Setting up the maximum speed at which the structural elements of the FUS 25 milling machine resist after remanufacturing.

Daraba, Dinu ; Cosma, Marius ; Alexandrescu, Ioan Marius 等

1. INTRODUCTION

Using the applicative research presented in this paper, we observed the setting up and verification of a methodology for diagnosing the dynamic parameters, which are characteristics of the elastic structures of the machine tools that are proposed for remanufacturing, before starting this process. The proposed methodology allows the exact determination of the natural frequencies, which occur within the limits of the work frequencies of the analyzed technologic equipment, and thus, the speeds at which the mechanic system could enter the resonance domain can be calculated (Chiriacescu, 2004).

This methodology, which can be used for all types of machine tools, satisfies the request of the remanufacturer and beneficiary of the future remanufactured machine-tool to know, even in the projecting stage, the main characteristic of the machine: the maximum speed of the main spindle.

The proposed methodology allows the determination of the speeds at which the machine tool could enter the resonance domain, warning the remanufacturer about them. The numerical controls of the remanufactured machine tools will be able to eliminate these speeds, not allowing the operator to use them, even by mistake.

A milling machine was used for this research, because the milling process for generating surfaces is highly generating forced vibrations, whose frequencies are comparable to the speed of the main spindle, and also having relative high amplitudes. The dynamic characteristics seen at the milling machines can be considered as covering most of the machine tools. So, the presented methodology can be used for all types of machine tools. (Ispas et al, 2008).

2. PRESENTATION OF THE METHODOLOGY

Almost all structures have the tendency of vibrating at certain frequencies, named vibration or resonance natural frequencies (Zapciu & Bisu, 2007).

When touching that frequency, each frequency is associated to a deformation mode. When a structure is dynamically loaded with stresses that overlap its natural frequencies, this structure undergoes considerable deformations. The natural frequencies and vibration modes are very important parameters during the projecting stage, as they supply information about the dynamic regimen behavior of the analyzed structure(Daraba, et at, 2008).

The purpose of the research, using the finite elements method, is:

* To determine the natural frequencies of the FUS 25 milling machine elastic structures;

* Calculation of the maximum speed borne by the elastic structure, before remanufacturing.

These are the steps made in this methodology, using the finite elements analysis:

* Creating the model using CAD programs;

* Discretization, using an optimum amount of finite and differently sized elements (Fig. 1);

* Setting up the loading conditions (Fig. 2);

* Running the analysis with a specialized software;

* Generating a report, which includes a description of these steps and the conclusions following the finite elements analysis.

We set up the type of the elements and the discretization meshing, in conformity with the shape of the structure and the stress underwent by it. More types of discretizations can be carried out for the same structure.

When choosing the optimum discretization method, we took into account that the precision degree of the obtained results increased with the augmentation of finite elements, but even this figure could not have been of any amount, as the number of unknown degrees of liberty would be increased, and their calculation period would consequently increase.

For an efficient correlation of these two aspects, the structure discretization with finite elements of different dimensions was performed. Thus, for the areas where the movements and tensions have not high variations, we used high dimension finite elements, and in the areas where the values of the movements and tensions highly differ from one element to another, we used a much more dense finite elements meshing.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

In order to ensure some results as near as the real ones, the passing from the finite elements with high dimensions to those with small dimensions, had to be progressively performed.

The first natural frequency is of 4.6 Hz and it appears due to some aspects linked to the static rigidity of the machine-tool table; all these elements will have to be eliminated during remanufacturing.

The resonance phenomenon appears at a natural frequency of 20.6 Hz (Fig.3.), due to which the machine tool elastic structure bears deformations. These deformations do not affect the processing precision, as there are no observed displacements between the main spindle (tool) and the machine table (part).

The natural vibration of 112.3 Hz brings the machine into a resonance condition, which concurs to the displacements of the machine structure elements, with the severe effect on the processing precision. In Fig.4. we notice a rotation of the machine trunk and table around the Z axis, but in different senses, leading to the positional modification of the tool against the part.

The own mode presented in Fig.5 at the frequency of 140,84 Hz marks out a bending of the machine body and table within the X-Z plane, catastrophically affecting the tool-part position(Daraba, 2008).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

3. RESULTS ANALYSIS

Tab.1. sums up the natural frequencies obtained using the graphic analysis of the vibration mode (Daraba, 2008).

Using the formula n = 60 f [Hz], and considering the natural frequencies, we calculated the speeds that are shown in Tab.1. At these speeds, the remanufactured machine tool will resonate.

Subsequently analyzing the natural modes of vibrations, we find that the deformations of the structure elements will certainly influence the processing precision, at frequencies higher than 112 Hz. The resonance danger does not appear anymore due the 112 Hz natural frequency of the analyzed structure, as the subsequent frequencies are off the domain of the work frequencies.

Thus, we set up that the frequency of 112 Hz is the maximum natural frequency until the structure elements bear deformations, which will not influence the processing precision of the would be remanufactured milling machine. Thus the maximum speed will be 6700 rpm. We found that after remanufacturing, the maximum speed of the FUS 25 shop universal milling machine--initially manufactured at 2200 rpm, can increase up to 6700 rpm.

4. CONCLUSIONS

The proposed methodology, for setting up the limits of the elastic structures of the technologic equipment before the remanufacturing process, is based on the finite elements analysis method and it is verified using the universal experimental modal analysis.

The studied machine tool can undergo a remanufacturing process, following which the performances of the machine can be substantially increased.

Main spindle speeds, which are much more superior to those used by the original machine can be used; these speeds can reach up to 6000 rpm without any problem. In case of the fulfilled analysis, we observed the determination of the vibration behavior mode of the structure elements of the non-operational milling machine, as before starting the remanufacturing process the work parameters of the old equipment do not interest anymore.

5. REFERENCES

Chiriacescu, S. T. (2004). Dinamica masinilor unelte. Prolegomene (Dynamics of Machine Tools. Prolegomena, In Romanian), Editura Tehnica, ISBN 973-31-2206-8, Bucuresti

Daraba, D.(2008). Analyses of technological equipment structural elements for remanufacturing using the finite element method, MicroCAD 2008 International Scientific Conference. Machine and Construction Design, pp.19-24, ISBN 978-963-661-812-4, University of Miskolc, March 2008, Miskolc

Daraba, D.; Cotetiu, R.; Ungureanu, N. & Alexandrescu, M. (2008). Experimental Determination of own Vibration Mode Machine Tools Before Remanufacturing, The 9th International Scientific Conference for PhD Students, pg. 52-57, ISBN 978-80-89276-11-0. University of Zilina, May 2008, Turcianske Teplice, Slovak republic

Ispas, C.; Bausic, F.; Zapciu, M.; Parausan, I. & Mohora, C. (2008). Dinamica masinilor si utilajelor (Dynamics of Machines and Equipment, In Romanian), Editura AGIR, ISBN 978-973-720-147-8, Bucuresti

Zapciu, M. & Bisu, C.F. (2007). Dynamic issues and procedure to obtain useful domain of dynamometers used in machine tool research area, The 7th International Multidisciplinary Conference, pp. 735-742, ISSN-1224-3264. North University of Baia Mare, May 2007, Baia Mare

Tab. 1. The calculated speeds relative to the natural frequencies

Figure of frequency F7 F8 F9

Frequency f [Hz] 4.6 20.6 26.1
Speeds [rpm] 276 1236 1571

Figure of frequency F10 F11 F12

Frequency f [Hz] 54.1 112.2 140.8
Speeds [rpm] 3249 6737 8450