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By the end of the lesson, students should be able to;
State and explain the system of measurement
Explain the term lobbing in measurement
Use the engineers try square to check for squareness.
Use engineers blue and surface plate for checking flatness.
Use angle gauges to measure angles.
Demonstrate a test for roundness
Use slip gauge to calibrate tolerance, limits and fits
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In the engineering industry accuracy is critical. Manufactured parts must fit
and do exactly what they are designed to do. For example, a piston must fit
exactly into the cylinder bore for an engine to work properly. Therefore it is
important that all drawing measurements are accurate.
You may be required to calculate the perimeter,
circumference, area and volume of the components that you manufacture.
To do this you will need to take linear and angular measurements in your
day to day work.
http://metal.brightcookie.com/shared/glossary/glossary_p.htmhttp://metal.brightcookie.com/shared/glossary/glossary_c.htmhttp://metal.brightcookie.com/shared/glossary/glossary_a.htmhttp://metal.brightcookie.com/shared/glossary/glossary_v.htmhttp://metal.brightcookie.com/shared/glossary/glossary_v.htmhttp://metal.brightcookie.com/shared/glossary/glossary_a.htmhttp://metal.brightcookie.com/shared/glossary/glossary_c.htmhttp://metal.brightcookie.com/shared/glossary/glossary_p.htm7/30/2019 Engineering Measurement1
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MEASUREMENT
Measurement is defined as the process of numerical evaluation
of a dimension or the process of comparison with standard
measuring instruments. The elements of measuring system
include the instrumentation, calibration standards,
environmental influence, human operator limitations and
features of the work-piece.
The basic aim of measurement in industries is to check whether
a component has been manufactured to the requirement of a
specification or not.
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There are two major systems of measurements used
world wide. These are;1. The imperial system and
2. The metric system of measurement.
THE IMPERIAL SYSTEM OF MEASUREMENTThis is often called the English system of measurement, which is
widely used in the United States and Canada. The unit of length in
the inch system is the INCH, which may be divided into fractional
or decimal fraction divisions.
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The decimal fraction system has a base of ten (10), so any
number may be written as a product of ten or a fraction of ten
as shown in the table below.
Value Fraction Decimal
One tenth 1/10 0.1
One hundredth 1/100 0.01
One-thousandth 1/1000 0.001
One-tenthousandth
1/10000 0.0001
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Linear metric dimensions are expressed in multiples and sub-
multiples of the meter. In the machine tool trade, the millimeter
(mm) is used to express most metric dimensions. Fractions ofthe millimeter are expressed in decimals.
Most often in the machine shop, dimensions are given in
millimeters (mm). Very large dimensions are given in meters
(m).
A brief comparison of common inch and metric equivalent is
shown below.
1 yard 36 inch1 m 39.37in
1000m 1 kilometer
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Length
STEEL RULES: are used for measuring lengths in the
workshop. The various types of steel rules include;
1. The metric steel rule
2. The fractional steel rule.
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THE METRIC STEEL RULE
The metric steel rule is
usually graduated in
millimeter (mm) and half-millimeter, which are used for
making linear metric
measurement and do not
require great accuracy. A widerange or variety of metric
rules are available in lengths
from 15cm (150mm) to
1m.Fig 1a & 1bFairly accurate measurements
can be made using steel rules.
Fig. 1a Metric steel rule.
Fig. 1b Metric steel rule.
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FRACTIONAL STEEL RULE
The fractional steel rule is mostly the
inch steel rule. Several varieties of
these rules may be used in the
machine shop work, such as spring
tempered, flexible, narrow and hook
rules. As shown in Fig1c.
Their length ranges from about 1-72
inch. These rules are also used formeasurement which do not require
great accuracy.Fig. 1c. inch steel rule
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FLATNESSA straight edge is used to check surface for
flatness and to act as a guide for scribing
long, straight lines in layout work.
Straight edges are generally rectangular
bars of hardened and accurately ground
steel, having both edges flat and parallel.
They are supplied with either plain or
beveled edges.Fig.3 shows a sketch of a
straight edge.
Long straight edges are generally made of
cast iron with ribbed construction.
