Engineering-metrology

Microsoft Office Excel 2007.lnk

MQAR Metrology, Quality Assurance & Reliability

Text Books : Engg. Metrology by R.K.Jain

Reference Books : 1.Statistical Quality Control by M. Mahajan,

2.Reliabity Engg. by L.Srinath .

1.Need of Inspection

2 . Standards of Measurement

3. Angle Measurement

4. Inspection of Screw-thread elements

5. S.Q.C.

6.Reliability Data Analysis

TOPICS TO BE COVERED


from greek “metron” (measure) and –logy.

metrology is the science of measurements and that to measure is to

compare with something (a unit) which is taken as the basis for

comparison. (Measurement standard)

includes all theoretical and practical aspects of measurement.

Quality:

a product’s fitness for use.

the totality of features that bear on a product’s ability to satisfy a given

need.

the ability of a system or component to perform its required functions

under stated conditions for a specified period of time.

– Failure: the inability of an equipment to perform its required

function

– Reliability: the probability of no failure throughout a prescribed

operating period.

Metrology:

Reliability


This is the set of actions taken to develop primary standards of

measurement for the base units and the derived units of the International

System of Units (SI).

Legal metrology

Scientific metrology

It is that part of metrology which treats units of measurement, methods of

measurement and the measuring instrument, in relation to the statutory,

technical and legal requirements.

It assures security and appropriate accuracy of measurement.

Industrial metrology

The function of industrial metrology is mainly the proper calibration, control

and maintenance of all measuring equipment used in production,

inspection and testing. The purpose is to guarantee that

the products will comply with quality standards.

For convenience, a distinction is often made between the several fields of

application of metrology


Process of Measurement

set of operations having the objective of determining a value of a quantity

Measurand: particular quantity subject to measurement

Reference/Standard of Measurement:

Comparator: Fixed Gauges / Measuring Instrument:

Needs of Inspection To ensure that part and components are confirmed to

required standards.

To meet the need of Interchangeability of parts.

To maintain good customer relationship by ensuring that No

faulty product reaches the customer.

The result of inspection are forwarded to the manufacturing

department, thus helps in improving the quality.

It helps to purchase good quality raw material, tool and

equipment.

It led to development of precision measuring instruments.


High Quality Product

performs its functions reliably

performs its functions for a long time

performs its functions conveniently

Low Quality Product

does not perform its function reliably

fails or breaks after short time of use

is difficult to use

GOAL

Continuous Quality Improvement

(functionality, reliability, durability, …)

Inspection (Measurement)

What? When? How?


Inspection specific to PRODUCTS

Electronic parts (circuits, chips, etc.)

Machine elements (engines, brakes, gears, etc.)

Heat and thermodynamic components (engines, fuel injectors, etc.)

Medical and Bio-related products (implants, dental devices, surgical

parts, etc.)

…

Inspection specific to PROCESSES

Chip removal processes (turning, milling, drilling, etc.)

Chipless manufacturing (casting, molding, forging, etc.)

Non-traditional methods (EDM, ECM, ultrasonics, etc.)

…


Inspection AFTER production

costly production steps already complete

high cost of rejection or rework

difficult to test for all possible defects

difficult to identify responsibility for defect

Inspection DURING production

defects found early, at each production step

reduced cost of rejection or rework

facilitates continuous process improvement


Measurement of DIMENSIONS

Linear measurements (length, thickness, etc.)

Angular measurements (taper, angle, etc.)

Measurement of surface texture (roughness, waviness, etc.)

Measurement of geometric shape (roundness, flatness,

squareness, etc.)

Measurement of screw threads and gears

Inspection for DIMENSIONAL ACCURACY

post-process (traditional)

in-process (modern trend)

DIMENSIONAL TOLERANCES

permissible variation in dimensions

directly affects product quality and cost


SOURCES OF ERRORS

an error is defined as real (untrue, wrong, false, no go) value at the output

of a measurement system minus ideal (true, good, right, go) value.

Error = Ŧ(MV – TV)

Classification of errors:

1.Absolute Error:- It is the algebraic difference between the result of

measurement and the value of comparison.

