
No full textTodays market of machined products is characterized by an ever increasing demand in high-quality products, which can be assured by low levels of variability of process conditions. This mostly concerns precise and ultraprecise machining operations, but also more traditional operations are interested, which are far to be completely optimized in terms of obtainable quality of cut surfaces. The paper presents a system developed in order to monitor in-line the quality of cut surfaces in interrupted machining operations, in particular devoted to sawing of metal tubes. The presence of burrs above an acceptable limit at the cut surfaces entrance and exit is an index of the fact that the blade teeth have reached unacceptable levels of wear and therefore have to be changed to still guarantee high quality products. The automatic recognition of unacceptable presence of burrs can provide the machine controller with feedback about the need of blade change without the operators aid and, moreover, can reliably predict the residual tool life. In the paper, first the problem will be outlined, then the concept of the system presented together with the software developed for the cut surfaces geometry acquisition; later on, the control chart developed for reliable and automatic monitoring of the process is proposed and main results presented and discussed
An experimental-numerical procedure to identify heat transfer coefficient in hot stamping processes
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On Development of an Optimal Control System for Real-time Process Optimization on Milling Machine Tools
No full textDeveloping an intelligent machine tool means to augment its level of automation. This augmentation, in turn, requires a machine
controller able to perform actions and to implement attributes that are currently demanded to, and hold by, human operators. The
present paper describes how this issue is being faced by a large Italian national research project, funded under the Industria 2015
initiative. Considering the case of milling machines, human operators are currently in charge of supervising the cutting process by
acting on spindle speed and feed override controls in order to compensate for undesired process conditions (e.g. excessive vibration
or power absorption) caused by a wrong choice of process parameters during the design of the part-program, by tool wear, by
unexpected work material properties, or by machine tool dynamics. The first part of the paper proposes the architecture for an
augmented-automation machine tool. Rather than revolutionizing the well-established architecture of a conventional machine tool,
the concept is based on an additional controller that implements a supervision and optimization loop. This additional controller gets
process state information from the CNC and from dedicated measurement systems, and closes a feedback action on the CNC as a
human operator would do: by acting on feed and spindle speed overrides. The second part of the paper illustrates how the additional
controller works: following the optimal control theory, it is based on a dynamic process model, a set of state variables (i.e.
measurements), and a set of controls. Exploiting a simplified process model and efficient optimization algorithms, it performs a
real-time optimization of the controls (i.e. the overrides named above) on the basis of a weighted multi-objective target function and
a set of measurements taken from the cutting process (power, forces, accelerations). In particular, the target function takes into
account the following objectives: cutting time, work-piece surface finish, tool wear rate and vibration mitigation in general. The
third part of the paper details the strategies concerned with tool vibrations prediction, monitoring and mitigation, which are
integrated into the optimization loop. A vibrations prediction module based on a simplified cutting process model allows the
estimation of the vibration level and/or chatter occurrence during a pre-processing phase: thus, through the computation of the
Stability Lobes Diagram along the tool path, the more stable spindle speeds can be identified. The pre-processing phase is
complemented with an in-process chatter monitoring algorithm based on a recursive dynamic model identification: detecting real-
time self-excited vibrations onset, and distinguishing them from forced vibrations, this module allows the controller to properly
update the vibration estimation
Formability Limits and Microstructural Features of Parts Made by Single-Point Incremental Forming
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