rus
Issue #7/2017
S.Stepanov, S.Tarasov, A.Petrov, S.Stepanov
Investigation of error of UKM-1000 comparator for calibration of gauge blocks of length over 100 mm
Investigation of error of UKM-1000 comparator for calibration of gauge blocks of length over 100 mm
A study of the accuracy of the Russian UKM-1000 comparator for calibration of gauge blocks of length over 100 mm was made on the basis of the international method of calibration of gauge block comparators.
The gauge blocks (GB) are intended to be used as working gauges for the adjustment and calibration of measuring equipment and for the direct measurement of the linear dimensions of industrial products, as well as reference measures for transferring a unit of length from a primary standard to gauge blocks of lower accuracy and for calibration and graduation of measuring instruments.
At machine-building enterprises, where large parts are machined, measuring instruments with large measuring ranges are used. For verification and adjustment of these devices, GB longer than 100 mm are used. Standard GB kits No. 8 and No. 9 include blocks of 100–1000 mm in length. To verify (calibrate) such gauge blocks, appropriate reference measurement tools are required.
Special instruments for checking and calibrating gauge blocks of this size are not currently available, therefore, as a rule, universal horizontal instruments are used, for example, developed by Mahr (Germany). However, because of their versatility and multifunctionality, such devices are very expensive. In Russia, enterprises and centers for standardization and metrology use optical-mechanical machines, interferometers and horizontal optometers that were manufactured earlier and adapted for checking large gauge blocks.
The Mikro engineering and metrology center together with Peter the Great St.Petersburg Polytechnic University within the program of import substitution developed and manufactured the first in Russia specialized UKM-1000 system for checking GB over 100 mm in length. The system is a workplace of the verifier and consists of a laboratory table, a comparator, a vacuum station and a computer. At present, this project is at the stage of testing.
The state verification scheme is applied to the GB in accordance with GOST R 8.763-2011 (Part 3), which sets for the working standards the grades from the 1st to the 4th. The values of the δ-confidence limits of the absolute errors with a confidence probability of 0.99 (± 3σ) determine the accuracy of the standards by the grades. However, the methods for checking GB and other standards do not provide for the calculation of σ. In addition, the accuracy of transferring the size of the standard from grade to grade is given by δ-, which is not defined in the standard.
At the same time, verification schemes are not applied in the western countries and the determination of the capabilities and purpose of measuring instruments is performed by means of calibration with the calculation of the value of its uncertainty. Thus, the standardization and evaluation of the accuracy of similar measuring instruments in our country and in the west are different.
In connection with the current situation, the accuracy of the UKM-1000 system was tested using the international calibration method "Calibration of gauge block comparators" used by all leading manufacturers of comparators for the certification of gauge blocks up to 100 mm in length. The method describes the calibration process of the comparator in measuring the central length and the plane-parallelity of the gauge being verified. It is intended for comparators for calibration by comparison, by contact method and using two modifiers in contact with the gauge block from opposite sides. UKM-1000 corresponds to the specified conditions, but verified gauge blocks from 100 to 1000 mm long are located on the comparator table horizontally.
The developed calibration technique is based on the use of four pairs of gauge blocks of the 1st grade (Table 1).
Before the calibration, the system was prepared in accordance with GOST 8.336-78.
COMPARISON OF CENTRAL LENGTHS OF GAUGE BLOCKS
Gauge blocks of 1.0, 1.01 and 1.04 mm were stacked in pairs in a special device, and blocks of 100.0 and 1000.0 mm were placed on the comparator table, aligned at the end and clamped with a clamp. Then, five comparative measurements of the central lengths of all four pairs were performed with adjusting to 0 using gauge block A. Then the series of measurements was repeated, but with adjusting to 0 using gauge block B. For each series of measurements, mean values and standard deviations were calculated.
MEASUREMENT
OF PLANE-PARALLELISM
A 100.0 mm gauge block was fixed on the comparator table and a series of four measurements was taken from the central length of the block to the corners A, B, D, E according to the scheme shown in Fig.2. Measuring series were repeated after turning the gauge block to 180º in the vertical plane. For each of the eight series, the mean values and standard deviations were calculated.
EVALUATION OF RESULTS
The evaluation of the results depends on the accuracy of gauge blocks, which must be calibrated using a comparator. In the methodology, the acceptance criteria are considered from the condition that the comparator is used to calibrate gauge blocks with extended measurement uncertainty U = 0.1 + 1L for k = 2, which corresponds to the gauge blocks of the 3rd grade. Under these conditions, the following criteria are established:
repeatability should not exceed 0.05 μm;
standard deviations should not exceed 0.03 μm;
deviations of the average values and the greatest deviations of the plane-parallelity from the values of the reference gauge blocks specified in the verification protocol should not exceed 0.05 μm.
