Basically aspects in order to reduce the vibration of machine tools with nonconventional materials structure.

Paun, Loredana ; Predincea, Nicolae ; Avramescu, Valeriu 等

1. INTRODUCTION

The modern tools-machines match overwhelmingly demands flexibility, increased productivity, simple cinematic structure, modern drives made on CNC equipment with a large number of axles (5 or more). Also, they are specific: high rigidity, high accuracy, good dynamic and thermal behavior.

The paper presents a study regarding the dynamic behavior of the structure elements which are made of composite materials of a CNC machines for multiple operations of turning, milling, drilling, boring, toothing, etc.

In order to reduce the weight of machine, the frame is made of composite material, adopting compensating solutions to reduce vibrations and providing the necessary rigidity. These composite materials are formed by pultrusionate profiles made of fiber glass embedded in a mass of thermal resin (www.fiberline.com).

During the processing function of cutting conditions parameters, appear some unbalanced forces that cause vibrations that contribute to precision processing and final quality of the processed part. Besides the vibrations which are generated during the processing may occur also some vibrations from the surrounding area (other machine tools, equipment, sources generating vibrations etc.). To attenuate this, the technique solution is to mount isolators on the multifunctional machine foundation--vibration shock--to prevent the transmission of vibrations from the source of harmful to the environment and the environment in multifunctional machine tool.

2. ALGORITHM FOR OPTIMIZATING THE ABSORBTION ELEMENTS OF VIBRATIONS

In isolation anti_vibrators study, is considered a multifunctional machine tool with 8 axes, which can be considered a rigid body composed of 12 pillars, 11 sleepers and 2 carriers, which are strutting by 180 metal corner units. This assumption has the significance that its frequencies of cutting and manufacturing elements are different than parts of the stiff under isolation, so practically not influenced it. To determine an algorithm for calculating need to use modeling--figure 1.

[FIGURE 1 OMITTED]

The machine is considered as one rigid body of mass m, elastic seated on the floor. This model has 6 degrees of freedom--so 6 own pulsations--namely 3 translations along the axes x, y, z and 3 rotations around these axes.

It is known that the amortizations had little influence on own pulsations and on their modes of vibration. Therefore, to study the own pulsations, the mathematic model will be characterized by mass m, mass moments of inertia from the three axes of coordinates and elastic constants of items absorption vibrations. In equations of the rotation motion, are intervening the moments of inertia of the masses. The moment of inertia of a body of mass m from the zy plane has the expression:

J = [integral] [x.sup.2]dm (1)

As the multifunctional machine can be similar to a parallelepiped of mass m, with [a.sub.x], [a.sub.y] and [a.sub.z] sides, then the moments of inertia of the rigid towards the tried axles x, y, z are the following:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

For the 6 degrees of freedom, if the main elastic axes of the damping elements coincide with the axes of coordinates, the constants are: [k.sub.x], [k.sub.y], [k.sub.z], [k.sub.[phi](x)], [k.sub.[phi](y)], [k.sub.[phi](z)].

The body is lean on a certain number of elastic supports. It may be a bearer 1, which has on the directions of its elastic axes, the main constants [k.sub.p1], [k.sub.r1] and [k.sub.q1]. These elastic axes have the directories cosine [[lambda].sub.xp], [[lambda].sub.xq], [[lambda].sub.xr], [[lambda].sub.yp], [[lambda].sub.yq], [[lambda].sub.yr], [[lambda].sub.zp], [[lambda].sub.xq], [[lambda].sub.zr] toward the xyz tried. Another bearer 2, may be represented by its elastic constants [k.sub.x2], [k.sub.y2], [k.sub.z2] which are determined on the parallel directions with the coordinate axes. It notes, too, by [a.sub.x], [a.sub.y], [a.sub.z] the coordinates binding point between the rigid body and the elastic bearers.

3. VIBRATION INSULATORS APPLICATION

The multifunctional machine is designed to perform various cutting operations which means some features of dynamic action. The issue of vibrations amortization to the machine involves solving three sequences: to reduce vibrations to a minimum the operator's own group, reduced to a minimum the transmission of vibrations in the machine and reducing to a minimum the transmission of vibrations from the machine to the floor and back hall (Buzdugan, 1980).

Vibration behavior of machine tools, for modes of vibration where occurring the relative movements between tool and piece of work can be improved through amortization. It can improve the behavior at vibrations by choosing the corresponding vibration of the cutting, the tool geometry and much less the geometry of processing piece. The cutting system and the tool geometry influence the self excitation vibrations that result in inadequate processing. It is well described in literature (Harris & Crede, 1969) that, during processing of a piece may appear self excitation vibrations--that there are areas of the processing machine in which vibration is amplified although far from the resonance of the structure. The cutting should be operated so as to be outside the self excitation vibrations because during this system the surface quality of the piece that process falls below the projected level. In this sense should be well controlled the technological process parameters.

