Hydrostatic spindle for ultra-precision two-sided machining

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Hydrostatic spindle for ultra-precision two-sided machining

Urgency of the research. Modern development of mechanical engineering creates constantly increasing demands con-cerning maintenance of quality indicators of detail processing. So, indicator of roughness, admissions of linear sizes and angular are decreased constantly. Application ofprecision machine tools is an effective way of decision of the tasks in view.

Target setting. Today ultra-precision processing of materials is widely used for manufacture of electronic components of techniques, telecommunication, medical, automobile, optical equipment. Therefore, the designing of ultra-precision ma-chine tools is an urgent task for ensuring the high efficiency of the processes of machining.

Actual scientific researches and issues analysis. Basic component of precision machine tool which defines quality of machining is a spindle. Quality indicators of a spindle are mostly provided with a correct choice of type and design of its bearings. It is showed that hydrostatic bearings together with increase of machining reliability and productivity allow providing of exclusively high indicators of detail machining quality compared with other types of spindle bearings. Besides, they are the most perspective type of spindle bearings for realization of high-speed machining and ultra-precision machining. However hydrostatic bearings are characterized by relatively big power losses on friction and, as a consequence, vigorous heating at increased rotational frequencies. Therefore lubrication of hydrostatic bearings with low viscous liquids, in partic-ular water, presents separate interest.

Uninvestigated parts of general matters defining are designing of new design of a spindle with combined hydrostatic bearing based on special water lubrication for increased precision and efficiency of two-sided ultra-precision machining.

The research objective of this article is designing of new design of a spindle unit with direct drive, clamping fixture and combined hydrostatic bearing, which based on special water lubrication, for increased precision and efficiency of two-sided ultra-precision machining.

The statement of basic materials. New design of a spindle with combined hydrostatic bearing based on special water lubri-cation and clamping fixture is proposed for increased precision and efficiency of two-sided ultra-precision machining. The de-sign of this precision spindle with direct drive by hollow-shaft torque motor provides two-sided machining of disk-shaped work-pieces. The regularities of the formation of stiffness, flow rate and power losses in the combined spindle bearing, depending on viscosity of the working fluid, clearances in the journal and thrust bearings, pump pressure, and size of the bearing lands are defined. As a result of mathematical and CFD modeling rational parameters of combined spindle bearing were identified that provide minimum power losses at operation at simultaneous insuring of the high accuracy of a spindle. For increase of accuracy of clamping advanced design of the clamping fixture with a screw clamping of workpieces on the basis of use ofPVC paste as pressure-transmitting environment is offered. The article introduce the use of water lubrication as an effective way of solving issues of increase concerning spindle unit efficiency, the reduction of operating costs due to simultaneous cooling of bearings and spindle drive, and also the increase of environmental friendliness of the design as a whole.

Conclusions. The new design of a spindle unit for two-sided ultra-precision machining with direct drive and clamping de-vice was presented in this article. The main advantage of this solution is the compact structure of the spindle. The direct drive of the spindle with hollow-shaft torque motor can effectively be used for two-sided ultra-precision end turning of the workpieces. The regularities of the flatness deviation formation of the end surface of the workpiece during clamping are established. It is shown that clamping forces acting on the workpiece do not have a significant impact on accuracy of the surfaces machined. The rational geometrical and operational parameters of the clamping for maintenance of high operational reliability are received.

Urgency of the research. Modern development of mechanical engineering creates con-stantly increasing demands concerning maintenance of quality indicators of detail processing. So, indicator of roughness Ra should be Ra < 10 nm, admissions of linear sizes Tl < 1 p,m and angular Ta < 1". Application of precision machine tools for machining is an effective way of decision of the tasks in view.

Target setting. Today ultra-precision processing of materials is widely used for manufac-ture of electronic components of techniques, telecommunication, medical, automobile, optical equipment. Therefore, the designing of ultra-precision machine tools is an urgent task for en-suring the high efficiency of the processes of machining.

Actual scientific researches and issues analysis. At Fraunhofer Institute for Production Systems and Design technology IPK and the Institute for Machine Tools and Factory Mana-gement IWF of the TU Berlin the new concept of turning machine for two-side machining of the front surfaces of optical elements was developed [1; 2], see Fig. 1.

