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A bore gauge is a convenient term for the measuring or transfer tools that are used in the process of accurately measuring holes.
Telescopic feeler gauges
These are a range of gauges that are used to measure a bore's size, by transferring the internal dimension to a remote measuring tool. They are a direct equivalent of inside calipers and require the operator to develop the correct feel to obtain repeatable results.
The gauges are locked by twisting the knurled end of the handles, this action is performed to exert a small amount of friction on the telescopic portions of the gauge (the smaller diameter rods found at the T head of the gauge). Once gently locked to a size slightly larger than the bore, the gauges are inserted at an angle to the bore and slowly brought to align themselves radially, across the hole. This action compresses the two anvils where they remain locked at the bores dimension after being withdrawn.
Small hole gauges
Small hole gauges require a slightly different technique to the telescopic gauges, the small hole gauge is initially set smaller than the bore to be measured. It is then inserted into the bore and adjusted by rotating the knurled knob at the base, until light pressure is felt when the gauge is slightly moved in the bore. The gauge is then removed and measured with a caliper or micrometer.
Beam Gauges for Larger Diameter:
To measure the larger diameter you may use extended beam gauges. It is designed to measure internal and external diameter. The major challenge is handling these gauges is slightly difficult compare to other bore gauges. It should be lightweight, low co-efficient of thermal expansion, high modulus and stiffness. Mostly we use low carbon fiber or aluminium. It has stem with ball at both ends the length of the stem can vary depends upon the width of the part. If you want to measure the diameter set the gauges required diameter with master. In order to measure the diameter of the bore, place the beam at one end and slide the other end on the diameter and watch the dial indicator, the values goes on increase at one point it will start to decrease stop that point, that point is called max point on the circle. That is the diameter of the component. Repeat the procedure in 3 different places and get the average value of the diameter of the component. This is the most common way to measure the larger bore. It is available from different range; normally you can use this from 300 mm to 4000 mm for external diameter.
sources: Take the picture from (http://www.directindustry.com/prod/bowers-metrology/analog-bore-gauges-large-diameters-9223-1229881.html)
Dial bore gauge
A dial or vernier bore gauge measures a bore directly. The gauge has three symmetrical anvils that protrude from the gauge body that are connected to the dial or micrometer mechanism. As the knob is rotated it moves the anvils in or out with respect to the measurements. The knob usually has a slipping mechanism to take the feel out of the device and increase reliability between measurements. The measurement given is the mean diameter of the three anvils, and is usually good to 0.001 mm (3.9×10−5 in).
Electronic and Wireless Bore Gauges
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