(a) True absolute error: algebraic difference between result of

measurement and conventional true value.

(b) Apparent absolute error: if a series of measurements are made, the

difference between one of the measurement and the arithmetic mean.

1.Relative Error:- It is the ratio of absolute error and the value of

comparision used for measurement.

Relative error = (Absolute error/True value)

TYPES OF ERRORS

Static Error

Reading Error

Environmental Error

Characteristic Error

Dynamic Error

Systematic Error

Random Error

Instrumental loading Error

1. Static Error: these result from the physical nature of the various

components of the measuring system as the system responds to a

fixed Measurand input.

Due to intrinsic imperfections in the hardware and apparatus

compared to the ideal instrument.

(a) Reading Error:

i. Parallax Error: Possibility of Error due to parallax (Read out). Use of

mirror behind the read out or pointer virtually eliminates such type of

error.

ii. Interpolation Error: It can be tackled by increasing Optical

resolution by using a magnifier or using digital read out devices.

(a) Environmental Error: This error is due to the effect of surrounding

temperature, pressure and humidity on measuring system. External

influences also include Magnetic or electric fields, nuclear radiation,

vibration or shock etc… these factors affects both measuring system

and measurand.

(b) Characteristic Error: The deviation of the output of the measuring

system under constant environmental conditions from the theoretical

predicted performance or from nominal performance specification.

2. Instrumental loading Error: This result from the change in measurand

itself when it is being measured. It is thus the difference between the

value of the measurand before and after the measurement system has

measured.

1. Dynamic Error: (Related with time)

This error caused by time variation in the measurand and results from

the inability of a measuring system to respond faithfully to a time

varying measurand. Usually dynamic response is limited by inertia,

damping, friction or other physical constraints in the sensing, read out

or display system.

Systematic Errors Come from the measuring instruments.

Something is wrong with the

instrument or its data handling system,

or instrument is wrongly used by the

Experimenter.

The errors in temperature

measurements because of poor

thermal contact between the

thermometer and the substance.

Errors in measurements of solar

radiation astrees or buildings shade

the radiometer.

Random Errors Caused by unknown and

unpredictable changes in the

experiment.

May occur in the measuring

instruments or in the

environmental conditions

(humidity, temperature, etc.)

The errors in voltage

measurements because of an

electronic noise in the

circuit of electrical instrument.

irregular changes in the heat

loss rate from a solar collector

due to the wind.

A thermometer that always

Reads 3ºcolder than the

actual temperature

A thermometer that gives

random values within 3º either

side of the actual temperature

Systematic Errors Reproducible between

measurements.

In principle, they can be

eliminated partially or

completely.(Controllable error)

Accuracy is often reduced by

systematic errors, which are

difficult to detect even for

experienced researchers.

We must define their size

To estimate what confidence

We have in our measured

value.

Random Errors Not reproducible, but fluctuate in

magnitude and sign between

measurements.

We can only know the probable

range over which a random error

lies.

Precision is limited by the

random errors. It may usually be

determined by repeating the

measurements.

They can be estimated so

that the measured value

can be adjusted to allow

for them.

Accuracy and Precision

Precision is defined as the repeatability of the measuring

instrument. It shows how close the measured values are to

each other.

The precision of a measurement is the size of the unit you

use to make a measurement. Ex: 12 s and 12 day

The number of decimal places in a measurement also affects

precision. 10,10.1, 10.12, 10.1237…..

Accuracy is how close a measured value is to the actual

(true) value. The accuracy of a measurement is the

difference between your measurement and the accepted

correct answer. The bigger the difference, the less accurate

your measurement.

Mistake of 5 cm in measurement of 100 cm or 1000cm…

Difference between Accuracy and Precision

Low Accuracy

High Precision

High Accuracy

Low Precision

High Accuracy

High Precision

apply a

systematic

adjustment

need to change the

equipment or

methodology used

If the instrument measures in "1"s then

any value between 6½ and 7½ is

measured as "7"

If the instrument measures in "2"s then

any value between 7 and 9 is

measured as "8"

Degree of Accuracy

Accuracy depends on the

instrument you are

measuring with. But as a

general rule: The degree of

accuracy is half a unit each

side of the unit of measure

Factors affecting Accuracy:

1. Standard: ambient influence, stability with time, elastic property,

Position of use……

2. Work piece: ambient influence, cleanliness, surface condition,

Elasticity, support arrangement, defining datum.