As it is marked in works [3], at designing of ultra-precision machine tools the following should be provided: high values of stiffness and damping in technological processing system; minimization of thermal deformations, isolation from external influences. Basic component of precision machine tool which defines quality of machining is a spindle [3]. Accuracy, specific speed, carrying capacity, static stiffness, dynamic quality, thermal stability, reliability which, in turn, depend on their design features, bearing section work capacity of a spindle and ways of lubrication of bearings are basic indicators of quality of spindle. Accuracy of a spindle is characterized by spindle rotational accuracy. Precision machine tools have radial, axial and face error motion of a spindle within 0.01-0.1 p,m [3; 4]. Quality indicators of a spindle are mostly provided with a correct choice of type and design of its bearings.

Aerostatic bearings work very well at high rotational frequencies (to 400,000 rpm) and provide exclusively high rotational accuracy (up to 12 nm) [4], but have very low stiffness and damping. Basic feature of air bearings are small power losses on friction. Technological restrictions of machine tool by processing with insignificant force of cutting are lack of such constructive decision. It does not allow carrying out two-sided machining of workpieces ef-fectively with high accuracy and productivity. The alternative solution is use of spindle hy-drostatic bearings. In the works [5, 6] it is stated that hydrostatic bearings which together with increase of machining reliability and productivity allow providing of exclusively high indica-tors of detail machining quality compared with other types of spindle bearings. Besides, they are the most perspective type of spindle bearings for realization of high-speed machining and ultra-precision machining. Only hydrostatic spindle combines ultra-high accuracy and high effectiveness in vibrations suppression. However, unlike of aerostatic bearings, hydrostatic bearings are characterized by relatively big power losses on friction and, as a consequence, vigorous heating at increased rotational frequencies [4; 5]. Lubrication of hydrostatic bearings with low viscous liquids, in particular water, presents separate interest considering constantly increasing requirements concerning specific speed of spindles [7; 8].

Therefore, the uninvestigated parts of general matters defining are designing of new design of a spindle unit with combined hydrostatic bearing based on special water lubrication for increased precision and efficiency of two-sided ultra-precision machining.

The research objective of this article is designing of new design of a spindle unit with di-rect drive, clamping fixture and combined hydrostatic bearing based on special water lubrica-tion for increased precision and efficiency of two-sided ultra-precision machining.

The statement of basic materials. For conditions of ultra-precision two-sided turning of workpieces, a spindle has to provide axial stiffness approximately three times more than radial stiffness. Only annular thrust and cylindrical journal combination with independent supply of fluid in each recess satisfies the requirement of not uniform bearing stiffness. This design of the bearings when using adjustable throttling device at inlet of each recess allows providing different values of axial and radial stiffness in a wide range of their changes. Based on the recommendations of the [4; 9] the number of recesses accepted for journal bearing equals four and one recess for thrust bearing.

Obviously, the most promising solution for double-sided machining is use of built-in torque motor design as spindle drive. In this case, placing rotor of the motor between spindle bearings, we get a compact and symmetric design relative workpiece with free access of tool to the processed surface. Such schematic solution will allow reducing influence of spindle thermal deformation on machining accuracy.

Mathematical Model for Calculation of Main Parameters of Spindle. For conditions of ultra-precision machining, normally there is a small cutting force (up to 2N) and negligible rotor offsets along both radial and axial directions. Then it's fair assumption about the film thickness constancy in the journal and thrust spindle bearings at machining. It is commonly known that in such case the maximum stiffness of hydrostatic bearings with restrictors at low speeds of spindle rotation is achieved when the pressure ratio is pk/pH = 0,5 [9]. Taking into account above mentioned assumptions, the mathematical model of journal hydrostatic bearing with four recesses will be [4; 9]:

where b - recess length in axial direction, Dp - journal bearing diameter, 8p - radial clearance in the bearing, 9k - recess length in circular direction, L - journal bearing length, la - length of ax-ial land, lt - length of circular land, p - coefficient of dynamic viscosity of working fluid, np - number of the recesses, P_w - viscous friction loses, Sefi - effective area of the journal bearing.

Similarly, given the previous considerations the mathematical model of annular thrust hy-drostatic bearing with one recess is the following [9; 10]:

where 8o - clearance in the thrust bearings, Dpo - thrust bearing diameter, Lo - thrust bearing width, lo - width of thrust bearing land, lk - recess width, ri...r4 - radii defining position of lands and recess in radial direction, P^o - viscous friction loses, Sef2 - effective area of thrust bearing, Qo - fluid flow rate in the thrust bearing.

