Skip to main content


Three-Point Internal Micrometer

 The Three-Point Internal Micrometer, also known as a three-point bore gauge or three-point inside micrometer, is a precision measuring instrument used to accurately measure the inside diameter of cylindrical objects, such as bores, cylinders, and holes. It is commonly employed in various industries, including manufacturing, engineering, and quality control, where precise measurements of internal dimensions are crucial.

The design of a three-point internal micrometer consists of three equally spaced measuring anvils or contact points mounted on a frame or a measuring head. These contact points are usually made of carbide or hardened steel to ensure durability and resistance to wear. The measuring anvils are spring-loaded, allowing them to retract when the instrument is inserted into a bore or hole.

To measure the internal diameter, the three-point internal micrometer is inserted into the object being measured, and the contact points make contact with the inner surface. The instrument is then gently rotated or rocked back and forth to ensure proper alignment and contact with the entire circumference of the bore. As the micrometer is moved, the contact points push against the surface, and the resulting movement is transferred to a calibrated dial or electronic display, providing the measurement reading.

One of the key advantages of the three-point internal micrometer is its ability to measure irregularly shaped or out-of-round bores accurately. Since the three contact points provide a triangular support system, they can adapt to variations in the shape of the bore and maintain contact with the inner surface. This feature makes it particularly useful for measuring bores with taper, ovality, or other deviations from perfect circularity.

Three-point internal micrometers are available in various sizes to accommodate a wide range of bore diameters. They can measure small bores with diameters as little as a few millimeters up to larger bores with diameters of several hundred millimeters. The instrument's precision typically ranges from a few micrometers to a fraction of a micrometer, depending on its design and quality.

To ensure accurate measurements, it is important to handle and use the three-point internal micrometer properly. The instrument should be cleaned and calibrated regularly to maintain its accuracy. It is also essential to choose the correct size of micrometer for the specific bore being measured and apply consistent and gentle pressure during the measurement process.

In conclusion, the three-point internal micrometer is a valuable tool for accurately measuring the inside diameter of bores, cylinders, and holes. Its design allows it to adapt to irregular shapes and provide precise readings even in the presence of taper or ovality. With proper handling and calibration, this instrument is an essential asset in industries where precise internal measurements are essential for quality assurance and manufacturing processes.