metrology and computer aided inspection 1 a

AE 224

Metrology and

Computer Aided Inspection [CAI]Part: I

AE 224 Metrology and Computer Aided Inspection

• Syllabus:• Introduction to Metrology • Fundamentals of dimensional Measurement• Length Standards• Application of light Interference for precision measurements• Fits and tolerances• Concepts and practice of gauging• Comparators and their applications• Linear and angular measurements• Thread and gear inspection• Form, flatness, straightness and alignment measurements• Surface metrology• Co-ordinate metrology• Laser applications in metrology; • Vision inspection• Micro and nano metrology.


Text Books:• Reference Books

Anand K Bevoor, Vinay A Kulkarni, Metrology and Measurement, Tata Mc Graw-Hill.

• Jain R.K., Engineering metrology, Khanna Publications• Busch., Fundamentals of Dimensional Metrology,

Delmar Publishers (1998)• Shotbolt, C.S. and Galyer. J., Metrology for Engineers,

5th ed., Cassell Publ. (1990).


Reference Books:• Graham T. Smith., Industrial Metrology, Surface and

Roundness.• D. J. Whitehouse., Handbook of Surface Metrology.• Bala Muralikrishnan, Jay Raja., Computational Surface

and Roundness Metrology.

Introduction

• What is Metrology?• Quality in Manufacturing.• Why Precision Length Measurements?• Need for reliable Standards.• Standard of Length.• Shop floor standards.• Traceability.

What is Metrology ?Metro - logy

from Greek 'metron' [measure], and –logy Meaning Measurement Science

In English, the term Metrology is often used for Linear Measurements

Engineering MetrologyIndustrial MetrologyDimensional Metrology

Manufacturing Metrology

Quality in Manufacturing

Changing Concepts on quality and their significanceInspection ProductProcess Control ProcessQuality Assurance DesignTQM People & SystemsStrategic Quality Management Product Life

Quality covers all activities from the concept to thefinal disposal of the product.

Inspection

• Passive - Inspect - Accept - Reject• Active - Inspect - Infer - Act• Dynamic - Inspect – Act

All three modes of inspection are still practiced in industries

Tolerance and Inspection

• Wide tolerances - Passive Inspection• Medium Tolerances - Active Inspection• Close Tolerances - Dynamic Inspection

• Passive Inspection – Sampling• Active inspection – SQC• Dynamic Inspection – In-process measurement

Metrology- Applications

Why Precision Length Measurements?

Majority of manufacturing measurements deal withdimensions which are in linear units.

Though the dimensions are normally given in mms, thetolerances on these dimensions are given inmicrometers and of late even in nanometers.

All measurements are comparisons.Comparison with reference to a standard

Measurement Accuracies

Measurement accuracies should be better thanthe accuracy expected in the measured entity.As a thumb rule, the measurement accuracyshould be an order better than the measuredvalue.

Systematic and Random Errors

• Systematic errors are caused by assignable factors such as setting errors, temperature, humidity, …

• Random errors are caused by unknown sources, including human errors.

Accuracy and Precision


1

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2

1

n

xx

n

xx

n

ii

n

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Average ------Mean Variation ----- Standard deviation

Precision of a Measurement device


Take an example of a dimension:10mm + 5 mm (tolerance 10 mm)

For checking this we need an accuracy of 1 mm.That instrument is to be calibrated to 0. 1 mmThat in turn has to be calibrated to 0.01mmThat in turn has to be calibrated to 0.001mmAnd so on …………………. …..Where to stop?

Tolerance Trend

Length Standards

• Basic Standards of Length – Meter ; Yard• Auxiliary Standards• Shop floor reference• Slip Gauges and accessories

• 1889 Physical

Meter Standard

• 1960 Wavelength Wavelength of Krypton- 86

was accepted to define the meter.

1 650 763.73 wavelengths in a vacuum of the radiation of krypton-86 is one Meter

(m)

Meter Standard

• 1983 In terms of time. New definition of Meter in Seconds!

Meter is the length travelled by light in

1 / 299 792 458 s.

Meter Standard

Laser Standard

It was also in 1960 that the first laser was constructed and by the mid 1970s lasers were being used as length standards. This is now realized by iodine-stabilized helium-neon lasers.

Line and End Standards

• In line standard, the distance between two lines marked on it, is the specified length.Original meter was a line standard.

• In End standard, the distance between the ends of a bar or a block is the specified distance.