3. Instrument: hysteresis, backlash, friction, zero drift, error in

Amplification, calibration error etc…..

4. Personal: Improper training for handling instrument, skill, sense of

Precision and accuracy, attitude……

5. Environmental: temperature, vibration, lighting, pressure….

SENSITIVITY OF MEASUREMENT Smallest difference in a dimension that an instrument can distinguish or

detect. It may be defined as the rate of displacement of the indicating

device of an instrument, w.r.t the measured quantity.

In other words, sensitivity of an instrument is the ratio of the scale

spacing to the scale division value.

For example, if on a dial indicator, the scale spacing is 1.0cm and the

scale division value is 0.01cm, then sensitivity is 100.

It is also called as amplification factor or gearing ratio.

Environmental changes affect instruments in two main ways, known as

zero drift and sensitivity drift.

Zero drift describes the effect where the zero reading of an instrument is

modified by a change in ambient conditions.

Sensitivity drift (also known as scale factor drift) defines the amount by

which an instrument's sensitivity of measurement varies as ambient

conditions change.

CALIBRATION

Calibration is the set of operations that establish, under specified

conditions, the relationship between the values of quantities indicated by

a measuring instrument and the corresponding values realized by

standards.

Calibration is the process of establishing the relationship between a

measuring device and the units of measure. This is done by comparing a

device or the output of an instrument to a standard having known

measurement characteristics.

When the instrument is made to give a null indication corresponding to a

null value of the quantity to be measured, the set of operation is called

zero adjustment .

Calibration can be called for:

with a new instrument

when a specified time period is elapsed

when a specified usage (operating hours) has elapsed

when an instrument has had a shock or vibration which potentially

may have put it out of calibration

whenever observations appear questionable

Calibration

Adjusting or setting of an instrument to

obtain accurate readings within a

reference standard.

Readability

Susceptibility of an instrument for having

its indications converted to a meaningful

number.

Precision

Degree of agreement in the

measurements of the same quantity.

Repeatability

Ability to do the same thing over & over.

Error between a number of successive

Attempts to move a machine to the same

position.

Terminology Accuracy

Degree of agreement of the

measured dimension with its true

magnitude.

Sensitivity

Smallest difference in a dimension

that an instrument can distinguish

or detect.

Resolution

Smallest dimension that can be

read on an instrument.

Reproducibility

Degree of agreement in the

individual results using the same

method and the same test

substance, but a different set of

laboratory conditions.

1 75

2 35

3 50

4 85

5 95

6 92

7 45

8 56

9 86

10 71

Mean 69.0

1 74

2 73

3 72

4 64

5 65

6 66

7 69

8 68

9 70

10 69

Mean 69.0

Standard Deviation A measure of the spread of a probability distribution, random variable, or

multiset of values.

More formally, it is the root mean square deviation of values from their

arithmetic mean.

In practice, it is often assumed

that the data are from

an approximately

Normally distributed

population.

According to this,

confidence intervals are:

σ: 68.26894921371% 4σ:99.99366575163%

2σ:95.44997361036% 5σ:99.99994266969%

3σ:99.73002039367% 6σ:99.99999980268%

Interchangeability An interchangeable part is one which can be substituted for similar part

manufactured to the same drawing.

The required fit assembly can be obtained in Two ways.

a)Universal or full interchangeability

b)Selective assembly Full interchangeability means any component will mat with any other

mating component without classifying Manufactured components into sub

groups or Without carrying out minor alteration for mating Purpose. It

requires precise machines or processes whose Process Capability is equal

or less than the manufacturing Tolerances

allowed for that part. So every component

produced will be with in desired tolerances

and capable of mating(Fitting) with any

other mating components to give the

required Fit.

