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Introduction to Metrology

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Introduction,
Definition,
Accuracy and Precision,
Standards of Measurement,
Limits, Fits,
Tolerances
Go and NOGO gauges,
Taylor's Principle,
IS919

Published in: Engineering
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Introduction to Metrology

  1. 1. Page 1 Metrology The Science of Measurement By, Afaqahmed M J AIKTC
  2. 2. Page 2 Syllabus TH + PR + OR Module1 1.1 Introduction to Metrology, Fundamental principles and definitions, measurement standards / primary and tertiary standards, distinction between precision and accuracy. 1.2 Limits, fits and tolerances, Tolerance grades, Types of fits, IS919, GO and NO GO gauges- Taylor’s principle, design of GO and NO GO gauges, filler gauges, plug gauges and snap gauges. Module 2 2.1 Comparators: Constructional features and operation of mechanical, optical, electrical/electronics and pneumatic comparators, advantages, limitations and field of applications. 2.2 Principles of interference, concept of flatness, flatness testing, optical flats, optical interferometer and laser interferometer. 2.3 Surface texture measurement: importance of surface conditions, roughness and waviness, surface roughness standards specifying surface roughness parameters- Ra, Ry, Rz, RMS value etc., surface roughness measuring instruments – Tomlinson and Taylor Hobson versions, surface roughness symbols.
  3. 3. Page 3 Module 3 3.1 Screw Thread measurement: Two wire and three wire methods, floating carriage micrometer. 3.2 Gear measurement: Gear tooth comparator, Master gears, measurement using rollers and Parkinson’s Tester. 3.3 Special measuring Equipments: Principles of measurement using Tool Maker’s microscope, profile projector & 3D coordinate measuring machine. Module 4 Quality Control Introduction, definition and concept of quality & quality control, set up policy and objectives of quality control, quality of design and quality of conformance, compromise between quality & cost, quality cost and planning for quality. Module 5 SQC and SQC tools Importance statistical methods in QC, measurement of statistical control variables and attributes, pie charts, bar charts/ histograms, scatter diagrams, pareto chart, GANT charts, control charts, X chart, X bar charts, R charts, P charts, np charts their preparation, analysis and applications. Elementary treatment on modern SQC tools.
  4. 4. Page 4 Module 6 Sampling Techniques Sampling inspection and basic concepts, OC curves, consumer & producer risk, single & double sampling plans and use of sampling tables.
  5. 5. Page 5 What is metrology? The science of measurement (not weather!) Metrology establishes the international standards for measurement used by all countries in the world in both science and industry. Examples: distance, time, mass, temperature, voltage, values of physical and chemical constants
  6. 6. Page 6 Let’s take a trip back in time…
  7. 7. Page 7 Prehistoric people didn’t have time to measure
  8. 8. Page 8 But over time….
  9. 9. Page 9 People started growing food
  10. 10. Page 10 And developing permanent settlements
  11. 11. Page 11 So, they desired a system of measurement…
  12. 12. Page 12 Or there would be pandemonium (chaos) !
  13. 13. Page 13 History of Measurement Wall and Leather Painting
  14. 14. Page 14 Fast-forward 5000 years to current measurement systems
  15. 15. Page 15 Career in Metrology 1.Scientific Metrology – Organization and development of measurement standards and their maintenance (highest level) – NIST Atomic Clock Accurate up to 1s / 20 million years National Institute Standard & Technology
  16. 16. Page 16 Industrial Metrology – Adequate functioning of measurement instruments used in industry as well as production and testing processes
  17. 17. Page 17 Who Needs Accurate Measurements? Pharmaceutical Industry • Metrology laboratories test weights and volume standards for pharmaceutical companies • Products include medicines like aspirin, antibiotics, vaccines, insulin, & vitamins
  18. 18. Page 18 Defense Industry • Metrology laboratories test standards for many military and defense companies • These companies make the guidance systems for the Patriot missiles and other things that are top secret
  19. 19. Page 19 Space Station, Satellites…. • Metrology laboratories test standards for many companies that provide parts of the space shuttle • These parts include the metal, heat shield, electronics, fabrics, o-rings, optics, and tires
  20. 20. Page 20 Standards of Measurement A standard is defined as “something that is set up and established by an authority as rule of the measure of quantity, weight, extent, value or quality”.
  21. 21. Page 21 1. Primary standards: They are material standard preserved under most careful conditions. These are not used for directly for measurements but are used once in 10 or 20 years for calibrating secondary standards. Ex: International Prototype meter, Imperial Standard yard.
  22. 22. Page 22 1. Secondary standards: These are close copies of primary standards w.r.t design, material & length. Any error existing in these standards is recorded by comparison with primary standards after long intervals. They are kept at a number of places under great supervision and serve as reference for tertiary standards. This also acts as safeguard against the loss or destruction of primary standards.
