linear metrology...2

Measurement Standards

“Precision is a Religion & Measurement Standards make it happen” (Arun Kudale)

• History of Measurement Standards:

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

• 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:

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.

• 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.

End Bar

• 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

Types of Slip Gauges

• 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.

Subdivision of Standards

1. Primary Standards

2. Secondary Standards

3. Tertiary Standards

4. Working Standards• Reference Standards• Calibration Standards

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.

• 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

Status of Calibration

1. Active

2. Calibration Before Use (CBU)

3. Only for indication (OFI)

4. Rework

5. Reject

6. Write-off

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

Assignments

• Case Study:

1. Dial calibration Tester2. Calibration of Gauge block measuring Unit

Linear Metrology

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.

• 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

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)

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.

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.

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.

Vernire Caliper

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

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

VERNIER DEPTH GAUGE

BaseGraduated beamClamping screwFine adjustment

mechanism Vernier scale

Digital Vernier Caliper

Micrometer

• 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

Digital measuring Instrument for Internal & External Dimensions