comparators (1)

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Metrology

Mr. Amit Pancharya MNIT Jaipur

Inspection by Measurement

Direct Measurement

◦ Vernier Caliper, Vernier Height gauge, Verneir Depth gauge

◦Outside Micrometer, Inside Micrometer, Depth Micrometer,

◦ Slip gauges (gauge blocks), length

bars,etc.

Indirect Measurement

◦ Comparators

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Slip gauge Box

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Comparators

Comparing the manufactured part to the master part envisaged by the designer

Dimensional variation from the master part can be amplified and measured so as to get the deviation and the instruments for doing that are called comparators

◦ Mechanical Comparators

◦ Optical Comparators

◦ Pneumatic Comparators

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Design Considerations

The instrument must be sufficiently robust

The system response should be faster

The contact plunger should be provided with a hardened steel or diamond to minimize wear & tear

Means should be provided for easy lifting of the plunger

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Design Considerations

Magnification should be as high as possible, but it should also be capable of measuring the wide range

The recording scale should be linear

The system should have good repeatability

Comparator should be versatile

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Mechanical Comparators Dial Indicators

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Johansson Mikrokator

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Sigma Comparator

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Optical Comparator

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Zeiss Ultra optimeter

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Pneumatic comparator

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Pneumatic comparator

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Angular Measurements

Angle Measuring Devices: Bevel Protractors & Angle Gauges measure the angle between the surfaces of a part or assembly

Bevel Protractor: It normally can read about 5 minutes of a degree. It consists of a sliding blade which can be set at some with respect to the stock. The reading can be obtained with the help of main scale and Vernier scale.

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Angle Gauges

A series of fixed angles are used for comparative assessment of angle between two surfaces

By making use of different permutations & combinations, we could set an angle nearest to 3”. The dimensions of the angle gauges are 75 mm length && 16 mm width.

The set consists of 10, 30, 90, 270 & 410

1’, 3’, 9’, 27’ and 3”, 6”, 18”, 30”.

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Angular Gauges

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Sine Bar

A sine bar is a high precision angle measuring instrument. It is used in conjunction with other accessories such as slip gauges, angle gauges, etc.

It consists of a bar carrying two rollers set at a known centre distance. The options are 100 mm, 250 mm, etc.

They are available in several designs.

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Sine Bar

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Sine Bar Contd..

Design Requirements are:

◦ The rollers must be of equal diameters and true geometric cylinders

◦ The distance between the roller axes must be precisely known and it should be same

◦ The upper surface of the bar must be flat , parallel with the roller axes and equidistant from each other

◦ The accuracy of the sine Bar depends on the above factors.

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Sine Bar Contd..

Sine bars can not be used for setting angles more than 450

Sin q = h/L (1)

Cos q dq = (L dh-hdL)/L2 (2)

dq / tan q = dh/h- dL/L

dq = (dh/h- dL/L) tan q

Hence, sine bar can not be used beyond 450

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Sine Centre

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Autocollimator

Autocollimators are used to detect small misalignments

If a parallel beam of light is projected from the collimating lens and if a plane reflector R is set up normal to the direction of the beam, light will be reflected back along the same path and light will be brought back to focus exactly at the position of the light source


Autocollimator contd..



If the reflector is tilted through a small angle (q), the parallel beam will be reflected through twice the angle (2q) and would be brought to focus in the same plane as the light source but to one side of it. The image will not coincide but there will be a distance equal to focal length times the angle of reflection (2f q) where f is the focal length of the collimating lens.



The distance between the reflector and the lens has no effect on the separation between the source and the image.

For high sensitivity, a long focal length is required

Although the distance of the reflector does not effect the reading, if it is moved too far back, reflected rays will miss the lens completely and no image will be formed.


Types of Gear Inspection

Element by Element Inspection of Gear: Laborious, useful in error analysis, can’t be used by the gear manufacturers.

◦ Tooth thickness measurement

◦ Base pitch measurement

Composite Error Checking: Used in Gear Inspection by conducting only one test

Parkinson Gear Tester


Gear Tooth Vernier


Base Pitch Measuring Instrument


Involute Property



Parkinson Gear Tester

Thread Inspection - Terminology

Pitch: It is the distance measured parallel to the axis between the corresponding points on adjacent surfaces in the same axial plane.

Lead: It is the axial distance advanced by the thread in one revolution. Lead is equal to the same multiples of pitches as the number of starts.


Thread Terminology Contd..

Included angle: It is the angle between the flanks of the thread measured in an axial plane.

Pitch diameter: It is the diameter of tee imaginary coaxial cylinder which intersects the surface of the thread in such a manner that the intercept on the generator of the cylinder is equal to half the pitch.


Thread Terminology Contd..

Major Diameter: It is the diameter of imaginary coaxial cylinder that just touches the crests

Minor Diameter: It is the diameter of imaginary coaxial cylinder that just touches the roots

Helix Angle (q): tan q = p/p d


Thread Inspection

Measurement of Major Diameter: Bench micrometer is normally used for this measurement.

It has measuring anvils so that the thread can be held between them.

It has fiducial indicator so that the required pressure can be applied for all the measurements


Bench Micrometer


Thread Inspection

Measurement of Minor Diameter: Bench micrometer can not be used for Minor Diameter. So, another one, Floating Carriage micrometer is normally used for this measurement.

In this instrument, the thread is held between the centres so that the influence of helix angle can be nullified in this arrangement.


Thread Inspection

Measurement of Pitch (effective) Diameter: Again Floating Carriage micrometer is normally used for this measurement.