Process capability of a machine is

defined as its ±3σ spread of

dimensions of components produced

by it.

Advantages of Interchangeability

1.Assembly time is reduced considerably.

2.There is an increased output with reduced production cost.

3.It facilitates production of mating components at different places

by different operator.

1.The replacement of worn out or defective parts and repair becomes

very easy.

2.The cost of maintenance and shutdown period is also reduced to

minimum.

Selective Assembly: In selective assembly components produced are classified into groups

according to their sizes by automatic gauging. This is done for both Holes

and Shafts and then corresponding parts will be matched properly.

It reduces chance of defective assembly and also the cost of assembly as

parts may be produced in wider tolerances.

Ex: Assembly of piston with cylinder bores.

Bore size = 50 mm

clearance required for assembly= 0.12 mm

Tolerance in both bore and piston = 0.04 mm

Dimension of bore diameter = 50 ±0.02 mm

Dimension of piston = 49.88 ±0.02 mm

By grading and marking the bores and pistons, they can be selectively

assembled as follows…

Cylinder Bore= 49.98mm 50 mm 50.02 mm

Piston = 49.86mm 49.88 mm 49.90 mm

Limits, Fits and Tolerances: 1.It is not possible to make any part precisely to a given dimension due to

variability of elements of production processes.

Man Machine Material

2. If by chance the part is exactly to a given dimension, it is impossible to

measure it accurately enough to prove it.

3. If attempts are made to achieve perfect size, the cost of production will

increase.

For a given system of Limits and fits to be successful following conditions

are to be satisfied:

It must be based on same standard so that every body alike

and a given dimension has the same meaning at all places.

The range of sizes covered by the systems should be sufficient

for most purposes.

Each basic size of hole and shaft must have a range of tolerance

values for each of the different fits.

Both unilateral and bi lateral methods of tolerances and hole

basis or shaft basis system should be acceptable.

The fundamental deviation required to give a particular fit must

increase with the basic size.

Size Designations Shaft: It refers not only to the

diameter of a circular shaft but Also

to any external dimension of a

component. (Male surface)

Hole: It refers not only to the diameter of a circular Hole but also

to any internal dimension of a component. (Female surface)

Basic Size or Basic dimension: It is the theoretical size worked

out by purely design consideration, from which limits of size are

derived by the application of allowances and tolerances.

Actual Size: is the measured size of the finished part.

Zero line: It is the straight line drawn horizontally to represent

the basic size. All the dimensions are shown w.r.t the Zero line.

Some Definitions Limit: Due to inevitable inaccuracy of manufacturing methods, it is

not possible to make a part precisely to a given dimension and may only be made to lie between to extremely permissible sizes called the limits for the actual size.

Upper/Lower limit: Largest/Lowest size permitted

Tolerance: The permissible variation in size or dimension of a part is called Tolerance. It is the difference between U.L and L.L of dimension.

It is the amount by which the job is allowed to go away from accuracy, with out causing any functional trouble.

Tolerance is always +ve.

Unilateral Tolerance: In this, the dimension

is allowed to vary only in one direction of

Basic Size, either above or bellow it.

Bilateral Tolerance: In this the dimension of part is allowed to vary in both

the sides of the basic size.

Deviation: It is the algebraic difference between the actual

size and the corresponding basic size.

Upper Deviation: It is the algebraic difference between the

upper (Max) limit and the corresponding basic size.

Denoted by “ES” for Hole and “es” for shaft.

+ve when UL> Basic size & -ve when UL< Basic size.

Lower Deviation: It is the algebraic difference between lower

limit and corresponding Basic size.

Denoted by “EI” for Hole and “ei” for shaft.

+ve when LL> Basic size & -ve when LL< Basic size.

So, Tolerance = IT

For Shaft: IT = es – ei For Hole: IT = ES - EI

Fundamental Deviation: (FD)

It is one of the two deviations (Either UD or LD) which is

conventionally choosen to define the position of tolerance

Zone in relation to the zero line.

It is one of the two deviations (Either UD or LD) which is

Nearest to the zero line for either hole or shaft.

