Unit-1 Measuring Instruments

Mechanical Measurement – 15ME33T 2020-21

DEPARTMENT OF MECHANICAL ENGINEERING GOVT POLYTECHNIC KALGI -585312

Unit-1 Measuring Instruments

1.1 Definition

The measurement of a given quantity is essentially an act or result

of comparison between a quantity whose magnitude (amount) is

unknown, with a similar quantity whose magnitude (amount) is known,

the later quantity being called a standard.

Since the two quantities, the amount of which is unknown and

another quantity whose amount is known are compared, the result is

expressed in terms of a numerical value. This is shown in fig. 1.1

Measurement is the basis of all scientific study and

experimentation. The process of comparison between measuring quantity

(an unknown magnitude) to the standard predefined fixed quantity, the

result obtained magnitude after comparison may or not be exactly same

with the standard quantity. May be acceptable deviation from the

standards.

1.2 Methods of Measurement

Two methods of measurement:

1) Direct comparison

2) Indirect comparison

1.2.1 Direct Comparison Method

In this method, measurement is made directly by comparing an

unknown magnitude with the primary or secondary standard.



Example: To measure the length of the metal bar, will measure it with

the help of the measuring tape or scale that acts as the secondary

standard.

1.2.2 Indirect Comparison Method

There are number of quantities that cannot be measured directly

by using some instrument. Hence in the indirect comparison method of

measurement, measurement is made by comparing an unknown

magnitude with a standard through the use of calibrated system.

The indirect method of measurements comprises of the system

that senses, converts, and finally presents an analogues output in the form

of a displacement or chart.

Example: To measure the strain in the machine member, a component

senses the strain and another component transforms sensed signal into an

electrical quantity which is then processed suitably before being fed to a

meter or recorder.

1.3 Terms Applicable to Measuring instruments:

1. Accuracy:

Accuracy of measuring instrument shows the closeness of the

measured value with the actual or true standard value. It is expressed in

the form of the maximum error (= measured value – true value) as

percentage of full scale reading.

Accuracy of the instrument depends on factors like static error, dynamic

error, reproducibility error etc.

Example, Obtained value of weight measurement is 6.3 kg for a given

quantity of

Substance, but actual weight is 9 kg, and then measurement is not

accurate. In this case, measurement is not close to the standard actual

value.



2. Precision:

Precision is defined as the repeatability of a measuring process.

The precision of an instrument indicates its ability to reproduce a certain

set of readings within a given accuracy.

OR

Precision refers to the closeness of two or more measurement results to

each other.

Fig 1.2

Example: Measured weight showing result same each time as 6.3 kg f o r

t h e given substance five times, then the measurement is very precise.

Precision is independent of accuracy

The fig. 1.2 illustrates the difference between accuracy and precision.

3. Sensitivity: It is the ability or capability of the measuring device to

detect small difference in a quantity being measured.

Example: Measuring the length in a device up to smallest division

0.001mm, then the device sensitivity is 0.001mm.

4. Repeatability: It is the ability of a measuring system to reproduce

output readings when the same input is applied to it repeatedly under the

same conditions and in same directions.

It may be expressed as the maximum difference in results between the

different output readings for the same input.



5. Range (or span): It represents the highest and lowest possible value

that can be measured by an instrument. It is the difference between the

largest and smallest results of measurement.

Or

It defines the maximum and minimum values of the inputs or the

outputs for which the instrument is recommended to use.

6. Threshold: The minimum value of input signal that is required to

make a change or to start from zero in the output is called as threshold.

Or

It is the minimum value of input below which no output can

be detected.

7. Back lash: It is the maximum distance through which one part of the

instrument may be moved without disturbing the other part.

8. Hysteresis: It is defined as the magnitude of error caused in the output

for a given value of input, when the value is measured from opposite

directions. i.e. from ascending order and then descending order.

This is may be because backlash, elastic deformations, magnetic

characteristics, but it is mainly caused by the frictional effects.

