Consequently, in general two different classes of thermal state control systems for metal-cutting machine tools are defined, targeting an increased precision and based on previous defined relations and thermal behaviour equivalence criteria:
1st class – structural thermal state control system, caused and defined by the
action on one or more structural model parameters, resulting in the output val- ue for relative displacement (linear or angle) of interacting elements (tool and work piece) in metal-cutting machine tools;
2nd class – computerized numerical state control system, caused and defined by
the additional action – not on structural model parameters resulting in the out- put value for relative displacement (linear or angle) – but on the output value from a CNC control system (or more significant values for geometrical dimen- sion creation) for elements and axes drives in machine tools, resulting and de- fining relative position or orientation:
a) Computerized numerical thermal position control of thermo-active ele- ments and quasi thermo-stable links;
b) Computerized numerical thermal state control (temperature and/or thermos-elastic) of thermos-active elements and/or quasi thermo-stable links.
Defined standard functions for the thermal deformation and displacement con- trol and specified machine tool structure (see table 1, 2) in three Cartesian co- ordinates are calculated with operational spindle speed and process time pa- rameters. The resulting value for a defined thermal spindle displacement func- tion is automatically converted into axis drive commands for same direction and same value. Thus, the relative position / orientation between machine axes units keep constant. A spindle stop leads to a machine cooling with defined thermal spindle displacement, depending on operational machine stand-still time. A reverse spindle displacement is calculated and converted into automatic compensation through reverse axes drive commands.
Figure 3a shows a scheme for the proposed compensation method of the thermal spindle displacement in a CNC machine tool.
Fig.3. a) Automatic compensation of the thermal spindle displacement in a numerical controlled met- al cutting machine; b) Scheme for the compensation of the thermal spindle displacement r in time ;
Standard functions for the thermal deformation and displacement control have been calculated for a variety of spindle speeds in heating and cooling operation. We now consider the automatic compensation of the spindle’s thermal dis- placement in the machine tool (see fig. 3b curve K) : a) in the machining of a single part at spindle speeds n1, n2, n3, with standard functions 1, 2, 3, and 4; b) in cooling of the machine tool, when the spindle speed is changed (time 2) and the tool is replaced (time 4) and when the final part is replaced and a new blank is installed (time 6). Automatic compensation of the spindle’s thermal displacement (according to curve K) is undertaken at intervals i for each con- trolled axis, in heating and cooling of the machine tool. (Determination of i is based on the required machining precision.) At startup of the machine tool and the onset of machining at spindle speed n1, the spindle’s thermal deformation corresponds to standard function 1 (see fig. 3b). When the thermal displace- ment is i, it is compensated by the corresponding displacement of the working component in the specified direction. After time 1, the spindle speed is switched to n2, and then the thermal displacement corresponds to standard function 2. During the speed change, the spindle does not turn, and machine tool cools according to standard function 4. During operation at spindle speed n2, the thermal displacement corresponds to standard function 2. When the thermal displacement reaches i, it is compensated. Automatic compensation proceeds analogously in operation at spindle speed n3, when the thermal dis- placement corresponds to standard function 3. When the finished part is re- moved and a new part is inserted, the spindle does not turn. The machine tool cools, and the thermal displacement of the spindle varies in accordance with standard function 4 over a time 6. When the thermal displacement is equal to i, it is compensated by displacement of the machine tool working component
by an amount i in the opposite direction. In subsequent operation (cooling), the spindle’s thermal displacement is compensated analogously.
a) b)
Fig. 4. a) Probabilistic scheme for automatic compensation of the thermal spindle displacement in a metal-cutting machine; b) Implementation on the machine tool TAJMAC-ZPS MCFV 1060 (curve 1- Simulation of thermal displacement of the spindle, 2-The model for compensation of thermal dis- placements during the machining, 3-The real displacement of the spindle,4-the required range dis- placement of the spindle,5- the air temperature.)
Implementation of the automatic compensation for the spindle’s thermal dis- placement is undertaken periodically, during the heating and cooling of the ma- chine tool, in terms of the quantity i determined for each controllable coordi- nate axis. Where necessary, i and the probability Pi( ) that it will be attained are established for each newly machined part, on the basis of the required pre- cision and its maintenance over time, and are entered in the memory of the numerical control system for the metal-cutting machine (Fig. 4 a). At startup of the machine tool and the onset of machining, data regarding the operating time i, the spindle speed ni, the current position of the working components (with respect to the controlled coordinates), and the ambient temperature are en- tered in the numerical control computer. The thermal displacement of the spin- dle with respect to each coordinate axis is calculated continuously at specified intervals i (Fig. 4 b). When the thermal displacement in any direction is equal to i, with specified probability Pi( ), it is compensated by appropriate dis- placement of the machine tool working component in the specified direction. After time 1, the spindle speed is changed to n2. That will also change the thermal displacement of the spindle in accordance with the parameters for that spindle speed. During the change in speed, the spindle does not turn, and the
machine tool is cooled in accordance with the cooling function. In operation at speed n2, there will be further thermal displacement of the spindle, which is again calculated continuously at specified intervals i. When the thermal dis- placement in any direction is equal to i, with specified probability Pi( ), it is compensated by appropriate displacement of the machine tool working com- ponent in the specified direction. If the required probability Pi( ) is not met, the calculation of the spindle displacement continues. As soon as the probability reaches Pi( ), the working components are moved accordingly.
