1. Measuring Instruments
Dr. N.K. Singh
Associate Professor (Workshop)
Department of ME&MME
Indian School of Mines
Dhanbad

2. Measurement
Measurement is the comparison of an unknown dimension to a known standard.
Good measuring instruments are 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.

3. 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)

4. Classification of Measuring Instruments
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.

5. Classification of Measuring Instruments
Line measuring devices
measurement along the length (steel rule)
End measuring devices
Measurement between the two ends (vernier caliper)

6. 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

7. Principal Measuring Instruments
Surface Measurement
Straight edge
Surface plate
Surface gauge
Optical gauge
Profilometer

8. 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.
Measurement

10. A complete vernier caliper
Measurement

12. 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.
Measurement

14. 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.
Measurement

15. 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 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.
Measurement

16. 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
Measurement

17. How to read a vernier scale
This reads 3.3
Measurement

18. How to read a vernier scale
This reads 5.6
Measurement

19. Main Scale plus a vernier scale
Measurement

20. 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.
Measurement

21. Classes of Vernier calipers
Measurement

22. 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.
Measurement

23. 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.

24. 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.

25. Outside caliper

26. 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 measurement. Horizontal threaded rod contains a thumbwheel for adjusting the width of the two measuring arms.
Measurement

27. Outside caliper

28. 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.
Measurement

29. Inside Caliper

30. 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.
Measurement

31. Different parts of a micrometer
Measurement

32. Common types of micrometers
Measurement

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

35. Thank you