JQR131.02 Precision Measurement Instruments

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131.02

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 Use and Care of  Micrometers (Continued)

Inside micrometer

The inside micrometer measures the diameter of a bore, or cylinder or can be used to measure the accuracy of parallel planes. It is more difficult to obtain an accurate reading from this instrument than from the outside micrometer.

The inside micrometer must be positioned properly to make accurate measurements; several readings should be taken.

When using an inside micrometer, it is important to locate and guide the instrument carefully so that it will measure the largest diameter at the correct spot. Unless the micrometer is held square across the diameter, an accurate measurement cannot be made.

 

Click on image to enlarge.
Figure: Inside Micrometer

Inside Micrometer

 
 

Depth micrometer

The depth micrometer is used to measure the depth of holes and slots and the length of shoulders and projections.

When using a depth micrometer, ensure that the base is seated firmly on the two parallel surfaces. Extend the extension rod carefully until it touches the bottom of the area to be measured.

Ensure that the extension rod is perpendicular. Read as shown in the first figure.

Care of micrometers

Remember that a micrometer is a precision instrument. It must be handled and used with care or the accuracy will be destroyed.

Keep your mike clean. Occasionally put a drop of light oil on the screw threads. Wipe the micrometer with an oily cloth to prevent tarnish and rust.

Before using a micrometer, clean the faces of the spindle and anvil. To clean them, close the spindle lightly on a piece of paper and draw the paper out. See if the zero line on the thimble coincides with the zero on the index line. If it does not, loosen the cap at the end of the thimble. Reset the thimble to zero; then, holding it carefully in position, tighten the cap.

When taking a measurement, use very light pressure. If you squeeze the workpiece, you will get an incorrect reading and strain the threads. This damages your micrometer.

When it is necessary to close or open a micrometer a considerable distance, hold it by the frame. Roll the thimble along your forearm or the palm of your hand.

 

Click on images to enlarge.
Figure: Depth Micrometer       Figure: Depth Micrometer

Depth Micrometer

 
Review Questions
If you can correctly answer the questions below, you have learned the key points in this lesson. Choose the correct answer for each question, then look at your score below the questions. Review the lesson if you miss any questions.

1. When using an outside micrometer, you should hold it ONLY by the ........
barrel.
thimble.
frame.
ratchet.

2. The spindle on a micrometer is cut to ........ threads per inch.
20
40
100
120

3. When you need accuracy to ten-thousandths of an inch, you would use a/n ........
inside micrometer.
depth micrometer.
vernier micrometer.
variable micrometer.

4. Micrometers should be cleaned ........
daily.
after each use.
when calibrated.
never.

5. Using a 3 - 4 inch micrometer, the sleeve shows the number 7 and three marks, and the thimble has the number 4 at the index line. What is the size of the workpiece?
3.779 inches
4.779 inches
3.729 inches
4.729 inches

Score =
Correct answers:

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