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EN
Thermal errors are one of the leading causes for positioning inaccuracies in modern machine tools. These errors are caused by various internal and external heat sources and sinks, which shape the machine tool’s temperature field and thus its deformation. Model based thermal error prediction and compensation is one way to reduce these inaccuracies. A new composite correlative model for the compensation of both internal and external thermal effects is presented. The composite model comprises a submodel for slow long- and medium-term ambient changes, one for short-term ambient changes and one for all internal thermal influences. A number of model assumptions are made to allow for this separation of thermal effects. The model was trained using a large number of FE simulations and validated online in a five-axis machine tool with measurements in a climate chamber. Despite the limitations, the compensation model achieved good predictions of the thermal error for both normal ambient conditions (21°C) and extreme ambient conditions (35°C).
EN
Machining accuracy reliability as a key index of CNC machine tools is seriously influenced by the geometric and thermal errors. In the paper, a spindle unit thermal error modeling and machining accuracy reliability analysis method is proposed. By analyzing the heat generation mechanism, a thermal error model was developed to describe the thermal deformation of the electric spindle. Based on the immune algorithm (IA), the heat generation power and the heat transfer coefficient were optimized, and the thermal error was obtained by finite element thermal-mechanical coupling. By adopting the multi-body system theory (MBS), a dynamic machining accuracy model was put forward including the geometric and thermal errors. Based on the Latin hypercube sampling Monte Carlo method (LHSMC), a machining accuracy reliability analysis method was proposed to characterize the machining accuracy reliability considering the geometric and thermal errors. The method was employed to a machine tool, and the experimental results indicate the verification and superiority of the method.
EN
Today, cooling systems are increasingly applied to the structure of machine tools (MT). Unfortunately, to date there have been few efforts to precisely control these cooling systems, which inhibits the full utilisation of their potential to improve MT thermal behaviour (to reduce thermal errors). Moreover, the effects of cooling systems, especially the effects of cutting fluids, on thermal error compensation models are often omitted. This paper deals with the effects of fluid cooling systems on the thermal behaviour of MT and thermal error compensation models. It provides a detailed review of the state of the art, followed by the authors’ recent research on these issues. Firstly, the sensitivity of thermal error compensation models based on transfer functions (TF) to modification of fluid cooling systems and cutting fluid presence is discussed. Secondly, gradient regulation of the cooling unit to improve MT accuracy is presented.
EN
In machining up to 75% of the geometrical variations of work piece, features are caused by thermally induced deformations of machine components [1]. Since in dry cutting up to 80% of the thermal energy is stored in the chips [2], we expected a significant effect of these process-dependent heat sources on the machine accuracy. Based on preliminary simulation results, we systematically applied determined quantities of heated chips to a machine table to understand their impact on the temperature field. Temperature sensors where used to measure the temperature change on the tables surface and in the structure. Length measuring probes measured the corresponding deformations at 24 points distributed over the table. The measurements show a temperature change of 4 K at the surface and 3 K in the structure near the heat source after 6 minutes of exposure to 500°C chips. In this case, the impact on the temperature field is local but causes the bending of the table. We recorded 8 micron of thermo-elastic deformations. The results suggest that high-accuracy processes with large energy input, such as hard turning, require the heat induced into the machine structure by hot chips to be implemented into compensation methods and correction algorithms.
EN
In numerous papers it is proposed to use IR measurements of feed axis ball screw temperature distribution in order to compensate CNC machine tool thermal errors. The paper aims to validate reliability of the IR measurements in application to the feed axes ball screws. The identification of key factors influencing the accuracy of the IR measurements of ball screw temperature distribution has been conducted. A test-bench utilizing a ball screw assembly with built-in temperature sensors was introduced and the experimental data are presented along with conclusions.
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.
PL
W artykule zaprezentowano innowacyjny system kompensacji odkształceń cieplnych śrub pociągowych obrabiarek CNC. Dzięki zastosowaniu bezprzewodowego układu przesytu danych uproszczono proces montażu systemu kompensacji oraz zwiększono jego trwałość mechaniczną. W artykule przedstawiono także metodologię szacowania wartości odkształceń cieplnych wprowadzanych przez ciepło powstające w wyniku ruchu osi posuwów liniowych za pomocą metody elementów skończonych. Słowa kluczowe: odkształcenia cieplne obrabiarek, toczne śruby pociągowe.
EN
The paper presents a novel set for the compensation of the CNC ball screw thermal error. With use of wireless measuring set the mounting procedure is simplified and the durability enhanced. Moreover, the paper presents the methodology of calculating the thermal error induced by the heat generated during movement of a CNC axes by means of a FEM analysis.