Thus we obtain two different values of output for the same input

under increasing and decreasing conditions. In case of instruments which

are used on both sides of zero i.e input applied on both positive and

negative sides, the variation of output is as shown in figure.

Fig 1.3



9. Calibration: A known (output of this input is known) input is supplied

to the measurement system and the system output is noted, if the system

output deviates with respect to the given known value, corrections are

made in the instrument so that the output matches the input. This process

is called calibration.

The calibration of the instrument is made to find its accuracy,

precision of the instrument. The calibration procedure compares an

"unknown" or test item(s) or instrument with reference to standards

(Primary and Secondary standards)

Standard procedure for calibration

Measuring instrument are calibrated with following standard procedures.

a) Cleaning of instruments: Instrument should be cleaned

thoroughly before calibration.

b) Determination of error: Determine the errors in the instruments

by various applicable methods.

c) Check for tolerable limits: The errors are to be compared with

the allowable tolerances.

d) Minor Changes: To minimize the errors in the readings if

possible made some minor changes in the instruments.

e) Allotment of calibration setup: Each instrument is allowed to

setup as per its standards.

f) Next Calibration date: Allot the next calibration date to calibrate

the instruments.

1.4 Errors in measurements

An error may be defined as the difference between the measured

value and the actual value.

No measurement can made with perfect accuracy; therefore it is

important to find out what accuracy actually is and what different errors



that have entered into the measurement are. A study of errors is a first

step in finding ways to reduce them. Errors may arise from different

sources and are classified as

1.4.1 Types of Errors in Measurement System

Usually errors are classified into three types:

1. Systematic errors or fixed errors.

a) Instrumental Errors

b) Environmental Errors

c) Calibration errors

d) Observational Errors

e) Theoretical Errors

f) Human errors

g) Loading errors

2. Random errors or Accidental errors

Errors by fluctuating environmental conditions.

Judgement errors

Insufficient sensitivity of the measuring instrument

3. Illegitimate errors.

Blunders or mistakes

Computational errors

Chaotic errors

1. Systematic Error

The Systematic errors that occur due to fault in the measuring device

are known as systematic errors. These errors can be detached by

correcting the measurement device. These errors may be classified into

different categories.



Instrumental Errors: Instrumental errors occur due to wrong

construction of the measuring instruments. These errors may occur due to

hysteresis or friction. These types of errors include loading effect and

misuse of the instruments. In order to reduce the gross errors in

measurement, different correction factors must be applied and in the

extreme condition instrument must be recalibrated carefully.

Environmental Errors: The environmental errors occur due to some

external conditions of the instrument. External conditions mainly include

pressure, temperature, humidity or due to magnetic fields. In order to

reduce the environmental errors

Observational Errors: As the name suggests, these types of errors occurs

due to wrong observations or reading in the instruments particularly in

case of energy meter reading. The wrong observations may be due to

parallax. In order to reduce the parallax error highly accurate meters are

needed: meters provided with mirror scales.

Theoretical Errors: Theoretical errors are caused by simplification of the

model system. For example, a theory states that the temperature of the

system surrounding will not change the readings taken when it actually

does, then this factor will begin a source of error in measurement.

2. Random Errors

Random errors are caused by the sudden change in experimental

conditions and noise and tiredness in the working persons. These errors

are either positive or negative. An example of the random errors is during

changes in humidity, unexpected change in temperature and fluctuation in

voltage. These errors may be reduced by taking the average of a large

number of readings.

3. Illegitimate errors.



These are errors which occur by mistakes in reading or computing

the results. Chaotic errors are those introduced in the measurements due

to high vibrations, shock to the instruments or electrical noise.

1.5 Factors in selecting the measuring instruments.

The following factors are considered in selection of measuring instruments

1. Accuracy expected from the instrument.

2. 2 .The time of taking final data required for the measurement.

3. Cost of measuring instrument.

4. The type of data displayed, i.e., indicating, recording, photograph

etc.

