Angular Measurement

Chapter’s key points

1. Introduction (international meter)

2. Classification of Angular Measurement instruments (line and end measurement instruments)

3. Line Measurement Instruments (Protractors, Clinometers and Dial Bevel Protractors)

4. End Measurement Instruments (Slip Gauges and Sine Bar)

1. Introduction This field of measurement is important in the area

of ship, airplane, car …etc., design.

It is generally concerned with the measurement of individual angels on gauges, tools as well as small deflections and angular changes.

“Angel” is defined as the opening between two lines that meet at a point.

It differs from linear measurement as it does not require absolute standard or datum.

2. Bevel Protractor Type: This is a low-resolution, line-measuring ,

Direct comparison , instrument. It has accuracy of + 5 minutes of arc.

Operating principle: It works on the basic measuring technique of comparing an unknown angel to a previously calibrated one.

Construction: It consists of a plate of hardened steel having angular graduations etched or engraved at intervals of fraction of standard unit of length. These graduations may not be uniform all throughout its length. This allows for multiple use for particular range as per accuracy required.

Basic desirable qualities:

(1)   Clearly engraved lines,

(2)   Minimum thickness,

(3)   Low coefficient of thermal expansion.

Degree of accuracy affected by:

(1)   Quality of rule,

(2) Skill of user in estimation the parts of mm.

3. Vernier Bevel Protractor

These are accessories to scales and help measuring directly those parts that cannot be measured directly by the scale.

Construction & Use: It consists of a base plate attached to the main body and an adjustable blade which is attached to a circular plate containing vernier scale. The adjustable scale can move freely about the center of the main scale which is engraved in the body of the instrument and can be locked at any position. It is capable of measuring angles from 0 to 360 degrees. It has an accuracy of reading up to 5 minutes of a degree.

• How to Read

The main scale is divided up into degrees from 0-90 each way.

The vernier scale is divided such that 12 divisions on it occupy same space as 23 degrees on main scale.

One vernier division = 23/12 = 1 11/12 degrees, i.e. 1/12 or 5 minutes less than 2 degrees.

• Example :

Read off directly the number of whole degrees between the zero on the main scale and that of the vernier scale.

Count, in the same direction, the number of the division on the vernier scale that is inline with a division on the main scale, and multiply it with 5 minutes to get the minutes of the reading. In this case :

Reading is = 52o + 9 * 5’ = 52O 45’

Precaution :

1) The protractor blade must always be perpendicular to the surface being tested.

2) Check the reading of the protractor after the clamping screw has been locked up.

Sources of Errors :

1) In the graduated scale.

2) In the reading of the scale.

3) Due to the non-eccentricity of the scale and pivot.

4) Movement of blade if not locked properly.