The following examples of measuring instruments , based on the application of the micrometer screw as the measuring member, should illustrate the wide uses of the micrometer principle.
Indicator micrometers . The narrow range indicator , whose mechanism is coupled with the movable anvil of the micrometer frame, has graduations in 100-microinch or 50-microinch increments. The smallest graduation of the thimble is in thousandths and the sleeve of the illustrated type carries no venier graduations. There are other models that have verniers on the sleeve ; in these the indicator acts as a fiducial gage.
When making measurements with the instrument without vernier, the micrometer screw is advanced to the thousandth mark nearest to the final size as signaled by the movement of the indicator pointer from its rest position . At this point , the measured size is read by combining the size shown on the thimble and the position of the indicator pointer..
The built-in indicator improves the repeat accuracy of the micrometer measurements and adds to the versatility of the instrument applications because of the following:
a. The constant measuring force , as checked by the pointer movement , is particularly valuable for comparative and repetitive measurements;
b. Measuring errors caused by mistakes in evaluating the venier position are eliminated;
c. The micrometer can be used as an adjustable snap gage with added indicating potential , when the range of size variations does not exceed the measuring spread of the indicator . A lever permits retracting the anvil while the object is introduced so as not to mar the work surface. The adjustable indexes on the indicator sector can be set to the limit sizes of the object; and
d. False diameter measurements due to a slanted position of the micrometer , Instead of it being strictly normal to the work axis, are avoided.( The minimum reading is observed on the indicator, as the true diameter value when measuring in the axial plane of a cylindrical object.)
Depth micrometers are used to measure the distance of an object feature from a flat reference surface . Examples of application are the measurement of steps , the depth of flanges or the bottom surface in a bore , and the height of an object feature in relation to a reference surface, when accessibility perm its it to be simultaneously contacted by the base member and the spindle face.
Bench micrometers can substantially improve the precision of micrometer measurements. Particularly when the objects to be measured are small. The stable position of the instrument during the measurements permits a more precise locating of the work and the heavy base adds to the rigidity of the instrument. Bench micrometers are usually equipped with both large-diameter spindles and thimbles, permitting a finer pitch thread for higher sensitivity, and direct reading in ten-thousandths , or smaller.
Micrometer heads without a frame as an integral part have wide applications in the instrument building field. One of the more frequent uses is the controlled displacement of slides, either single or cross slide . Commonly, the micrometer will move the slide against a moderate spring pressure, which is applied to assure a positive contact between the micrometer spindle face and the contact point on the slide face. Springs exerting a uniform force along the entire slide traverse are preferred.