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RESEARCH REGARDING THE DYNAMIC BEHAVIOUR OF AUTOMATIC TOOL FEEDING SYSTEMS FOR DIFFERENT TRANSITORY REGIMES CLAUDIU OBREJA1*, GHEORGHE STAN1 1
“Vasile Alecsandri” University of Bacau, Engineering Faculty, Calea Marasesti 157, Bacau, 600115, Romania
Abstract: The total machining time for a part produced on machining centers is influenced, besides the actual machining time, by the auxiliary times which include the automatic changing of the tools. In order to increase the productivity of machining centers, the research presented in this paper has focused on enhancing the movement speed of an automatic tool feeding system. Increasing the movement speed of the mechanism`s components has led to the appearance of unwanted side effects, such as shocks and vibrations. In order to minimize these effects, we have proposed and tested a new solution to adjust the braking regime.
Keywords: automatic tool feeding system, tracking valve, transitory regime, deceleration
1. INTRODUCTION The continuous trend in the manufacturing industry using machining centers is to increase the productivity, leading the researches towards the development of new methods and solutions in order to minimize the nonproductive auxiliary times [1, 2]. The researches aimed at finding new solutions for reconfiguring tool changing systems in order to reduce the number of movements needed to perform the tool change [3]. A method of structural synthesis of tool changing systems based on three graphs is able to minimize the number of movements needed to change the tool from the spindle to the tool magazine of an existing machining center [4]. In [5] Turkay developed a new method based on the genetic algorithms, for the redistribution of the entering order of the tools in the machining process, in order to minimize the non-productive machining time. Ecker and Gupta [6] presented a new method of minimizing the auxiliary times by using the balanced graphs. In this paper, using the proposed and researched solution of adjusting the deceleration path, the movement speed of the mobile elements was highly increased by improving the dynamic behavior at the end of the stroke. 2. EXPERIMENTAL SETUP 2.1. Method description Because the tool feeding systems which equip machining centers can perform translational and rotational movements in order to exchange the tool in the machining process [7, 8], the main objective followed in this paper is to increase their functional performances. One of the components which can be adapted to increase the functional performances of these tool feeding systems is to reduce the times consumed with the tool exchange, which imposes a speed increase for each linear and rotational displacement sequence of the feeding system [9].
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Corresponding author, email:
[email protected] © 2013 Alma Mater Publishing House
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In this paper we considered that the tool feeding system is hydraulically driven through the use of hydraulic cylinders; we also considered that the deceleration of the hydraulic cylinder is assured through its construction. The original braking device of the cylinder is replaced with the proposed solution which consists in a tracking valve, this replacement having the role to improve the dynamic behavior at the end of the hydraulic cylinder`s piston stroke. The researches followed the speed variation in time of the moving element regarding the dynamic parameters of the component elements: vibration, load, pressure, etc. Because the tool feeding systems perform movements during and outside the machining process the level of vibration range was recorded in the system`s bed and needed to be held between 0.002 ÷ 0.004 m/s according to IRD 10816 Vibration Severity Standard [10]. 2.2. Experimental set-up and data acquisition tools The experimental set-up was built to analyze the possibilities of time reduction of the mobile component linear displacement performed by a hydraulic drive system.
Fig. 1. The experimental set-up: 1-Hydraulic linear cylinder; 2-Exhaust pipe; 3-Tracking valve; 4-Moving element; 5-Cam with variable angle; 6-Tracking valve`s exhaust pipe; 7- Electric motor; 8-Pump; 9-Manometer; 10-Tank; 11-Command panel. In Figure 1 the experimental set-up is presented, the proposed solution, namely the tracking valve 3, is mounted in the hydraulic circuit on the exhaust pipe of the linear hydraulic cylinder 1 (at the piston`s withdrawal). The hydraulic oil which flows through the exhaust pipe of the cylinder is routed to the tracking valve through pipe 2 and then to the tank through pipe 6. The construction of the tracking valve is normally open, the transitory regime being assured when the cam 5, mounted on the mobile element 4, steps on the valve`s rod; thus the exhaust circuit is gradually closed, obtaining the deceleration transitory regime. In order to obtain different deceleration paths of the transitory regime, and also different deceleration times, besides varying the input flow, the cam 5 position can be modified through rotating it at different angles (other than the original angle). In the case of the classic hydraulic cylinder deceleration (Figure 2) the transitory regime is made by making use of an hydraulic resistance 1 and the directional valve 2 which are mounted in the cylinder`s construction.
Fig. 2. Hydraulic cylinder`s built-in deceleration: 1-Hydraulic resistance; 2- Directional valve; 3-Piston extension; 4-Piston.
Fig. 3. Data acquisition tools display on the experimental set-up: 1-Laser source; 2, 3-optic prism; 4-DX10 interface; 5-Pressure transducer, 6-Data acquisition board; 7-Vibration analyzer; 8-Vibration magnetic sensor; 10-Laptop.
