Slip gauges are calibrated by direct comparison with calibration grade gauges using a comparator. Slip gauges need to be calibrated at regular intervals, since even a slightly worn out slip gauge can create havoc at the quality control stage, resulting in increased scrap losses. NPL has recommended schedules for the calibration of different grades of slip gauges. Notwithstanding regular calibration schedules, a slip gauge is a candidate for recalibration under the following conditions:
1. Visual inspection shows wear beyond permissible level.
2. Wringing becomes difficult.
3. An unusual increase in rejections occurs during quality inspection.
Working slip gauge blocks are calibrated using master sets. The master gauges, in turn, are calibrated by grand masters, which are maintained by the National Bureau of Standards. In addition, usually all manufacturers of gauge blocks provide calibration services. In most of the advanced countries, there are independent metrology laboratories that mainly deal with providing calibration services. Such a service is conspicuous by its absence in India.
Fig.1 Calibration of Slip Gauge
It is of academic interest to know the different types of comparators used for calibrating slip gauges. The popular ones are the Brook-level comparator, Eden-Rolt millionth comparator, and the NPL-Hilger interferometer. The working principle of the Brook-level comparator is explained to give flair of the significance of instrumentation for calibration of slip gauges.
The Brook-level comparator works in a way similar to a spirit level, but is more sophisticated and highly sensitive. It compares differences in the levels of two objects to a sub-micron-level accuracy. The comparator comprises a sensitive level mounted in an aluminium casing. The underside of the casing has two steel balls, each of which rests on top of the two objects whose heights are to be compared. Whenever there is a variation in height, the bubble moves over a graduated scale, which has a resolution of 1 μm or 0.1 μm. The slip gauge to be calibrated (test gauge) and the reference calibration gauge (standard) are kept on top of a surface plate. It is needless to say that the surfaces of the two slip gauges and the surface plate should be thoroughly cleaned and positioned so that there is no additional layer of dust, oil, etc., which will reduce the accuracy.
Fig.2 Brook-level comparator
The Brook-level comparator is kept on top of this pair of slip gauges such that one steel ball rest on the test gauge and the other on the standard, as shown in Fig. 2. The reading on the graduated scale on the top face of the comparator is noted. Let us say this reading is 12.2.
Now, the comparator is lifted up and the surface plate is given a 180° rotation. The Brook-level comparator is again lowered onto the two slip gauges, such that the two contact points are reversed. The new reading is noted down. Let us say the second reading is 12.0. If the resolution of the comparator is 0.00001 mm, then the difference in length is calculated as follows:
Difference in length = (0.0001) × (12.2 − 12.0)/2 = 0.00001 mm