  23. 23. Page 23 Tertiary standards: The primary or secondary standards exist as the ultimate controls for reference at rare intervals. Tertiary standards are the reference standards employed by National Physical laboratory (N.P.L) and are the first standards to be used for reference in laboratories & workshops. They are made as close copies of secondary standards & are kept as reference for comparison with working standards.
  24. 24. Page 24 4. Working standards These standards are similar in design to primary, secondary & tertiary standards. But being less in cost and are made of low grade materials, they are used for general applications in metrology laboratories. Sometimes, standards are also classified as; • Reference standards (used as reference purposes) • Calibration standards (used for calibration of inspection & working standards) • Inspection standards (used by inspectors) • Working standards (used by operators)
  25. 25. Page 25 Seven base units: Length: meter (m) Mass: kilogram (kg) Time: second (s) Electric current: ampere (A) Thermodynamic temperature: kelvin (K) Amount of substance: mole (mol) Luminous intensity: candela (cd)
  26. 26. Page 26 • Accuracy: – How close you are to the actual value – Depends on the person measuring – Calculated by the formula: % Error = (YV – AV) x 100 ÷ AV Where: YV is YOUR measured Value & AV is the Accepted Value
  27. 27. Page 27 • Precision: – How finely tuned your measurements are or how close they can be to each other – Depends on the measuring tool – Determined by the number of significant digits
  28. 28. Page 28 • Accuracy & Precision may be demonstrated by shooting at a target. • Accuracy is represented by hitting the bulls eye (the accepted value) • Precision is represented by a tight grouping of shots (they are finely tuned)
  29. 29. Page 29 ACCURACY ACCURACY with PRECISION PRECISION without ACCURACY No ACCURACY, No PRECISION
  30. 30. Page 30 LIMITS, FITS & TOLERANCES
  31. 31. Page 31 TERMINOLOGY • NOMINAL SIZE: It is the size of a part specified in the drawing. • BASIC SIZE: It is the size of a part to which all limits of variation are determined. Or It is the theoretical size from which limits of size are derived by the application of allowances and tolerances. • ACTUAL SIZE: It is the actual measured dimension of a part. Nominal and basic size are often the same.
  32. 32. Page 32 DEVIATION • LOWER DEVIATION: It is the algebraic difference between the minimum limit of size and the basic size. • UPPER DEVIATION: It is the algebraic difference between the maximum limit and the basic size.
  33. 33. Page 33 LIMIT OF SIZES • There are two extreme possible sizes of a component. • The largest permissible size for a component is called upper limit and smallest size is called lower limit.
  34. 34. Page 34 BASIS OF LIMIT SYSTEM • SHAFT BASIS SYSTEM: • In this system, the shaft is kept as constant member and different fits are obtained by varying the hole size.
  35. 35. Page 35 BASIS OF LIMIT SYSTEM HOLE BASIS SYSTEM: In this system, the hole is kept as a constant member and different fits are obtained by varying the shaft size.
  36. 36. Page 36 ZERO LINE • It is the straight line corresponding to the basic size. The deviations are measured from this line.
  37. 37. Page 37 Tolerance • Tolerance is the total amount that a specific dimension is permitted to vary; • It is the difference between the maximum and the minimum limits for the dimension. • For Example a dimension given as 1.625 ± .002 means that the manufactured part may be 1.627” or 1.623”, or anywhere between these limit dimensions.
  38. 38. Page 38
  39. 39. Page 39 Tolerances The Tolerance is 0.001” for the Hole as well as for the Shaft
  40. 40. Page 40 POSITIONAL TOLERANCES • Two types of positional tolerances are used: 1. Unilateral tolerances 2. Bilateral tolerances • When tolerance is on one side of basic size, it is called unilateral and if it is both in plus and minus then it is known as bilateral tolerance.
  41. 41. Page 41 Specifications of Tolerances 1. Limit Dimensioning The high limit is placed above the low limit. In single-line note form, the low limit precedes the high limit separated by a dash
  42. 42. Page 42 Cumulative Tolerances
  43. 43. Page 43 International Tolerance Grade (IT): They are a set of tolerances that varies according to the basic size and provides a uniform level of accuracy within the grade. `
  44. 44. Page 44
  45. 45. Page 45 IS919 ACT ( REVISED)
  46. 46. Page 46 Fits Between Mating Parts Fit is the general term used to signify the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in mating parts. There are four types of fits between parts 1. Clearance Fit: an internal member fits in an external member (as a shaft in a hole) and always leaves a space or clearance between the parts. Minimum air space is 0.002”. This is the allowance and is always positive in a clearance fit
  47. 47. Page 47 2. Interference Fit: The internal member is larger than the external member such that there is always an actual interference of material. The smallest shaft is 1.2513” and the largest hole is 1.2506”, so that there is an actual interference of metal amounting to at least 0.0007”. Under maximum material conditions the interference would be 0.0019”. This interference is the allowance, and in an interference fit it is always negative.