• Line standards need auxiliary set ups to use them for measurement.[eg. Microscope, precision linear movement etc]

• End stands need only simple accessories for their applications.[eg. Dial indicator]

Shop floor measurements

Auxiliary Standards:-Length BarsBlock or Slip Gauges

A box of Slip gauges

Reference (‘00’)Calibration (' 0 ')Inspection (‘ I’ )Workshop (‘II’)


Slip gauge accessories


Auxiliary Standards:-Length Bars

A box of Length Bars

Slip Gauges Available in different series, in a box.a) Millimeter series: 1,2,3, 5,10,15,20,25 etc; max:100 mmb) Tenth (of a mm) series: 1.1,1.2, 1.3 - - - 1.9 mmc) Hundredth (of a mm) series: 1.01, 1.02- - - 1.09 mmd) Micrometer series: 1.001, 1.002,1.003 - - - 1.009 mmWith these series any dimension could be built up.Example : Say 34.732 mm . For this first set the micrometer value 1.002Then set the hundredth value 1.03Then set the tenth value 1.7Balance needed is 31.000 mmThis is set from mm series (30+1)=31.00

Total 34.732

Airy Points

• Definition: The best points for supporting a bar horizontally so that the end slopes become zero. If the bar is of length L and there are n supports, the supports should be separated by distance L/sqrt(n2-1). For two support this is 0.5773 L .

Support points

Line and End standards need the end portion of the standard to be horizontal and the ends to be parallel. This is possible when the slope at the ends is zero.Airy points of support allows this.For straight edges used for checking the straightness, the support points should be such that the total deflection is minimum.

Traceability

For international trade there is the need toadhere to the standard scrupulously – especiallywith diminishing tolerances.

This means that all linear measurements doneare to be traceable to the standard of length –the meter.

Traceability

It is practically impossible to directly refer to the meterfor all measurements. So it is done by reference tosecondary or auxiliary standards which are calibratedwith reference to the standard meter.

Like wise all other measuring instruments are to becalibrated with secondary standards, ensuringtraceability to the meter.

ISO 9000 recommends this.

Light Interference for Precision Measurements

• Interference of light.• Optical flat.• Simple set-up for interference.• Measurement of length by comparison.• Slip gauge comparison.• Distance measurement using interference.• “Absolute” Measurement of Length. • Laser Interferometer.

Whitworth’s flat surface generation

Production of Master Surfaces and Squares

For making the master optical flat of high quality, three of them are made together. Then they are checked for their flatness, in pairs, using light interference. i.e, A with B, A with C and B with C. If in all the three cases the fringes seen are 0 or near that then all the three surfaces are perfectly flat. The same method is applied for an engineer’s square. Here interference approach is notused; only the gap between the vertical portions are checked and minimized.

Light Interference

Principle of Distance Measurement

It is evident from the figure that the shape or pattern of interference fringes can be obtained in a set up using an optical flat as shown above by taking imaginary sections parallel to the optical flat at /2 intervals. If the surface is perfectly flat then these fringes will be straight and equally spaced as shown.

Surface and Form Measurements

Here one can note that the fringe pattern on a spherical surface will be circularwith decreased spacings, at the periphery. This is due the change in the slope.

On the right, two slip gauges of the same size are shown in an interference setup. The one on the left is a “00” grade and the other, a used grade “1” slip. Identify the surface defects on it by looking at the pattern of the fringes.

Laser Interferometer

Interference can be used for distance measurements i.e, movements.Laser is the preferred source of monochromatic light and due to its coherrance one can obtain interference for long distances. The laser beam is split into two parts by the beam splitter and after reflection from the two cube corner mirrors they are combined to observe the interference. One of the mirror is fixed. Hence the fringes will move as the other mirror is moved. This allows the distance to be measured.

Laser Interferometer

The Laser, sensors, and one mirror are all enclosed in one unit. The fringe counting sensor is connected to the electronic unit that subdivides the fringes further and displays the distance based on the wave length of the laser, either in mm or in inch units.The other mirror is placed on the table which is moved. This movement causes the fringes to move and the interferometer counts this movement as well as its direction (+ or - ) and displays it. Laser interferometer is used for calibrating table movements in CNC Machines, Coordinate measuring machines and other precision units for precision measurement of their movements.

Slip Gauge Calibration• Slip gauges could be calibrated for their length directly using

wave length of light. This is called absolute calibration of the slip. Here again the same interference set up is used. The procedure adopted is known as method of coincidence.

• If one measures the length of a an object using three different units then three different integers and fractions will be obtained.

• Eg., a length of 124 mm is measured using three units having 8 mm, 9 mm and 10 mm.

• The values will be for 8 : 15.5; for 9 : 13.777; for 10 : 12.4• They are all - an integer + a fraction. These fraction values will

only be correct for this length of 124 mm when checked by these three units. Any small change in the length will change these fractions. Hence they coincide only for this length.

Observation of the fraction by which the slip gauge height exceeds a full multiple of /2

Observe the fringe offset between the fringes on the base and the slip gauge surface. Between the Blue and Red fringes there is no off-set as the height is /2. This will be true for any multiples of /2 . However between the blue and the green there is an off-set. Measure them in mm and find a the fraction. Here this fraction is 0.6 . That is the amount by which the height exceeds /2 .

