Influence of guideways misalignment on the positioning precision of kinematic feed chains used on CNC machine tools

FUNARU Marian1, a, STAN Gheorghe1,b 1 “Vasile Alecsandri” University of Bacau, Department of Industrial Engineering, Calea Marasesti

Street, No. 157, 600115 Bacau, Romania amarian_funaru@yahoo.com, bghstan@ub.ro

Keywords: CNC machine tool, kinematic feed chain, guidance system, alignment errors, positioning precision

Abstract. The main goal of the kinematic feed chains used on numerically controlled machine tools is to ensure the relative position between the manufactured part and the cutting tool tip with a high accuracy. Therefore, the positioning precision is a major performance criterion for CNC machine tools. The guidance system of the machine table, together with the actuation system and the closed-loop control system, represents a key factor in manufacturing high precision machine tools. Mounting and alignment errors of guideways, both in vertical and horizontal directions, directly affect the table positioning precision and implicitly, the manufacturing precision of the machine tool. Guideways misalignment also generates stresses, plays and excessive run-out. The present paper contains a method of experimental analysis regarding the influence of guideways alignment errors on the positioning precision of the kinematic feed chains, which takes into account the alignment of the two guide rails, both in vertical and horizontal plans. The feed axis used in the experimental setup is closed-loop controlled, making use of an indirect position measurement system.

Introducere

Guidance systems used for achieving the linear motion of the moving table, together with the driving system and the control system, represent a key factor in designing and building high precision CNC machine tools. Mounting and alignment errors, both in vertical and horizontal plans, of the linear guiderails, which are part of the kinematic feed chain mechanical structure, directly affect the positioning precision of the moving table and implicitly, the manufacturing precision of the machine tool. Also, guideways misalignment leads to the appearance of stresses, plays and excessive wear [1].

Geometric errors of guideways have a significant influence on the difference which occurs between the real position of the moving table and the programmed one and also on the tool orientation in relation to the workpiece. These errors are part of the volumetric error of the machine tool as a whole. Beside the geometric errors, the volumetric error is given by the temperature increase of the machine tool structure, which leads to thermal deformations, by the elastic deformations of the elements which form the mechanical part of the kinematic feed chains, induced by the resistant forces and also by the performances of the numerical control equipment [2,3].

Knowing the optimum domain within which the guideways misalignment errors range is very useful for developping and implementing a software compensation algorithm, leading to an increased positioning precision of the moving table. The efficiency of the software compensation method was proved especially for large machine tools, having the table length bigger than 1200mm [4].

Measuring the machine tool geometric error can be done directly, using a laser interferometer or an electronic level, in which case the elements of the measuring system are placed on the moving table, or indirectly, by using the ballbar test. Such a method was developed by Yang (2004), where the end of the ballbar was placed on different points of the table [5].

The present paper contains a method of experimental analysis regarding the influence of guideways alignment errors on the positioning precision of the kinematic feed chains, both in vertical and horizontal plans. During the experiments, different values of the parallelism error in both plans were introduced, measuring the positioning precision of the table for each case. The feed axis used in the experimental setup is closed-loop controlled, making use of an indirect position measurement system.

Experimental design and setup

Experimental researches have been conducted on a test stand, having the structure presented in figure 1. The kinematic feed chain is mainly composed of the electromechanical actuator 1, having the rod 5 fixed against the moving element 8. The table has the dimensions of 250x400mm and the axial guiding is achieved using two linear rolling cylindrical guiderails 4, with the dimensions of Ø20x1000mm, provided with several supporting elements 3, with the purpose of assuring a high stiffness of the table-guideway assembly.

The structure of the electromechanical actuator is mainly composed of a precision ballscrew (precision class 7, in conformity with ISO 3408), having the diameter of 20mm and a 5mm pitch. The ballscrew is supported by two sets of angular contact ball bearings on the motor side. Using this type of bearing allows the development of high axial forces, reaching a maximum of 9300N, in both moving ways and also leads to obtaining a minimum backlash when reversing the moving direction. On the side opposite to the motor, the ballscrew is supported by a polymer sliding bearing, which has the advantages of a high service life and a vibration free functioning.

Fig.1. General view of the experimental test stand

Description of the experimental method

Several international standards are used for determining the positioning precision of the numerically controlled axes from CNC machine tools, from which the most commonly used in the present time are VDI/DGQ 3441 and ISO 230:2. These standards establish the methodology regarding the testing, testing conditions and evaluation procedure for processing the measuring results. Testing procedure is based on repeated measurements of the effective position of the tested feed axis, discreted in several points (target positions), placed at equal distances along the table stroke.