When tolerance zone is above the zero line, LD is the FD.

When tolerance zone is bellow the zero line UD is the FD.

Maximum Metal Limit (MML): At this limit the part has

maximum possible amount of metal.

UL for Shaft and LL for Hole.

Least Metal Limit (LML): At this limit the part has minimum

possible amount of metal.

LL for Shaft and UL of Hole.

Basic Shaft (h)

It is the shaft whose upper deviation is Zero. UL= basic size.

Basic Hole (H)

It is the hole whose lower deviation is Zero. LL= basic size

Tolerance Zone: It is the zone bounded by two limits of size

of a part.

Tolerance grade (IT):

It is the degree of accuracy manufacture and is designated

by the letter IT followed by a number.

There are 18 grades of tolerances – IT01, IT0, IT1 to IT16

Larger the number, greater will be the tolerance. IT01 to IT4 - For production of gauges, measuring instruments

IT5 to IT 7 - For fits in precision engineering applications

IT8 to IT11 – For General Engineering

IT12 to IT14 – For Sheet metal working or press working

IT12 to IT14 – For Sheet metal working or press working

IT15 to IT16 – For processes like casting, general cutting work

Standard Tolerance Unit (i)

A unit, which is a function of Basic size and which is common

To the formula defining the different grades of tolerances.

It is denoted by letter “i” and expressed in Microns.

It serves as a basis for determining the standard tolerance (IT)

Of the system. (Micron)

where, D (mm) is the geometric mean of the lower and upper diameters of

a particular diameter step within which the chosen the diameter D lies.

Clearance:

This is the difference between the sizes of the Hole and shaft

before assembly when this difference is positive.

Maximum size of Hole-Minimum size of shaft=Max. clearance

Minimum size of Hole-Maximum size of shaft=Min. Clearance.

Size:

A number expressing the numerical value of a length in a

particular unit.

Allowance:

It is the prescribed difference between the dimension of two

mating parts (Hole and Shaft)

It is the intentional difference between lower limit of hole and

Higher limit of shaft.

Allowance= LLH-HLS

It may be +ve or –ve. +ve allowance = clearance

-ve allowance = Interference

Tolerance Allowance

• Permissible variation in dimension of a part.

• Tolerance= UL – LL • It is provided to the dimension

of a part. • It has Absolute value with out

sign.

• Prescribed difference between the dimension of two mating parts.

• Allowance = LLH - ULS • Provided on the dimension of

mating parts to obtain the desired type of fit.

• It may be +ve. or –ve.

“Go” limit and “NOGO” limit:

“GO” limit refers to UL of shaft and LL of Hole.

Thus it corresponds to MML.

“NOGO” limit refers to the LL of a shaft and UL of a hole.

Thus it corresponds to LML.

Fits:

It is the degree of tightness or looseness between two mating

Parts to perform a definite function when they are assembled

Together.

A fit may result either in a movable joint or a fixed joint.

Ex: Shaft in Bearing, Pulley on a Shaft.

Classification

Clearance fit

a) Slide Fit

b) Easy Slide fit

c) Running fit

d) Slack running fit

e) Loose running fit

Transition fit

a) Push Fit

b) Wringing fit

Interference fit

a) Force Fit

b) Tight fit

c) Shrink fit

Clearance fit:

In this type of fit Shaft is always smaller than the Hole i.e. UL of shaft is

smaller than LL of Hole.

Clearance fit exists when the shaft and the hole are at their MML.

The Tolerance zone of hole will be above the shaft tolerance.

Allowance is +ve.

Ex: Shaft can rotate or slide in a bearing with different DOF according to

purpose of mating part.

a)Slide Fit: Tail stock spindle of Lathe

b)Easy Slide fit: Spindle of lathe & dividing head, Pistons &

Slide Valves, Spigots etc.

c)Running fit: Gear Box Bearings, Shaft Pulleys

d)Slack running fit: Arm Shaft of IC Engine, Shaft of CF

Pump

e)Loose running fit: Idle Pulley on their shaft (Quick Return

Mechanism)

Interference fit In this type of fit, LL of shaft is larger than UL of Hole.

