Increasing of the Stern Tube Bushes Precision by On-Line Adaptive Control of the Cutting Process

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1 Increasing of the Stern Tube Bushes Precision by On-Line Adaptive Control of the Cutting Process LUCIAN VASILIU, ALEXANDRU EPUREANU, GABRIEL FRUMUŞANU, VASILE MARINESCU Manufacturing Science and Engineering Department Dunarea de Jos University of Galati Domneasca str ROMANIA Abstract: - Among the manufacturing operations required to be realized into the ship building domain, stern tube bushes machining is one of the most important, because it has a high influence onto the good functioning of the ship propulsion system in time. Problems may appear because we are talking about pieces with very large dimensions, which are adjusted after their assembly by welding on the ship hull; additional errors may occur during or after machining, caused by ship body thermal or mechanical induced deformations. In this paper, we try to find an appropriate model for these errors, in order to be able, then, to realize an on-line adaptive control system of the cutting process, which will enable a superior precision of stern tube bushes machining process. Key-Words: - stern tube bushes, precision, cutting process, adaptive control, errors model. 1 Introduction In ship building domain, the stern tube bushes are used as support for the stern tube, which is the hole in the hull structure for accommodating the propeller shaft to the outside of the hull. The propeller shaft is supported in the stern tube by two bearings one at the inner end and one at the outer end of the stern tube called stern tube bearings [1, 2, 3]. The bushes are first fixed by welding on the hull, and then they are adjusted by boring, before introducing the stern tube, Fig.1, [5]. Fig.1 Stern tube frame adjusting, by boring [5] Stern tube bushes inner surfaces machining is done by using a boring bar and a special device (Fig.2, [5]). The results obtained by using this system are often poor, because of more specific inconvenient: Although the manufacturing system has a relative good stiffness, because the bar is long (over 4 meters) and because it is impossible to use a middle bearing for the bar, elastic deformations do appear; Vibrations appear during the machining process, with negative effects onto the generated surface quality; The clearances variation in the sliding bearings, along the boring bar, have also a negative influence concerning the geometric shape of the generated surface; The two bushes, assembled by welding on the ship hull, are the subject of deformations their own or induced by diverse external factors (thermal or mechanical); if boring process is realized when the deformations are high, the impact on the generated surfaces is very bad. The manufacturing system used to stern tube bushes inner surfaces machining may be considered as a special one, not only because of the dimensions of the manufactured pieces, but also because the place of a traditional machine-tool bed is taken by the manufactured workpiece a structure more or less stable and, anyway, in connection with other structures, through the ship hull. The boring device includes in itself the tool holder feed mechanism (Fig.2), in connection with the spindle rotation motion. The boring bar is driven by the opposite side of the gears for the feed motion, through a Hooke s coupling, from an electric motor and a reduction transmission. To change the bar rotation speed (together to the feed rate), some of the gears must be manually changed. ISSN: ISBN:

2 Axial feed shaft Feed gear Tool holder Feed screws Screw driving gears Tool position regulation Bearing Spindle Satellite gears Bearing Gear on the spindle Gear on the axial feed shaft Fig.2 The boring bar device The goal of this paper is to draw up a map of the errors which appears when machining the stern tube bushes. There are two types of errors to be considered: errors due to the manufacturing system (system errors) and errors due to the cutting process (process errors). The total machining error results as sum of both system and process errors. Once the errors map known (in connection with the conditions of the machining process development), after a first, roughing pass, it will become possible to find the corrections to be made at the finishing pass, in order to obtain the required quality of the final surfaces. The other parts of the paper are treating the following aspects: 2 Problem formulation and experimental plan; 3 Experimental results, including measurements of bushes inner surfaces and boring bar outer surface; 4 Conception of an on-line adaptive control system; 5 Analysis of the results and conclusions. 2 Problem Formulation The starting point of the researches presented in this paper is the fact that errors (sometimes unacceptable high errors) occur after machining the stern tube bushes holes by using the current solution the boring bar device upper described. There are two types of errors: system errors and process errors. First are produced by: - Incorrect positioning of the boring bar device, relative to the stern tube bushes axis real position; - Clearance in the boring bar bearings; - Insufficient stiffness of the boring bar, while the others are a consequence of: - Cutting tool wearing; - Thermal induced dimensional modifications; - System instability, leading to vibrations appearance. The idea of realizing an on-line adaptive control of the cutting process, in order to improve the stern tube bushes machining precision, requires a model of the errors that appear as a consequence of the conditions on which the process takes part. To find the errors model, we must build, firstly, a data base, by recording each cutting process parameters (e.g. the cutting force variation, the tool position etc.) together to the errors map of the surface generated under those conditions. Then, we ll be able to anticipate the corrections to be made at the finishing pass, by using a dedicated adaptive control system. To realize the errors map, we measured the surfaces resulted after machining with the boring bar device, by using the piano wire method. Because of its length, corrections of the measuring results had to be done (taking count of the wire deflection under its own weight). We also measured the surface of the boring bar spindle (as a potential important source of errors). Finally, we imagined a solution to introduce the required corrections of the machining process. 3 Experimental Results 3.1 The Measurement of Stern Tube Bushes Inner Surface The inner surface of the stern tube bushes was measured, after machining it by boring. The measurements were done in 15 transversal sections, along their common ISSN: ISBN:

