A bore gauge is a convenient term for the measuring or transfer tools that are used in the process of accurately measuring holes.
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). To use, the gauge is inserted at a slight angle to the bore and gently locked to a size slightly larger than the bore while at that angle. Then, rocking the handle side-to-side, slowly move the handle across the bore to the other side. The rocking will first align the gauge with the bore axis and the act of moving the handle to the other side of the bore will bring it to the exact bore diameter. This action compresses the two anvils where they remain locked at the bores dimension after being withdrawn.
The gauge is then removed and measured with the aid of a micrometer or caliper. To accurately detect the maximum distance between the two anvil heads, move the head of the gauge around while making the measurement to insure you get the maximum reading. Grasp the gauge near the head to aid in your maneuvering of the gauge while adjusting the micrometer so it just stops the gauge's motion at one spot only. A bit of practice will quickly give you the idea.
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. To accurately detect the maximal distance between the two halves of the gauge head, move the head of the gauge around while making the measurement to insure you get the maximal reading. Grasp the gauge near the head to aid in your maneuvering of the gauge while adjusting the micrometer so it just stops the gauge's motion at one spot only. A bit of practice will quickly give you the idea.
There are two styles of small-hole gauges: full-ball and half-ball. The full-ball gauges are easier to set correctly and maintain, under the pressure of measurement, a better representation of the bore. Half-ball gauges tend to spring just a little bit, and this may be enough to make a measurement incorrect. A lighter "touch" is required to accurately use the half-ball gauges.
Beam gauges for larger diameters
To measure larger diameters, you may use extended beam gauges. It is designed to measure internal and external diameters. The major challenge is handling these gauges is slightly difficult compared to other bore gauges. It should be lightweight, have low coefficient 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 range of the stem depends upon the width of the part. mm to 4000 mm for external diameter.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 initially increases and at one point starts decreasing; stop that point, it 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 in different ranges; normally you can use this from 300
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).