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Ball screw inspection setup with high accuracy laser interferometer

ABSTRACT In the following paper we describe arrangements of laser interferometer for investigation of screws and for inspection of ball screws. We have constructed two of them, namely: the technological setup for investigations of screw in process of production and the ball screw inspection setup. The former one is used to measure the pitch of screws. The data gathered during measurement is used to calculate the parameters for grinding machine. The later setup is used for testing parameters of complete ball screws. The software supporting this setup makes calculation of parameters of tested ball screw and creation of reports possible. Additionally, the inspection setup is the one that the torque measuring arrangements have been integrated on. Both the arrangements and the software allow for measurements of all parameters during movement of nut in full travel length of the ball screw and make charts and reports. Keywords: ball screw inspection, laser interferometer, pitch inspection, torque/pitch inspection.

1. INTRODUCTION

The aim of the research project was to improve the quality of produced ball screw, speeding up the preparation of the correction parameter for grinding machine in process of production as well as shortening the time of final inspection of produced ball screw. The preparation of the correction parameter for grinding machine and the inspection of the produced ball screw play the key role in quality of production. The preparation of correction parameter based on point-by-point measuring of pitch of screw and elaboration of gathered data usually took more then thirty minutes per one screw. It means that the automatization of that process is crucial for productivity. On the other hand the final inspection

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of complete ball screw consists of measuring several parameters and preparing reports for all produced ball screw. Another thing is that this process had to be shortened by automatization. The screw and the ball screw inspection arrangements were worked out in frames of the project entitled; "Elaboration of methodology and of arrangement for measurements of screw in process of production and inspection arrangement of ball screw according to norms ISO/DIN" No 8 T10C 063 2000C/5124 of Polish Committee of Science.

2. METHODOLOGY OF MEASUREMENT The methodology of measurements was developed according to following norms: PN-84/M-55275, DIN 69051-3(1-4), ISO 3408-3. The measuring procedures and the form of reports have been consulted with the division of quality inspection of Star (Germany) a worldwide producer of screws and of ball screws. Fig.1 presents parameters of ball screw. It was established that after measurements all parameters would be automatically calculated and presented in the form of reports and charts

Terms ; L0 – Nominal travel direction ∆L0 – Travel deviation Lu – Useful travel c – Specified travel for compensation. Customer determines this value as it depends on different application requirements. ep – Permissible mean travel deviation. vup – Permissible travel deviation in useful travel Lu v300p – Permissible travel deviation in random 300 mm v2πp – Permissible travel deviation in random 1 revolution 2π rad Cumulative representative lead - a straight line representing the tendency of the cumulative actual lead. This is obtained by a least square method and measured is by laser system. Figure 1. The graph of travel deviation of screw The main requirements of the arrangements were as follows:

1. The time of preparation of the data for grinding machine, including the time of mounting of the screw on the arrangement should not exceed fifteen minutes.

2. The time of final inspection of ball screw should be within twenty minutes.

Cumulative representative lead

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3. Three temperature sensors placed on the ball screw should assure the accuracy of temperature measurement of 0.1oC in the range of 10 – 40 oC.

4. The repeatability of the measurements should be not less than 0.6 µm for manual measurements and 1.2 µm for dynamic measurements in full measuring range.

5. The measuring range should assure the measurements of the screw up to 5 meters long.

6. The laser head and the measurement optics should be mounted fixedly. 7. The measurements should be carried on without necessity of additional adjusting of

optical path. 8. The optical elements should be mounted in the place where there is no risk of

interruption of the laser beam. 9. The cosine and abbe errors should be taken into account and minimized. 10. The measuring platform should be light, as it will be moved along the screw. 11. The driving gear of the measuring platform should assure the synchronous

movement of the platform with the lead of the screw. 12. The velocity of movements should be easily programmable according the lead of

the screw. 13. On the base on measurement the ball screw should be classified in the grade of

tolerance.

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Following table shows the tolerance parameters and classification.

Lu

ep (µm) Grade of tolerance

vup (µm) Grade of tolerance

> ≤ 1 3 5 1 3 5 315 6 12 23 6 12 23

315 400 7 13 25 6 12 25 400 500 8 15 27 7 13 26 500 630 9 16 30 7 14 29 630 800 10 18 35 8 16 31 800 1000 11 21 40 9 17 35

1000 1250 13 24 46 10 19 39 1250 1600 15 29 54 11 22 44 1600 2200 18 35 65 13 25 51 2000 2500 22 41 77 15 29 59 2500 3150 26 50 93 17 34 69 3150 4000 32 62 115 21 41 82 4000 5000 - 76 140 - 49 99 5000 6300 - - 170 - - 119