EN
The challenge to be addressed by this paper is to extend the common machine tool thermal compensation models considering only internal heat sources to include the effects of the cutting process. Although real-time software compensations of thermal errors exist, the majority of these models only presume machine tools under load-free rotation of the main spindle without any reference to rough machining. The machining process is completely neglected in spite of the fact that it represents a significant heat source and causes workpiece inaccuracy. This paper presents a real-time compensation of three-axis vertical milling centre thermal errors including the cutting process effect. The simulation is based on dynamic modelling using transfer functions. The model was implemented into a standard CNC controller of a three-axis vertical milling centre to compensate for thermal errors in real time. The inputs of the compensation algorithm are the spindle rotational speed, the 5 temperatures of the machine structure and spindle power. A reduction of thermal errors achieved by using the new approximation TF model including cutting process impact is up to 79% for steel cutting tests with different cutting conditions.
EN
Thermal errors caused by internal and external heat sinks and sources may cause more than 50% of machine tool (MT) total error. Demands on machining accuracy and machine time utilization are still increasing. Therefore, research on thermal behaviour of MT structures is crucial for successful manufacturing. Direct (measuring between tool and work-piece) and indirect (predictive models) approaches to compensation of thermo-dependent tool centre point (TCP) displacement can be used to reduce sensitivity to heat flow or temperature control of MT and its environment. Thermal error indirect compensation is one of the techniques widely employed to reduce thermal errors due to its cost-effectiveness and ease of implementation. This paper is a continuation of scientific work on the modelling of complex MT thermal behaviour using a dynamic method with a physical basis: thermal transfer functions (TF). Previously published works focusing on middle-sized MTs are extended here to include heavy-duty MT application. Particular attention is paid to issues concerning compensation of thermal errors caused by interchangeable spindle heads.
PL
W prezentowanej pracy przedstawiono system kompensacji odkształceń cieplnych śrub pociągowych konwencjonalnych osi posuwu obrabiarek sterowanych numerycznie. Położenie osi napędowych prezentowanej obrabiarki jest korygowane o modelowaną wartość odkształceń cieplnych. Model odkształceń cieplnych oparto na architekturze sieci neuronowych. Przedstawiono porównanie wyników kompensacji osiągniętych przy zastosowaniu modelu neuronowego oraz modelu analitycznego. Zaprezentowany system umożliwia korektę położenia w trybie on-line. Poprawnie koryguje on odkształcenia cieplne napędów posuwu w układzie ułożyskowania: węzeł swobodnyustalającym jak i układów z napięciem wstępnym, najczęściej spotykanych w nowoczesnych obrabiarkach CNC.
EN
The paper presents CNC axis control system with build-in procedure for compensating thermal errors of ball screws. The axis position accuracy is improved by adding the computed correction value. In order to compute the correction value artificial neural networks were implemented. The comparison of compensation accuracy achieved by implementing ANN model and analytic model is shown. The presented system was designed to compensate online the thermal errors. The presented system was successful in reducing thermal errors in both: locating - nonlocating bearing arrangement, and, most commonly applied in novel machine tools, pretentioned ball screw arrangement.
EN
Thermal errors caused by the influence of internal and external heat sources in machine tool structure can cause more than 50% of total machine tool inaccuracy. Therefore, research on thermal behavior of machine tool structures is crucial for successful manufacturing. This paper provides an insight into the modeling of highly nonlinear machine tool thermal errors using thermal transfer functions (TTF). The method is dynamic (uses machine tool thermal history) and its modeling and calculation speed is suitable for real-time applications. The method does not require interventions into the machine tool structure, uses very few additional gauges and solves separately each influence participating on thermal error. The paper focuses on the development of a robust thermal model sensitive to various machine tool thermal behavior nonlinearities with the help of minimum input information. Model was applied on real machine tool (portal milling centre) and was verified within a complicated electro-spindle revolution spectrum. Moreover, the said compensation model was applied on another machine tool to prove its robustness and portability among machines of the same type set and the results of the TTF model were compared with a model obtained via multiple linear regression (MLR) as a case study.
EN
The paper presents a set allowing for on-line monitoring of machine tool feed screw temperature. Indirect supervising of the feed screw thermal elongation protects ball bearings from excessive loads. Moreover, the usage of an appropriate model in the control system of the CNC machine tool allows for the reduction of ball screw thermal errors.
PL
Prezentowany w pracy układ pozwala na monitorowanie „on-line” temperatury śruby pociągowej obrabiarki. Pośredni nadzór wydłużenia cieplnego śruby zabezpiecza przed nadmiernym obciążeniem łożysk tocznych na skutek wydłużenia cieplnego śruby pociągowej. Dodatkowo zastosowanie odpowiedniego modelu w systemie sterowania obrabiarki CNC pozwala na redukcję „on-line” błędów pozycjonowania wynikających z odkształceń cieplnych śrub pociągowych obrabiarki.
