Sine bar, Sine centre & Sine table
Sine Bar
The sine bar is the most accurate instrument for
measuring angles. It consists of a accurate straight rectangular bar made of
high quality steel and having two accurately lapped cylindrical plugs or
rollers at the ends. Sine bar is specified by the distance between the centres
of two rollers i.e., 100mm, 200mm, 250mm, or 300mm, two rollers must be of same
diameters. The various parts of sine bar are hardened before grinding and
lapping. The rollers are so fixed that when sine bar is placed on surface
plate, the surface of the bar is exactly parallel to surface plate.
Principle of
Working of Sine Bar:
The principle of operation of the sine
bar is based upon the application of trigonometry. In a right angle triangle
ABC shown in Fig. The ratio of the length BC (perpendicular) to that of the
length AB (hypotenuse) is referred to as the sine of the angle θ.
The ratio will always be the same
irrespective of the length of sides of the triangle. By placing the slip gauges
under the one roller, the bar surface can be made parallel to any angular
surface of work.
Then
by measuring the total height made by slip gauge, the angle can be found by
using the formula:
Measurement of
Angle for Small Component:
For
checking the angle of small size components, a sine bar is set up approximate
(Nominal) angle on a surface plate by suitable combination of slip gauges. The
component being checked is placed over the surface of a sine bar as shown in
Fig. A dial gauge is mounted upon a suitable stand such as universal surface
gauge.
It
is moved over the component throughout its length. If there is a variation in
parallelism of the upper surface of the component and the surface plate, it is
indicated by the deflection of pointer. Now, we tried to adjust the slip gauges
height so that the dial indicator reads zero position.
Then in this
situation, the angle of the component is given by:
Where,
θ = is the angel to be measure
h = height of the slip gauges.
L = Length of the sine bar (between centres)
Measurement of
Angle for Large Component:
When
component is too large to be mounted on the sin-bar, the sine bar can be
mounted on the component as shown in figure.
The height over the rollers is
measured by means of a vernier height gauge. A dial gauge is also used to check
the measuring pressure. Now, adjusting the height gauge until the dial gauge
reads zero reading each time. The difference of the two height gauge reading
being the rise of the sine-bar as shown in fig.
The angle of the
component is given by:
Uses of Sine Bar:
A
sine bar has to be used in conjunction with a surface plate and slip gauges.
Sine bar is used for checking angle of components, taper of work pieces, etc.
It is also used for setting a work to a known required angle.
Accuracy
Requirement of Sine Bar:
Sine
bar is more accurate for angles between 15° to 45°, below and above to this
limit, angles measured are not much more accurate.
If a sine bar is to be accurate than the following
properties must exist:
(i)
The distance between centres (L) must be precisely known.
(ii)
The axes of rollers must be parallel to each other.
(iii)
The upper surface of the sine bar must be flat and parallel.
(iv)
The rollers must be of identical diameters and round to within a close
tolerance.
Sources of
Errors in Sine Bar:
1. Constant Angle Error: This is caused if the working surface
and the cylinder axes are not parallel.
2. Progressive Angle Error: This is due to the error in cylinder
centre distance.
3.
Gauge block tolerance accumulation is also a source of progressive error.
Sine centre
·
This is a sine bar with block
holding centre which can be adjusted and rigidly clamped in any position.
·
Generally used for inspection of
conical objects between centre.
·
These are used up to inclination
of 60 degree.
·
Principle used here is same as of
the sine bar.
Sine Table
·
It
is a convenient and accurate design for heavier components.
·
In
that one roller of the sine bar is hinged on its datum surface.
·
So
it can be swung at any angle from zero degree to ninety degree about its hinged
end.
·
Very
useful in measuring angles of taper plug gauge.
