Research on high accuracy low-rigid details manufacturing process.

Bagimov, Igor ; Kramar, Vadim ; Szabelski, Jakub 等

1. INTRODUCTION

The analysis of constructions of mechanisms in the area of machine building shows that shafts, sleeves, cylinders, axis and rings are over 40% of all parts of mechanisms consist of. All of these parts are rotary bodies manufactured mainly from high-strength steel or alloys. Nearly 12% of parts of them can be qualified as low-rigid elements (shafts--rotors, suspension spring, flexible and torsion shafts, etc.). High requirements for geometric accuracy, mutual position of surfaces, linear dimensions, surfaces roughness are demanded for such parts.

The commonly used method of manufacturing such details is straight turning. In this case accuracy of machining during turning operations should be between 8th and 11th standard tolerance grade of accuracy and roughness [R.sub.a] = 0,63 -2,5 [micro]m. Further realization of grinding operations as well as results of whole manufacturing cycle depend on accuracy of these turning operation. In most of the cases turning is the final operation (Taranenko & Swic, 2005).

Functional need for such parts (elastic, compensating and vibration isolation), together with tendency to decrease amount of material used for producing mechanisms and machines (miniaturization) resulted in introducing low-rigid elements group. Such parts are characterized by disproportionate overall size (length to diameter ratio higher than 5) and/or low rigidity in certain sections and/or directions. Decision to build the automatic control system of low-rigid elements shaping as well as providing of demanded surface quality is disturbed by the fact that during turning process--tool, units of lathe and the detail itself are in relative movement, creating complicated dynamic system for which it is impossible to predict any parameters without undertaking theoretical and experimental research.

It is possible to provide stability of the turning process and in some cases to control accuracy and details surface roughness by lowering the cutting parameters--cutting speed, depth and feed. But in this case considerable loss of productivity because of multiple-cut machining occurs.

Increasing the accuracy and productivity of low-rigid details manufacturing process cannot be executed on the automatic lathe for straight turning because of limited technological abilities and high time loss for resetting.

Usage steadies on a lathe is also not effective in this case because they eliminate only static deformations which are 1.2 to 1.7 times lower than dynamic ones. Usage of steadies on a lathe demands more time for auxiliary operations, steadies and detail resetting therefore there is no place for automation of turning accuracy control.

Usage of vibration dampers allows reducing the oscillations intensiveness, particularly to absorb energy of oscillation movement. There are some constructions of vibration dampers, based on addition of artificial resistance (hydraulic, friction) and also dynamic dampers of oscillations. But complexity of vibration dampers constructions and lack of flexibility during resetting on different dimension-types of details doesn't allow to use them widely (Taranenko & Swic, 2006).

2. RESEARCH PROJECT

One of the most effective methods for increasing the accuracy of low-rigid details manufacturing is the method based on controlling their elastic-deformable state. Central straining was selected as a controllable factor (Abakumov et al., 1999).

Research taken on the mathematical models of objects with such controllable influences showed that object is characterized by much smaller inertia in comparison to control of feed rate. As a result of that it is possible to obtain much better quality parameters of dynamic control in automatic system controlling the elastic-deformable state.

The aim of present investigations is development of automatic control system, which will allow obtaining specified roughness and precision parameters in low-rigid details machining process. This system will control tensile force and tool movements (depth and feed).

Experimental stand (Fig. 1) was developed and assembled by authors in order to obtain static and dynamic characteristics of system (Draczow et al., 2007). The stand consists of numerical control machine tool EMCO Concept Turn 55 with designed and built tailstock (Fig. 2). The tailstock (Taranenko et al., 2008) is equipped with pneumatic drive which allows applying axial tensile force up to 1000 N to the elastic deformable detail (diameter 2-3 mm and length up to 100 mm). Tensile force depends on pressure of compressed air and diameter of piston of pneumatic drive. Control of force is carried out by supplying the pneumatic drive with compressed air using pneumatic valve control.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Investigated elastic deformable detail is fastened in clamp 1 and strongly fixed by turning the clamp nut 2 with sleeve 3 (Fig.2). Rotation of detail is provided by presence of ball bearing 4 mounted inside of moving element 5 which has possibility to move inside of fixed sleeve 6 with the help of bar 7 connected with the coupling rod of pneumatic drive 8. All devices are mounted in body of standard tailstock 9 for EMCO machine tool.