EN
This paper presents thermal analysis of micro milling machine tool SNTM-CM-ZUT-1. Infrared camera images are utilised to identify major heat sources in the machine tool. The influence of the heat generated in linear motors and frictional heat generated in spindle on the machine tool body temperature is examined. Basing on the CAD geometry and IR measurement, the thermal expansion of the machine is evaluated by means of the FEM analysis. The study indicates places where constructional improvements can be made in order to decrease thermal error influence. Moreover, the time period ensuring the thermal stability is identified.
PL
W pracy przedstawiono analizę odkształceń cieplnych obrabiarki do mikrofrezowania SNTM-CM-ZUT-1. Celem zidentyfikowania głównych źródeł ciepła maszyny wykorzystano pomiar kamerą termowizyjną. Zbadano wpływ ciepła wydzielanego podczas pracy napędów liniowych oraz wrzeciona na temperaturę korpusów mikrofrezarki. Na podstawie zmierzonych temperatur oraz znajomości geometrii obrabiarki opracowano model MES. Pozwolił on oszacować odkształcenia cieplne jakim poddawana jest obrabiarka podczas pracy. Wskazano potencjalne elementy konstrukcji, których modyfikacja może zredukować wielkość odkształceń cieplnych. Dodatkowo, wyznaczono cieplne stałe czasowe maszyny.
PL
W artykule zaprezentowano układ umożliwiający kompensowanie odkształceń cieplnych śruby pociągowej obrabiarki. Opracowane rozwiązanie przeznaczone jest dla napędów konwencjonalnych ze sterowaniem numerycznym, obrotowo-impulsowym przetwornikiem obrotowym, śrubą toczną ułożyskowaną w układzie: węzeł ustalający – węzeł swobodny. Przedstawiono schemat konstrukcyjny rozwiązania, ulepszony algorytm sterowania obrabiarki, stanowisko badawcze oraz wykonane na nim pomiary potwierdzające skuteczność funkcjonowania zaproponowanego układu.
EN
The paper presents a novel set allowing to compensate the thermal error of a CNC machine feed axis screw. Invented solution is designed for a screw driven CNC axis, with encoder, in locating and non-locating bearing arrangement. Mechanical layout, modified control algorithm, test bed and results of conducted experiments proving correctness of introduced method were demonstrated.
EN
The article presents a criterion supporting the selection of the proper number of sensors in a system compensating the effect of thermal deformation of the feed screw on the positioning accuracy of the CNC axis. Moreover, it presents an experimental verification of the criterion and a calculation example along with the tips allowing for the practical use of the criterion.
PL
W artykule przedstawiono opracowane kryterium wspomagające dobór liczby czujników w układzie kompensującym wpływ odkształceń cieplnych śruby pociągowej na dokładność pozycjonowania osi CNC. Prowadzono weryfikację doświadczalną dokładności pozycjonowania dla opracowanego kryterium. Omówiono przykład obliczeń wraz ze wskazówkami pozwalającymi na praktyczne użycie opracowanego kryterium.
16
Content available remote Compensation of thermal deformations of the feed screw in a CNC machine tool
EN
The paper presents a novel system facilitating continuous compensation for thermal deformations of the feed screw during its work in the workshop conditions. Screw temperature measurement ensues in an automatic way and enables compensation of thermal deformations, and thus, the removal of one of the main components of thermal volumetric errors. NTC Sensors (thermistors) are situated along the screw. Data concerning the thermal state of the screw is used to determine thermal deformations by means of the analytic model. Quantitative determination of thermal deformations allows for the introduction of compensations in the control system of a CNC machine, increasing the accuracy of tool positioning in relation to the machined object.
PL
W pracy przestawiono nowy układ prowadzący ciągłą kompensację odkształceń cieplnych śruby pociągowej podczas jej pracy w warunkach warsztatowych. Pomiar wartości temperatury śruby odbywa się w sposób automatyczny. Umożliwia kompensację odkształceń cieplnych i tym samym usunięcie jednej z głównych składowych błędu - składowej cieplnej błędu wolumetrycznego. Czujniki rozmieszczone są wzdłuż śruby. Wielkości określające stan cieplny śruby są stosowane do określenia odkształceń cieplnych za pomocą modelu analitycznego. Ilościowe wyznaczenie odkształceń cieplnych pozwala na wprowadzenie kompensacji w układzie sterowania obrabiarki CNC, zwiększającej dokładność pozycjonowania narzędzia względem przedmiotu obrabianego.
EN
The goal of this paper is to present the effective methods of decreasing thermal deformations of a turning centre's structure in order to reduce the thermal error. The design of a machine tool as well as the scope of analyses are described, and calculation methods are specified. The paper also presents the results of computer simulations, concerning studies on the influence of type and parameters of a headstock-cooling liquid on the reduction of spindle displacements. Moreover, the results of feed mechanism's thermal studies are shown, describing the influence of heat generated during the operation of ball screws and their inner liquid cooling, on tool's thermal displacements.
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