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Micrometer

What is a micrometer?

A micrometer is a highly precise handheld measuring instrument and, along with the caliper, is one of the most commonly used measuring tools in machine shops. A micrometer allows for measurement accuracy down to 0.01 millimeters, making it an ideal tool for measuring ISO tolerances in machining operations.

Using a Micrometer

A micrometer is primarily used to measure external and internal distances as well as depths with high precision. Objects measured with it can include, for example, the outer diameters of shafts and the internal diameters of holes:

  • Frame micrometer: Specifically designed for measuring external distances, such as the outside diameters of shafts.
  • Three-point micrometer: Used for measuring the internal diameters of holes and is particularly useful for detecting the triangularity of a hole.
  • Depth micrometer: Accurately measures depths.

Structure and Operation of the Micrometer

The micrometer operates on a screw thread, where the pitch is designed so that the screw moves the spindle by either one millimeter or half a millimeter for each revolution. The micrometer has several key components that together enable accurate measurement:

  • Frame: The main body of the micrometer, to which all other components are attached.
  • Anvil and Measuring Surfaces: The measuring surfaces are usually made of carbide or hardened, ground steel.
  • Spindle: The movable part that moves the measuring surface via the sleeve and thimble.
  • Sleeve and Reference Line: The sleeve holds the spindle in place, and the reference line serves as the zero line for the scale.
  • Scale on Sleeve and Thimble: In analog micrometers, measurement results are read from these scales.
  • Thimble: Used to rotate the spindle, and measurement results are read from the thimble scale.
  • Ratchet Stop: Ensures consistent measuring force during the measurement process.
  • Heat Insulating Plate: Ensures the warmth of the operator’s hands doesn’t transfer to the instrument and cause thermal expansion.

Different Types of Micrometers

There are various special models of micrometers available, each suited for specific purposes:

  • Frame micrometer: Models with curved, round, disk-shaped, interchangeable, or reduced measuring surfaces allow for a wide range of external measurements.
  • Three-point micrometer: A better tool for checking the internal diameter and form of holes.
  • Inside two-point micrometers and Stick Micrometers: Used for precise measurements, particularly for checking the roundness (ovality) of holes.
  • Thread Micrometers: A frame micrometer with interchangeable measuring tips designed for different thread pitches to measure the pitch diameter of threads.

Digital Micrometers

Modern digital micrometers often feature easy-to-read displays and several advanced features, such as:

  • Wireless Data Transfer: Enables measurement results to be transferred and saved directly to a computer or database.
  • Tolerance Function: For example, Mahr's Micromar micrometers have a built-in function that immediately indicates whether the measured dimension falls within the specified limits.

Proper Use and Maintenance of a Micrometer

There are several important maintenance and operating guidelines related to micrometer use:

  • Inspection: The accuracy of the micrometer should be regularly checked by closing the measuring surfaces together and ensuring the scale reads zero. For larger micrometers, a calibration rod or gauge block is used. When measuring very precise dimensions, it’s advisable to verify the reading with a gauge block of similar size to the object being measured.
  • Cleaning: Clean the micrometer of dust and dirt before and after use with a soft, dry cloth. Pay special attention to cleaning the measuring surfaces.
  • Oiling: Apply a thin layer of oil to the measuring surfaces after use to protect them from rust and wear.
  • Storage: Store the micrometer in its protective case to avoid damage. Do not store the micrometer in your pocket.
  • Use: Only use the micrometer for measuring; do not use it for any other purpose.
  • Temperature Control: Use the micrometer at the official measuring temperature (20 °C) to avoid measurement errors caused by thermal expansion.

Summary

The micrometer is an essential tool for machinists, and its precision and reliability are key features that improve the accuracy and quality of measurement results. Digital models and special micrometers provide additional options for precise measuring. Proper maintenance and correct use ensure the micrometer’s lifespan and accuracy, making it an indispensable tool for high-precision measurements in everyday machine shop work.