The measurement protocol in accordance with the described calibration procedure is shown in Tables 2 and 3.
CONCLUSIONS
The conducted researches showed that the UKM-1000 system with its accuracy characteristics on the basis of calibration corresponds to comparators for verification of the gauge blocks of the 3rd grade.
The method of "Calibration of gauge block comparators", applied by all leading manufacturers of comparators for certification of gauge blocks, is more informative than previous methods. ■
At machine-building enterprises, where large parts are machined, measuring instruments with large measuring ranges are used. For verification and adjustment of these devices, GB longer than 100 mm are used. Standard GB kits No. 8 and No. 9 include blocks of 100–1000 mm in length. To verify (calibrate) such gauge blocks, appropriate reference measurement tools are required.
Special instruments for checking and calibrating gauge blocks of this size are not currently available, therefore, as a rule, universal horizontal instruments are used, for example, developed by Mahr (Germany). However, because of their versatility and multifunctionality, such devices are very expensive. In Russia, enterprises and centers for standardization and metrology use optical-mechanical machines, interferometers and horizontal optometers that were manufactured earlier and adapted for checking large gauge blocks.
The Mikro engineering and metrology center together with Peter the Great St.Petersburg Polytechnic University within the program of import substitution developed and manufactured the first in Russia specialized UKM-1000 system for checking GB over 100 mm in length. The system is a workplace of the verifier and consists of a laboratory table, a comparator, a vacuum station and a computer. At present, this project is at the stage of testing.
The state verification scheme is applied to the GB in accordance with GOST R 8.763-2011 (Part 3), which sets for the working standards the grades from the 1st to the 4th. The values of the δ-confidence limits of the absolute errors with a confidence probability of 0.99 (± 3σ) determine the accuracy of the standards by the grades. However, the methods for checking GB and other standards do not provide for the calculation of σ. In addition, the accuracy of transferring the size of the standard from grade to grade is given by δ-, which is not defined in the standard.
At the same time, verification schemes are not applied in the western countries and the determination of the capabilities and purpose of measuring instruments is performed by means of calibration with the calculation of the value of its uncertainty. Thus, the standardization and evaluation of the accuracy of similar measuring instruments in our country and in the west are different.
In connection with the current situation, the accuracy of the UKM-1000 system was tested using the international calibration method "Calibration of gauge block comparators" used by all leading manufacturers of comparators for the certification of gauge blocks up to 100 mm in length. The method describes the calibration process of the comparator in measuring the central length and the plane-parallelity of the gauge being verified. It is intended for comparators for calibration by comparison, by contact method and using two modifiers in contact with the gauge block from opposite sides. UKM-1000 corresponds to the specified conditions, but verified gauge blocks from 100 to 1000 mm long are located on the comparator table horizontally.
The developed calibration technique is based on the use of four pairs of gauge blocks of the 1st grade (Table 1).
Before the calibration, the system was prepared in accordance with GOST 8.336-78.
COMPARISON OF CENTRAL LENGTHS OF GAUGE BLOCKS
Gauge blocks of 1.0, 1.01 and 1.04 mm were stacked in pairs in a special device, and blocks of 100.0 and 1000.0 mm were placed on the comparator table, aligned at the end and clamped with a clamp. Then, five comparative measurements of the central lengths of all four pairs were performed with adjusting to 0 using gauge block A. Then the series of measurements was repeated, but with adjusting to 0 using gauge block B. For each series of measurements, mean values and standard deviations were calculated.
MEASUREMENT
OF PLANE-PARALLELISM
A 100.0 mm gauge block was fixed on the comparator table and a series of four measurements was taken from the central length of the block to the corners A, B, D, E according to the scheme shown in Fig.2. Measuring series were repeated after turning the gauge block to 180º in the vertical plane. For each of the eight series, the mean values and standard deviations were calculated.
EVALUATION OF RESULTS
The evaluation of the results depends on the accuracy of gauge blocks, which must be calibrated using a comparator. In the methodology, the acceptance criteria are considered from the condition that the comparator is used to calibrate gauge blocks with extended measurement uncertainty U = 0.1 + 1L for k = 2, which corresponds to the gauge blocks of the 3rd grade. Under these conditions, the following criteria are established:
repeatability should not exceed 0.05 μm;
standard deviations should not exceed 0.03 μm;
deviations of the average values and the greatest deviations of the plane-parallelity from the values of the reference gauge blocks specified in the verification protocol should not exceed 0.05 μm.
The measurement protocol in accordance with the described calibration procedure is shown in Tables 2 and 3.
CONCLUSIONS
The conducted researches showed that the UKM-1000 system with its accuracy characteristics on the basis of calibration corresponds to comparators for verification of the gauge blocks of the 3rd grade.
The method of "Calibration of gauge block comparators", applied by all leading manufacturers of comparators for certification of gauge blocks, is more informative than previous methods. ■
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