INFORMATION SHEET

ON

MICROMETER

SUBJECT: Read The Micrometer

INTRODUCTORY INFORMATION:

In many trades the micrometer is an instrument used to make very precise meas$-$

surements. To read the micrometer the operator must note three separate items.

Two of these items or readings are taken from the barrel of the micrometer and the

third is taken from the thimble. Accuracy in the reading of the micrometer is as

important as taking the actual measurement.

GENERAL INFORMATION:

Spindle Threads

There are $40$ single threads per inch on a micrometer spindle. One turn of the thim$-$

ble on this screw moves it $\frac{1}{40}$ of an inch. To change $\frac{1}{40}$ to a decimal, we write

it thusly: $\frac{1}{40}={\mathrm{.}025}^{\text{'}\text{'}}.$

Barrel

The barrel is marked in such a manner that a line is scribed for each $\frac{1}{40}$ or ${\mathrm{.}025}^{\text{'}\text{'}}.$

To aid in keeping track of these lines, every time four lines are passed a number

appears. For example, when the micrometer is opened from $0$ through four com$-$

plete turns of the thimble, the figure $1$ appears on the barrel. This figure is read as

$.1"$ or one$-$tenth of an inch. To assist further in reading the "mike," the thimble is

graduated into $25$ equal parts. Each line on the thimble is $\frac{1}{25}$ or $\frac{1}{40}$ or $\frac{1}{1000}.$

The fraction $\frac{1}{1000}$ is written as a decimal thusly: $\mathrm{.}001".$

Read The "Mike"

To read the mike, first observe the number of divisions on the barrel, the line on the

barrel, and the number of thousandths on the thimble. Take a reading from the mike

by counting the number of divisions visible on the barrel and add to this amount the

number of thousandths on the thimble coinciding with the line on the barrel, The

reading on the mike parts illustration sheet is ${\mathrm{.}225}^{\text{'}\text{'}}.$

$12$

$14$

$15$