Measuring and measuring tools

Accurate measurement is the basis of good engineering and crafting practice. The accuracy of any measuring device depends on the user as much as on the design of the tool.

Measuring is not only checking the length, width or thickness of an objects but also checking of the shape – things like the flatness, straightness, roundness or squareness. Measuring tools are also used for inspecting a finished or partly finished product.

All measuring tools are precision tools. You must take good care of them to keep them in good shape to maintain accuracy.

There are two systems of measurement. The traditional system used in the United States is the US customary system. It is based on the English imperial system of measure. The second is called the SI system (Metric system). The metric system is an international decimalised system of measurement, first adopted by France in 1791, that is the common system of measuring units  used by most of the world. All measuring tools have metric or imperial graduations or a combination of both. One big advantage of the metric scale is that it eliminates the necessity for a range of fractional sizes. The markings on a metric rule are every millimetre with the figures marked at 10mm intervals. Fractions are not used in the SI system.

There are many different types of measuring tools.

Steel ruler

The most common, and the best known, piece of measuring equipment

– Reading a metric ruler

– Reading an imperial ruler

These tools are comparators, used for transfering a dimension from one place to another.

Caliper

Vernier calipers are used for more accurate measuring than can be achieved with a measuring rule or a slide caliper.

Vernier caliper

Micrometers are used for more accurate measuring than can be achieved with a measuring rule or a slide caliper.

Micrometer

Reading a Metric Micrometer Caliper

Vernier callipers

Vernier callipers work like slide caliper.

Vernier calipers are used for more accurate measuring than can be achieved with a measuring rule or a slide caliper. It is capable of measuring internal and external dimensions and can also be used as a depth gauge. Vernier calipers are available with metric and imperial graduations.

Vernier caliper - Parts

1. External jaw

2. Intenal jaw

3. Sliding jaws

4. Metric Vernier scale

5. Locking screw

6. Main scale

7. Depth gauge

Measuring with a Vernier caliper

1. Measuring external dimensions

2. Measuring internal dimensions

3. Measuring depth dimensions
MEASURING WITH  METRIC VERNIER CALIPER TO WITHIN 0,02mm

Each division on the main scale is 1mm.

The metric vernier scale is 49mm long and divided into 50 equal parts. Each division is 49/50, which is equal to 0,98mm. The difference between one division on the main scale and one division on the metric vernier scale is 1/50 or 0,02mm.

Measuring with metric Vernier caliper to within 0,02mm

Read the measurement as shown above.

To read the measurement note the main scale measurement immediately preceding the zero line on vernier scale.

The zero of the vernier scale immediately preceding  28mm.

To this (28mm) must be added the decimal reading on the vernier scale. Note the line on the vernier scale which is coincident with a line on the main scale.

30th line concides with a line on the main scale.

So the reading is  28mm plus 30 divisions of 0,02mm

TOTAL: 28mm + 30 * 0,02 = 28,6mm

MEASURING WITH  IMPERIAL VERNIER CALIPER TO WITHIN 0,001in

The main scale on the vernier is graduated and numbered in inches with each inch graduated and numbered in tenths (0,1in). Each tenth is divided into four giving 0,1in. divided by 4 = 0,025in. On the vernier scale  0,6in is divided into 25 parts. Each of these has a length of 0,6in divided by 25 = 0,024in. The difference in length between a  small division on the main scale and the vernier  scale is 0,025- 0,024 = 0,001in.

Measuring with imperial Vernier caliper to within 0,001in

Read the measurement as shown above.

To read a measurement note the position of the zero line on the vernier scale in relation to the main scale. This is shown as 3,00in plus  0,300in plus 2*0,025in which equals 3,35in. To this must be added the number of divisions from the zero line on the vernier scale to the line which is coincident witha line on the main scale, in this case 15 divisions which equals 0,015in the total reading is therefore:

Main scale = 3,35in

Vernier scale = 0,015in

So the reading is 3,35in + 0,015 = 3,365in

The vernier caliper should not be used to measure the outside diameter of a cylinder having a radius greater than the depth of the caliper opening.

 Vernier caliper opening

CARE OF CALLIPERS:

1. Store calipers in separate containers provided.

2. Keep graduations and markings on all callipers clean and legible.

3. Do not drop any caliper. Small nicks or scratches can cause inaccurate measurements.

4. Protect caliper points from damage.

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