Here, the prisms used in the measurement of minor diameter are replaced by the steel wires whose size is chosen such that they pitch at the effective diameter.


Thread Inspection

Two wire Method: The two wires used should be identical in diameter and should pitch properly between the flanks.

Reading of the floating carriage micrometer is taken over these two wires so that the effective diameter can be calculated using the formula developed.


Thread Inspection

Diameter of the best wires: If the wires used make contact exactly at the pitch diameter, then such wire is called the best wire. The diameter of such wire can be estimated from the formula.

Compression & Rake correction.


Thread Inspection

Measurement of flank angle: Flank angle may be measured using Tool Maker’s Microscope with a goniometric head. This consists of glass screen with datum lines which can be rotated through 3600. The thread is mounted on centers and illuminated from below.

The microscope is mounted above the thread in such a way it can be swiveled to be in line with the thread helix and avoid interference of the image


Thread Inspection

The datum lines in the microscope head are set to zero and the table is rotated until the crests of the thread coincide with the horizontal datum and the angle is measured.


Measurement of Internal Threads

Measurement of Major Diameter: The major diameter of internal thread is normally measured using horizontal comparator fitted with ball end styli of radius less than the root radius of the thread to be measured.

When the thread is mounted on a comparator, it aligns itself with the axis of the machine and measurement would be done along the helix of the thread as indicated above.


Measurement of Int. Threads


Measurement of Internal Threads Measurement of Minor Diameter: Calibrated

rollers and slip gauges are used for this measurement as shown below.


Measurement of Internal Threads

Measurement of Effective Diameter: The effective diameter of internal thread is measured using the horizontal comparator using ball ended styli of the best size wire.

Measurement of Flank angle: The semi cast of the thread form may be made using dental plaster and measurement is obtained as done for external thread.


Thread Gauges

Thread inspection can also be carried out using gauges designed according to the Taylors Principles:

Limit gauges for Internal threads:

◦ Full form Go Gauge

◦ Truncated Effective Diameter NOGO gauge

◦ Minor Diameter NOGO gauge


Go & NoGo Gauges


Thread Gauges

Limit gauges for External threads:

◦ This is usually carried out using caliper type gauges

◦ Full form Go Gauge

◦ Truncated Effective Diameter NOGO gauge

◦ Major Diameter NOGO gauge


Limit gauges for External threads


Limit gauge for Major Diameter


Interferometry

Huygens theory proposes that light is considered as an electro-magnetic wave of sinusoidal form. The maximum disturbance is called amplitude and the velocity of transmission is represented by frequency.


Light wave


Interferometry Contd..

Interferometry is the branch of science which deals with the manner in which the monochromatic rays of light are combined by the lens system, usually the eye.

Let us see the effect of combining two rays A & B which are of same wave length



When they happen to be in phase, it results into the increased amplitude i.e. if the two rays of equal intensity are in phase, they augment each other and produce increased brightness.

If they are out of phase i.e. the path differs by the half the wave length, the combined effect is zero if the amplitudes are equal which results in darkness. This situation refers to the occurrence of interference.



Interference of two rays may be demonstrated by considering the two rays emanating from the same source and travelled different distances so as to create the required path differences as shown below:



Another simple method of obtaining fringes is by illuminating an optical flat over a plane reflecting surface.

An optical flat is a disc of glass or quartz whose faces are highly polished and within few microns



Consider a case when an optical flat is kept on the surface of a workpiece.

Due to some reason, it would not make an intimate contact but rests at some angle to the surface. If the optical flat is illuminated by monochromatic light, interference fringes would be observed.



Consider the two rays, one reflected from the bottom of the optical flat and other reflected from the top of the surface. If the path difference is half the wave length, then dark fringe is formed and again the dark fringe is formed when the path difference is 3 times half the wave length.



The fringe pattern consists of alternate dark & bright straight bands in case of flat surface.

If the angle of inclination is small, ab=bc=l/4 and de=ef=3l/4

Change in separation between the optical flat and the surface is the difference between ab & de i.e. l/2.

Total change in elevation from the point of contact to the outer fringe= n l/2.



When the optical flat is kept on a spherically convex/ concave surface, the fringe pattern consists of concentric circles. To distinguish between these two conditions, optical flat is pressed at a point away from the centre and if the centre of the fringe pattern gets shifted, then it is convex or if the fringe spacing reduces, it is convex.


NPL Flatness Interferometer

The flatness of the surface under testing is measured by comparing it with an optical flat as discussed.

Mercury 198 is used as Mono-chromatic source of light.

The system consisting of optical flat, the surface under test, base plate, etc. are shown below:



The instrument was designed by National Physical Laboratory.

It consists of Mercury vapor lamp whose radiations are passed through a green filter giving a monochromatic light. This light is focused on to a pin hole giving an intense point source of light which is the focal plane of collimating ;lens.



The parallel beam is directed on to the component to be tested via an optical flat so that interference fringes are formed which can be viewed directly above by means of thick glass plate semi-reflector set at 450 to the optical axis.

It should be noted that the optical flat is mounted on an adjustable tripod.



This instrument can be used to test parallelism between the two surfaces of the components.

The component to be tested is wrung on the surface plate.

The interference fringes are formed due to the rays reflected from the underside of the optical flat and due to those reflected from the surface of the component.



The fringes are also formed due to the rays reflected from the underside of the optical flat and due to those reflected from the surface of the base plate.

If the component is flat and parallel, the optical flat being equally inclined to both the surfaces, the fringe pattern from both the component & base plate are similar & equally spaced.



When the component is flat but not parallel to the base plate, fringe pattern is straight, parallel but not equally spaced.


comparators (1)

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