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Determining machine condition--assessement of dynamic behavior of the high speed milling machine tool spindle.

Abstract: The assessment of machine condition requires usually evaluating of a variety of operating parameters. Vibration characteristics provide an early indication of defects on components such as rolling element bearings and gears; the level of vibration could be a very important parameter in the predictive maintenance concept. The main objective in this paper is to find the dynamics of the milling machine tool spindle based on the procedure type tracking that is the most appropriate for obtaining the real dynamic condition. This work was validated on the machine Mikron HSM 600U.

Keywords: vibration of spindle, condition, stiffness matrix.

1. INTRODUCTION IN DYNAMIC CONDITION ANALYSIS

The main focus in analysis of the dynamic condition of machines and equipments is to find appropriate parameters of the manufacturing process in order to assure the proper quality of the product and proper maintenance and service for the machines. Studying machines or a part of them, the dynamic behaviour is analysed in the following situations: constant operating speed, variable speed into a limited operating domain, imposed speed inside the domain (e.g. rotational frequency 1 ... 1000 Hz). In all of the mentioned cases, the behaviour of the system under the effect of external excitation is evaluated (fig. 1).

In the literature, the transfer function is evaluated like the ratio response of the system / dynamic excitation. In machine diagnose the main characteristics offered by transfer function are (Sutter et al, 2005): dynamic rigidity or compliance, resonance frequencies, damping factor, natural modes of vibration.

Sub-synchronous vibration (where the dominant excitation is less than the rotating frequency) can be caused by six factors: instability, looseness, structural resonance, belt defects, bearing defects and vibration transmitted structurally from nearby machinery. The instability may be observed from approximately 35% up to something above 50% of the running speed (Fig. 2).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

In a simple time--domain display, the sub-synchronous instability will cause the running frequency to have an irregular, wandering appearance.

The entire utilization of vibration characteristics for condition monitoring requires measuring amplitude, frequency and phase. With some exceptions, amplitude is the primary indicator of the quality of condition--how long a machine can operate.

The primary symptom for the machine malfunction is radial vibration at rotating frequency. The most important causes for these malfunctions are: unbalance, bent or thermally bowed shaft, misalignment and coupling problems, bearing clearance, looseness and resonance frequency.

Actual diagnosis instrumentation automatically forms the applicable measured value from the sensor signal, e.g. the RMS value of vibration velocity in the case of absolute bearing vibrations. Comparing the Overall value with some limit values allows having a conclusion about the condition of the machine.

The table 1 shows the associations between evaluation criteria and measured variables and their importance in machine condition analysis (Ispas et al., 2004), (Zapciu et al., 2004)

As long as the behaviour of the machines is unknown and previous experience or data exists, the evaluation of the machine condition can be made by comparing the current measured values with limits recommended by some Guideline or Standard. This method is quick and simple to execute.

2. RIGIDITY MATRIX MODEL

By the characterization of the machining system, we determine the elastic behaviour of the machine and the parameters which influence the appearance of vibration in processing. Knowing that the appearance of the vibrations is strongly influenced by the stiffness of the system (Bisu, 2007), an experimental protocol is set up to provide that the model of the stiffness is necessary. The adopted experimental method is based on the matrix stiffness development. The deformation of an element of the structure is represented by displacements of the nodes determining this element. Displacements that act as these nodes in the structure, they have a correspondent in "associated forces". The matrix of transformation that connects the generalized displacements of an element to the forces associated is the matrix of stiffness. Considering the assumption of proportionality between the forces and displacements, they are connected by the following equations:

[q] = [k] x [[DELTA]] (1)

where [q] represents the matrix of forces, [k] the matrix of stiffness and [[DELTA]] the matrix of generalized displacements. According with the relation (1) we introduce the notations:

{T} = [K] x {D} (2)

where {T} represents the mechanical torque of the actions, [K] the stiffness matrix and {D} the torque of small displacements. At the time of this operation the total matrix is given according to the correlation with the eigen frequencies of the spindle of the machine tool. The static determination of the total stiffness matrix allows the determination of the couplings existing and thus, of the parameters of stiffness closer to those found during dynamic process.

