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Transcript of metrology
STANDARDISATION AND INTERCHAGEABILITY copy
Component selected randomly should assemble
correctly with any other mating component.
This is interchangeability. Possible when certain
standards are strictly followed‐‐‐international
standards
Reqd fit in an assembly can be obtained by:
• Universal or full interchangeability
• Selective assembly
Full interchangeability :‐
Any component will match with any other mating part
without classifying manufactured components into
subgroups or carrying out minor alterations.
Selective assembly:
In this parts are graded according to size and only matched
grades of mating parts are assembled e.g.
Mating of piston in car cylinder – The bore size is 63.5mm
and the skirt clearance of piston is 0.13mm on the
diameter, The tolerance on bore and on piston skirt each is
0.04mm.
HL of bore – 63.5+0.02 = 63.52mm
LL of bore ‐ 63.5‐0.02 = 63.48mm
Piston bore is 63.5‐ 0.13 = 63.37 mm
HL of piston – 63.37+0.02=63.39
LL of piston ‐ 63.37 – 0.02=63.35
Max clearance = HL of bore – LL of piston =
63.52‐63.35=0.17mm
Min clearance= LL of bore‐ HL of piston =
63.48‐ 63.39=0.09mm
By grading bore and piston, selective assembly will give :
Cylinder bore 63.48 63.5 63.52
Piston 63.35 63.37 63.39
Comparators:
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
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.
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.
Mechanical Comparator:
• In Mechanical type, the required magnification is obtained by
using mechanical linkages, levers, gears and other
mechanical devices.
• Magnification (M): It is the ratio between the movement of
the plunger and the resultant movement of the pointer
M= P(pointer movement)/p Plunger movement
SIGMA COMPARATOR( Mechanical Comparator):
It has magnifications in the range of 300 to 5000.
It has a plunger attached to a rectangular bar which is
supported at its upper and lower ends by flexure plates.
A knife edge is fixed to the side of rectangular bar which
bears on a moving block.
The moving block and the fixed block are connected
by flexible strips at right angles to each other.
Scale
Drum
Bronze band
Forked arm
Plunger
Flexure plate
Fixed block
Flexible strips
Flexure plate
Pointer
Y X
Knife edge
Moving block
If an external force is applied to the moving block, it would pivot
about the hinge .
A forked arm or Y-arm attached to the moving block transmits the
rotary motion to the indicator driving drum through a bronze band
wrapped around the drum.
Magnification: If Y is the length of forked arm and X is the distance
from the knife edge to hinge, then first stage magnification is Y/X
If the pointer length is R and the radius of the drum is r the second
stage magnification is R/r such that the total magnification is
(Y/X)* (R/r)
Advantages of Mechanical Comparators
• They are cheaper compared to other amplifying
devices
• Do not require electricity or air and such the
variations in the outside sources do not affect the
accuracy.
• They have a linear scale, robust and easy to handle.
Disadvantages of Mechanical Comparators
• They have more linkages due to which friction is more and
accuracy is less.
• Mechanisms used have more inertia and hence they become
sensitive to vibrations.
• The range of the instruments is limited as the pointer moves
over a fixed scale.
• It is also difficult to incorporate arrangement for adjusting
magnification.
O
d
C
A
ScreenLamp
Pivot
Mirror
Plunger
NormalO
C
A
Screen
Lamp
Mirror
Normal
d
2
h angleof tilt
PRINCIPLE OF OPTICAL LEVER
OPTICAL COMPARATORS
OPTICAL COMPARATORS
• An optical comparator works on one of the following two main principles:
1)Use of optical lever 2) Use of enlarged image
– If a beam of light AC is directed on to a mirror as shown in fig, it will
be reflected onto the screen at O as a dot.
The angle of incidence = angle of reflection
– When the plunger moves up, the mirror tilts by . Then the reflected
beam moves through 2 . i.e. the illuminated dot moves to B.
– Also the increase in distance CO of the screen from the tilting mirror
will increase the magnification.
In this comparator, small displacements of the
measuring plunger are first amplified by a mechanical
lever. It is equal to (L2/L1).
The amplified mechanical movement is further
amplified by optical system due to tilting of mirror by .
