READING THE STAR GAGE VERNIER
1 VERNIER SCALES
2 The measurement obtained with a star gage is the variation of the gun diameter from the diameter of a ring gage of a known dimension. Since the ring gage diameter corresponds to the original design diameter of the gun, the reading then is the variation from the original gun diameter. As indicated earlier, this variation is read on the star gage vernier.
3 The vernier is composed of two parts: the fixed scale (internal) on the sleeve of the vernier assembly and the movable vernier (external reads 0 +/- 10 ) on the slide (see below). Two verniers, with a common zero in the center, make up the movable vernier. If the zero on the movable vernier is to the left of zero on the fixed scale, the left-hand vernier is used. The gun diameter in this case is larger than the diameter on the ring gage by the amount indicated by the vernier reading. Conversely, if the zero on the vernier is to the right on the zero on the fixed scale, the right hand vernier is used and the gun diameter is smaller than the ring gage diameter.
4 Each minor graduation on the fixed scale represents a change of 0.010” in the diameter being measured by the star gage. Thus, the variation between the 100 and 200 major graduations on the fixed scale amounts to a change of 0.100”.
5 Each graduation on the movable vernier represents a change of 0.001” in the diameter of the measuring points. Alignment of a mark on the movable vernier with a mark on the fixed scale determines which mark on the movable scale is to be used on the reading. For example, if the second mark on the movable vernier lines up with ANY fixed scale mark then the measurement is 0.002. Similarly, if the sixth mark on the movable vernier mark lines-up with any fixed scale mark, the value is 0.006.
6 The vernier reading for a diameter measurement is the sum of the fixed scale major and minor readings and the mark on the movable vernier.
7 EXAMPLE READINGS.
8 Below, illustrates the vernier reading and shows the calculations for a bore diameter larger than the ring gage diameter. The zero on the movable vernier is to the left of the zero on the fixed scale. The movable scale is also to the left of the 100 mark on the fixed scale (the major reading), and to the left of the 110 mark (the minor reading), which is 1/10th the distance between the 100 and 200 marks. Further, the fifth mark on the movable vernier is lined up with a mark on the fixed scale. Hence, as calculated in below, the variation from the ring gage diameter is 0.115”. The bore is the ring diameter plus 0.115”.
Major: 0.100 Bore diameter is Minor: 0.010 the ring diameter Vernier: 0.005 plus 0.115”. Variation: 0.115
Created by: GM1 Mercurio, USCG, 04/04/18
9 Below, the zero on the movable vernier is to the right on the fixed scale. This indicates that the bore diameter is smaller than the diameter of the ring gage. This is not a normal reading. When it occurs, an investigation should be made to determine the cause. Usually, this can be attributed to metallic build up in the bore caused by projectile rotating bands. Proper preparations for star gaging will normally reveal any build up, which should be removed prior to commencement of gaging operations.
Major: 0.000 Bore diameter is Minor: 0.010 the ring diameter Vernier: 0.008 minus 0.018”. Variation: 0.018
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Created by: GM1 Mercurio, USCG, 04/04/18