linear metrology...2
TRANSCRIPT
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Measurement Standards
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“Precision is a Religion & Measurement Standards make it happen” (Arun Kudale)
• History of Measurement Standards:
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Standards• Definition:• The word Standard means something that is set up & established
by authorities as a rule for the measurements of quantity, weight, value etc.
• Types of Standards:1. Line Standard2. End Standard3. Wavelength Standard
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• Line Standard:• According to line Standard the yard or meter is defined as the
distance b/w inscribed lines on a bar of metal under certain condition of temperature % support.
• This is legally authorized & an act of parliament.
a) The imperial Standard Yard:• The yard is defined as the distance b/w central transverse line on
the plugs.• When the temperature is constantly at 62F & the bar is
supported on rollers.• As shown in Figure:
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a) International standard Prototype meter:
• The international bureau of weights & measurements establish meter as Linear measuring standard in 1875.
• It is the distance b/w the center portion of two lines engraved on the polished surface of a bar.
• That is made of Platinum-iridium alloy having a unique (web) cross section as shown:
• Meter is also defined as the path traveled by light in a vacuum during a time interval of 1/299792458 of a second.
• The examples of line standard includes meter, Yard, steel Rule (Scale) etc.
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• End Standard:• The need of line standard arises as the use of line standard was
difficult at various places in workshops.• End standards can be made to a high degree of accuracy.
a) End Bar:• It is made of steel having cylindrical cross sections.
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End Bar
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• Slip Gauges:
• They are practical end standards and can be used in linear measurement in many ways.
• Theses were invented by a Swedish engineer C E Johnson.
• Slip gauge sets are made according to the following conditions:
IS 2984-1981, Metric BS-4311: 1968Imperial BS.888.1950, DIN: 861-1988JIS B 7506-1978
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Types of Slip Gauges
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• Wavelength Standard:
• High sensitivity length measurements are very important as theses are widely used & they are of type that have highest accuracy after time frequency measurements.
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Subdivision of Standards
1. Primary Standards
2. Secondary Standards
3. Tertiary Standards
4. Working Standards• Reference Standards• Calibration Standards
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Calibration• It is a comparison of Instrument performance to standards of
Known accuracy.• Calibrations directly link customers measurement equipment
to national & international standards.• According to ISO, Calibration is the quantitative determination
of errors of measuring instruments & adjusting them to a minimum.
• In other words, calibration means to find out whether the instrument gives correct reading or not.
• It also includes minor adjustments to the instrument.
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• Calibration Labs are to be set at the following different levels:
a. In-house Calibration Lab
b. Professional Calibration Labs
c. National Accreditation board for testing (NABL) Certification to professional Labs
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Status of Calibration
1. Active
2. Calibration Before Use (CBU)
3. Only for indication (OFI)
4. Rework
5. Reject
6. Write-off
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Standard Procedure for Calibration
1. Cleaning of Instrument
2. Determination of Error
3. Check for Tolerable Limit
4. Minor Changes
5. Allotment of Calibration Set up
6. Next Calibration date
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Assignments
• Case Study:
1. Dial calibration Tester2. Calibration of Gauge block measuring Unit
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Linear Metrology
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Linear Metrology• It is defined as the science of linear measurement, for the
determination of the distance b/w two points.
• It is applicable to all external and internal measurements i.e. distance, length, diameter, thickness, straightness etc.
• Its principle is to compare the dimension to be measured & aligned with standard dimensions marked on the measuring instrument.
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• Linear Metrology follows the following two approaches:
1. Two-Point Measuring-Contact-Member Approach• Vernier Calipers, Micrometer for Measuring Distance
2. Three-Point Measuring-Contact-Member Approach• Out of three measuring contact members, two are fixed• E.g. to measure the diameter of a block held in V block
• The Instruments used in Linear Metrology are Classified as:
1. Non Precision Measuring instruments e.g. Steel Rule2. Precision Measuring Instruments e.g. Vernier Calipers
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STEEL RULE (Scale)• It is the Simplest & most Commonly used linear Measuring
instrument.
• It measures an unknown Length by comparing it with the one previously Calibrated.
• It is used to measure components of lower accuracy.
• The Figure Shows different Steel Rules (scales)
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CALIPERS• A caliper is used to measure the distance b/w two points.
• It use a precise slide movement for inside, outside, depth or step measurement.
• It consist of two legs hinged at the top, wit the ends of the legs spanning the part to be measured.
• Calipers should be held gently near the joint & square to the work by applying light gauging pressure to avoid disturbance during setting for accurate measurement.
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Types of Calipers1. Inside Calipers:• It has Made with straight legs which are bends outwards.• Used for measuring inside diameter.• Opening can be checked by rule or micrometer.
2. Outside Calipers:• It has two legs which are bends inwards.• Mainly used for measuring outside calipers.
3. Spring Calipers:• It is the improved variety of ordinary friction-joint caliper.• the two leg carry a curved spring at the top fitted in notches used to force the spring apart.
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4. Center Measuring Calipers• It has conically pointed jaws designed to measure the distance b/w center
of the holes.
5. Gear Tooth Calipers:• It has adjustable tongue designed to measure the thickness of gear teeth
at the pitch line.
6. Machine Travel Calipers:• It is designed to measure the position change of a machine.
7. Rolling mill Calipers:• It is a simple device used for quick gauging of a stock in production
Environment.
8. Sliding Calipers:• It use a precise slide movement for inside, outside, depth or step
measurements.
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Vernire Caliper
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Example
Main scale reading =35mm
The vernier division coinciding with the main scale is the 20th division. Value=20 multiplied by 0.02=0.40mm.
Total reading is 35mm+0.40= 35.40mm
35.40 mm
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VERNIER HEIGHT GAUGE
The main parts of a vernier height gauge and their function are given.
1.base 2. beam 3.vernier slide 4. fine setting device 5. vernier plate 6. locking screws 7. scriber
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VERNIER DEPTH GAUGE
BaseGraduated beamClamping screwFine adjustment
mechanism Vernier scale
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Digital Vernier Caliper
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Micrometer
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• A micro meter is a precision instrument used to measure a job, generally within an accuracy of 0.01mm.Micrometer used to take the outside measurements are know as outside micrometer.
Least Count =
• Micrometers are classified into following types:
1. Outside Micrometers2. Inside Micrometers3. Depth gauge Micrometers
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Digital measuring Instrument for Internal & External Dimensions
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