Thus, the shaft and holes are attached permanently and used as a solid

Component.

Elastic strains are developed during the process of assembly.

Allowance is –ve. (Interference)

Ex: Bearing bush, Small end in connecting rod, Gear in intermediate

shafts in trucks.

a)Force Fit: Gears on the shaft

b)Tight fit: Stepped pulley on drive shaft of a conveyor,

Cylindrical Grinding M/C.

A)Shrink fit/

Heavy Force fit: Metallic rim on the wheels of a cart.

Transition fit:

It lies midway between the clearance and interference fit.

In this, tolerance zone of hole and shaft overlap completely or

in part.

UL of hole > LL of shaft but LL of hole < UL of shaft.

Ex: Spigot in Mating parts, Coupling rings etc.

a)Push Fit: Change gears, Slip bushings

b)Wringing fit: Parts which can be replaced with out difficulty

during minor repairs.

Hole Basis System:

The size of hole is kept constant and shaft sizes are varied to

Give various types of fits.

In this, lower deviation of the hole is Zero i.e. LL = Basic size.

Hole basis system is commonly used as it is convenient to

make a hole of correct size due to availability of standard drills,

Reamers, with less cost.

Shaft Basis System:

The size of the shaft is kept constant and sizes of hole are

varied to get the required type of fit.

In this, Upper Deviation of the shaft is zero i.e. UL= Basic size.

This system is not suitable for mass production because it is

Time consuming and costly to make a shaft of correct size.

Recommendation for limits and fits for Engineering:

For universal Interchangeability it is essential to follow a

uniform standard Through out the world.

Indian standards (IS) are in line with ISO recommendations.

It consists of 25 Holes designated by capital letter

A, B, C, D, E, F, G, H, JS, J, K, M,N, P, R, S, T, U, V, X, Y, Z,

ZA, ZB, ZC

It consists of 25 shafts designated by small letter

a, b, c, d, e, f, g, h, js, j, k, m, n, p, r, s, t, u, v, x, y, z, za, zb, zc

Each of holes and shafts has a choice of 18 Grades of

Tolerances Designated as:

IT01, IT0, IT1, IT2, IT3, ……… IT15, IT16. IT01 – 0.3 + 0.008D IT0 – 0.5 + 0.012 D IT1 – 0.8 + 0.020D

IT2 – 2.7i; IT3 - 3.7i; IT4 – 5i; IT5 – 7i;

IT6 – 10 i; IT7 – 16i; IT8 – 25i; IT9 – 40i;

IT10 – 64i; IT11 – 100i; IT12 –160i;

IT13 – 250i; IT14 – 400i; IT15 – 640i;

IT16 – 1000i.

The value of IT for Hole and shaft

Using the value of “i” as

Where D=Geometric Mean Diameter of the lower and upper diameters of

A Particular diameter step in which diameter lies in mm.

The seven Tolerance grades IT01, IT0, IT1, IT2, IT3, IT4, IT5 covers diameter

Sizes up to 500 mm and rest eleven grades i.e. IT06 – IT16 covers diameter

Sizes up to 3150 mm.

Fundamental Deviation are obtained from Empirical Formula (Table/Given in

Question) for shaft and hole respectively up to 500 mm.

FD for Hole A – H are same as that of Shaft a – h but opposite in direction.

They provide clearance fit.

FD for hole “H” and shaft “h” are Zero.

Now IT = ES – EI (Hole)

IT = es – ei (Shaft)

Basic size followed by symbol Φ30 H7/h8

Hole with tolerance Grade IT7 = 16i

Shaft with tolerance grade IT8 = 25i.

If Hole basis system FD for hole = 0

FD for shaft can be found out from the table or given in the question.

Example #1

Evaluate limits and fits for a pair of – Diameter 6 H7/g6. The

size 6 mm lies in the diametral step of 3-6. Standard

tolerance for hole H7 is 16i and shaft g6 is 10i. Fundamental

deviation for g shaft is µ.

