measuring instruments lecture

Dr. N.K. SinghAssistant Prof. (Workshop)ME&MME DEPARTMENT

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Measurement

Measurement is the comparison of an unknown dimension to a known standard.

Good measuring instruments were a key to high volume production. Without them, parts could not be built accurately enough to be interchangeable. Each assembly had to be hand fitted together.

Today, measuring tools are essential for most machining operations from initial part layout to final inspection.

Classification of Measuring Instruments

Precision instrument Ability to measure within the accuracy of 0.01 mm

or more Non-precision Instrument

Limited to the measurement of parts to a visible line graduation on the instrument (ex Scale)


Direct measuring Instruments Determine the actual dimension and size of a

work piece Indirect measuring Instruments (comparison

instruments) Transfer the measurement from the work piece to

the direct measuring instrument, then comparison is made.


Line measuring devices measurement along the length (steel rule)

End measuring devices Measurement between the two ends (vernier

caliper)

Principal Measuring Instruments

Linear Measurements Steel rule Calipers Divider Depth gauge Micrometers Vernier caliper Height gauge Slip gauge Comparator

Angular Measurements Protractors Adjustable bevel Engineering's’ square Bevel protractor Dividing head Sine bar Angle gauge Clinometers Autocollimators

Principal Measuring Instruments

Surface Measurement Straight edge Surface plate Surface gauge Optical gauge Profilometer

Vernier Caliper

A vernier caliper (or vernier) is a common tool used in laboratories and industries to accurately determine the fraction part of the least count division.

The vernier is a convenient tool to use when measuring the length of an object, the outer diameter (OD) of a round or cylindrical object, the inner diameter (ID) of a pipe, and the depth of a hole.

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A complete vernier caliper

Vernier calipers

Two scales on Vernier calipers: Main Scale and Vernier Scale

Vernier calipers have two graduated scales. The main scale is like a ruler.

The second scale, called the vernier scale, slides parallel to the main scale and enables readings to be made to a fraction of a division on the main scale.

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Some brief definitions

Vernier scale : the scale on the smaller sliding portion of the caliper.

It gives the least significant digits in the reading, and sub-divides a mark on the main scale into 10, 20, or 50 subdivisions.

Read the vernier scale at the point where a vernier line and a main scale line best line up.

Combine the main scale reading with the vernier scale reading to get a final reading.

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Some brief definitions

Instrument least count : the size of the smallest division on a scale.

For the main scale on the common vernier caliper this is probably 0.1 cm.

With the vernier scale the least count might be 0.002 cm.

Main Scale The scale on the larger, fixed portion of the caliper. It gives the most significant digits in the reading.

Make the reading to the nearest least count of the main scale opposite the zero of the vernier.

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How to read a vernier scale

The scale is read by first taking note of where the zero mark on the vernier scale falls on the main scale.

This is the number of complete divisions on the main scale.

This is the whole number that should be noted. The fraction or decimal is then read from the vernier scale.

This number is taken as the line on the vernier scale that aligns with any line on the main scale

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This reads 3.3

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This reads 5.6

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Main Scale plus a vernier scale

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Classes of Vernier calipers

Vernier calipers (Manual) Electronic calipers

Electronic calipers are relatively expensive compared to the standard vernier style.

They are quite accurate, quick, and easier to read, however.

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Classes of Vernier calipers

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Digital Electronic Calipers These have an electronic readout rather than a

vernier scale. Typically an electronic caliper will have three

buttons, an on/off button, an English/Metric button, and a Zero button.

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Digital Electronic Calipers

The procedure of using an electronic caliper is: Turn the caliper on. Select English (inches) or Metric (millimeters)

reading mode. Close the caliper jaws and press the Zero button. Measure the desired object and read the digital

readout.

2. Calipers

A caliper is used to transfer and compare a dimension from one object to another.

Spring-loaded calipers do not have calibrations. You obtain measurement by placing the measuring arms against a rule (stainless steel ruler with english and metric scales shown). As you can see the accuracy of this tool and method does not compare with the accuracy of the vernier or electronic caliper.

Outside caliper

Outside caliper Spring-loaded OD Calipers are an inexpensive

alternative to vernier calipers. They do have limitations - primarily in accuracy and

versatility. Advantages other than cost may be in the increased

measuring capacity or range, and heat tolerance when making rough measurements.

The horizontal threaded rod contains a thumbwheel for adjusting the width of the two measuring arms.

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Outside caliper

Inside Caliper

Spring-loaded ID Calipers have the same features and limitations of the OD Calipers. The difference is in the shape/orientation of the measuring arms allowing access to the inside of the object.

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Inside Caliper

Micrometer

The micrometer is a precision measuring instrument used to measure distances between surfaces in thousandths of an inch.

Most micrometers have a frame, anvil, spindle, sleeve, thimble, and ratchet stop.

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Different parts of a micrometer

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Common types of micrometers

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Micrometers

Measure: Outside diameters Inside diameters; The distance between parallel surfaces; The depth of holes, slots, counter bores, and

recesses; and

Thank you

measuring instruments lecture

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