Engineering Measurement1

7/30/2019 Engineering Measurement1

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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.
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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
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Engineering Measurement1