What Is Three-point Inside Micrometer

what-is-three-point-inside-micrometer?

• Basic Concepts of this item

  A three-point inside micrometer (also known as a three-jaw inside micrometer or an internal diameter measuring instrument) is a precision measuring tool used to accurately measure internal dimensions, such as the diameter of inner holes and the width of grooves. It is particularly suitable for checking the roundness error of holes.

   

  Key features: The three measuring jaws are evenly spaced at 120° and have a self-centering function, automatically aligning with the center of the hole during measurement and eliminating the need for the operator to rock the device to find the inflection point.
• 

  Structure and working principle

  Measuring head: Three retractable jaws evenly spaced at 120°, directly contacting the wall of the bore being measured.
  Main body: Contains a guiding mechanism and a display device (either a scale or a digital display).
  Fine-tuning mechanism: Controls the extension and retraction of the jaws through a differential thimble and a force-measuring device.
  Extension rod (optional): Used for measuring deep holes.

• Working principle

  Three-point contact centering measurement is used: A rotating force-measuring device drives the micrometer screw to rotate. The square conical thread at the end of the micrometer screw pushes the three measuring jaws to move radially in sync. The three jaws contact the hole wall simultaneously, determining the axis position of the inner hole and enabling precise measurement.

  Measurement principle: The radial pitch of the square conical thread is 0.25 mm, meaning that one rotation of the micrometer barrel increases the measuring diameter by 0.5 mm. An accuracy of 0.001 mm or 0.005 mm is achieved through a 50-division scale.

 

 

Technical parameters of three points internal micrometer

Common measurement range: 2–300 mm, divided into multiple sub-ranges (e.g., 6–8 mm, 8–10 mm, 10–12 mm, etc.)
Graduation value: 0.001 mm (small range), 0.005 mm (large range)
Accuracy: Up to ±0.003 mm or better
Repeatability error: ≤0.005 mm
Applicable temperature: 20 °C (the instrument must be brought to the same temperature as the workpiece for 30 minutes before use)

 

 

How to use the Three-point Inside Micrometer

Preparation before use

Clean the measuring jaws and the surface to be measured, removing any oil or impurities.

Check that the measuring jaws extend and retract smoothly.

Calibration: Perform zero-point calibration using a standard ring gauge.

Calibration steps:

Insert the micrometer into the ring gauge of the corresponding size.

Rotate the measuring device until a "click" sound is heard.

Adjust the micrometer thimble to match the nominal value of the ring gauge.