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3-point micrometer

3-Point Micrometer

A 3-point micrometer, also known as a three-point micrometer, is a precise handheld measuring instrument used to measure internal diameters. Unlike traditional micrometers, the 3-point micrometer uses three measuring anvils positioned at the end of the body, working together with a ratchet mechanism. This design enables accurate measurements of inner diameters.

The Importance of the Micrometer in Machining

The micrometer is one of the most common measuring tools in machine shops, essential for making precise measurements. A micrometer can achieve a measuring accuracy of up to 0.01 millimeters, making it ideal for checking machining ISO tolerances. Micrometers are used to measure both external and internal dimensions, such as the outer diameters of shafts and the inner diameters of holes.

Using the 3-Point Micrometer

The 3-point micrometer is specifically designed for measuring the inner diameters of holes. Thanks to its three anvils, it can also detect the triangularity of a hole, for example in thin-walled bushings, which can be critical for precision. Because measurements are often taken in hard-to-reach areas, the 3-point micrometer provides a reliable and accurate measuring method.

Micrometer Structure and Operation

The micrometer operates based on a screw thread: a screw moves the measuring spindle either one or half a millimeter per revolution. Key components include:

  • Frame: The main body of the micrometer, to which all other components are attached.
  • Measuring faces: Usually made of carbide or hardened and ground steel.
  • Spindle: The movable part that moves the measuring face via the sleeve and thimble.
  • Sleeve and datum line: The sleeve holds the spindle in place, and the datum line serves as the zero-mark on the scale.
  • Thimble: Rotating the thimble moves the spindle, and the measurement is read from the thimble scale.
  • Ratchet: Ensures constant measuring force, which is especially important for obtaining accurate results.
  • Heat insulation plate: Prevents heat from the user's hands from transferring to the instrument and causing thermal expansion.

Different Types of Micrometers and Their Applications

There are many types of micrometers, each designed for specific purposes:

  • Frame micrometer: Suitable for measuring external distances, such as the outer diameters of shafts.
  • Three-point micrometer: Especially for checking hole internal diameters and form accuracy.
  • Depth micrometer: For precise measurement of depths.
  • Two-point micrometers and stick micrometers: Used especially for measuring ovality.
  • Thread micrometers: For measuring the pitch diameter of threads using various anvils.

Digital Micrometers

Modern digital micrometers offer several advanced features, such as:

  • Wireless data transfer: Enables measurement results to be transferred and stored directly to a computer or database.
  • Tolerance function: For example, the Micromar micrometers from Mahr have a built-in feature that immediately indicates whether the measurement is within the specified tolerance.

Proper Use and Maintenance of the Micrometer

Proper micrometer use involves some important maintenance actions and instructions:

  • Checking: Regularly check the accuracy by bringing the measuring faces together and ensuring the scale reads zero.
  • Cleaning: Clean the micrometer of dust and dirt before and after use with a soft, dry cloth.
  • Oiling: Apply a thin layer of oil to the measuring faces after use to protect against rust and wear.
  • Storage: Store the micrometer in its protective case to prevent damage.
  • Temperature control: Measurements should be made at 68°F (20°C) to avoid measurement errors caused by thermal expansion.

Summary

The 3-point micrometer is a critical and precise measuring tool in machine shops and manufacturing workshops, especially when measuring the internal diameters of holes. Its accuracy and reliability improve measurement quality and help ensure that parts meet tolerance requirements. When used and maintained properly, the micrometer is an indispensable tool for machinists in their daily work.