Fringes are geometrically obtained by taking imaginary section at /2 intervals parallel to the optical flat surface as shown.Blue lines show the fringes on the base.Red lines show the fringes on a slip gauge of size /2 size ( Not possible practically)Green lines show the fringes on a slip gauge which is 1.6 /2 height.

Slip Gauge Calibration• Let the Nominal size of the slip gauge be G• G= (N+ F) /2 Where N is the full multiple of /2 and F the

fraction by which it exceeds N.• G can be measured by 3 different unit as explained earlier.

These units are three different wavelengths.• Hence • G= (N1+F1) 1/2; G= (N2+F2) 2/2 ; G= (N3+F3) 3/2 • F1,F2,F3 can be observed in an interference set up using the

three wave lengths. If the observed values of these fractions and the calculated values are the same, then the actual size of the slip gauge is exactly the nominal size.

Slip Gauge CalibrationThe procedure mentioned earlier can be simplified by finding out

the error in the slip gauge, directly.“A” be the actual size of the gauge. “G” its nominal size.A = (N+F) /2; G= (n+f) /2A-G = [(N-n)+(F-f)] /2] (same as the general equation (N+F) /2)A-G is the error in the size of the slip gauge.N-n is a very small integer number, say 1,2 , etc(F-f )is the fraction difference ( Observed – Theoretical fraction)With 3 different wave lengths three different fraction differences

are obtained.For each wave length, find (A-G) for say (N-n) 0, 1, 2, 3 or -1, -2 etc

Slip Gauge CalibrationWave length N-n F-f A-G

1/2 1 (F1-f) X1 2 X2 3 X3

2/2 1 (F2-f) X1’2 X2’3 X3’

3/2 1 (F3-f) X1’’2 X2’’3 X3’’

A-G has to be the same for all three wave lengths. Search for the same value in the table under A-G. If there is no convergence, repeat with (N-n) as -1, -2 etc till the same error appears for all the three wave lengths.

Slip Gauge Interferometer

Refreshing on Tolerancing

• The following slides explains the concept of tolerance and fits.

• The IT numbers that are indicators for the precision achieved is connected with the process of manufacturing as given in the following slide.

Tolerancing

Fundamental Deviations

Specifying Fits

Quality of fits

Types of Fits

Hole and Shaft Basis System

Preferred Fit Families• For hole tolerances, tolerance zones H7, H8, H9 and H11

are used preferably.• For shaft tolerances, tolerance zones h6, h7, h9 and h11

are used preferably. • Preferred Fits • Clearance fits: H11/c11, H9/d9, H8/f7, H7/g6, H7/h6,

C11/h11, D9/h9, F8/h7, G7/h6Transition fits: H7/k6, H7/n6, K7/h6, N7/h6Interference fits: H7/p6, H7/s6, H7/u6, P7/h6, S7/h6, U7/h6

Standard Fits• Loose Running H11/ c11• Free Running H9/ d9• Loose Running H11/ c11• Easy Running - Good quality easy to do-H8/ f8• Sliding H7/ g6• Close Clearance - Spigots and locations H8 f7• Location/Clearance H7/ h6• Location- slight interference H7/ k6• Location/Transition H7/ n6• Location/Interference- Press fit which can be separated H7/ p6• Medium Drive H7/s6• Force H7/u6

Manufacturing Processes and the Quality achieved

Limit Gauging

• When a dimension is given a tolerance, the actual dimension has to be within the tolerance specified. Hence we need not find out the exact dimension.

• It is sufficient to check whether the dimension lies within the tolerance limit.

• This can be done faster than by measurement.• This process is called Limit Gauging

Limit Gauging

Limit Gauging

• Tolerance is not on the dimension. It is on the form.

• A Go gauge checks the form of the part.• Hence it is the a unique inspection.• However there are many limitations for

gauging.

Taylor’s Rules on Gauge Design

Gauges

Plug Gauge

Gauges

Adjustable Gap Gauge

Ring Gauge

Fundamentals of Linear Measurements

Abbe Principle: Deals with the error in measurement when the measured entity and the measuring unit are not aligned.

Measuring force and Measurement- Force exerted during measurement can change its value. Hence standard forces are to be used.

Environment: All measurements done are affected by the environment. Factors include temperature, pressure and humidity.

Contact and Non-contact measurements: Non contact measurements do not apply any force. Hence the results are different.

Measurement and Sensing: Sensing is a quick way to assess value in an indirect fashion. There should be a calibration procedure to asses the measured value. Here the precision is not very high.

Abbe Principle

• The scale of a linear measuring system should be co-linear with the spatial dimension or displacement to be measured or else the measurement must be corrected for the associated Abbé error.

Abbe Error

metrology and computer aided inspection 1 a

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