Total characteristics of the combined bearings are calculated by the following dependen-cies [4; 10]:

where Cb - specific heat capacity of the lubricant, AT - temperature drop between entry and exit from the bearing which is estimated for a single pass of lubricant, n - efficiency factor of spindle drive, Kb - power ratio, Pcut - maximum value of cutting power at end turning of the workpiece, Pq - pumping loses, Psum - total power losses in the combined bearing, P^ - total viscous friction loses, Qsum - total flow rate in the combined bearing, Qr - fluid flow rate in the journal bearing, p - density of the working fluid, Pdr - spindle drive power.

The average value of clamping force is defined as:

where Npr - average value of stress on the workpiece outer surface, Spr - area of the work-piece outer surface.

The CAD/CAE system SolidWorks is used to determine characteristics of the area, volu-me and stress on the part outer surfaces.

Results of Analysis of Hydrostatic Bearings. High values of bearing stiffness can be achieved by rational choice of clearances and increase of pressure pk in the bearing recesses while ensuring ratios pk/pH = 0,5. Establishment of rational values of static clearance in jour-nal 5p and thrust 5o bearings and also viscosity v of working fluid are of key importance for minimization of both power losses due to friction P^ and fluid flow rate Q (Fig. 3).

It is established that static value of clearance 5 has the biggest impact on fluid flow rate in journal Qr and thrust Qo bearings. However, in case of friction power losses in the bearing the opposite tendency is observed. The biggest influence on friction power losses has viscosity of working fluid. Length la of axial journal bearing lands and width lo of thrust bearing lands have also significant and multidirectional influence on pumping Pq and friction losses P^ (Fig. 4). It is necessary to search for rational values of clearance 5, viscosity v, sizes of lands, la and lo, with the aim of increase of efficiency of spindle bearing.

Fig. 4. Total flow rate Q a) and power losses PSum b) in combined hydrostatic bearings

Rational values of these parameters are set as a result of simulation: 5p = 25 p,m; 5o = 25 p,m; v = 1 sSt; la = 6 mm; lo = 9 mm (Fig. 3). This allowed to minimize values of total flow rate Q to 62.3 l/min, Qr = 12 l/min, Qo = 19.15 l/min; power losses in the bearing Psum = 3,400 W, Pq = 1,560 W, P^ = 1,846 W. Calculated value of maximum power of the spindle drive is 2,073 W (when n = 2,730 rpm; radial stiffness Cp1 = 350 N/^m, axial stiffness Cp2 = 1,425 N/p,m (when pH = 15 bar); temperature drop between entry and exit from the bearing AT = 1.42 °C. It is established that flow pattern of the fluid on the lands is laminar (Re = 857 when n = 2,730 rpm) and in the recesses is mixed.

Modeling of combined hydrostatic bearings by means of FlowWorks Simulation package was performed to check main geometrical and operational parameters obtained above. As a result of simulation pressure distribution on the recesses, lands and drainage channels were defined. The obtained values of pressure in the recesses of the journal bearing (7.47 bar) and thrust bearing (7.42 bar) correspond to the calculated values (7.5 bar) according to the results of mathematical modeling. The values of the pressure in the drainage system of the bearing were used for reasonable selection of seals.

Clamping System for the workpiece. Simulation of the workpiece clamping was divided into two stages: backlash elimination in the coupling of expansion sleeve - workpiece and, in fact, the clamp on the outside diameter D = 70±0.005 providing required clamping force of the fixture. Maximum clearance in the coupling of expansion sleeve - workpiece is 10 gm. At finishing end turning the cutting force does not exceed the values 2 N, then, considering safe-ty factor to be 2.5 clamping force can be equal to 5 N.

Simulation was performed by means of SolidWorks Simulation. Securing sleeve in the as-sembly unit was simulated by two connections - fixed geometry at its ends. External load created by pressure of PVC paste was simulated by option of pressure in the inner cavity of the sleeve.

It is established that maximum stress under deformation of the sleeve during diametric backlash elimination 10 gm is more than three times less than yield point of the sleeve. Addi-tionally, stresses and displacements of the sleeve clamping surface were defined. Moving of the clamping surface of the sleeve occurs according to the parabolic pattern with reaching the highest value in the middle of the clamping surface.

Dependence of flatness deviation Dev of end surfaces from clamping force was defined. Increase in clamping forces by 10 times to 50 N leads to a linear increase of deviation from flatness up to 3.6 nm, which is almost three orders less than tolerance of the flatness of the workpiece end surfaces (2 ^m).

One of the main drawbacks of thin-walled clamping devices is the problem of ensuring of set parameters of fatigue strength. For example, basic variant of the expansion sleeve has val-ue of load factor ks = 0,4 and number of cycles to failure Nc = 1,739 that doesn't satisfy opera-tional reliability of clamping devices. Moreover, fatigue failure occurs on the fillet radius of internal cavity of the sleeve.

