Micrometer Screw Gauge

In previous lesson, we have discussed vernier caliper. In this lesson, we will focus on micrometer, where we will discuss the sensitivity of micrometer, the label of the parts of a micrometer and the function of some of the parts. We will also discuss how to take readings from a micrometer such as how to read the main scale, how to read the thimble scale and how to determine the zero error.

This is a micrometer screw gauge. One of the famous question related to micrometer screw gauge in SPM exams is naming the part of the micrometer. Therefore, make sure that you memorise the name of all the parts of a micrometer.

Let’s see the anvil and spindle. Both of these are to hold the measured object. On the sleeve there is the main scale. Each division on the main scale equivalent to 1mm. The range of the main scale is from 0 – 25mm.

There is a thimble. The scale on it is called the thimble scale. Each division on the thimble scale represents 0.01mm.

The ratchet knob. The function of the ratchet knob is to exert appropriate amount of pressure on the measured object so that the object is not compressed. Asking about the function of the ratchet knob is also a famous question in SPM exam, especially in the structure questions in paper 2.

The lock. Its function is to tighten and hold the spindle stationary so that a reading can be taken easily.

Last but not least, the frame, a C-shaped body that holds the anvil and sleeve in constant relation to each other.

A micrometer scale comprises a main scale and a thimble scale. The reading of a micrometer is equal to the sum of the main scale and the thimble scale.

Let’s see the main scale. Each division in the main scale represent 1 mm. Below the line are the marks for 0.5 mm. Therefore, starting from zero, there is 0.5 mm, there is 1.0 mm, there is 1.5, 2.0, 2.5, 3.0 and so on.

The reading of the main scale is taken by referring to the edge of the thimble. For example, in the micrometer above, the edge of the thimble shows that the reading of the mains scale is 5.5 mm.

Let’s see the thimble scale. Each division on the thimble scale represents 0.01 mm. The reading of the thimble scale is taken by referring to the straight line on the main scale. In this case, the reading of the thimble scale is 0.28mm.

As we mentioned before, the reading of a micrometer is equal to the sum of the main scale and the thimble scale.  Therefore, the reading is 5.5 + 0.28 = 5.78 mm.

As all other measuring instruments, micrometers are also subjected to zero error.

When the anvil and spindle are firmly closed, the reading of the micrometer should be 0.00mm. However, in some micrometers, the reading does not start from zero and we call this zero error.

Let’s see the  image above. The reading is slightly higher than zero. This is a positive zero error. The reading of the main scale is zero and the reading of the thimble scale is 0.02mm. The zero error is +0.02mm.

Image above shows an example of negative zero error. The reading of the thimble scale is 0.47mm. However, this is not the zero error. If you still remember, the negative zero error must be read from the back of the scale. In this case, it must be read from the “0” mark. Therefore, the zero error is -0.03mm.

The correct reading of a micrometer is equal to the reading obtained – zero error. For example, if the reading obtained = 2.34 mm, the zero error is +0.02 mm, then the correct reading is 2.34 – (+0.02), which is equal to 2.32 mm. If the reading obtained is 2.34 mm and the zero error is -0.03 mm, then the correct reading is 2.34 – (-0.03), minus a negative become plus, and the answer is 2.37 mm.