5. The measurement quantity to be measured has constant value or

is it a time variant. (Ex: Linear or Parabola)

6. Safety in use.

7. Adaptability to different sizes of inputs.

1.6 Thread measurements.

1.6.1 Thread Gauge Micrometre.

Fig 1.4



A thread gauge micrometer is a device used to measure the pitch

diameter of screw thread. It is just like an ordinary micrometer and it has

a pointed spindle and a double V- anvil, both are correctly shaped to

contact the screw thread of the being gauged as shown in fig 1.4

The anvil is not fixed which is free to rotate. Thus V- anvil can

accommodate itself to any rake angle of thread. When the conical spindle

is brought into contact with the Vee shape anvil, micrometer reads zero.

Different set of anvils are provided for different types of threads. It

directly reads in terms of pitch diameter.

The angle of Vee anvil and the conical point at the end of

the spindle corresponds to the included angle of the profile of the thread.

The micrometer spindle is moved over a screw whose pitch diameter is to

be measured. When double Vee shape anvil and conical shape of

spindle contacts with the thread flanks, the micrometer reading should

be taken, which is the measure of pitch diameter of screw thread.

1.6.2 Bench Micrometer

Fig 1.5 Bench Micrometer

A bench micromere is used for linear measurement and it is a best

suitable instrument for thread measurement where the work must be



brought to the gauge. The simple bench micrometer diagram is shown in

the fig. 1.5

The work is held between the anvil and spindle on an adjustable

table which can be raised to a selected height and locked in position by

turning a knurled thumb screw on back of the base made of cast iron with

black wrinkle finish. The base is heavily proportioned to sustain gauge

accuracy and assure stability in use.

When the job is locked to a required height then the micrometer is rotated

to move the spindle along with anvil to have a gental touch on the point

of the workpiece where measurement is to be done for major diameter.

Then note down the reading on the micrometer which is the measurement

of linear dimension. The dial indicator is used when the micrometer is

used as a comparator.

For measuring the minor diameter the arrangement of the anvils

over the thread as shown in figure 1.5

For checking the minor diameter the anvil end and spindle end have to

reach roots on opposite sides. But it doesn’t happen. Therefore the wedge

shaped pieces are held between the anvil face root of the thread and

spindle face root of the thread. And minor diameter is measured by

touching the tip of the wedges as shown in the fig 1.5

1.7 Angle measurements

1.7.1 Vernier Bevel Protractor (Universal Bevel Protractor)

Fig 1.6



It is a simplest instrument for measuring the angle between two

faces of a component. It consists of a base plate attached to a main body

and an adjustable blade which is attached to a circular plate containing

Vernier scale.

The adjustable blade is capable of sliding freely along the groove

provided on it and can be clamped at any convenient length. The

adjustable blade along with the circular plate containing the Vernier can

rotate freely about the center of the main scale engraved on the body of

the instrument and can be locked in any position with the help of a

clamping knob. The adjustable blade along with the circular plate

containing the Vernier can rotate freely about the center of the main scale

engraved on the body of the instrument and can be locked in any position

with the help of a clamping knob.

The main scale is graduated in degrees. The vernier scale has 12

divisions on either side of the center zero. They are marked 0-60 minutes

of arc, so that each division is 1/12th of 60 minutes, i.e. 5 minutes. These

12 divisions occupy same arc space as 23 degrees on the main scale,

such that each division of the vernier = (1/12)*23 = 1(11/12) degrees

If the zero graduation on the vernier scale coincides with a

graduation on main scale, the reading is in exact degrees. If some other

graduation on the vernier scale coincides with a main scale graduation,

the number of vernier graduations multiplied by

5 minutes must be added to the main scale reading.

1.7.2 Sine Bar

Fig. 1.7



Sine bar is a tool used to measure angles in metal surfaces. Sine

bars are made from high carbon, high chromium, corrosion resistant steel

which can be hardened, ground & stabilized. Two cylinders of equal

diameters are attached at the ends as shown in fig. The distance between

the axes can be 100, 200 & 300 mm. The Sine bar is designated basically

for the precise setting out of angles and is generally used in conjunction

with slip gauges & surface plate. The principle of operation realies upon

the application of Trigonometry.