To improve operational reliability of the devices fatigue analysis of the sleeve was ful-filled with varying parameters of radius r and wall thickness h of the clamping surface of the sleeve. As a result of modeling the most rational value of the wall thickness (h = 0,5 mm) is obtained. When h = 0,5 mm the value of fatigue load factor increases to 2 and the value Nc equals to 95010 what corresponds to purpose of use of the fixture.

Analysis of Spindle Conception for Two-Sided Machining. Design of the spindle with combined hydrostatic bearings built for two-sided machining of workpieces was developed according to analysis results (Fig. 5). Special lubricant based on water is used as working fluid. Use of water can effectively solve issues of increase of spindle efficiency, reduce operating costs and increase environ-mental friendliness of the design as a whole. Use of the mentioned lubrica-tion also allows solving the issue of the spindle drive cooling by fluid supply from one hydraulic power unit.

Spindle has built-in direct drive.

Rotor of the motor is placed between two rotating sleeves of the bearings.

Bearing sleeves and rotor are fixed by

bolts with both sides. Stator of the motor is fixedly mounted on the mo-tor cover. Both bearing housings are based on end surfaces of the stator providing operating clearances in radial and axial direc-tions. Labyrinth seals are applied to prevent fluid leak between the moving and stationary

parts of the spindle. Spindle bases on outer prismatic surface of the stand with abutment to the housing end and fixing by bolts. In this case the influence of overturning moment on accuracy of position of the spindle axis is eliminated. During assembly of the unit concentricity of outer surface of the stator to the inner diameter of the rotor within 0.1 mm should be provided.

The fixture unit is based on bearing aperture with abutment to the end of the right cover of the bearing arrangement. There is a special pin to ensure the correct position of the clamping fixture relatively to the spindle at change of the workpiece. To improve fixture accuracy the clamping surface of the extension sleeve should be finally grinded ready-fitted with bearing arrangement.

Expansion sleeve is installed into the clamping housing and fixed with both ends by screws (Fig. 6).

The cavity formed between the clamping housing and expansion sleeve is filled with PVC paste. Screws with plunger and plugs with seals seal working cavity with PVC paste. For fix-ing of the workpiece a screw drive is used. Necessary fixing effort to the workpiece is created by rotation of the adjusting screw. Thus, the plunger receives linear moving, compressing PVC paste.

Pressure upon PVC paste made by the plunger is spread in regular intervals, de-forming concentrically the thin-walled part of the expansion sleeve. The backlash between the clamping housing and the workpiece is thus liquidated.

As a result the workpiece is clamped and aligned on external diameter with high accuracy. Unclamping of the workpiece is carried out by turning of the adjusting screw in the opposite direction.

Conclusions

To conclude the combined hydrostatic bearings provide a compact spindle unit design simultaneously ensuring high degree of stiffness in axial, over 1,400 N/^m, and radial direction, around 350 N/^m. The use of low-viscous lubrication based on water allows providing low friction power losses (1,846 W) at the maximum rotation speed of the spindle. Besides, the application of water for both lubrication of bearing and cooling of the spindle drive reduces operating costs and increases environmental friendliness of the spindle design. The direct drive of the spindle with hollow-shaft torque motor can effectively be used for two-sided ultra-precision end turning of the workpieces. The main advantage of this solution is the compact structure of the spindle. For the increase of accuracy of clamping advanced design of the clamping fixture with a screw clamping of workpieces on the basis of PVC paste as pressure-transmitting environment is offered. The regularities of the flatness deviation formation of the end surface of the workpiece during clamping are established. It is shown that clamping forces acting on the workpiece do not have a significant impact on accuracy of the surfaces machined. As a result of the analysis of strain-stress state and fatigue strength of the expansion sleeve rational geometrical and operational parameters of the clamping for maintenance of high operational reliability are received. Taking into account the obtained value of water temperature drop inside the bearing would be necessary for further analysis of the thermal stability of the spindle during machining. Acknowledgement. The presented results are part of the project "Development of a spin-dle with hydrostatic bearings for ultra-precision machining" (ID 57210233) funded by the DAAD. The authors expresses sincere gratitude to colleagues from Fraunhofer Institute for Production Systems and Design technology IPK and the Institute for Machine Tools and Fac-tory Management IWF of the TU Berlin who provided equipment as well as recommendation and expertise that greatly assisted the research.

References

hydrostatic development machining

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