Fig. 3 Straight edge
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STRAIGHTNESS A spirit level fig 4a&b, is used to check for
straightness. The spirit level consist
essentially of a glass vial containingspirit and filled except for a bubble.
The inside surface of the glass
container is curved to a large radiuseither by curving the whole tube or by
shaping its inside to the form of a
barrel. The vial is set in a base and is
adjusted so that when the base is
horizontal, the bubble rests at the
centers of a scale which is engraved on
Fig.4.spirit levels
(a)
Fig 4(b) Spirit level
SQUARENESS
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SQUARENESS The square is a very important tool used by the
machinist for layout, inspection and set up
purposes. Squares are manufactured to various
degrees of accuracy, ranging from semi-precision
to precision squares as shown in fig 5. Precision
squares are hardened and accurately ground.
The most commonly used standard for squareness
in the workshop is the engineers squares or try-
square. The square must be checked against a
standard to ensure that its accuracy is maintainedand the try square is the standard of squareness for
checking squares.
Fig. 5 Try square
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Slip Gauges: Slip gauges are rectangular blocks of steel having a
cross-section of about 32mm by 9mm which before being
finished to size are hardened and carefully matured so that they
are independent of any subsequent variation in shape and size.
Slip gauges are used to check the accuracy of micrometers,
vernier and other gauges. These consist of blocks of different
thickness, which are made to such a fine degree of accuracy and
flatness on their measuring faces. In the workshop, these blocks
can be used to build up any required length.
Th li i d t
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The slip gauge is used to
calibrate micrometers and
other gauges. This is done by
summing or packing the
blocks to a specific length and
then compared with the
reading of the micrometer or
any other length measuring
tool.
The maximum permissible
errors a slip gauge can give
/unit = 1/100,000 mm.
Fig 2 shows a set of slip
gauges.
Fig. 2 Set of slip gauges
E ample 1:
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Example 1:
Choose block to assemble a length of 48.51 mm.
solution
To assemble 48.51mm, we may use the following blocks. (mm).and summing them.
10.00
26.00
10.02
2.31
--------------
48.51 mm
---------------
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Engineers blue: It is a highly pigmented paste used to assist in
the mating of two or more components.
Engineers blue is prepared by mixing Prussian blue with a non-
drying oily material (for example grease).
Surface plate: A surface plate is a rigid block of granite or cast
iron, the flat surface of which is used as a reference plane for
layout and inspection work. A surface plate is a reference
surface which are hardened for testing other flat surface.
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The surface to be checked is first carefully cleaned and any
burrs are removed with an oil stone.
Engineers blue is then smeared lightly over its surface. The blued surface is then inverted over an inspection grade
surface and gently moved in series of small circles. This will
transfer the blue to the high spots of the scraped reference
surface lightly and uniformly.Fig6a
The surface to be checked is now cleaned of the blue and
brought once more into contact with the surface plate.
Blue will now be transferred back to the high spots
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Blue will now be transferred back to the high spotson the surface under test. Fig 6b.
Fig 6a(workpiece being gently moved in series of small circles).
Fig6b (engineers blue on high spot).
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Lobbing is a form of out-of-roundness which can occur when a
capstan lathe roller tool box is incorrectly set or when centerless
grinding machine is incorrectly set. Lobbing can never occur when machining components between
centers.
The simplest lobbed figure has three lobes and is based on an
equilateral triangle and tangential radii R and r are struck fromthe corners of the triangle as indicated fig7a below.
The diameter (R+r) appear to be the diameter of the figure
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The diameter (R+r) appear to be the diameter of the figure
which will behave as though it were truly cylindrical when
tested between the jaws of the vernier caliper as shown in the
fig7b below.
Fig7a (Simple lobbed figure). Fig 7b.(vernier caliper and component)
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Roundness can be tested by using a dial test indicator(dial
gauge).
The dial test indicator consist of a circular case which isclamped to a suitable rigid base when the instrument is in use.
To test for roundness,(for instance a solid component with two
diameters with the same center holes);
1. One diameter is supported in a vee block, whilst
2. The dial test indicator rest upon the other diameter.
3 If the diameters are concentric the dial test indicator reading
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3. If the diameters are concentric, the dial test indicator reading
will remain constant.