The reflected ray D (image of index line) will be turned
through an angle 2, and hence optical amplification
=2(L4/L3)
The overall magnification = (L2/L1)* 2(L4/L3)
L1 L2 L3
L4
PivotMeasuringPlunger
Mecahnicallever
Screen with scale
Light source
Mirror pivot
Mirror
Projection lens
Glass plate carrying indexline
Collimating lens
D
Mechanical-Optical Comparator
Advantages & Disadvantages of Optical comparators
• Advantages:
1) Few moving linkages and hence no friction & wear.
2) High range of measurements and no parallax error
3) Magnification is usually high.
• Disadvantages:
1) Heat from the source of light, transformers etc., may cause the setting to drift.
2) An electric supply is required to operate these comparators.
3) The size is large and costly.
4) Use of eyepiece to view is inconvenient for prolonged use. Also a dark room is essential to take readings.
Electrical& Electronic Comparators
R1 R2
R3R4
Battery
Coils
Arm
Measuring
PlungerCoils
IronArmature
Electrical& Electronic Comparators
• These comparators depend on the principle of balancing the
Wheatstone bridge,
(R1/R2) = (R3/R4) applicable for only to direct current
obtained from a battery.
• In actual instruments, one pair of inductances takes the form
of a pair of coils in the measuring head of the instrument.
• An iron armature inside these coils moves along with the
measuring plunger, and upsets the balance of the circuit and
causes the deflection of the pointer meter which is calibrated
directly in linear units.
• Magnifications of the order of 30,000 are possible.
Pneumatic Comparators (Solex Gauge):
Principle:
It works on the principle of pressure difference generated by the
air flow.
Air is supplied at constant pressure through the orifice and the air
escapes in the form of jets through a restricted space which
exerts a back pressure.
The variation in the back pressure is then used to find the
dimensions of a component.
Working:
As shown in Figure (a) the air is compressed in the compressor at
high pressure which is equal to Water head H.
The excess air escapes in the form of bubbles. Then the metric
amount of air is passed through the orifice at the constant
pressure.
Due to restricted area, at A1 position, the back pressure is
generated by the head of water displaced in the manometer
tube.
To determine the roundness of the job, the job is rotated along the
jet axis, if no variation in the pressure reading is obtained then we
can say that the job is perfectly circular at position A1.
Then the same procedure is repeated at various positions A2, A3,
A4, position and variation in the pressure reading is found out.
Also the diameter is measured at position A1 corresponding to the
portion against two jets and diameter is also measured at various
position along the length of the bore.
Fluid Displacement Comparators
In this comparator, a fine bore capillary tube is arranged so that its
lower end is placed in a chamber of relatively large cross-sectional
area and containing a fluid of low viscosity.
At the bottom of the chamber, a diaphragm is fitted, which gets
deflected by pressure transmitted to it by the measuring plunger.
Due to deflection of diaphragm, a small quantity of liquid rises up
in the capillary tube.
A scale is arranged at the side of tube and limit pointers are set
relative to this to indicate the high and low limits of the dimension
being measured. These limits are obtained by master setting.
Instrument body comprises of a cone shape with a thin base
containing a small button centrally. Any movement of the
base causes a displacement of liquid up the tube to indicate
actual dimension.
The magnification of this type of comparator is usually of the
order of 1500. Various disadvantages of the instrument are :
(1) It is too much dependent upon the temperature as the fluid
changes its volume with change in temperature.
(2) The deflection of the diaphragm per unit measuring force is
not constant.
(3) There is certain passivity in the instrument due to the
characteristics of diaphragm and the viscosity of the fluid.
Automatic Gauging Machines.
These machines incorporate comparator amplifying
methods and are similar to multicheck devices.
They eliminate manual inspection and parts are
inspected for all the dimensions simultaneously and
segregated and classified automatically.
Coordinate Measuring Machine (CMM) is a 3-dimensional measuring
device that uses a contact probe to detect the surface of the object.
The probe is generally a highly sensitive pressure sensing device that
is triggered by any contact with a surface.
The linear distances moved along the 3 axes are recorded, thus
providing the x, y and z coordinates of the point.
CMMs are classified as either vertical or horizontal, according to the
orientation of the probe with respect to the measuring table.
Advantages and Disadvantages of CMM
Advantages:
1) High precision and accuracy.
2) Requires less labor.
3) Accurate dimensions can be obtained just by knowing
the coordinates and distance between the two reference
points.
Disadvantages:
1) The Coordinate measuring machines are very costly.
2) The CMMs are less portable.
3) If the operating software cracks down it is difficult to
restart the entire system.
1) It needs to construct some feature on its own as some
parts of the work piece are unreachable by the probe.