Fig. 1.8

Use of sine bar

In the above fig, the standard length AB (L) can be used & by

varying the slip gauge stack (H), any desired angle θ can be obtained as,

θ=sin-1

(H/L).

For checking unknown angles of a component, a dial indicator is

moved along the surface of work and any deviation is noted. The slip

gauges are then adjusted such that the dial reads zero as it moves from

one end to the other.

Advantages of sine bar:

1. It is used for accurate and precise angular measurement.

2. It is available easily.

3. It is cheap.



Disadvantages:

1. The application is limited for a fixed center distance between two

plugs or rollers.

2. It is difficult to handle and position the slip gauges.

3. If the angle exceeds 45°, sine bars are impracticable and inaccurate.

4. Large angular error may results due to slight error in sine bar.

1.8 GAUGES

Gauges are inspection tools of rigid design, without a scale, which

serve to check the dimensions of manufactured parts. Gauges do not

indicate the actual value of the inspected dimension on the work. They

can only be used for determining as to whether the inspected parts are

made within the specified limits or not. i.e Plain gauges are used for

checking plain (Unthreaded) holes and shafts.

Plain gauges may be classified as follows

1. According to their type:

a. Standard gauges

b. Limit gauges

2. According to their purpose:

a. Work shop gauges

b. Inspection gauges

c. Reference or Master Gauges

3. According to the form of tested surface:

a. Plug gauges

b. Snap & Ring gauges

4. According to their design:

a. Single limit & double limit

b. Gauges Single ended and

c. Double ended gauges Fixed &

d. Adjustable gauges



1.8.1 Plain Plug gauges

Plug gauges are the limit gauges used for checking holes and

consist of two cylindrical wear resistant plugs. The plug made to the

lower limit of the hole is known as 'GO' end and the plug made to the

upper limit of the hole is known as 'NO GO' end.

If GO end doesn’t go inside the hole, the hole is undersize and

component is rejected.

If the hole size is within the limits, the GO gauge should go into the hole

and

NOGO end should not enter into the hole.

The plugs are arranged on either ends of a common handle. Plug

gauges are normally double ended for sizes up to 63 mm and for sizes

above 63 mm they are single ended type. The handles of heavy plug

gauges are made of light metal alloys while the handles of small plug

gauges can be made of some non metallic materials

Fig. 1.9

For smaller through holes, both GO & NO GO gauges are on the same

side separated by a small distance. After the full length of GO portion

enters the hole, further entry is obstructed by the NO GO portion if the

hole is within the tolerance limits.

1.8.2 Ring gauges

Ring gauges are used for checking the diameter of shafts. The ring

gauges are having a central hole, which is the gauging surface and is heat

treated ground and lapped. The other surfaces are finished smooth and



its periphery is knurled. Ring gauges are available separately as GO

ring and NOGO ring as shown in the figure. The GO ring made to the

lower limit size of the shaft and NOGO ring made to the upper limit size

of the shaft

Fig. 1.10

1.8.3 Snap or Gap gauges

Fig. 1.11

A snap gauge usually consists of a plate or frame with a parallel

faced gap of the required dimension. Snap gauges can be used for

both cylindrical as well as non-cylindrical work as compared to ring

gauges which are conveniently used only for cylindrical work. Double

ended snap gauges can be used for sizes ranging from 3 to 100 mm. For

sizes above 100 mm up to 250 mm a single ended progressive gauge

may be used.

1.9 Surface finish

It is defined as the allowable deviation from a perfectly flat

surface that is made by some manufacturing surfaces.

https://en.wikipedia.org/wiki/Flatness_%28mathematics%29



Surface finish, also known as surface texture or surface

topography, is the characteristics of a surface. It has three components:

lay, surface roughness, and waviness.

Terminology

1. Lay: It is the measure of the direction of the predominant machining

pattern and it reflects the machining operation used to produce it.

2. Surface roughness: Surface roughness commonly shortened to

roughness is a measure of the finely spaced surface irregularities. In

engineering, this is what is usually meant by "surface finish".

