The supreme discipline of length measurement technology

The globalized economy increasingly demands interchangeable production parts. To achieve this, however, producers, dealers and users must "measure with the same yardstick". In order to guarantee this, measuring and testing equipment is brought back to national standards through calibration in certified and accredited testing and calibration laboratories (2). Precision measuring instruments used in industrial production, such as calipers and outside micrometers, as well as measuring equipment such as gauges and gauge blocks, must be calibrated regularly (usually annually) in DAkkS-accredited calibration laboratories. Companies that do not maintain their own DAkkS laboratories commission external service providers for this purpose.

One such service provider is the Kassel-based testing and calibration laboratory of PMK-GmbH (Prüfen+Messen+Kalibrieren). PMK-GmbH (Prüfen+Messen+Kalibrieren) calibrates both their measuring instruments and final masses on behalf of a wide range of companies. The owner-managed company is accredited by the Deutsche Akkreditie-rungsstelle GmbH (DAkkS) for the measurands length and torque (accreditation number D-K-17519-01-00) and offers the ISO calibration of nearly all length measuring instruments according to the corresponding standards, guidelines (VDI/VDE/DGQ 2618), manufacturer specifications and PMK calibration instructions. PMK's quality ISO calibrations are optimized within the framework of quality assurance according to DIN EN ISO 9000ff. Of course, all test equipment used by PMK is traceable to national and international standards. "We calibrate the measuring equipment and devices in such a way that they have a clear status after calibration. This tells the customer whether the test equipment complies with the manufacturer's specifications or the standard specifications," explains Dipl.-Ing. Uwe Schmahl, managing partner of PMK-GmbH (Prüfen+Messen+Kalibrieren).

Challenge gauge block
The calibration of the measuring equipment used in production is carried out using special gauges and gauge blocks. Gauge blocks are available in various forms, including parallel, angular, spherical or cylindrical gauge blocks, as well as in the four accuracy classes K, 0, 1 and 2 (see info box). Parallel gauge blocks are used primarily for the calibration of length measuring equipment. They are standardized cuboidal measuring bodies - usually made of steel with a honed surface of the highest quality, often also made of hard metal, ceramics or glass. Parallel end masses embody, within the scope of their classification, the 100-percent accuracy of a given length. They are usually offered in sets of 32 to 121 individual gauge blocks of various lengths and gradations.

To calibrate a micrometer or outside micrometer or a caliper gauge, the calibration technician pushes together the number of gauge blocks required to obtain the desired measurement. Since the gauge blocks are subject to a certain amount of wear due to permanent use, they must also be calibrated regularly. PMK-GmbH (Prüfen+Messen+Kalibrieren) uses for this calibration the gauge block test station EMP II from Feinmess Suhl (FMS), one of the oldest manufacturers of mechanical and electronic precision measuring technology in Germany.

Calibration of the final mass
The comparative gauge block test station EMP II from FMS for accuracy classes 0, 1 and 2 - class K is achieved exclusively by the Physikalisch-Technische Bundesanstalt using laser testing methods - consists of a heavy cast base with positioning and measuring unit, a motorised switching unit and an interface unit. Its decentralized design minimizes thermal influences on the measurement result. The additional cover for rear ventilation of the temperature-stable measuring stand also contributes to this. The calibration of the gauge block is carried out by comparing the gauge block to be calibrated with a reference gauge block which should be at least one accuracy class more accurate than the gauge block to be calibrated. For calibration, the gauge block to be calibrated is set up in the front test station of the EMP II. The rear test station takes the reference gauge block. The gauge blocks are lifted and not - as with other test stations - pushed back and forth on a guide rail. The lifting prevents mechanical abrasion and avoids scratching of the gauge block surfaces.

The measurement uncertainty of the FMS final mass inspection station is 0.03 µm + 0.002∙D (for a length difference of D ≤ 10 mm). "A pre-cision of only 30 nanometers - that is, of only 30 millionths of a millimeter - in a tactile measurement process is enormous," says Dipl.-Ing. Uwe Schmahl. However, with these high measurement accuracies, the environmental conditions in the calibration laboratory must be exactly right and consistent. In addition, the calibration objects and reference gauge blocks are tempered the day before. Last but not least, the measuring station must be completely protected - even against the operator's breath. Since touching the gauge block by hand would heat it up and thus destroy the calibration, the gauge blocks are inserted into and removed from the test station exclusively with the appropriate tongs (supplied with the EMP II). In this way, falsification of the measurements can be prevented.

Time expenditure is often underestimated
"The effort of calibration is often underestimated. But it simply takes time. This is necessary to achieve the excellent results that German industry demands," explains Dipl.-Ing. Uwe Schmahl and continues: "Precisely because calibration is so time-consuming, many companies outsource this task to external service providers like us.

In total, two and a half to three days must be estimated for the entire process, depending on the size of the final mass set. Although the EMP II gauge block test station is the test station with the fastest handling thanks to its precise splitter guide and the actual calibration takes only one day, the preparation of the calibration objects requires a lot of time and care. This is because the final masses are usually oily or dirty and must first be cleaned in a time-consuming process before calibration. Afterwards, the final masses are entered into the test equipment monitoring system, whereby the identification number of each individual final mass is recorded. Only then are the gauge blocks calibrated. Finally, they are greased and thus protected against corrosion. Thus, depending on the design, about 150 set calibrations per year are possible per gauge block test station.

Uncomplicated handling and good service
PMK-GmbH (Prüfen+Messen+Kalibrieren) has been using an FMS comparative gauge testing station in its calibration laboratory since 2013, but has already ordered another EMP II to cope with the high volume of calibrations. In addition to the EMP II, the Kassel calibration laboratory also uses other calibration systems from FMS, including the BMG 3000 strip mass tester, the MFP 30 BV fully automatic dial gauge and fine pointer tester and the KLM 60 small length gauge. The team at PMK-GmbH (Prüfen+Messen+Kalibrieren) particularly appreciates the simple, uncomplicated handling of the devices, which are compatible with many common test equipment management software programs, and the good service from FMS.