A micrometer is defined as the measuring device and is also known as a micrometer screw gauge. It is used for taking accurate measurements of the components in the fields of mechanical engineering and machining. Other fields where micrometers find applications are telescopes and microscopes to measure the apparent diameter of the heavenly bodies and microscopic objects.

Invention of Micrometer

The micrometer was invented by William Gascoigne who was an English astronomer in the year 1638. It is considered to be the enhancement of the vernier and was mainly used in the telescope for measuring the angular distances between the stars and the other planets. The word micrometer is derived from the two Greek words micros and metron which means small and measure. 

What is the Least Count of Micrometer?

The least count of micrometers is defined as the smallest increment in the reading on the micrometer scale. Following is the formula used for calculating the least count:

Least count = Pitch length/total number of divisions of the circular scale

Hence, using the above formula, the micrometer least count is given as:

Least count of micrometer = 1mm/100 = 0.01 mm

Types of Micrometers

  • Universal micrometer sets: This micrometer has an interchangeable anvil, for instance, it could be either flat, spherical, disk, blade, point, spline, or knife-edge.
  • Blade micrometers: The blades and the narrowing tips of this micrometer has a matching set. A narrow o-ring groove can be measured by using a blade micrometer.
  • Pitch-diameter micrometer: These are also known as the thread mics since they have thread-shaped tips that are used for measuring the pitch diameter of a screw thread.
  • Digital mics: An encoder is used for measuring the distance and the values are displayed on the digital screen.
  • V mics: This micrometer finds application when the diameter of the circle needs to be measured from three different points such that it gets spaced evenly.

Components of Micrometer

The following are the components or parts of micrometer:

  • Frame: It is the C-shaped body that holds the anvil and barrel which is covered with an insulating plastic plate. The insulating material is used for reducing the heat transference as the material used in the frame has a high thermal mass.
  • Anvil: It is the shiny part on which the sample rests. Also, the spindle moves towards the anvil.
  • Spindle: It is the cylindrical component that is shiny in nature. 
  • Thimble: It is the graduated markings on the micrometer on which the thumb rests.
  • Screw: It is considered to be the heart of the micrometer which is present inside the barrel. 
  • Thimble lock: This is also known as lock nut or lock-ring which is used for tightening of the spindle.

Calibration of Micrometer

Zeroing, testing and adjustment are the three ways of calibrating the micrometer.

  • Zeroing: This is done so that the zero lines is repositioned such that it is in line with the markings on the thimble. A pin spanner is used for adjusting the zero line which is inserted into a small hole in the sleeve. This method of calibration is used for canceling the zero error. 
  • Testing: The main sources of error are dirt, abuse, and low operating skills. In this method of calibration, the accuracy of the micrometer is measured by measuring the gauge blocks. If the gauge block has 0.75000 and the reading on the micrometer should match this. If the reading is 0.7503, then the micrometer needs to be calibrated.
  • Adjustment: Adjustment of micrometer means after the instrument has been zeroed and tested and still the inaccuracy persists. This occurs when the parts of the micrometer are worn out. These parts need to be replaced.

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