Fig. 1.12

3. Waviness: Waviness is the measurement of the more widely spaced

component of texture. These usually occur due to warping,

vibrations, or deflection during machining. Flaws: Irregularities that

occur occasionally on the surface. It includes cracks, scratches,

inclusions, and similar defects in the surface.

4. Roughness height: It is the height of the irregularities with reference

to a reference line. It is measured in mm or microns.

https://en.wikipedia.org/wiki/Surface_roughness

https://en.wikipedia.org/wiki/Waviness

https://en.wikipedia.org/wiki/Wood_warping

https://en.wikipedia.org/wiki/Wood_warping

https://en.wikipedia.org/wiki/Machining_vibrations



Fig. 1.13

5. Roughness width: It is the distance parallel to the nominal surface

between successive peaks or ridges which constitute the predominate

pattern of the roughness measured in mm.

6. Waviness height: Waviness height is the peak to valley distance of

the surface profile, measured in millimetres.

7. Real surface: It is a surface limiting the body and separating it from

the surrounding surface.

8. Geometrical Surface: It is the surface prescribed by the design

of manufacturer, neglecting the errors of form and surface roughness.

9. Effective surface: It is the close representation of real surface

obtained by instrumental means.

10. Surface texture: Repetitive or random deviations from the nominal

surface which form the pattern of the surface. It includes roughness,

waviness, lay and flaws.

11. Traversing length: it is the length of the profile ecessary for the

evaluation of the surface roughness parameters. It may include one or

more sampling length.

12. Sampling length(l): It is the length of profile necessary for the

evaluation of the irregularities to be taken in to account. It is also

known as cut off length of a measuring instruments.

13. Mean line of the Profile: It is the line that divides the effective

profile such that, within sampling length the sum of squares of



distances (y1, y2, ….yn) between effective points and mean line is

minimum.

14. Center line of the Profile: It is the line for which the area

embraced by the profile above or below the line is equal.

15. Spacing of irregularities: It is the mean distance between the more

prominent irregularities of the effective profile, within the sampling

length.

16. Maximum height of irregularities: It is defined as the average

difference between the five highest peaks and the five deepest

valleys within the sampling length measured from a line parallel to

the mean line.

1.9.1 Talysurf Surface Roughness Tester

Fig 1.14

This is an electronic instrument works on principle of carrier

modulation. The measuring head of this instrument consists of a diamond

stylus of about 0.002 mm tip radius and skid or shoe which is drawn

across the surface by means of a motorized driving unit (gearbox), which

provides three motorized speeds giving respectively x 20 and x 100

horizontal magnification and a speed suitable for average reading. A



neutral position in which the pick-up can be traversed manually is also

provided. In this case the arm carrying the stylus forms an armature

which pivots about the centre piece of E-shaped stamping as shown in

Fig. On two legs of (outer pole pieces) the E-shaped stamping, there are

coils carrying an a.c. current. These two coils with other two resistances

form an oscillator. As the armature is pivoted about the central leg, any

movement of the stylus causes the air gap to vary and thus the amplitude

of the original a.c. current flowing in the coils is modulated. The output

of the bridge thus consists of modulation only as shown in Fig. This is

further demodulated so that the current now is directly proportional to the

vertical displacement of the stylus only.

The demodulated output is caused to operate a pen recorder to

produce a permanent record and a meter to give a numerical assessment

directly. In recorder of this statement the marking medium is an electric

discharge through a specially treated paper which blackens at the point of

the stylus, so this has no distortion due to drag and the record strictly

rectilinear one. Now-a-days microprocessors have made available

complete statistical multi-trace systems measuring several places over a

given area and can provide standard deviations and average over area-

type readings and define complete surface characterization.

1.10 Coordinate measuring machine (CMM)

Fig. 1.15



A coordinate measuring machine (CMM) is a device for measuring

the physical geometrical characteristics of an object. This machine may

be manually controlled by an operator or it may be computer controlled.

