Strojniški vestnik - Journal of Mechanical Engineering 53(2007)11, 742-746 UDK - UDC 621.941 Kratki znanstveni prispevek - Short scientific paper (1.03) Preizkusna raziskava sinhronizacije dvovretenske stružnice Experimental Investigation into the Synchronization of a Double-Spindle Lathe Jan Kosmol1 - Krzysztof Lehrich1 - Robert Jastrzebski2 ('Silesian University of Technology, Gliwice, Poland; 2Machine Tool Research and Design Centre, Pruszków, Poland) V prispevku so prikazani rezultati preizkusne raziskave stružnice VENUS 350, izdelane v CBKO iz Pruszkowa (Poljska). RK stružnica je opremljena z dvema vretenoma. Merili smo vrtilne hitrosti obeh vreten, sinhronizacijo napake in pogonske navore motorjev, glavnega in nasprotnega. To omogoča ovrednotenje postopka sinhronizacije. © 2007 Strojniški vestnik. Vse pravice pridržane. (Ključne besede: RK stružnice, sinhronizacija, vrtilne hitrosti, servopogoni, pogonski navor) In this paper we show the results of an experimental investigation of a VENUS 350 lathe, manufacture by CBKO in Pruszkow, Poland. This CNC lathe is equipped with two spindles. We have measured the rotational speeds of both spindles, the synchronization error and the driven moments of the motors, main and opposite. With this information it was possible to evaluate the process of synchronization. © 2007 Journal of Mechanical Engineering. All rights reserved. (Keywords: CNC lathe, synchronization, rotational speeds, servo-drives, moment of inertia) 0 INTRODUCTION Modern CNC lathes are very often equipped with two spindles: the main spindle and the opposite spindle. Such a feature should make complete machining possible, as it means being able to machine first on the right-hand side of the workpiece and then on the left-hand side of the workpiece without stopping the machine tool. The opposite spindle chucks the workpiece on the right-hand side, while the main spindle chucks the workpiece on the left-hand side. This means that there is a short time when the workpiece is chucked on both sides. Of course the spindles, main and opposite, have to have exactly the same rotational speed, i.e., they should be synchronized. But what is important from the point of view of productivity of the machine tool and the quality of the workpiece? First, the time of synchronization should be as short as possible. Second, the moment of chucking the workpiece with the opposite spindle is important from the point of view of the quality of the workpiece due to the possibility of slip between the chuck and the workpiece (if the rotational speed of the workpiece and the opposite spindle differ). The paper shows the results of an experimental investigation of a lathe, manufactured in Poland. Such a CNC lathe is equipped with two spindles. We have measured the rotational speeds of both spindles, the synchronization error and the driven moments of the motors, main and opposite. This made it possible to evaluate the process of synchronization. 1 THE BASIC PROBLEMS OF SYNCHRONIZING A TWO-SPINDLE CNC LATHE In the newest lathes and turning centers the idea of automatic workpiece reclamping, called the intercept spindle, is used. The idea of such a spindle does not seem to be complicated. There are two spindles: the main (M) and the auxiliary (A), as is shown in Fig. 1a). The main spindle rotates and a workpiece is clamped in the chuck. The second auxiliary spindle may rotates too, and it also has the possibility to move along the lathe axis. 742 Strojniški vestnik - Journal of Mechanical Engineering 53(2007)11, 742-746 There are two main problems to be solved if such a concept is to work successfully: - Coaxiality, between the axis of the main spindle and the direction of movement of the auxiliary spindle, - Synchronization of the rotation speeds of the main and auxiliary spindles. If the coaxiality of both spindles is insufficient the workpiece may be destroyed during reclamping. The coaxiality of both spindles depends first of all on the geometric accuracy of the machine tool. The newest lathes and centers belong to the high-accuracy group of machine tools and in most cases there is no problem with coaxiality. More important is the second technical problem, connected with the synchronization of the rotating spindles. Both spindles