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At the displacement of the piston, the oil exhaust orifice from the cylinder is obstructed by the piston`s 4 extensions 3 which are diametrical opposite. Thus the hydraulic oil will be routed through the resistance 1 which can be adjusted. For acquiring the measured data, three different measurement tools were used. The way the three data acquisition tools are mounted on the experimental set-up and also their components are presented in Figure 3. We used a laser interferometer, a pressure transducer and a vibration analyzer, all synchronized to acquire the data simultaneously.
3. RESULTS AND DISCUSSION The experimental tests when the transitory deceleration regime was assured through the built-in braking device of the cylinder aimed to obtain a minimum total time to cover the whole displacement range by the mobile element, taking into account the range of the vibration speed which occurs on the experimental set-up bed. In order to obtain minimum times the input flow was varied with the following values: 26, 30, and 34 L/min; the load on the mobile element was held constant at 12 kgf. In Table 1 we present the values obtained in the case of braking through the built-in system of the cylinder.
Pressure (MPa) 12 16 20
Table 1. Experimental test values at constant load of 12 kgf. V ib.acc Flow Load Tacc Tfrp Tdec Ttot V it (m/s) (m/s) (l/min) (kgf) (s) (s) (s) (s) 26 12 0.023 0.782 0.388 1.197 0.598 0.021 30 12 0.026 0.665 0.392 1.079 0.685 0.021 34 12 0.027 0.661 0.234 0.916 0.807 0.025
Vib.dec (m/s) 0.027 0.028 0.028
In Figure 4 we present by overlapping the three speed variations against time which correspond to the input flows. We can notice that all three diagrams have a quasi-similar characteristic, but with important implications on increasing the speed of the mobile element. -3
v 10 (m/s)
Time (s)
Fig. 4. Speed variation against time for linear displacement at variable input flow and constant flow of 12kgf.
-3
v 10 (m/s)
Time (s)
Fig. 5. Speed variation of vibration range against time that occurs in the experimental set-up`s bed.
In Figure 5 were overlapped the speed variation of vibration range that occur in the experimental set-up bed which correspond to the displacement speed obtained for the mobile element. Following this sets of tests one can emphasize that the time of the transitory regime at deceleration (Tdec) and acceleration (Tacc) increased once the input flow is increased, because of the inertia forces which occur at start and braking. The time of the constant speed regime (Tfrp) decreases with the increase of the input flow; the total time, given by summing the times corresponding to each transitory regime, decreases once the input flow is increased.
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In Table 2 are presented the values of the test set where the deceleration is carried out through the tracking valve.
Pressure (bar) 120 160 200
Table 2. Experimental test values at constant load of 12 kgf. Flow Load Tacc Tfrp Tdec Ttot V it V ib.acc (l/min) (kgf) (s) (s) (s) (s) (m/s) (m/s) 26 12 0.011 0.773 0.104 0.888 0.551 0.003 30 12 0.009 0.631 0.105 0.745 0.652 0.004 34 12 0.012 0.569 0.207 0.653 0.719 0.007
Vib.dec (m/s) 0.005 0.004 0.003
By analyzing the graphs presented in Figure 6 and Figure 7 we can say that we have obtained an improvement regarding the total time and a decrease of the vibration range for the deceleration regime obtained using the tracking valve. We can remark a transitory deceleration regime which has a continuously improved behavior once the mobile element`s speed is increased. -3
v 10 (m/s) -3
v 10 (m/s)
Time (s)
Fig. 6. Speed variation against time for linear displacement at variable input flow and constant flow of 12 kgf.
Time (s)
Fig. 7. Speed variation of vibration range against time that occurs in the experimental set-up`s bed.
4. CONCLUSIONS The new constructive solution, which is materialized in the tracking valve, brings better results from the point of view of the deceleration transitory regime in comparison with the built-in braking device of the hydraulic cylinder. By making use of the tracking valve in braking the mobile element along with the cam, which assures the command, can provide an adequate adjustment of the space on which the deceleration occurs. This constructive solution is the only one which offers the possibility to adjust the deceleration slope in wider limits. In other words, this constructive solution can be applied successfully at mobile elements, such as the one presented, where the load varies between medium and large.
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[6] Ecker, K.H., Gupta, J.N.D., Scheduling tasks on a flexible manufacturing machine to minimize tool change delays, European Journal of Operational Research, vol. 164, 2005, p. 627-638. [7] Jan, P., Problematic of fast automatic tool change by working machinery, Journal of MM Science, 2011, p. 274-278. [8] Mutell, A., Jurat, M., Zheng, L., The kinematics simulation to the ATC for machining center based on IDEAS, International Journal of Computer Integrated Manufacturing, vol. 22, 2003, p. 40-46. [9] Dundas, B., A new mechanism for tool changes, modern machine shop, ProQuest, vol. 76, no. 6, 2003, p. 5864. [10] http://www.mobiusinstitute.com/Alaram.aspx?id=2046 (20.05.2013).
