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Electronic Instruments In Height Gage Applications

The test indicator type electronic gaging head attached to a pivoting, extendable , and tiltable cross bar of a gage stand , with means for controlled height adjustment and complemented with a fine adjustment device , results in a versatile gage, commonly designated as an electronic height gage. Although adaptable for varied applications , the term height gage seems warranted in view of the major use of this instrument : transferal of height dimensions and the comparison of the vertical separation between specific part surfaces, by referencing the measurement from a common datum plane. For the function of the datum plane, a surface plate is most commonly used.
A typical electronic height gage consisting of three essential members: a ) the gage stand ; b) the test indicator type sensing head; and c) the amplifier unit with the meter.
The height gage stand ID designed to be supported on a surface plate and for this purpose the bottom surface of the base has precisely machined contact pads , which must be flat and contained in a common plane within very close limits .The height gage stand for electronic measurements must be properly dimensioned for a high degree of rigidity and to provide a stable support for the gage head even in the most extended position of the cross bar. The positive locking of the joints is also critical . The dimensioning of the gage stand , however, must not result in excessive weight, which could defeat one of the functional purposes of the stand , that is the transfer of height dimensions by the sensitive sliding of the stand on the surface plate.
For the vertical adjustment of the joint block that is holding the cross bar, the columns of height gage stands are often equipped with a rack and pinion device or with a friction roller guided in a groove. One model is that the fine adjustment screw is part of the gaging head support. Other models are , particularly those that instead of a cross bar , are equipped with a short horizontal arm only, achieve the fine adjustment by means a flexure spring in the base of the stand that, when actuated by a thumb screw, imparts a tilt motion to the gage column. In other designs the turning of the thumb screw produces a sensitive vertical movement of the column.
Electronic height gages are generally used for the comparative measurement of linear distances on objects whose configuration satisfies the following requirements: a) the surface being measured must lie in the horizontal plane; and b) the distance to be determined must be referenced from a surface plate representing a plane parallel to the part surface on which the measurement is being carried out. The locating surface on the part is not required to rest directly on the surface plate, but may be supported on an intermediate parallel element of known thickness. Such an arrangement is needed for locating a surface that is in a setback position . When adequate support blocks are used, the indirect referencing will not distract from the resulting correctness of the height measuring process.
The linear dimension being measured in this process is designated a height because of the arrangement of the gaging process . The size of the dimension being measured is determined by comparing it to the geight of a gage block stack , or to a level established by means of adjustable step blocks.
Modern digital electronic technology permits absolute height measurement, with the push of a button, the digital display an be zeroed out at any position of the measuring probe ( that is , the surface plate, parallels or a part feature). From the zero position , the probe is moved to measure a given height , with the difference in distance being displayed in absolute terms on the digital readout.
Another Important application for electronic height gages in surface plate work is the verification of angles in conjunction with a sine plate. By referencing the electronic height gage at one point of that surface element, and then comparing the other points to that reference position, it is possible to detect and to measure sensitively any variations in the actual position of the explored surface element in relation to a horizontal plane.
It is possible in principle, to use electronic height gages for comparator type measurements also. In such applications , particularly when small- or medium-size parts are measured , it is practical to retain the height gage stand in a fixed position , while the master and the parts are brought under the sensing tip of the gage. This operation could be carried out in any of several arrangements, such as: a) sliding the part while it rests directly on the surface plate ; b) setting up a stage under the gage tip, similar to a comparator table; the stage may have guide rails and an end stop to reduce the gaging time in repetitive measurements; c) holding the part in a locating support setting the reference size; and d) holding the part on a precision slide with traverse movement actuated by a fine thread screw.