  48. 48. Page 48 3. TRANSITION FIT: In this type of fit, the limits for the mating parts are so selected that either a clearance or interference may occur depending upon the actual size of the mating parts.
  49. 49. Page 49 PLAIN GAUGES Gauges are inspection tools which serve to check the dimensions of the manufactured parts. Limit gauges ensure the size of the component lies within the specified limits. They are non-recording and do not determine the size of the part. Plain gauges are used for checking plain (Unthreaded) holes and shafts.
  50. 50. Page 50 Plain gauges may be classified as follows; According to their type: (a) Standard gauges - are made to the nominal size of the part to be tested and have the measuring member equal in size to the mean permissible dimension of the part to be checked. A standard gauge should mate with some snugness (Comfort). (b) Limit Gauges These are also called ‘go’ and ‘no go’ gauges. These are made to the limit sizes of the work to be measured. One of the sides or ends of the gauge is made to correspond to maximum and the other end to the minimum permissible size. The function of limit gauges is to determine whether the actual dimensions of the work are within or outside the specified limits.
  51. 51. Page 51 LIMIT GAUGING Limit gauging is adopted for checking parts produced by mass production. It has the advantage that they can be used by unskilled persons. Instead of measuring actual dimensions, the conformance of product with tolerance specifications can be checked by a ‘GO’ and ‘NO GO’ gauges.  A ‘GO’ gauge represents the maximum material condition of the product (i.e. minimum hole size or maximum shaft size) and conversely a ‘NO GO’ represents the minimum material condition (i.e. maximum hole size or minimum shaft size)
  52. 52. Page 52 1.Plug Gauge Plug gauges are the limit gauges used for checking holes and consist of two cylindrical wear resistant plugs. The plug made to the lower limit of the hole is known as ‘GO’ end and this will enter any hole which is not smaller than the lower limit allowed.  The plug made to the upper limit of the hole is known as ‘NO GO’ end and this will not enter any hole which is smaller than the upper limit allowed.
  53. 53. Page 53 1. Plug Gauge
  54. 54. Page 54
  55. 55. Page 55
  56. 56. Page 56
  57. 57. Page 57 2. Ring Gauge Ring gauges are used for gauging shafts. They are used in a similar manner to that of GO & NO GO plug gauges. A ring gauge consists of a piece of metal in which a hole of required size is bored.
  58. 58. Page 58
  59. 59. Page 59 SNAP (or) GAP GAUGES:  A snap gauge usually consists of a plate or frame with a parallel faced gap of the required dimension. Snap gauges can be used for both cylindrical as well as non cylindrical work as compared to ring gauges which are conveniently used only for cylindrical work. Double ended snap gauges can be used for sizes ranging from 3 to 100 mm. For sizes above 100 mm upto 250 mm a single ended progressive gauge may be used.
  60. 60. Page 60
  61. 61. Page 61 Taylor’s Principle of Gauge Design: GO LIMIT  According to Taylor, ‘Go’ and ‘No Go’ gauges should be designed to check maximum and minimum material limits which are checked as below; ‘GO’ Limit. This designation is applied to that limit of the two limits of size which corresponds to the maximum material limit considerations, i.e. upper limit of a shaft and lower limit of a hole. The GO gauges should be of full form, i.e. they should check shape as well as size.
  62. 62. Page 62
  63. 63. Page 63 No Go Limit: This designation is applied to that limit of the two limits of size which corresponds to the minimum material condition. i.e. the lower limit of a shaft and the upper limit of a hole.  ‘No Go’ gauge should check only one part or feature of the component at a time, so that specific discrepancies in shape or size can be detected. Thus a separate ‘No Go’ gauge is required for each different individual dimension.
  64. 64. Page 64 Wear Allowance: The GO gauges only are subjected to wear due to rubbing against the parts during inspection and hence a provision has to be made for the wear allowance. Wear allowance is taken as 10% of gauge tolerance and is allowed between the tolerance zone of the gauge and the maximum material condition.
  65. 65. Page 65 Filler Gauges L, W Use for precise spacing inspection by inserting into the gap between two flat surfaces  Crucial instrument for measuring gap between piston and cylinder of automotive engine
  66. 66. Page 66 REFERANCES- For Video https://www.youtube.com/watch?v=X5xEE6YTpqI

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