3. MEASUREMENT OF THE SPINDLE VIBRATION FOR A HSM MILLING MACHINE

In order to know the proper domain for High Speed Cutting for specific milling processes (new in this paper) the authors propose to begin the research with the vibration of the spindle machine tool (Fig. 3) using Tracking analysis module of Vibroport 41 (Zapciu & Bisu, 2007), (Zapciu & Paraschiv, 2007) and after that to modify rigidity matrix spindle model.

In the figure 4 it is presented the tracking signal acquired using Vibroport 41. Eigen frequencies of the assembly spindle-bearings (no cutting process) were: 29130 rpm (first harmonic of 242.5 Hz), 14550 rpm (242.5 Hz) and 12.600 rpm (210 Hz).

[FIGURE 3 OMITTED]

[FIGURE 4a OMITTED]

[FIGURE 4b OMITTED]

[FIGURE 5 OMITTED]

The figure 5 presents the first eigen frequencies of the table of the milling machine tool (determined using transfer function module of the Vibroport 41). On the direction Y-Y is important to deeply analyse the values of the eigen frequencies of spindle because it is possible to use the process speed close to the value of 600 Hz (36000 rpm).

4. CONCLUSION

The main objective in this research paper was to find the dynamics of the machine tool spindle, based on the procedure type tracking that is the most appropriate to obtain the real dynamic condition of the machine spindle. This work was validated on the machine Mikron HSM 600U based on the measuring of the level of vibration using the Tracking module from Vibroport 41 and accelerometers type AS020.

Further research will aim to find a correlation between the rigidity matrix of the mechanical system and the experimental results concerning eigen frequencies, based on the vibration signal obtained from the real system studied.

Actual research is new and important because it can help to elaborate a proper model for the cutting process. We take into account the separation of the frequencies having like cause the structure of the machine tool and the frequencies caused by the cutting process.

5. REFERENCES

Zapciu, M.; Bisu, Cl.Fl. (2007). Dynamic issues and procedure to obtain useful domain of dynamometers used in machine tool research aria, Proceedings of 7th International Multidisciplinary Conference, pp. 735-742, ISSN -1224-3264, Baia Mare, Romania.

Zapciu, M.; Paraschiv, M. (2007). Predictive maintenance and use of tracking concept to analyze dynamics of machine tool spindle. The 11th International Conference--TMCR'2007, pp.512-516, May 31th-June 3th, 2007, Chisinau, Modavia.

Sutter, G.; Molinari, A. (2005). Analysis of the Cutting Force Components and Friction in High Speed Machining. Journal of Manufacturing Science and Engineering, Trans. of the ASME, May 2005, Vol.127, pp. 245-250.

Zapciu, M.; Anania, Fl.D.; Tanasescu, A.; Bisu, Cl.Fl. (2004). Analyzing the fondation to have accurate dynamic behavior for industrial robots included into FMS. Proceedings of the International Conference ICMaS 2004, Romanian Academy Edition, pp. 425-428, ISSN 0035-4074.

Ispas, C.; Gerard, A.; Zapciu, M., Mohora, Cr. (2004). Condition monitoring and dynamic behavior for CNC turn main spindle turning with 6000 rpm. Proceedings of the International Conference ICMaS 2004, Romanian Academy Edition, pp. 113-116, ISSN 0035-4074.

Bisu, C. (2007). Study of Self-Maintained Vibration in 3D Cutting--A new modelling applied on turning, PhD Thesis, IMST faculty, University POLITEHNICA of Bucharest
Table 1. Evaluation criteria and measured variables

 Evaluation criterion

 Guideline Manufacturer Operator
 limits limits experience Trending

Measured variable * ** *** ***
Bearing vibration * ** *** ***
Shaft vibration * * * ***
Bearing condition *** * *
Displacement ** *** ***
Sound *** ** **
Temperature

These are: * usable; ** suitable; *** preferable criterion.
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Article Details
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Author:Zapciu, Miron; Bisu, Claudiu Florinel
Publication:Annals of DAAAM & Proceedings
Article Type:Technical report
Geographic Code:4EUAU
Date:Jan 1, 2007
Words:1340
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