Figure 2. Grade of tolerance of ball screw

3. BALL SCREW INSPECTION ARRANGEMENTS

Two arrangements have been constructed, on which modified interferometers LP 30 have been used. The arrangement consists of 6 m long solid optical base on antivibration groundwork and the support of the screw. The retroreflector was placed on the moving platform near the probe. The optics of the interferometer and the laser head were placed on the optical base. The signals from rotary encoder were sent to the electronics of the interferometer and used to control the torque. Eight measurements of translation on one turn of screw were taken. Other arrangement, which was used for completed ball screw inspection, was additionally equipped with setup for torque measurements. Both arrangements were equipped with laser interferometer LP30. The interferometers were used in linear displacement measurement configuration with dynamic gathering of the data. The signal from rotary encoder was used to control the rate of measurements. The moments of the measurements were synchronized with turn of the screw. For manual measurements the remote control of the moment of the measurement was supplied. The source of radiation of the interferometer was He-Ne Zeeman laser with 1 mW exit power. In frequency stabilization loop the cell with surface stabilized ferroelectric liquid crystal (SSFLC) was used [1]. The frequency stabilization system assures repeatability of laser frequency on the level of 2x10-8. The interferometer is two-frequency interferometer with frequency difference of about 1.2 MHz. The arrangements of PLL in measuring and reference path multiply by 32 the frequency shift; in this way the resolution of 10 nm is obtained.

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Figure 3. The measuring stage with the probe The system was equipped with the environment parameter compensation station. The environment station measured the temperature with accuracy of 0.1 oC, the pressure with accuracy of 1 hPa and the humidity with accuracy 5%. Additionally, three small size temperature sensors were placed on the screw. The accuracy of temperature of screw measurements was 0.1oC. The accuracy of measurement linear displacement measurements was 0.6 + 0. 001 x L, where L is the screw length in mm. The repeatability of manual measurements with the use of probe positioned in the grave of the thread of the screw was 0.6 µm (Fig.6). The accuracy of manual measurement depended on technical skill of the operator. The tested screw was measured with the probe with appropriate curvature. The accuracy of the dynamic measurements depended on the precision of mounting of screw on the optical base and on the quality of the thread of the screw. Fig. 5 presents the inspection setup of ball screw with integrated arrangement for resistant moment measurements.

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Figure 4: The ball screw inspection arrangement.

Figure 5. The inspection setup of ball screw with torque measurement arrangement This arrangement allows for the measurements of the resistant moment in the range from 0.3 to 15 Nm. The measurements are carried on in two directions of movements. The velocity of movements is monitored and could be changed. The chart of measured resistant moment is drawed in real time.

4. THE MEASUREMENTS

The investigations of screws in process of production are conducted on the technological setup with manual measurements of basic parameters of screws. The main goal of the measurements is preparation of correction parameters for grinding machine. The results of manual measurements are presented in fig. 6. The measurements were carried on by translating the stage with measuring probe by the step of the thread and after that the probe was positioned in the thread of the screw. The measurement was accepted by remote control. The basic parameter measured on arrangement was the lead of screw. The software automatically calculates: the v300p -permissible travel deviation in random

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300 mm, Lu - useful travel, ep - permissible mean travel deviation and vup -permissible travel deviation in useful travel Lu. The temperature, the pressure and the humidity of the air and temperature of the screw are registered.

Figure 6. The report of the screw measurements The measurements of mounted ball screw were carried on second arrangement for the inspection of completed ball screw. Measuring system of the interferometer was synchronized with rotation of the ball screw. Every entire turn eight values of measuring displacement of the nut were registered. The strength of laser beam is monitored and any misalignment or interruption of it is signalized. The software automatically generates the chart and calculates the parameters like in manual measurements. Figure 7.The report of the ball screw inspection.

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Figure 8: The report of torque of the screw measurement The conditions of measurement and parameter of the tested ball screw are registered. Developed database of produced ball screw allows quick accesses to all reports. The choice of the ball screw matching the requirements of the client is easy. The link to the internet simplifies the shipping of the reports. The measurements of torque were carried on at the same moment. Fig. 8 presents a sample report of such measurements. The displacement is measured by interferometer. The velocity of the movement of the nut is controlled and registered.

5. CONCLUSIONS

The development of the arrangements for inspection of screw and ball screw was a significant step forward to high quality production. The time of preparation of the parameter for grinding machine was shortened two times. The production of the ball screw in grades 1 and 3 was enlarged by 40%. The development of the methodology of the inspection and software for calculation of parameters conforming international norms allow for comparison of the parameters of produced ball screw with parameters offered by other producers. The care must be taken when positioning the ball screw on measuring mount. The tilt of the screw during dynamic measurement is the significant source of error.

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