Calculate the limits of sizes for φ 25 S8/h7 and identify the fit.

The size 25 mm lies in the diametral step of 24-30. The

fundamental deviation S8 hole – IT7 + 0.4D. For grade 8 and

7 the standard tolerance is 25i and 16i respectively.

Calculate the limits of sizes for φ 60 H8/m6 and identify the

fit. The size 60 mm lies in the diametral step of 50-80 mm.

The fundamental deviation for m is IT7 – IT6. For grade 6

and 7 the multipliers are 10 and 16 respectively.

Calculate the limits of sizes for φ 32 S7/h6 and identify the

fit. The size 32 mm lies in the diametral step of 30-50 mm.

The fundamental deviation S is IT7+0.4D. For grade 6 and 7

the multipliers are 10 and 16 respectively.

GAUGES Gauges are scale less inspection tools at rigid design which are used to

check the dimensions of manufactured parts. Measurement by gauges is

Easy and rapid. So they are suitable in mass production. Instead of

measuring actual dimension of each part which is time consuming and

Costly, the conformance of part with tolerance specification can be

checked by gauges.

Measuring Instrument Gauges

1. They carry calibrated scales. 1. With out scales.

They are general purpose instrument.

They are made for a particular component.

Measures actual dimension of part.

Checks whether the dimensions of parts are with in the specified tolerance limit or not.

Time consuming and not suitable for mass production.

Easy and rapid, suitable for mass production.

Skilled labour to handle. No need of skilled labour.

Increased cost. Reduced cost.

Adjustment is required. No adjustment

PLAIN GAUGES Plain gauges are used to check plain, i.e. unthreaded holes and shafts.

Classification:

1.According to Type (a)Standard Gauge: If a gauge is made as an exact copy of the mating part

Of component to be checked, it is called standard gauge.

A standard gauge can’t be used to check interference fit.

It has limited application.

(a)Limit Gauge: Two gauges are used to check each dimension of the

Part i.e. upper and lower limit. These are “GO” and “NO-GO” gauges.

GO gauges check MML and NO GO gauges check LML.

These are widely used industries.

A part is considered to be good if the GO gauge pass through the work and

the NO GO gauge fails to pass under the action of its own weight. This

Confirms the actual dimension of part with in the specified tolerances.

If both the gauges fail, it indicates that hole is under size and shaft is

Oversize.

1. According to Purpose:

a) Workshop gauge

b) Inspection gauge

c) Reference or master gauge

d) Purchase inspection gauge

2. According to the form of the tested surface:

a) Plug gauges for checking holes

b) Snap or Ring or Gap gauges for checking the shaft

3. According to their design:

a) Single limit or double limit gauges

b) Single ended or double ended gauges

c) Fixed and adjustable gauges

Difference between work shop gauge and Inspection

gauge?

Work shop gauge:

1.Used by the operator during manufacture of a part in shop.

2.Usually have limits with in those of components being

inspected.

3.The tolerance is arranged to fall inside the work tolerance.

4.Some of the components which are in work tolerance limit

may be rejected under these gauges.

Inspection Gauge:

1.Used by inspector for the final inspection.

2.These gauges are made slightly larger tolerance than the

work shop gauges.

3.The tolerance on inspection gauges is arranged to fall

outside the work tolerance.

4.Some rejected parts may be accepted. IT of Inspection Gauge>Work tolerance>W/S Gauge Tolerance.

Gauge Tolerance/ Gauge Maker’s Tolerance/

Manufacturing Tolerance: In actual practice Gauges can’t be manufactured to the exact size (Due to

imperfection in the process). Some allowance must be provided to the

gauge maker known as gauge tolerance.

Gauge tolerance should be kept as small as possible but this will increase

the cost of manufacturing the gauges.

Gauge tolerance of limit gauges (GT)= 1/10th of Work Tolerance (WT)

Or Work shop Gauges (GO, NOGO Gauges) (10%)

Gauge tolerance for Inspection gauges (GT) = 5% of WT

(GO, NOGO Gauges)

Gauge tolerance for Master/Reference gauges (GT) = 10% of WT

Wear Allowance:

The measuring surfaces of GO gauges rub constantly against

the surfaces of work piece during checking. This results in

wearing of measuring surfaces of gauges.

