Q. Define Micrometer Screw Gauge and describe how it is used to measure length.

Micrometer Screw Gauge

            Screw gauge in extensively used in engineering field for obtaining precision measurements. Micrometer screw gauge is used for measuring extremely small dimensions.A screw gauge can even measure dimensions smaller than those measured by a Vernier Caliper.

            Micrometer Screw gauge works on the simple principle of converting small distances into larger ones by measuring the rotation of the screw. This “screw" principle facilitates reading of smaller distances on a scale after amplifying them .

 

Reading A Micrometer Screw Gauge

 

Step 1 Turn the thimble until the anvil and the spindle gently grip the object. Then turn the ratchet until it starts to click.

 

Step 2 Take the main scale reading at the edge of the thimble

 

Step 3 Take the thimble scale reading opposite the datum line of the main scale. Multiply this reading with least count i.e., 0.01mm

 

Step 4 Now add main scale reading to thimble reading. This will be the diameter of the object.

 

Zero Error of a Screw Gauge:When the plane surface of the screw and the opposite plane stud on the frame are brought into contact, if the zero of the head scale coincides with the pitch scale axis, there is no zero error.

 

Positive Zero:Error When the plane surface of the screw and the opposite plane stud on the frame are brought into contact, if the zero of the head scale lies below the pitch scale axis, the zero error is positive.

Negative Zero Error:

When the plane surface of the screw and the opposite plane stud on the frame are brought into contact, if the Zero of the head scale lies above the pitch scale axis,the zero error is negative.