Experimental assessment of the milling machine column before remanufacturing.

Daraba, Dinu ; Boca, Gratiela Dana

1. INTRODUCTION

Continuous real structures have an infinite number of degrees of freedom and an infinite number of modes of vibration. During experiments, due to time and cost constraints, FRF measuring is carried out for the degrees of freedom requested for the precise definition of the natural vibration modes, within the measuring frequency range intervals. In modal testing, frequency response functions measurements are made under controlled conditions, where the structure of the analyzed technologic equipment is artificially excited. The measurement of the frequency response function determines the natural frequency values that are the calculation basis of the speeds at which the machine tool can enter the zone of resonance (Ispas, 2008).

2. RESEARCH COURSE

The continuous real structures have an infinite number of degrees of freedom, and an infinite number of vibration modes. Using certain experimental research techniques, a mechanic structure can be put in a vibration condition using a sustained oscillatory movement.

Fig.1 presents the algorithm of the experimental research.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Based on this algorithm, a test stand for the requested measurements was made for identifying the natural vibration modes of a tool-shop FUS 25 universal milling machine (fig.2). The measuring points of the frequency response were chosen to obtain information regarding the dynamic behaviour mode of these structure elements which are appropriate to be remanufactured.

3. TEST STAND FOR RESEARCH

The basic component of the test stand used in our experimental research is the FUS 25 tool-shop milling machine.

The following instruments were used for making the experimental research (Schwarz, 1999):

* Impact hammer (ih) with a load cell attached to its head to measure the input force.

* Accelerometer (acc) to measure the response acceleration at a fixed point and direction.

* FFT (Fast Fourier Transform) analyzer with two or four channels to compute FRF (Frequency Response Function).

* VIBROEXPERT CM--S40 processing software, for the data processing and identifying the modal parameters.

4. MEASURING METHOD

For the modal test, the measurements of the frequency response functions are carried out under controlled conditions, where the structure of the analyzed technologic equipment is artificially excited (Zapciu & Bisu, 2007).

[FIGURE 3 OMITTED]

The graphic presentation of the excitation points and the accelerometer response recording points are shown in Fig. 3 (for measurement no.7) and summed up in Table 1.

The technologic equipment that would be remanufactured, are mostly non-operational. More than that, the remanufacturer determines the parameters of the newly remanufactured equipment at his headquarters, when the equipment is non-operational, and rarely at the location from which it is sent to be remanufactured.

If the measurements were made with the equipment in a certain operational regimen, these results would be affected by all the factors which influenced the machine-tool dynamic behavior.

The data acquisition was carried out without a previous preparation of the machine tool, as the inspection of the play between the component elements, the verification of the foundation fitting screws pressing, and the inspection of the structural elements fitting screws, etc.

An excitation was induced in the system with the impact hammer (impulse), and the structure subsequently suffered a move (response), which was perceived and measured with an accelerometer. The excitation made with the impact hammer was produced in the beforehand fixed points, taking into account the following considerations (Daraba, 2008):

* The measurements to comprise all structure elements which are to be reused for remanufacturing.

* The most vulnerable elements of the machine tool to be assessed from a dynamic point of view.

5. MEASUREMENTS RESULTS PROCESSING AND INTERPRETATION

Based on the experimentally obtained numeric results, using specialized software, we obtained the graphs of the transfer functions, which highlighted the vibration natural frequencies of the analyzed machine-tool elastic structures.

The experimental research allows the identification of the speeds that can bring about the machine-tool in the zone of resonance.

These speeds will be annulled from the speeds domain helped by the numeric control (CNC) that will equip the remanufactured machine-tool.

[FIGURE 4 OMITTED]

Analyzing the transfer function graphs of Fig. 4, the natural frequencies of every measurement were setup and represented on respective images.

The critical speeds of the milling machine were calculated, commencing from the experimentally determined natural frequencies, and the results are shown in Table 2.

6. CONCLUSION

The column is the component of the elastic structure which limits the maximum speed of the spindle, for the new technologic equipment obtained by remanufacturing.

This experimental research ascertained that the FUS 25 universal milling machine, which by manufacturing had the maximum speed of 2200 rot/min, could increase it up to 6000 rot/min, following the remanufacturing process.

The natural frequencies within the range of the operational frequencies of the surveyed technologic equipment were analyzed in this experimental research.

Analyzing the natural vibrations modes of the system, relevant conclusions were obtained about the dynamic condition of its structure. According to the presented algorithm, the testing is quick, suitable, and cost efficient.

7. REFERENCES

Daraba, D. (2008), Studies and Researches Regarding the Remanufacturing of the Technological Equipment, PhD. Thesis, North University of Baia Mare

Ispas, C.; Bausic, F.; Zapciu, M.; Parausan, I. & Mohora,C. (2008), Dynamics of Machine and Equipments, Editura AGIR, ISBN 978-973-720-147-8, Bucuresti

Schwarz, B.J.; & Richardson, H.M. (1999), Experimental modal Analysis, CSI Reliability Week, Orlando, FL, Available from: http://www.systemplus.co.jp/support/ Accessed: 2010-01-15

Zapciu, M.; Bisu, C.F. (2007), Dynamics Issues and Procedure to Obtain Useful Domain of Dynamometers user in Machine Tool Research Aria, Scientific Bulletin, Serie C, Vol. XXI, May 17-18, pp. 735-742, ISSN 1224-3264

*** (2010), www.lds-group.com, Basics of Modal Testing and Analysis, Accessed on: 2010-02-19

*** (2008), www.Bksv.com/doc, Bruel&Kjaer, An introduction to modal testing, Accessed on: 2010-03-23

Tab.1. The location of the accelerometer and impact hammer
on the column

 Impact
 Accelerometer hammer
Measurement no. location location Impulse on

3 3Yacc 3X ih column
 3Xih top slide
 3Yih column

5 5Xacc 5Xih column(backside)
 5Yih column-on side
 5Zih the top slide

7 7Zacc 7Xih column
 7Xih the base
 7Zih the top slide

Tab. 2. The natural frequencies and the critical speeds of the
column

 Natural frequencies [Hz]

Meas. no. Acc. Ih. Critical speeds [rot/min]

3 3Yacc 3Xih 50 112 150 175
 3000 6720 9000 10500
 3Xih 50 125 -- --
 3000 7500 -- --
 3Xih 62 100 125 162
 3720 6000 7500 9720

5 5Xacc 5Xih 50 112 -- --
 3000 6720 -- --
 5Xih 50 100 137 162
 3000 6000 8220 9720
 5Zih 50 100 162 200
 3000 6000 9720 12000

7 7Xacc 7Xih 50 112 137 162
 3000 6720 8220 9720
 7Xih 50 100 162 --
 3000 6000 9720 --
 7Zih 50 112 -- --
 3000 6720 -- --