2. The general principle of comparator is to indicate the
differences in size between the standard and the work
being measured by means of some pointer on a scale
with sufficient magnification
It thus does not measure the actual dimension but
indicates how much it differs from the basic
dimension
3. All comparators consist of three basic features
1)A sensing device
• which faithfully senses the input signal
2) A Magnifying or amplifying system
• to increase the signal to suitable magnitude.
Mechanical, Optical, Pneumatic,hydraulic and electronic
methods are used for this purpose.
3) A display system (usually a scale and pointer)
Which utilizes the amplified signal to provide a suitable
readout.
4. Need for a comparator
A comparator is used in mass production to inspect the
components to close tolerances with high degree
of precision and speed. Ex: Piston
• Use of line standards such as vernier and micrometer
calipers require considerable skill
• Many dimensions can be checked in a very short time
Comparators can be classified as
1. Mechanical 2. Optical 3. Electrical &Electronic
4. Pneumatic
5. Fluid displacement comparators
6. Mechanical optical comparator
7. Electro-mechanical comparator
8. Multi-check comparator.
5. Design Consideration
1.Ability to record variation of at least 0.0025 mm
2.Linear recording scale
3.Variance free (Instrument readings should not vary
when repeated measurements of given quantity are
made
4.Scale indicators clear and free from oscillations
5.Free from backlash, unnecessary friction &clearances
inertia should be low.
6.Measuring pressure should be low and constant
7.Supporting frame to be rigid and withstand reasonable
ill usage without permanent damage
8.Capable of measuring wide range.
6. Basic operation
1. The comparator is first adjusted to zero on its dial
with a gauge block in position
2. The work piece to be checked is then placed in
position
3. The comparator gives the difference in dimension
in relation to the gauge block.
7. Uses of comparator
1. Laboratory Standards:
to set the working or inspection gauges
2.Working Gauges
To prevent the work spoilage and to maintain required
tolerances
3. Final Inspection Gauges
4. Receiving Inspection Gauges
5. For checking newly purchased gauges
8. Essential characteristics of a comparator
1. Robust design and construction
2. Linear characteristics of scale
3. High magnification
4. Quick in Results
5. Versatility
6. Minimum Wear of contact Point
7. Free from Oscillations
8. Free from backlash
9. Quick insertion of work piece
10. Adjustable table
11. Compensation from temperature effects
12. Means to prevent damage
9. Working principle of a Mechanical Comparator
A mechanical comparator employees mechanical
means for magnifying the small movement of the
stylus
the magnification of the small stylus movement is
obtained by means of levers, gear trains, rack and
pinion or a combination
The usual magnification obtained by these
comparators ranges from about 250 to 1000
10. Types of Mechanical Comparators
1. Dial indicator (Dial Gauge)
2. Read Type
3. Johansson Mikrokator
4. Sigma Comparator
12. • The simplest type of mechanical comparator
• It consists of a base with a rigid column rising from
its rear
• An arm mounted on this column and it carries a dial
gauge at its outer end
• The indicator is set at zero by the use of slip gauges
• The part to be checked is placed below the plunger
• The linear movement of the plunger is magnified by
means of mechanical means to a sizable rotation of
the pointer
13. This type is generally used for inspection of small
precision machined parts
The dial gauge with various attachments are commonly
used
This comparator is ideal for the checking of components
with a tolerance of + 0.005 mm
16. Working of the comparator
A very thin metal strip at the centre carries a light
pointer made up of glass .
One end of the strip is connected to the adjustable
cantilever strip and the other end is to the spring
elbow, in turn connected to the plunger.
The slight movements of the plunger will make the
bell crank lever to rotate.
This rotation will create tension in the strip and causes
the strip to rotate thereby the strip start to untwist
resulting in the movement of the point
17. The spring ensures that the plunger returns when the
contact is removed
The length of the cantilever can be varied to adjust the
magnification
The mechanical magnification is obtained by
“Abramson Movement”
19. The gauging head is usually a sensitive, high quality,
dial indicator mounted on a base supported by a sturdy
column.
The reed mechanism is frictionless device for
magnifying small motions of spindle.
It consists of a fixed block A which is rigidly fastened to
the gauge head case, and floating block B, which carries
the gauging spindle and is connected horizontally to the
fixed block by reeds C
20. Vertical reed is attached to each block with upper
ends joined together. These vertical reeds are shown
in the figure by letter D. Beyond this joint extends a
pointer or target.
A linear motion of the spindle moves the free block
vertically causing the vertical reed on the floating
block to slide past the vertical reed on the fixed
block.
How- ever, as these vertical reeds are joined at the
upper end, instead of slipping, the movement causes
both reeds swing through an arc and as the target is
merely an extension of the vertical reeds, it swings
through a much wider arc.
21. The amount of target swing is proportional to the
distance the floating block has moved but of course
very much magnified.
The scale may be calibrated by means of gauge block
(slip gauges) to indicate any deviation from an initial
setting.
Sensitivities of the order of 0.25 micron per scale
division.
The mechanical amplification is usually less than 100,
but it is multiplied by the optical lens system. It is
available in amplifications ranging from x 500 to x
1000.
23. The plunger is mounted on a pair of slit diaphragms in
order to have frictionless linear movement. A knife
edge is mounted on it and bears upon the face of the
moving member of a cross strip hinge.
The cross strip hinge consists of the moving
component and a fixed member which are connected
by thin flexible strips alternately at right angle to each
other.
Thus if an external force is applied to the moving
member ; it will pivot, as would a hinge, about the
line of intersection of the strips.
To the moving member an arm of Y shape and having
effective length I is attached.
24. Advantages of mechanical comparators
1.Cheaper than all the other type of comparators
2.Does not require any external source of power or
air supply
3.These comparators use a linear scale that can be
easily understood.
4.Usually these comparators are robust and compact
but are very easy to handle
5.These are small in size and can are portable from one
place to other very easily without much difficulty
25. Disadvantages
1.Contains more number of moving parts so there
develops friction which in turn reduces the accuracy
2.Slackness in the moving parts reduces accuracy very
drastically
3.These have more inertia so the instrument is prone to
vibrational effects
4. Limited range of the instrument is another drawback
as the pointer moves over a fixed scale
5. Parallax error may also arise when proper scale is
used
27. In mechanical optical comparators small displacements
of the measuring plunger are amplified first by a
mechanical system consisting of pivoted levers.
The amplified mechanical movement is further
amplified by a simple optical system involving the
projection of an image.
The usual arrangement employed is such that the
mechanical system causes a plane reflector to tilt about
an axis and the image of an index is projected on a
scale on the inner surface of a ground-glass screen.
Optical magnification provides high degree of measuring
precision due to reduction of moving members and
better wear resistance qualities.
Optical magnification is also free from friction, bending,
wear etc.
