PROBE
Hot Rolled
- Joined
- Jan 23, 2003
- Location
- Tel Aviv, Israel
During my 50 years long professional career I performed and supervised over 1000 calibration and certification activities of CNC machines. In vast majority of cases these activities were executed in order to prove that machine accuracy is conforming with given standard. The machine geometric accuracy (straightness, squareness, flatness) is referred by machine producers to ISO standards: ISO230-1 and ISO10791-2. These standards determine the expected result of the test (10791-2) and how and using which tools the test should be performed (230-1).
Almost all known to me machine producers, and specially the BIG O-M SOCIETY, emphasize, that accuracy of their machines exceeds by far the ISO 10791-2 requirements.
Recently I had opportunity to witness the certification activity of one the BIG O-M machines, which I was supposed to approve. The activity was performed by engineering staff of producer’s official representative. The “Certificate of accuracy”, furnished by producer, states, that all tests should be performed in accordance with ISO230-1 directions. The straightness allowed error, although specified by ISO10791-2 to 15 micron over 800 millimeters, was narrowed by machine producer to 7 microns over 800 millimeters. The test was performed using 00 class 600 MM long granite straight edge and Mitutoyo model 2110S-10 dial indicator.
According to standard DIN 876 class 00 granite straight edge 600 mm length assures straightness accuracy of 2.6 microns.
The data below is taken from Mitutoyo specifications chart of 2110S-10 indicator:
GRADUATION: 0.001 MM
RANGE: 1 MM
ACCURACY OVERALL: +/- 5 MICRON
ACCURACY RETRACE: 3 MICRONS
ACCURACY 1/10 REVOLUTION: 2.5 MICRONS
ACCURACY 1 REVOLUTION: +/- 4 MICRONS
REPEATABILITY: .5 MICRON
The accuracy of measurement in range of 1/10 revolution (disregarding retrace error) is 2.5 microns.
According to ISO230-1, “The measuring instrument should not cause any error of measurement exceeding a given fraction of the tolerance to be verified”. It is well known to any machinist, that accuracy of measuring instrument should be at least 10 times higher then measured tolerance. This is well expressed in web site
WHAT WHEN HOW, section MEASURING INSTRUMENTS (METROLOGY) : “If a measurement is desired to an accuracy of 0.01 mm, then instrument with accuracy of 0.001 mm should be used for this purpose”.
Summarizing, in order to fulfill machine producer’s CERTIFICATE OF ACCURACY statement, that tests should be performed in accordance with ISO requirements, the combined accuracy of the Mitutoyo indicator and granite straight edge should not exceed 0.7 microns !!!
Interesting was the discussion with certification team of producer’s representatives, once the above was presented to them. “But all the world is doing certifications exactly this way, using this kind of equipment !”.
May be. But here I was asked to approve, and I refused to do so. Standards are to be observed, and if one can not do that, he should not refer to them.
Almost all known to me machine producers, and specially the BIG O-M SOCIETY, emphasize, that accuracy of their machines exceeds by far the ISO 10791-2 requirements.
Recently I had opportunity to witness the certification activity of one the BIG O-M machines, which I was supposed to approve. The activity was performed by engineering staff of producer’s official representative. The “Certificate of accuracy”, furnished by producer, states, that all tests should be performed in accordance with ISO230-1 directions. The straightness allowed error, although specified by ISO10791-2 to 15 micron over 800 millimeters, was narrowed by machine producer to 7 microns over 800 millimeters. The test was performed using 00 class 600 MM long granite straight edge and Mitutoyo model 2110S-10 dial indicator.
According to standard DIN 876 class 00 granite straight edge 600 mm length assures straightness accuracy of 2.6 microns.
The data below is taken from Mitutoyo specifications chart of 2110S-10 indicator:
GRADUATION: 0.001 MM
RANGE: 1 MM
ACCURACY OVERALL: +/- 5 MICRON
ACCURACY RETRACE: 3 MICRONS
ACCURACY 1/10 REVOLUTION: 2.5 MICRONS
ACCURACY 1 REVOLUTION: +/- 4 MICRONS
REPEATABILITY: .5 MICRON
The accuracy of measurement in range of 1/10 revolution (disregarding retrace error) is 2.5 microns.
According to ISO230-1, “The measuring instrument should not cause any error of measurement exceeding a given fraction of the tolerance to be verified”. It is well known to any machinist, that accuracy of measuring instrument should be at least 10 times higher then measured tolerance. This is well expressed in web site
WHAT WHEN HOW, section MEASURING INSTRUMENTS (METROLOGY) : “If a measurement is desired to an accuracy of 0.01 mm, then instrument with accuracy of 0.001 mm should be used for this purpose”.
Summarizing, in order to fulfill machine producer’s CERTIFICATE OF ACCURACY statement, that tests should be performed in accordance with ISO requirements, the combined accuracy of the Mitutoyo indicator and granite straight edge should not exceed 0.7 microns !!!
Interesting was the discussion with certification team of producer’s representatives, once the above was presented to them. “But all the world is doing certifications exactly this way, using this kind of equipment !”.
May be. But here I was asked to approve, and I refused to do so. Standards are to be observed, and if one can not do that, he should not refer to them.