4 Summary
This paper presents first results in creating systems that allow the control of spindle axis displacements without the direct application of a temperature sen- sor as information for control parameters. The system includes a probabilistic model of the thermal state of the machine. Practical test results show the actu- al feasibility and potential of this research direction.
Existing control methods of thermal deformations and temperature in machine tools require further development for their versatility in relation to different types of machines. In addition, all the prerequisites for the development of new control principles of the machine are energy - information systems. The com- plexity of the mathematical description of thermal processes and their depend- ence on number of thermal variables, determines the need to consider not only deterministic, but also probabilistic models of thermal machine control.
5 References
[1] Yto, Y., Thermal Deformation in Machine Tools,New York: McGraw_Hill, 2010.
[2] Thermal issues in machine tools/ J.Mayr, J.Jedrzejewski, E.Uhlmann, M.A.Donmez,
.Knapp, F.Hartig, K.Wendt, T.Moriwaki, P.Shore, R.Schmitt, C.Brecher, T.Wurz, K.Wegener / CIRP Annals – Manufacturing Technology, 2012, v.61. – pp.771–791.
[3] YangLi, WanhuaZhao, ShuhuaiLan, Jun Ni, Wenwu Wu , BinghengLu. A review on spin-
dle thermal error compensation in machine tools. InternationalJour- nalofMachineTools&Manufacture 95(2015)20–3822.
[4] Shuanqiang Yang, Jianxiong Chen and Shuwen Lin. A Software Method for Online
Thermal Error Inspection and Compensation. International Journal of u- and e- Service, Science and Technology, Vol.9, No. 2 (2016), pp.195-206
[5] G. Spur., G. Lechler., U. Heisel, Methods for Reducing Thermal influences on the Ac-
curacy o f Machine Tools Proceedings of the 3ed International Conference on Pro- duction Engineering.,1977, v 1. - p l0 -22.
[6] Sokolov Yu. N. Temperature calculations in the machine tool. M: NTO Mashprom, 1965.-77 .
[7] Weck M., McKeown P., Bonse R., Herbst U. Model reduction & compensation method
of thermal deformations in machine tools,1995.
[8] Attia M.H.Method for optimal real-time CNC control of thermal deformation compen-
sation in machine tools, 1999
[9] Kuznetsov, A.P., Teplovoe povedenie i tochnost’ metallorezhuchchikh stankov (Thermal
Behavior and Precision of MetalCutting Machines), Moscow: YanusK,2011.
[10] Kuznetsov, A.P., Teplovoi rezhim metallorezhushchikh stankov (Thermal Conditions in Metal Cutting Machines), Moscow: MGTU Stankin, YanusK, 2013.
[11] Kuznetsov A. P..Thermal Behavior of Components in Metal_Cutting Machines. - Russian
Engineering Research,2011,31, 4.- .351-358.
[12] Kuznetsov A. P. Patterns of Thermal Behavior of Metal-Cutting Machines. - Russian En-
gineering Research,2011, 10.- .975-985.
[13] Kuznetsov, A.P. and Kosov, M.G., Structural thermophysical analysis of metalcutting machines, Russ. Eng.Res., 2011, vol. 31, no. 6, pp. 599–606.
[14] Kuznetsov, A.P., Similarity of the thermal behavior of components in metal cutting ma- chines, Russ. Eng.Res., 2011, vol. 31, no. 4, pp. 351–358.
[15] Kuznetsov, A.P. and Kosarev, M.V., Classification of standard types of thermal defor- mation in metalcutting machines, Stanki Instrum., 2013, no. 9, pp. 13–19.
[16] Kuznetsov A. P.Temperature Control of Metal_Cutting Machines. Russian Engineering Research. Vol. 35, No. 1, 2015. - pp. 46–50.
[17] Vyroubal. J. Compensation of machine tool thermal deformation in spindle axis direc-
tion based on decomposition method. – Precision Engineering, 2012, 36. – .121–
127.
[18] Turek P., Jegrzejewski J., Modrzycki W. Methods of machine tool error compensation.
Journal of Machine Engineering, 2010, v.10, 4. – pp.5–25.
[19] Ni J. N machine accuracy enhancement through real-time error compensa- tion.Journal of Manufacturing Science and Engineering, 1997, v.119. – pp.717–725. [20] Yang H., Ni J.Thermal deformation simulation by use of neural networks, 2005
[21] Patent US 6,651,019 B2, 17/38 G01C, G06F 19/00. Method and device for calculating the correction of thermal deformations of the machine, 2003.
[22] Patent US 6,923,603. B23C 9/00; B23Q 11/12.Machine with the function of preventing thermal deformation, 2005.
[23] RF Patent 2499658/A. P. Kuznetsov, M. G. Kosov, Krohin O. I. Method of automatic
compensation of thermal displacement of the spindle machine tool with numerical
[24] RF Patent 2511075, B23B 25/06./ Kuznetsov A. P., Crachin O. I., Zenin, V. A. Method of automatic control of a thermal state of heat-loaded devices. Published: 10.04.2014, Bull. No. 10.
[25] Russian Federation Patent No. 2 573 854. B23B 25/06. Method of compensation of thermal deformation of machine tools c NC./ Kuznetsov A. P. Published: 27.01.2016 Bull. No. 3