4.If the diameters are eccentric, the dial test indicator will vary
cyclically as shown in the fig8 below.Fig8(vee block and test indicator)
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ANGLE GAUGES
fig. 9 (angle gauge)
Angle Gauges (fig. 9) are available forbuilding up combinations for angulartesting, as slip gauges serve for thechecking of lengths. These are wedge
shaped and their working faces arefinished in the same manner as lengthslip gauges, enabling them to be wrungtogether in combination. A full setcomprises twelve pieces as follows; 1, 3,9, 27, and 41 degrees; 1, 3, 9, and 27minutes; 0.1, 0.3 and 0.5 minute. Used inconjunction with a precision squareblock, these enable any angle between 0degrees and 360 degrees to be built up insteps of 6 seconds (0.1 degrees) bywringing blocks together in an additiveor subtractive manner.
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The angle gauge must be supplied in a kit (box), in which the
main blade, short blade, with a built-in magnifier, and a right
angle blade are included to prevent scattering of gauges and to
ensure accurate measurement.
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Inaccuracies: Inaccuracies are factors that can creep in and
degrade results of measurement. Inaccuracies include thefollowing;
Pressure or feel.
Tool alignment.
Dirt and burrs.
Parallax (The perspective problem)
Tool Wear
Heat Damage
Bias-the built-in error.
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This is the most common inaccuracy factor associated with
engineering measurement, especially those that are hand
calibrated.
That is, they must be used with the same touch or pressure in
measuring as that to which they were calibrated.
TOOL ALIGNMENT
The second most common inaccuracy factor is getting the
measurement along the axis intended. For example, when
measuring the thickness of any object, if the measuring tool is not
aligned (perpendicular to the sides), it could be measuring an
incorrect distance.
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Although this inaccuracy is listed as third, for
inexperienced mechanist this might be first. The smallest
burr can be from 0.0030.005 inch.
Dust and atmospheric grime can account for another
0.001 error.
This accuracy can be controlled by regular cleaning and
keeping the tools in their cases when not in use.
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This is a visual problem that is easy to eliminate in some
instances and difficult to avoid in other. It is caused from not
sighting the tool straight on when measuring and reading
it. Example, Think of yourself as a passenger looking at a
speedometer needle, would the speed appear at its true
value or would you see it as slower or faster from your perspective
That perspective, sighting the needle to the numbers would
make it appear as though the car was going slower than reality.
This is known as parallax error.
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When a tool is used for sometime, there will be an
undetected wear which affects its measurement.
Example, Faces of micrometers wear where they are
constantly rubbed against the work. They become chamferedor rounded.
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Machining causes heat, which expands metals. If a part is
significantly above room temperature, the measured size unit
will be larger than when it returned to a standard or normalroom temperature.
DAMAGE
Similar to wear, this inaccuracy factor is often hidden. For
instance, if the frames of a micrometer is closed using too muchforce, although tough, it can bend and the faces can become out
of parallel.
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This error occurs when the machinist starts out to prove the
part is right, rather than to measure it. This factor is
psychological but real.
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In conclusion, measurement plays a unique and essential role in
the field of engineering. We hope engineering students have
understood the basis of engineering measurement since we have
talked about the system of measurement, lobbing in
measurement, using engineers blue and surface plate for
checking flatness, etc.
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Engineering Measurement Dr. A. Shibl
Galyer J.F.W
Pitman's Publication Ltd. London
Chapman W.A.J.(1976) workshop technology P3.
www. Mechanicsupport.com Timings R.T(1980) manufacturing .tech 2
http://metal.brightcookie.com/1_calc/calc_t2/htm/calc2_
2_1.htm
Shotbolt C.R(1972) metrology for engineers
http://metal.brightcookie.com/1_calc/calc_t2/htm/calc2_2_1.htmhttp://metal.brightcookie.com/1_calc/calc_t2/htm/calc2_2_1.htmhttp://metal.brightcookie.com/1_calc/calc_t2/htm/calc2_2_1.htmhttp://metal.brightcookie.com/1_calc/calc_t2/htm/calc2_2_1.htm