Mechanical Comparators (Metrology)

5.5.
In these comparators, magnification is obtained by mechanical linkages and other
mechanical devices.
5.5.1.

Systems of Displacement Amplification used in Mechanical Comparators

(i) Rack and Pinion. In it the measuring spindle integral with a rack, engages a pinion
which amplifies the movement of plunger through a gear train. (Refer Fig. 5.1)
(ii) Cam and gear train. In this case the measuring spindle acts on a cam which
transmits the motion to the amplifying gear train. (Refer Fig. 5.2)
(iii) Lever with toothed sector. In this case a lever with a toothed sector at its end
engages a pinion in the hub of a crown gear sector which further meshes with a final pinion to
produce indication. (Refer Fig. 5.3)
Rack and pinion
Fig. 5.1. Rack and pinion.
Cam and gear train
Fig. 5.2. Cam and gear train.
Lever with toothed gear
Fig. 5.3. Lever with toothed gear.
Compound levers.
Fig. 5.4. Compound levers.
Twisted taut strip
Fig. 5.5. Twisted taut strip.
Lever combined
Fig. 5.6. Lever combined
with band wound around drum.
(iv) Compound Levers. Here levers forming a couple with compound action are
connected through segments and pinion to produce final pointer movement.
(v) Twisted Taut Strip. The movement of measuring spindle tilts the knee causing
straining which further causes the twisted taut band to rotate proportionally. The motion of
strip is displayed by the attached pointer.
(vi) Lever combined with band wound around drum. In this case, the movement
of the measuring spindle tilts the hinged block, causing swing of the fork which induces rotation
of the drum.
(vii) Reeds combined with optical display. In this case parallelogram reeds are used
which transfer measuring spindle movement to a deflecting reed whose extension carries a
target utilised in optical path.
(viii) Tilting mirror projecting light spots.
5.5.2.


Dial Indicator.

One of the most commonly used mechanical comparators is
essentially of the same type as a dial indicator. It consists of a robust base whose surface is
perfectly flat and a pillar carrying a bracket in which is incorporated a spindle and indicator.
The linear movement of the spindle is magnified by means of a gear and pinion train into
sizable rotation of the pointer on the dial scale. The indicator is set to zero by the use of slip
gauges representing the basic size of the part. This is generally used for inspection of small
precision-machined parts. This type of comparator can be used with various attachments so
that it may be suitable for large number of works. With a V-block attachment it can be used
for checking out-of-roundness of a cylindrical component.
5.5.3.

The Johansson ‘Mikrokator’.

This comparator was made by C.F. Johansson
Ltd. and therefore named so. It is shown diagrammatically in Fig. 5.7. This instrument uses
the simplest and most ingenious method for ob-
taining the mechanical magnification designed
by H. Abramson which is called Abramson Move-
ment. It works on the principle of a button spin-
ning on a loop of string. A twisted thin metal strip
carries at the centre of its length a very light
pointer made of thin glass. The two halves of the
strip from the centre are twisted in opposite
directions so that any pull on the strip will cause
the centre to rotate. One end of the strip is fixed
to the adjustable cantilever strip and the other
end is anchored to the spring elbow, one arm of
which is carried on the measuring plunger. As the
measuring plunger moves either upwards or
downwards, the elbow acts as a bell crank lever
and causes twisted strip to change its length thus
making it further twist or untwist. Thus the
pointer at the centre of the twisted strip rotates
by an amount proportional to the change in
length of strip and hence proportional to the
plunger movement.
The bell crank lever is formed of flexible strips with a diagonal which is relatively stiff.
The length of cantilever can be varied to adjust the magnification of the instrument. Since the
centre line of the strip is straight even when twisted, therefore, it is directly stretched by the
tension applied to the strip. Thus in order to prevent excessive stress on the central portion,
the strip is perforated along the centre line by perforations as shown in Fig. 5.7.
Johansson Mikrocator.
Fig. 5.7. Johansson Mikrocator.
tmpB-8_thumb
mid-point of strip with respect to the end, I is the length of twisted strip measured along its
neutral axis and w is the width of twisted strip and n is the number of turns.
It is thus obvious that in order to increase the amplification of the instrument a very
thin rectangular strip must be used. Further amplification can be adjusted by the cantilever
strip- which also provides anchorage. It increases or decreases effective length of strip. Final
setting of the instrument amplification is made by a simple adjustment of the free length of
cantilever strip.
A slit C washer as shown in Fig. 5.7 is used for the lower mounting of plunger. Thus
this instrument has no mechanical points or slides at which wear can take place. The
magnification of the instrument is of the order of x 5000.
5.5.4.

Reed Type Mechanical Comparator.

In mechanical comparator, the gauging
head is usually a sensitive, high quality, dial indicator mounted on a base supported by a sturdy
column. Fig. 5.8 shows the reed type mechanical comparator.
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. A 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. 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.
Comparators using this type of linkage have
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.
Reed type mechanical comparator.
Fig. 5.8. Reed type mechanical comparator.
5.5.5.

The Sigma Comparator.

