Angular Measurements
By
Dr. Siddhalingeshwar I.G.
Learning Objectives At the end of this chapter student should be able to:
Enlist the various instruments for measuring angles.
Define sine bar? How it is used for angle measurement?
What is angular measurement ?
Why is it so necessary?
Different instruments used for angular measurements.
Angular
measurements is concerned with the
measurements of
individual angles on gauges, tools as well as small angular changes and deflections. It
is necessary because of its precise angular measurements.
Different Types Of Measuring Instruments 1. Bevel Protractor. 2. Sine Bar (Sine Principle). 3. Sine Centre. 4. Angle Gauges. 5. Clinometers. 6. Interferometer, Autocollimator and Optical Flats.
Bevel Protractor
This
is probably the simplest instrument for measuring the angle between
two faces of the component.
It consists of a base plate, an adjustable blade, a Vernier scale and a main
scale. Vernier Least
scale contains 24 divisions coinciding with 23 main scale divisions.
count of the instrument is 5’(minutes).
Diagram showing improved bevel protractor and combinations of sets
Bevel Protractor A protractor is a device for measuring the angle between two intersecting lines.
The angle is measured in degrees, and a circle is defined as having 360 degrees of identical size.
A universal bevel protractor consists of a base plate attached to the main body, and an adjustable blade which is attached to a circular scale containing Vernier scale. The adjustable blade is capable of sliding freely along the groove provided on it and can be clamped at any convenient length. The adjustable blade along with the circular plate containing the vernier scale can rotate freely about the centre of the main scale engraved on the body of the instrument and can be locked in any position with the help of clamping knob. An acute angle attachment is provided for the purpose of measuring angles. The base plate so that it can be laid upon the work and any type of angle can be measured.
Measuring Acute Angles
Measuring Obtuse Angles
Using a protractor with a vernier height gauge
Fig 1
Fig 2
As with other Vernier measuring devices, the Vernier scale of the bevel protractor allows the tool to divide each degree into smaller increments. The Vernier scale is divided into 24 spaces, 12 spaces on either side of the zero (Fig 1). Each space on the Vernier scale is, therefore, one-twelfth of a degree. One-twelfth of a degree is equal to 5 minutes. To read the protractor, note where the zero on the Vernier scale lines up with the degrees on the dial in Figure 10. The degrees are read directly from the main scale. The zero on the Vernier scale is just pass the 85 degree mark. Now, reading in the same direction (counter-clockwise) , count, by five, from zero on the Vernier scale to the lines that match up on the dial (Fig 2). The main component of the bevel protractor is the main scale. The main scale is graduated into four 90-degree components. The main scale is numbered to read from 0 to 90 degrees and then back from 90 degrees to 0. Add this number of minutes to the number of whole degrees. The total number of degrees and minutes in Figure 2 would equal 85 degrees and 30 minutes.
Sine bar and the sine principle
It
is a simple and moderately accurate device, which is used to measure angular conjunction with gauge blocks. Made
from high carbon, high chromium corrosion resistant steel, hardened, ground and stabilized. Works
on the basis of sine principle i.e. the centre distance between the two rollers is taken as the length of the hypotenuse of the right angled triangle formed in measuring the angle.
Sine centre
This
is a sine bar with block holding centre which can be adjusted and
rigidly clamped in any position. Generally These
used for inspection of conical objects between centre.
are used up to inclination of
Principle
degree.
used here is same as of the sine bar.
Angle gauges
As
slip gauges are built up to give a linear dimension , the angle gauges can be built up to give required angle. They
are made of hardened steel and seasoned carefully to ensure permanence of angular accuracy. These
gauges are about 3 inch long and 5/8 inch wide with their faces lapped to within 0.0002 mm and angle between the two ends to +/- 2 seconds. All
these angle gauges in combination can be added or subtracted thus
Clinometer
The
clinometer is used as precision setting tool, to set a tool head or table at
specific angle. The
clinometer are also used for checking angular, and relief angles on large
cutting tools and milling cutter inserts. In
clinometer the spirit level is mounted on the rotary member carried in a
housing. On the housing, there is a circular scale. The clinometer is mainly used
Clinometer
Autocollimators The
autocollimator is a precision optical instrument for measuring very small
angular displacement over a significant distance. They
can be used for valuating alignment of machine surfaces, surface plate
flatness, square ness of one surface to another, straightness of shafts and a variety of other orientation measurements.
Autocollimator
costs $30,000 range for industrial units but vary based on
accessories, resolution and power of the light source which can influence the range of measurement. Lower priced test units with lower resolution (in the 5 arc minute range) are available for around $1000. These are still precision instruments and useful for educational and basic experiments in optics.
Different types of autocollimators
Optical Flats Optical flats are cylindrical in form with working surfaces flat. There are two types: 1)Type A- it has only one surface flat. 2)Type B- It has both the surfaces flat and parallel to each other. Type
B is used for testing measuring surfaces of micrometers , measuring
anvils and similar length measuring devices. Type
A is used for testing the flatness of precision measuring surfaces of
flats ,slip gauges and measuring tables.