The size of GO plug gauges is reduced and that of Ring or

Snap Gauges is increased.

Wear allowance is provided to the gauges in the direction

Opposite to that of the wear.

WA is not provided for NOGO gauges as they are not

Subjected to much wear compared to GO gauges.

GO plug gauges => WA is added.

GO snap or ring gauges => WA is subtracted.

WA = 5% of WT or 10% of GT.

WA may be avoided in clearance fit.

WA is applied to W/S GO gauges not to Inspection GO

gauges.

Providing WA, the GO gauge will reject more number of

acceptable parts as compared to gauge with only GT.

WA is provided when WT>0.09 mm.

Three basic size of Gauges:

1.Work shop gauge:

GT is with in WT, some accepted parts are rejected,

WA is given to W/S GO Gauge.

1.Inspection gauge:

GT is out side the WT, some rejected parts are Accepted,

As GO gauge for inspection is fairly slack, no WA is required.

1.General gauge:

To over come the draw back of w/s and inspection gauge,

general gauge has been recommended.

Tolerance zone of GO gauge placed inside Work tolerance.

Tolerance zone of NOGO gauge placed outside work

tolerance.

GO gauge of General gauge is taken same as W/S gauge.

NOGO gauge of General gauge is taken same as

Inspection gauge.

Taylor’s Principle of Gauge Design: It states that

1. “Go gauges should be designed to check the Maximum Metal Limit

(MML) while the NO GO gauge should be designed to check the Least

Metal Limit (LML).”

GO plug gauge should correspond to LL of Hole.

NOGO plug gauge should correspond to UL of Hole.

GO snap gauge should correspond to UL of shaft.

NOGO snap gauge should correspond to LL of shaft.

The difference between the GO and NOGO plug gauge as well as the

difference in size between GO and NOGO snap Gauge is approximately

equal to the work tolerance.

2. “GO gauges should check all the related dimensions (Roundness, size,

location, straightness etc).

NOGO gauges should check only one element of the dimension at a

time.”

GO plug gauge should have a full circular section and full

Length of the hole it has to check. It ensures that any lack

Of straightness or roundness of the hole will prevent the entry

Of full length GO gauge.

The length of GO plug gauge should not be less than 1.5

times the diameter of the hole to be checked.

Calculate the dimension of Plug and ring gauges to control

the production of a part 50H7d8. Given: 50 mm lies in the

step 30-50. For d shaft FD= - 16 D 0.44µ. IT6=10i and above it

tolerance magnitude is multiplied by 10 at each fifth step.

Determine actual dimension to be provided for shaft and

hole of 90 mm size for H8/e9 type of fit. Size 90 falls on

Diameter steps of 80 and 100. FD for “e” type shaft is

= - 11 D 0.41µ. Also design “GO” and “NOGO” gauges.

IT8=25i and IT9=40i.

Calculate the limits of size for inspection gauges conforming

to Taylor’s principle to check the rectangular hole . The limits

of size for a 50 mm H8 hole are low limit 50.000 mm and

high limit 50.039 mm. The limits of size for a 75 mm H8 hole

are low limit 75 mm and high limit 75.046 mm.

50 mm diameter step lies 30 – 50.

75 mm diameter step lies 50 – 80.

State Taylor’s Principle of Gauge Design of Limit gauges.

Design the “Work shop” “Inspection” and “General” type of

GO and NOGO gauges for checking the assembly

Φ 30 (mm) H7/f8. Fundamental deviation for “f” shaft is =

-5.5 D 0.41µ. Diameter step for Φ 30 is 18 – 30 mm.

Fundamental tolerance for IT7 and IT8 are 16i and 25i

respectively. Also determine

I. Type of fit

II. Allowance for the above fit

III. Other shafts giving the same type and same

degree of fit

IV. Equivalent fit in shaft based system

Part A Tolerance of

Part

B

Tolerance of B

Engineering-metrology

Engineering

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