Fig. 5.9 shows the constructional details of the Sigma
Mechanical Comparator.
The vertical beam is mounted on flat steel
springs A connected to fixed members, which in turn
are screwed to a backplate. The assembly provides a
frictionless movement with a restraint from the
springs.
The shank B at the base of the vertical beam is
arranged to take a measuring contact, selecting from
the available range.
The stop C is provided to restrict movement at
the lower extremity of the scale.
Mounted on the fixed members, is the hinged
assembly D carrying the forked arms E. This assembly
incorporates a hardened fulcrum (provided with
means for adjustment of controlling the ratio of trans-
mitted motion) operative on the face of a jewelled insert
on the flexible portion of the assembly.
The metal ribboni^, attached to the forked arms,
passes around the spindle G causing it to rotate in
specially designed miniature ball bearings. Damping
action to the movement is affected by a metal disc,
mounted on the spindle, rotating in a magnetic field
between a permanent magnet and a steel plate. The
indicating pointer H is secured to a boss on the disc.
Sigma mechanical comparator
Fig. 5.9. Sigma mechanical comparator.
The trigger J (opposite K) is used to protect the measuring contact. At the upper end of
the vertical beam, an adjusting screw is provided for final zero setting of the scale.
A new patented feature is shown at K. This is a magnetic counter-balance which serves
to neutralise the positive “rate” of springs reaching on the measuring tip. In this way a constant
pressure over the whole scale range is achieved.
The instrument is available with vertical capacities of 150 mm, 300 mm and 600 mm
and magnifications of500,1000,1500,3000 and 5000. The scales are graduated in both English
and Metric systems.
The least count which one division represents is of the order of 0.25 microns.
Advantage : It has got a bold scale and larger indicating pointer.
Disadvantages : (i) Due to motion of the parts, there is wear in the moving parts.
(ii) It is not as sensitive as optical or other types of comparators due to friction being
present in the moving parts.
5.5.6.1.

Diagrammatic sketch showing the movement of sigma comparator.

The
various movements in the ‘sigma’ comparator discussed in Art. 5.5.6, will be very clear with
the help of Fig. 5.10 which shows the various movements in diagrammatic form.
Movement of sigma comparator
Fig. 5.10. Movement of sigma comparator.
The plunger in Fig. 5.10 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. For details of cross strip hinge refer Fig. 5.11. 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. If
the distance of the hinge from the knife edge be a then the magnification of the first stage
tmpB-12_thumb The cross strip hinge used in sigma comparator
Fig. 5.11. The cross strip hinge used
in sigma comparator.
 Comparator Movement used in comparator
(a) Comparator (b) Movement used
in comparator.
Fig. 5.12. Mechanical comparator using
rocking prism.
In order to adjust the magnification, distance a must be changed by slackening and
tightening the two screws attaching the knife edge to the plunger.
Some of the interesting features of the instrument are :
(1) As the knife edge moves away from the moving member of the hinge and is followed
by it, therefore, if too robust movement of the plunger is made due to shock load, that will not
be transmitted through the movement.
(2) By mounting a non-ferrous disc on the pointer spindle and making it move in field
of a permanent magnet, dead beat readings can be obtained.
(3) The error due to parallax is avoided by having a reflective strip on the scale.
(4) The constant measuring pressure over the range of the instrument is obtained by
the use of a magnet plunger on the frame and a keeper bar on the top of the plunger. As the
plunger is raised the force required increases but the keeper
bar approaches the magnet and the magnetic attraction bet-
ween the two increases. Thus as the deflecting force increases,
the assistance by the magnet increases and total force remains
constant.
5.5.7.

Mechanical Comparator.

The diagram of a type
which is used for comparative measurement of external sur-
faces is shown in Fig. 5.12 (a). In this instrument the move-
ment of the measuring tip attached at the end of the spindle is
transmitted to the pointer through a mechanism shown in Fig.
5.12 (b). The upper end of the spindle bears against a rocking
prism (knife-edge). There is a frame member having two V-
slots offset to each other by distance a. The end of the spindle
rests against the first V-slot and its movement is transmitted
to this frame through a prism. A knife edge which is stationary
relative to the body of the instrument enters the upper V-slot.
The apex of the upper knife-edge is the centre for all the moving
parts of the comparator. The distance a between; the V-slots
forms the shorter lever arm of the system, whereas, the longer
Mechanical comparator using sector and pinion
Fig. 5.13. Mechanical comparator
using sector and pinion.
lever arm is the distance L from the centre of rotation of the system to the other end of hand,
which moves along the comparator scale. Thus the magnification of the instrument is L/a and
is of the order of 1000. The contact pressure is of the order of 300 to 400 grammes and is
provided by a spring. The use of knife-edge pivots in the comparator movement excludes the
influence of possible clearance in the pivots on the accuracy of this instrument.
Yet another type of mechanical comparator is shown in Fig. 5.13 in which the movement
of the plunger (contact member) is transmitted to the pointer through an angular lever which
ends in the form of a sector and pinion. It may be noted that a spring attached with the angular
lever avoids the play in comparator.

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