In the following paper we show the use of a compact laser interferometer to inspect ball screws. Two different systems have been developed, one to be used during ball screw manufacturing, the second one to inspect existing ball screws. The former one is used to measure the screw pitch. Data collected during the measurement are used to calculate the parameters for the screws grinding machine. The second setup is used to evaluate the parameters of the manufactured ball screw. 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
INTRODUCTION
The aim of the project was improving 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 setup of the correction parameters 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 for each single screw.
It means that the automatization of that process is crucial for productivity. On the other hand the final inspection 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.
MEASUREMENT METHODOLOGY
The methodology of measurements was developed on the base of the following norms: PN-84/M-55275, DIN 69051-3(1-4), ISO 3408-3. 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
Symbols
L0 – Nominal travel direction
DL0 – 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
v2Dp – Permissible travel deviation in random 1 revolution 2 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 screw travel deviations
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.
3. Three temperature sensors placed on the ball screw should assure the accuracy of temperature measurement of 0.1°C in the range of 10 – 40 °C.
4. The repeatability of the measurements should be not less than 0.6 micrometers for manual measurements and 1.2 micrometers 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 to the lead of the screw.
13. On the base on measurement the ball screw should be classified in the grade of tolerance.
In fig.2 the grade of tolerance is presented.
Lu ep (Dm)
Grade of tolerance vup (Dm)
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
BALL SCREW INSPECTION SETUP
Two different applications have been developed, 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 the 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. Torque was synchronized with the screw rotation. 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; allowing a resolution of 10 nm.
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 °C, 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 0.1°C. The accuracy of measurement linear displacement measurements was
0.6 + 0. 001 x L, where L - length of screw expressed in mm. The repeatability of manual measurements with the use of probe positioned in the thread groove of the screw was 0.6 μm. 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.
Ball screw inspection setup
The inspection setup of ball screw with torque measurement arrangement
This setup measures the torque in the range from 0.3 to 15 Nm. The measurements are carried on in both directions. Displacement speed is recorded and can be selected. The torque chart is displayed in rael time.
MEASUREMENTS
The investigations of screws in the manufacturing process are performed 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 300 mm, Lu - useful travel, ep - permissible mean travel deviation and vup -permissible travel deviation in useful travel Lu. Local temperature, air pressure and air humidity of the air, and temperature of the screw in three different points are continually registered and used for the laser compensation.
The measurements were carried on second arrangement for the inspection of completed ball screw. The laser measurement system was sinchronyzed with the rotation of the ball screw. Every entire turn eight values of measuring displacement of the nut were registered. The strength of the laser beam is monitored and any misalignment or interruption is immediately reported. The software automatically generates the chart and performs the evaluations.
The conditions of measurement and the parameters of the tested ball screw are continually registered. A specific report database of the manufactured ball screw units allows easy and fast access to all reports. The choice of the ball screw matching the requirements of the client is easy. Reports can be shared on Internet through a standard browser. The measurement of the torque was performed simultaneously. Fig. 8 presents a sample report of such measurements. The displacement is continuously measured by the laser interferometer. The velocity of the movement of the nut is controlled and registered.
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 according to international standards allows comparison of the parameters of produced ball screw with parameters offered by other producers. Particular care should 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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Luca Bochese 卢卡 13717978084
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