Structure
The bore gauge is a comparative measuring instrument for measuring internal dimensions, usually bore diameters. It consists of a measuring head and a shank, at the end of which the dial indicator is clamped.
On one side of the measuring head, a fixed measuring anvil is mounted. This can be exchanged to adjust the measuring range of the instrument, allowing large measuring ranges to be covered. Opposite to it is a movable measuring bolt, whose movement is transmitted to a rod in the shank and thus to the measuring bolt of the dial indicator. Depending on the model of the bore gauge, the travel of the movable measuring bolt is 1-3mm.
a movable measuring bolt
b fixed measuring anvil
c spring-loaded centering bridge with cradle-shaped ends
d shank
e deflection of the measuring bolt movement by lever
Presetting
With the bore gauge, measurements with very low error margins are possible, provided that the presetting and the measurement are carried out correctly. Before use, the bore gauge is preset as follows:
The interchangeable measuring anvil is mounted that most closely matches the length of the measuring head to the dimension to be measured. The dial indicator is inserted with some preload (approx. 1-2 turns) and clamped.
Since the dial indicator only determines the deviation from a known dimension, an exact internal dimension must be defined using a master setting. The master setting can be, for example, an outside micrometer, a setting ring, or another setting device. The dial indicator scale is now set so that the 0 on the scale aligns with the pointer's position at the reversal point. This should be done as accurately as possible to avoid measurement errors.
Measurement
The spring-loaded centering bridge on the side of the movable bolt centers the measuring head in the bore. In the horizontal position, the measuring head must be manually aligned to find the reversal point of the dial indicator: The measuring instrument is gently swiveled back and forth until the maximum deflection of the dial indicator's pointer (reversal point) is found. This is the measured value.
a Set the measuring instrument to the master setting with a setting ring or gauge block; set the dial indicator to zero.
b Insert the measuring head into the bore; determine the bore dimension at the reversal point of the pointer by swiveling movement; read the deviation.
Evaluation
The measured dimension must now be compared with the preset master setting. If the reversal point of the pointer is after the 0 on the scale, the dimension is smaller. Thus, the measured value is subtracted from the master setting. If the reversal point is before the 0, the dimension is larger, and the measured value is added.
Measured dimension = Master setting - Measured value
Example
The bore gauge was preset to 70mm, and a deviation of 0.02mm was measured. The diameter of the bore is therefore 70mm – 0.02mm = 69.98mm.
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