have to have the same speed at the moment of reclamping. This is a very hard to achieve condition because the drives of the main and auxiliary spindles are independent, and only the CNC has any influence on the actual speeds of the spindles. This problem is the main subject of our paper. We would like to present some results of an investigation performed in CBKO Pruszkow, Poland, where such an idea of reclamping was applied to the Venus 350 CNC lathe [1]. The mechanical and kinematic scheme for the synchronization system is shown in Fig. 1b). What is important from the point of view of the synchronization of the two spindles? - The rotational speeds ?M and ?A have to be the same at the moment of reclamping; - The time of synchronization should be as short as possible, for reasons of productivity; - Defining the moment of reclamping, which means to identify the time when the signal for clamping the chuck of the auxiliary spindle should be sent from the CNC to the chuck; - What the signal controlling both drives should look like. Both drives, from the main and auxiliary spindles, work as servodrives, which means that there are two encoders working as feedbacks and the CNC controller, which generates the signals ?Mgiv and ?Agiv as given values of the rotational motion of both spindles. Main spindel M Workpiece QW Chuck M Auxiliary spindel A Encoder M (direct measurement) Main spindel M ?Mist ?M Servo “M” 11 11 11 H a) Workpiece 9W Auxiliary spindel A ? Chuck M Chuck A Encoder A (direct measurement) i i i i i i H 0} Mgiv CNC 0} Agiv Servo “A” J Ein OlM tf *t roA b) Fig. 1. Concept of automatic workpiece reclamping using two spindles (a) and the kinematic scheme of the synchronization while reclamping the workpiece on the CNC lathe (b) Encoder M Preizkusna raziskava sinhronizacije - Experimental Investigation into the Synchronization 743 Strojniški vestnik - Journal of Mechanical Engineering 53(2007)11, 742-746 synchronous speed (3) spindle A (0)2= spindle synchronous control signal spindle synchronous speed command signal spindle synchronous speed control completion signal spindle phase synchronous control signal spindle phase synchronous control completion signal chuck close command signal chuck close completion signal velocity integral control signal /Spindle A chuck closed / spindle M chuck open J spindle A (cDf=synch. speed) (1) (1) (2) . (3) \ spindle M (bii=0) Time Fig. 2. Scheme of time performance of the control signals The quality of the synchronization process depends first of all on the time performance of the ?Mgiv (t) and ?Agiv(t) signals, on the stiffness properties kM and kA of the mechanical parts of the kinematic chains of both spindles and on the inertial moments ?M and ?A of the chucks and the workpiece. This means that a special schedule of time performance for all the signals has to be implemented in the CNC controller of the machine tools. Fig. 2 shows an example of such a schedule that we have used in our investigation. 2 EXAMPLES OF EXPERIMENTAL TESTS We made several experiments using a control algorithm like that in Fig. 2. During the investigation we measured parameters like the moments, M, of both servodrives, the rotational speed, co, of both drives, and the synchronization error, e, defined as the difference between the rotational speed of the main and auxiliary spindles. An example of the time performance of the measured parameters during the process of reclamping is shown in Fig. 3 and Fig. 4 (the rotational speeds of both servodrives are shown in rev/min, the moment, M, in Nm, but the synchronization error, e , is in specific units). We have made several experiments for different values of the rotational speeds and the time constants of the servodrives [1]. Fig. 3a) presents an example of the time performance of the rotational speed of the main spindle, coM, the auxiliary spindle, ccr, the moment of the main servodrive, M, the synchronization error, e, for the rotational speed of 1700 rev/min, and the time constant of the servodrives, 30 ms. Fig. 3b) presents another 744 Kosmol J. - Lehrich K. - Jastrzçbski R. Strojniški vestnik - Journal of Mechanical Engineering 53(2007)11, 742-746 example of the time performance of the rotational speed of the main spindle, 01 M, the auxiliary spindle,