3 axis; in each section the distance from the axis to 4 equidistant points (up, down, starbord, portside) was measured. The measurements were done by using a piano wire, drawn by a constant weight and a digital comparator with optic sensor to detect the moment when the wire is touched (Fig.3). There are three reference sections, two on the bottoms and an intermediate one, where the comparator indications were brought to zero. The primary results are presented in Table 1. The length of the used wire was 4500 mm (between the fix support and the fix roller), while the constant weight to draw the wire was 4.41 Kg. Table 1 Stern tube bushes inner surface measured errors Crt. Errors [mm] no. Up Starboard Down Portside Comparator Piano wire Table 2 Stern tube bushes inner surface errors (after corrections) Crt. Errors [mm] no. Up Starboard Down Portside Because the wire is relative length, the results of measurements are affected by its deflection, x y = p, (1) 2H where p means the uniform distributed gravitational load, H the wire drawing force and x the distance from the wire support. By calculating the deflections in all the 15 sections and by applying the corrections to the primary measurements, the data from Table 2 result. A graphical representation of bushes inner surface shape, as it really is after machining, is presented in Fig.4. In conclusion, the machined surface has the shape of a circular deformed cylinder, thinner and with deviations from circularity in its median section. The axis of the system of generator circles it is not rectilinear and it is unknown, as relative position. The quality of the inner surface is poor and requires subsequent finishing. 2 Stern tube bushes Constant weight Fig.3 The stern tube bushes measurement scheme Fig.4 The profile of the bushes inner surface ISSN: ISBN:

4 3.2 The Measurement of Boring Bar Spindle Outer Surface Because the deviations of the boring bar spindle outer surface from its theoretical shape and dimensions have a great influence onto the generated surface precision, the on-line adaptive control system needs information about these deviations. In the analyzed case, we checked the boring bar spindle outer surface. The spindle was fixed between centers, on a lathe (Fig.5). The measurements were done in sections placed from 300 to 300 mm along spindle axis, on the superior generator line of the cylinder; in each section, four measurements were done, from 90 to 90 degrees, with a balance level having a 0.02 mm/m accuracy. Because the deformation of the spindle under its own weight is also significant, by considering the middle deflected line equation p y = 2E I z 4 x l x l x + 12, (2) where in addition to notations upper explained, l means the middle line length and E I z the modulus of rigidity, corrections to the measured values had to be done. Table 3 Spindle deviations from cylindrical shape Spindle Deviations from cylindrical shape [mm] section 0º 90º 180º 270º The spindle dimensions are d = 370 mm, l = 7390 mm; it is made from steel, so E = MPa and ρ = 7850 kg/m 3. The sliding guides errors of the lathe used to fix the measured spindle, were also considered and measured with the same balance level. The final results, obtained after making all the corrections, are shown in Table 3. The analyze of the deviations shows that the boring bar spindle surface presents signs of wearing, more evident in its central zone, which leads to a clearance between the tool holder ring and the spindle surface. Obviously, this clearance has a direct influence onto generated surface accuracy and quality. 4 On-Line Adaptive Control System The experimental research that we made demonstrates the possibility of mapping with a reasonable accuracy the errors which appears when boring the stern tube bushes inner surfaces. The errors induced by the device used to do this machining can also be evaluated. Under these conditions, an on-line adaptive system to control the final boring operation, in order to increase the generated surfaces accuracy seems to be a solution to be considered. By using both a data base including results of previous machining processes (together with the conditions of realizing them) and the measurements made during the current process, it could anticipate the corrections to be done at the finishing pass, to fulfill the above mentioned aim. In Fig.6 there is presented the principle scheme of such an adaptive control system, to be attached to the currently used boring bar device. It is based on a system of piezoelectric actuators, placed under the tool holder device, on a sliding bush; they are connected to an electrical source through a mobile system of wires. Tool holder Satellite gear Bearings Sliding bush Adjusting to zero High accuracy balance level Fig.5 Measurement of spindle surface Piezoelectric actuators Electrical wires Fig.6 On-line adaptive control system ISSN: ISBN:

5 The magnitude of the voltage to feed the actuators is established by the control system, depending on the magnitude of the errors to be compensated, by moving the tool holder on radial direction. The required corrections are small, so the active strokes of this type of actuators could be sufficient. Acknowledgement The authors gratefully acknowledge the financial support of the Romanian Ministry of Education, Research and Innovation through Grant PN-II PC / Conclusion The machining of stern tube bushes inner surfaces is a difficult operation, because of a great number of specific factors and conditions. In the dedicated literature, there are few specifications about the solutions used by the shipyards to solve the problem. In the analyzed case, the machined surface has the shape of a circular deformed cylinder, thinner and with deviations from circularity in its median section. The axis of the system of generator circles it is not rectilinear and it is unknown, as relative position. The quality of the inner surface is poor and requires subsequent finishing. The device used to generate the surfaces by boring is affected by wearing and it constitutes a potential source of errors. A simple reshaping of the boring bar surface wouldn t eliminate the errors, because the process errors and the other categories of system errors still remain. A steady solution to improve the precision of stern tube bushes inner surfaces precision could be the use of an on-line adaptive control system, based on a system of piezoelectric actuators. References: [1] Carlton, J.S, Marine Propellers and Propulsion, Elseiver Ltd., [2] Harrington, R., Marine Engineering, Newport News Shipbuilding, [3] Molland, A.F., The Maritime Engineering Reference Book, Elseiver Ltd., [4] Okumoto, Y., Takeda, Y., Mano, M., Okada, T., Design of Ship Hull Structures, Springer Berlin Heidelberg, [5] ***, Technical Instructions, Galaţi, Tulcea, Mangalia Shipyards. ISSN: ISBN: