How to Use A Micrometer
CONSTRUCTIONThe spindle on a micrometer is moved in or out by rotating the thimble or ratchet.
The thread pitch on the spindle is 40 threads per inch. A single full rotation of the thimble extends or retracts the spindle one thread or 1/40 of an inch (0.025, 25 thousands).
The thimble is divided into 25 equal divisions. Rotating the thimble one division will extend or retract the spindle 1/25 of a thread.
(1/25) x (1/40)inch = 1/1000 inch (0.001), one division on the thimble is equal to 0.001 inches.
HOW TO READ
As the thimble is rotated out it uncovers a scale on the sleeve. Each major division on the scale is 1/10 of an inch (0.100)
Each major division is separated into 4 minor divisions, each equal to 25/1000 of an inch (0.025).
A full rotation will move the thimble exactly one minor division on the sleeve.
To read the micrometer you must add the measurement which is visible on the sleeve to the measurement which is showing on the thimble
In the example the sleeve measurement is 0.250 inches and the thimble measurement is 0.017 inches. The total measurement is then equal to the two measurements combined, 0.267 inches.
NOTES
The accuracy and calibration of a micrometer can be altered easily by applying too much pressure to the spindle.
- Use the thimble to quickly rotate the spindle in until it is near the rotor.
- Use the ratchet to rotate the spindle into contact with the rotor.
- The ratchet has a built in slip which prevents excessive stress or pressure from developing.
Calibrate the micrometer regularly. Most micrometers come with a precision dowel which allows for easy and accurate adjustments.