For measuring the positioning precision of the researched feed axis, a calibrated measuring system consisting of a Renishaw ML 10 laser interferometer was used, presented in figure 2. The evaluation of the positioning precision of the moving table, in conformity with VDI/DGQ 3441, is done through the following main parameters: positioning inaccuracy P, mean repeatability Ps and

mean reversal error U. It is necessary for the precision parameters to be determined using statistical methods, because of the large number of measuring points and measuring runs which are done for each point. This is required in order to evaluate the evolution of the position deviations with high accuracy.

Fig. 2. Renishaw ML10 laser interferometer used for experiments 1- laser source; 2- linear interferometer; 3- linear reflector; 4 - ball bearing; 5- rolling linear guiderail

Experimental results and discussion

During the experiments, two cases of guideways misalignment were taken into account: in the vertical and horizontal plan. Thus, different values of the parallelism deviation were introduced in both plans and measurements of the positioning precision were made for each case. The parallelism deviation is graphically represented in figure 3.

Fig. 3. Parallelism deviations of guideways: a. horizontal plan; b. vertical plan

In order to determine the influence of the guideways misalignment in vertical plan on the

positioning precision of the moving table, three different values of the parallelism deviation were introduced, by ranging the height of the guideways supporting elements. The values of the deviations, together with the working parameters of the kinematic feed chain and the resulted positioning precision parameters for each case are given in table 1. From the experimental data which was acquired, the positioning precision diagrams were plotted and are given in figure 4.

Table 1. Kinematic feed chain parameters for different parallelism deviations in vertical plan

dV

[mm]

Mm [Nm]

Fa [N]

I [A]

a [mm/s2]

V [m/min]

P [µm]

sP [µm]

U [µm]

0.3 0.197 225 0.164 100 6 30.548 2.640 0.830 0.5 0.197 225 0.164 100 6 50.394 2.671 1.103

0.7 0.197 225 0.164 100 6 56.068 2.722 1.963

In order to determine the influence of the guideways misalignment in horizontal plan on the positioning precision of the moving table, three different values of the parallelism deviation were

introduced, by ranging the distance between the guideways supporting elements. The values of the deviations, together with the working parameters of the kinematic feed chain and the resulted positioning precision parameters for each case are given in table 2. From the experimental data which was acquired, the positioning precision diagrams were plotted and due to the fact that the curves present the same trends as in the anterior case, they are not included in the paper.

Table 2. Kinematic feed chain parameters for different parallelism deviations in horizontal plan

dH [mm]

Mm [Nm]

Fa [N]

I [A]

a [mm/s2]

V [m/min]

P [µm]

sP [µm]

U [µm]

0.5 0.207 237 0.172 100 6 30.548 2.640 0.830 0.7 0.214 245 0.178 100 6 46.662 1.867 0.590 0.9 0.221 253 0.184 100 6 57.676 3.096 0.980

a.

b.

c.

Fig. 4. Positioning precision diagrams, for parallelism deviations in the vertical plan a. dV = 0.3mm, b. dV = 0.5mm, c. dV = 0.7mm

Conclusions

From the analysis of the obtained experimental data and diagrams in the case of guideways misalignment in the vertical plan, an increase of 20µm of the positioning inaccuracy was recorded, when increasing the parallelism deviation value from 0.3 to 0.7mm. The other two static parameters, the mean repeatability and the mean reversal error, respectively, have ranged in a very small interval and have reduced values, of maximum 3µm in the case of repeatability and 2µm in the case of mean reversal error.

In the case of the guideways misalignment in horizontal plan, the obtained positioning precision diagrams show a significant increase of the positional deviation, which can be seen especially in the values of the positioning inaccuracy, which varies between 30 and 57µm for a deviation of 0.5 and 0.9mm, respectively.

The obtained results form a useful experimental database for the designers and builders of CNC machine tools when establishing the maximum values of the guideways mounting errors. Also, the experimental results represent a guide for selecting the type of guideways which are to be used, the number of carriages, the number of circuits and also the type of rolling elements.

References

[1] Fujita, T., Matsubara, A., Yamazaki, K., Experimental characterization of disturbance force in a linear drive system with high-precision rolling guideways, International Journal of Machine Tools and Manufacture, vol. 51 (2011), pp. 104–111. [2] Paweł Majda, Modeling of geometric errors of linear guideway and their influence on joint kinematic error in machine tools, Precision Engineering 36 (2012), pp. 369– 378. [3] Brecher C, Utsch P, Klar R, Wenzel C. Compact design for high precision machine tools. International Journal of Machine Tools & Manufacture 50 (2010), pp. 328–34. [4] Schwenke H, Knapp W, Haitjema H, Weckenmann A, Schmitt R, Delbressine F., Geometric error measurement and compensation of machines – an update. CIRP Annals – Manufacturing Technology 57 (2008), pp. 660–75. [5] Yang SH, Kim KH, Park YK, Lee SG. Error analysis and compensation for the volumetric errors of a vertical machining centre using a hemispherical helix ball bar test. International Journal of Advanced Manufacturing Technology 23 (2004), pp. 495–500.