Protractor, Vernier bevel protractor & Optical bevel protractor
Protractor
A
simple protractor consists of a blade that pivots about a semi-circular head
that is graduated in angular units (e.g., degrees, radians). To use, the blade
is rotated to a position corresponding to some part angle to be measured and
the angle is read off the angular scale.
Vernier Bevel Protractor
It is probably the simplest instrument
for measuring the angle between two faces of component. 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 blade is capable of rotating freely about the centre of the main
scale engraved on the body of the instrument and can be locked in any position.
An acute angle attachment is provided at the top as shown in Fig. for the
purpose of measuring acute angles.
The
base of the base plate is made flat so that it could be laid flat upon the work
and any type of angle measured. It is capable of measuring from 0 to 360°. The
vernier scale has 24 divisions coinciding with 23 main scale divisions. Thus
the least count of the instrument is 5′. This instrument is most commonly used
in workshops for angular measurements till more precision is required.
Read
off directly from the main scale the number of whole degrees between 0
on this scale and the 0 of the vernier scale. Then count,
in the same direction, the number of spaces from the zero on the vernier scale
to a line that coincides with a line on the main scale; multiply this number by
5 and the product will be the number of minutes to be added to the whole number
of degrees.
For
example: Zero on the vernier scale has moved 28 whole degrees to the right of
the 0 on the main scale and the 3th line on the vernier scale coincides with a
line upon the main scale as indicated. Multiplying 3 by 5, the product,
15, is the number of minutes to be added to the whole number of degrees, thus
indicating a setting of 28 degrees and 15 minutes.
Different
angular measurements by vernier bevel protractor:
Universal Bevel Protractor
It
is used for measuring and laying out of angles accurately and precisely within
5 minutes. The protector dial is slotted to hold a blade which can be rotated
with the dial to the required angle and also independently adjusted to any
desired length. The blade can be locked in any position.
Optical Bevel Protractor
In
the case of optical bevel protector, it is possible to take readings up to
approximately 2 minutes of arc. The provision is made for an internal circular
scale which is graduated in divisions of 10 minutes of arc.
Readings
are taken against a fixed index line or vernier by means of an optical magnifying
system which is integral with the instrument. The scale is graduated as a full circle
marked 0—90—0—90. The zero positions correspond to the condition when the blade
is parallel to the stock. Provision is also made for adjusting the focus of the
system to accommodate normal variations in eye-sight. The scale and vernier are
so arranged that they are always in focus in the optical system
