Determining machine condition--assessement of dynamic behavior of the high speed milling machine tool spindle.
Zapciu, Miron ; Bisu, Claudiu Florinel
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.