In order to improve the processing, to reduce the vibrations, could increase a rigidity of the assembly made by tool and the processed piece or to the shocks amortization which are applied to the tool or to improve the tool geometry.

A special issue of amortization presents a vibration attenuation of the vertical, parallel pillars, which close to the main locker on the side. Because the two primary components from the main chanson of resistance do not enter in resonance, is proposed:

a) A simultaneous amortization solution--one against the other. The amortization is made through two rubber pads that can be charged with the task of relative distance by simultaneously using a threaded spindle. Force to be charged with the two pads will be determined experimentally. It is possible that the loading force to be uniform across all pairs of dampers or to vary according to a mathematical function. By the vibration measurements will determine the positions of the double dampers placement and their number.

b) Between the two vertical pillars must be a minimum mass difference for 1,41 for the two pillars do not enter in resonance at the same time.

Under machine structure we propose to place some isolators--elastic dampers--considering that the floor is rigid and immobile. In this case, the proportions which control the quality of isolation are the displacement amplitude of the multifunctional machine during the operations to achieve that was designed and sent ground forces.

Result that the structure of a multifunctional machine tool has a dual role: to provide some support by static pressure on the floor through the insulator and to contribute to the isolation anti_vibration.

4. THE CALCULATION OF RUBBER IZOLATORS VIBRATIONS

The amortization problem raises three issues: the dimensioning calculations, choice calculations and a location efficient insulator of vibration on the rubber foundation.

Many companies have launched on the market of vibration isolators, rubber (Buzdugan, 1986), but presenting basic information or summary. The only company that is respected in this area is firm Trelleborg (www.trelleborg.com) which provides users with a computer program and diagrams choice of types of insulators in their own catalogs. Entry data:

--Machine mass: 2000 kg

--Number of insulators--variable: 4, 6, 8, 10

--Rotation speed of the main engine: 0 - 5000 rpm

--The type of equipment that is mounted: machine tools, RAEM class, maximum performance from vibrations and noise.

Calculations were performed with RAEM chart. Summary of calculations is presented in table 1 (Broch, 1973).

In the first stage, the four vibration isolators rubber will be placed in the corners of the base frame that forms the foundation of the multifunctional machine. These measurements will be the reference.

In the second stage, the isolators will be move simultaneously on the foundation of the machine to the edge bars because different processing schemes have completely different characteristics and generates different vibrations. These measurements will show where to place, in the final, the vibration isolators and maximum degree of damping which can be obtained.

In the third stage, when it will be simultaneous processing operations, will make measurements for confirmation again, the correct position of the vibration insulator rubber.

5. CONCLUSIONS

Of all the possible variations, the best insulation to ensure a vibration insulator of 1500-40 which will achieve the performance of damping of 87% to a number of turns of 1200 rpm of the main engine and 99% when its number of turns passes 2400 rpm. Trelleborg diagrams also shown that if the frequency of the main engine of the multifunctional machine falls below 7 Hz value--equivalent to number of turns below 420 rpm--it's possible entry to resonate of insulators--rubber bumpers. The completed entry in resonate of the insulators--rubber bumpers is produced at a turns of the main engine of 5 Hz -300 rpm.

From the above we recommend the use of work range with the main engine over 420 rpm. From calculations show, confirming theoretical considerations and the equations above, the degree of damping of vibrations with isolators--selected bumper rubber increases with increasing engine speed.

6. REFERENCES

Harris, C.; Crede E.(1969). Shocks and vibrations handbook, McGraw-Hill, New York

Buzdugan, Gh., (1980). The antivibration isolation of the machine tools. Academiei Publishing RSR, Bucharest

Buzdugan, Gh., (1986). Material strength. Academiei Publishing, Bucharest

Broch, J., (1973). Mechanical vibration and shock measurements.

*** (2008) www.fiberline.com Fiberline composites, Accesed on 2009-02-20

***(2008) www.trelleborg.com Trelleborg Industrial AVS, Accessed on 2009-02-18

Tab. 1. The calculation of rubber izolators vibrations

Number Charge/ Types of Degree of Ridging
of insulator, insulators damping, damper,
insulators daN which may % mm
 be use

4 500 1500-40 87 2,8
 500 1500-60 70 6,0
6 333 800-40 81 4,5
 333 350-60 70 5,5
8 250 800-40 65 6,8
 250 350-60 45 3,0
10 200 350-40 82 6,8
 200 800-40 65 5,3
 200 350-60 45 3,0