The process of machining the investigated detail on turning machine tool while the experimental data are being gathered is shown on Fig 3.

[FIGURE 3 OMITTED]

To obtain parameters of surface roughness of investigated detail specified by experiment, the accuracy of machining of external diameter and surface quality are influenced by three parameters: [F.sub.X1]--tensile force which creates elastic deformable state, f--feed rate, [a.sub.P]--depth of cut. Scheme of information processing to control the investigated system is presented on Fig 4.

Mathematical first-order models were used (equations (1) and (2)) to estimate the influence of operated force, parameters of cutting during turning and mathematical formulation of investigated process:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where: [x.sub.1] = [a.sub.P], [x.sub.2] = f, [x.sub.3] = [F.sub.X1].

[FIGURE 4 OMITTED]

Complete factorial experiment of [2.sup.3] type was used to determine the coefficients in equations (1) and (2).

The main factors were chosen according to those used in real industry manufacturing processes for given material (stainless steel 12X18H10T of d = 2-5mm). Variability intervals were chosen from the range of real oscillation of factors values.

Matrix of experiment planning with calculated columns of factors interaction was created (Halas et al., 2008).

Every experiment should be repeated three times. As results of experiment the factor which the most influences accuracy parameters of turning operations and surface roughness is expected to be found.

3. CONCLUSION

Automatic control system for high accuracy machining of low-rigid details should be designed as a result of investigations. Such an automatic control system can be used for automation of turning process.

Authors plan to compare classic automatic control system and systems based on controllers which use fuzzy logic laws and artificial neural network to control turning process (Zubrzycki et al., 2007).

After receiving the experimental data it will be possible to check mathematical model of manufacturing low-rigid details by turning. Results of investigations will allow to obtain much higher parameters of geometric accuracy after turning and to reduce time loss of automated technology process during resetting.

4. REFERENCES

Abakumov A.M., Taranenko W.A., Taranenko G.W. (1999). Patent RU No 2130360 C1 Method of mechanical operation with low-rigid axisymmetric details and device for its realization. B23B 23/00, B23Q 15/00. Bulletin No 14, 1999.

Draczow O., Taranenko W., Zubrzycki J., Halas W. (2007). Stanowisko badawcze i uklad sterowania automatycznego procesu obrobki walow (A research position and steering shaft automatic cutting process), Przeglad Mechaniczny No.5/2007, Suplement. S. 53-55

Halas W., Taranenko W., Swic A., Taranenko G. (2008). Investigation of influence of grinding regimes on surface tension state. N.T. Nguyen et al. (Eds.): IEA/AIE 2008, LNAI 5027, pp 749-756, 2008. [c] Springer-Verlag Berlin Heidelberg 2008

Taranenko W., Swic A. (2005). Technologia ksztaltowania czesci maszyn o malej sztywnosci (Technology shaping machines for small parts rigidity), Politechnika Lubelska, ISBN 83-89246-84-8, Lublin

Taranenko W., Swic A. (2006). Urzadzenia sterujace dokladnoscia obrobki czesci maszyn o malej sztywnosci (Control units of part accuracy machines with small stiffness), Politechnika Lubelska, ISBN 83-7497-001-4, Lublin

Taranenko W., Swic A., Bagimow I., Taranenko G., Szabelski J.: Konik obrabiarki. Zgloszenie w sprawie uzyskania PATENTU P 386794 [WIPO ST10/C PL386794] z dnia 2008.12.12

Zubrzycki J.; Szabelski J.; Taranenko W.; Taranenko G. (2007). Using the artificial neural network to control the low stiffness shafts machining. Automation: problems, ideas, solutions: Materials of international scientific and technical conference. On September, 10-15, 2007/Sevastopol national technical university, Kopp Y. (Ed.), pp 93-95, ISBN 978-966-2960-13-6, Sevastopol, September 2007, Publishing house SevNTU, Sevastopol