Gauge blocks are the most accurate standards of length available outside a laboratory setting. Although the cost of a full set of the most accurate grade of blocks can run to near five figures, the price plummets if just a little less accuracy can be tolerated. A set of blocks of Federal Grade 2 accuracy , can be had for about $1,000, with sets of lesser accuracy even more reasonably priced. For a shop engaged in high precision work on a regular basis, such an expense may be justified.
The most accurate (and most an expense may be justified) blocks are Tresna Calipers, properly handled inside calipers are capable of returning a result more accurate than the 0.01-inch divisions on a rule, and unlike outside calipers, gauging the setting of the calipers does not introduce any particular problems, so a micrometer might be used for this purpose. It must be emphasized, however, that the initial gauging is not likely to be accurate to any closer than 5 thou, and often not even that; in general terms, calipers are best regarded as accessories to a steel rule.
Apart from the fact that calipers are not really precision measuring tools, by our definition, there is the additional problem of determining that tool is truly at right angles to the gap being gauged. Any slight amount of tilt will result in an error, always in the direction of exaggerating the dimension.
Also, when measuring a round hole, it is important to ensure that it is truly a diameter that is being gauged; If the tool is offset even slightly toward one side of the hole, an error that underestimates the true dimension will occur. Achieving decent results with calipers requires a very light touch, generally holding the instrument at the joint with just thumb and forefinger, patiently probing the gap until certain that the right dimension is being gauged. The correct technique is to use one leg as a fixed point and to rock the tool gently back and forth, gauging the drag of the other.
On place where this dirt-cheap, simple (indeed, almost primitive) device shines is in gauging the â€œtruthâ€?of a hole. A worker with a well-developed sense of feel can detect very slight amounts of taper or ovality and, with experience, can estimate the variation in roundness to within a few thou. As with outside calipers, there are both friction and spring-bow types, and ones with a third transfer leg that allows one of the gauging legs to be retracted to clear obstructions outboard of the measuring point, such as when measuring the depth of a snap ring groove within a bore. As with outside calipers, 6- and 12- inch sizes are available.