Measurements are defined by a probe attached to the third moving axis of

this machine. Probes may be mechanical, optical, laser, or white light,

among others.

A machine which takes readings in six degrees of freedom and

displays these readings in mathematical form is known as a CMM.

The typical 3D "bridge" CMM is composed of three axes, X, Y

and Z. These axes are orthogonal to each other in a typical three-

dimensional coordinate system. Each axis has a scale system that

indicates the location of that axis. The machine reads the input from the

touch probe, as directed by the operator or programmer. The machine

then uses the X, Y,Z coordinates of each of these points to determine size

and position with micrometre precision typically.

A coordinate measuring machine (CMM) is also a device used in

manufacturing and assembly processes to test a part or assembly against

the design intent. By precisely recording the X, Y, and Z coordinates of

the target, points are generated which can then be analyzed via regression

algorithms for the construction of features. These points are collected by

using a probe that is positioned manually by an operator or automatically

via Direct Computer Control (DCC). DCC CMMs can be programmed to

repeatedly measure identical parts, thus a CMM is a specialized form of

industrial robot

Coordinate-measuring machines include three main components:

1. The main structure which includes three axes of motion.

2. Probing system

3. Software.

https://en.wikipedia.org/wiki/Regression_analysis

https://en.wikipedia.org/wiki/Regression_analysis

https://en.wikipedia.org/wiki/Industrial_robot



1.10.1 Role of CMM

CMMs are particularly suited for the following conditions:

4. Short runs-We may be producing hundreds or even thousands of

a part, but the production run is not sufficient to justify the cost of

production inspection tooling.

5. Multiple features-When we have a number of features both

dimensional and geometric to control, CMM is the instrument that

makes control easy and economical.

6. Flexibility-Because we can choose the application of the CMM

system, we can also do short runs and measure multiple features.

7. High unit cost-Because reworking or scrapping is costly, CMM

systems significantly increase the production of acceptable parts.

8. Production interruption-Whenever you have to inspect and pass

one type of part before you can start machining on the next part, a

machining centre may actually be able to help a manufacturer save

more money by reducing down time than would be saved by

inspection.

1.10.2 Advantages of CMM

1. High precision and accuracy

2. Requires less labour.

3. Accurate dimensions can be obtained just by knowing the

coordinates and distance between the two reference points

4. Robustness against external force and error accumulation.

5. Reduction in set up time.

6. Uniform inspection quality.

7. Total flexibility.

8. Simplification i inspection procedure.

9. Reduces total cost.



1.10.3 Disadvantages of CMM

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.

4. It needs to construct some feature on its own as some parts of the

work piece are unreachable by the probe.

1.10.4 Applications of CMM

CMMs find applications in automobile, machine tool,

electronics, space, and many other large companies. These machines are

ideally suited for development of new products and construction of

prototype because of their maximum accuracy, universatility and ease of

operation

Because of high speed of inspection, precision and reproducibility

of coordinate measuring machines, these find application to check the

dimensional accuracy of NC produced workpiece in various steps of

production.

For safety components as for aircraft and space vehicles, 100%

inspection is carried out and documented using CMM.

CMMs are best suited for the test and inspection of test equipment,

gauges and tools.

CMMs can be used for determining dimensional accuracy of the

bought in components, variation on same and thus the quality of the

supplier.

CMMs can also be used for sorting tasks to achieve optimum pairing of

components within tolerance limits. A coordinate measuring machine can

replace several single purpose equipment with a low degree of utilisation



like gear tester, gauge tester, length measuring machine, measuring

microscope, etc.

CMMs are also best for ensuring economic viability of NC

machines by reducing their downtime for inspection results. They also

help in reducing reject costs, rework costs through measurement at the

appropriate time with a suitable CMM.

1.10.5 Uses of CMM:

1. Profile measurement.

2. Dimensional measurement.

3. Depth mapping.

4. Angularity or orientation measurement.

5. Shaft measurement.

6. Digitalizing or imaging activity

Unit-1 Measuring Instruments

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