Metrology & Statistical Quality Control

1. Metrology & Statistical Quality Control 5th Term Batch 2009 lec# 5 & 6

2. INTERCHANGEABILITY Concept: Interchangeability refers to assembling a number of unit components taken at random from stock so as to buildup a complete assembly without fitting or adjustment Interchangeable Part: is one which can be substituted for a similar part manufactured from the same drawing In practice it is found there are degrees of interchangeability lec# 5 & 6

3. INTERCHANGEABILITY(CONT..) Types: • Universal Interchangeability: it is assume that similar parts, derived from any source whatsoever, are interchangeable. • Local interchangeability:i.e parts made at the specific source (in particular factory, for instance) are interchangeable, but such parts are not necessarily interchangeable with similar parts manufactured else where lec# 5 & 6

4. INTERCHANGEABILITY (CONT..) Interchangeability of Component Parts depend on two factors: i.e, • It is necessary for the relevant mating parts to be designed incorporating specified limits of size. • Parts must be manufactured within the specified limits, which must be controlled rigidly lec# 5 & 6

5. INTERCHANGEABILITY (CONT..) Eg: a modern motor car, consists of many hundreds of separate components each of which is manufactured in large numbers. For complete interchangeability it should be possible simple to collect at random the constituent parts, then to assemble the hole without the use of any cutting tools & for the assembly to function satisfactorily lec# 5 & 6

6. INTERCHANGEABILITY (CONT..) Essential Characteristics when a Hole mates with a Shaft with a clearance fit: 1. Allowance (minimum clearance) is the functional requirement 2. Tolerance on (a) the Hole & (b) the Shaft is essential for manufacturing the feature concerned 3. The fit b/w the mating features is determined by the limits of (a) the two maximum metal limits & (b) the two minimum metal limits, which are known as the GO & NOT GO limits respectively lec# 5 & 6

7. INTERCHANGEABILITY (CONT..) Essential Characteristics when a Hole mates with a Shaft with a clearance fit: (cont..) • A correct fit is obtained only if the mating features are made correctly within the prescribed extreme limit of size • The required fit is not obtained if the mating features are made outside the prescribed extreme limits of size. Further, in practice , any feature made outside the prescribed limits may be unable to function with the mating feature, even if the later is within the correct limits lec# 5 & 6

8. INTERCHANGEABILITY (CONT..) Essential Characteristics when a Hole mates with a Shaft with a clearance fit: (cont..) 6. The conditions giving maximum metal limits to both features result in the minimum clearance 7. The conditions giving minimum metal limits to both features result in the maximum clearance lec# 5 & 6

9. METHOD FOR ACHIEVING PRECISION & ACCURACY • The terms precision & accuracy are associated with measurement • Precision: is defined as • the repeatability of measuring process or • The quality or state of being precise or • Degree to which an instrument gives repeated measurement of the same standard • Accuracy: • is the agreement of the result of a measurement with the true value of the measured quantity • the closeness with which the reading approaches an acceptable standard value or the true value. • It is numerically equal to the degree of error in the final result • In any experiment, accuracy of the measured quantity is influenced by the limits of the intrinsic error, limits of variation in the indication, accuracy of the observer & the environment lec# 5 & 6

10. METHOD FOR ACHIEVING PRECISION & ACCURACY (CONT..) • It is the precision which is of immense importance in most measurements. The chief concern is with comparing the dimension of measurement relative to each other, it being assumed that the scale used for measure is standard & accepted one. lec# 5 & 6

11. METHOD FOR ACHIEVING PRECISION & ACCURACY (CONT..) • Higher accuracy can be achieved by incorporating the magnifying devices in the instrument and these magnifying devices carry with them their own inaccuracies. By taking many precautions we can make these errors extremely small lec# 5 & 6

12. METHOD FOR ACHIEVING PRECISION & ACCURACY (CONT..) An accurate measurement instrument should fulfill the following requirements: • It should possess the requisite & constant accuracy • As for as possible, the errors should be capable of elimination by adjustment contained within the instrument itself • Every important source of inaccuracy should be known • When an error can not be eliminated, it should be made as small as possible • When an error can not be eliminated it should be capable of measurement by the instrument itself & the instrument calibrated lec# 5 & 6

13. SOURCES OF ERROR Error: • Difference b/w the measured value & the true value of a quantity is called static (or absolute) error or simply error of measurement • The error may be positive or negative • If the error is on positive side, the instrument reading is on higher side • where as if it is on lower side then the corresponding reading is on lower side lec# 5 & 6

14. SOURCES OF ERROR (CONT..) Error falls into two categories: • Controllable Errors 2. Random Errors 1. Controllable Errors: Such errors are called as system error & are controllable both in their magnitude & stress. These can be determined and reduced if attempts are made to analyze them lec# 5 & 6

15. SOURCES OF ERROR (CONT..) Controllable Errors can be due to: 1.1. Calibration errors: actual length of standard such as slip gauges and engraved scales will vary from the nominal value by small amount. Some times the instrument inertia & hysteresis effects do not let the instrument translate with complete fidelity lec# 5 & 6

16. SOURCES OF ERROR (CONT..) Controllable Errors can be due to: (cont..) 1.2. Ambient Conditions: The variations in the ambient conditions from internationally agreed standard value of 20oC, barometric pressure 760mm of mercury, or 10mm of mercury vapor pressure, can give rise to errors in the measured size of component. Temperature is by far the most significant of these ambient conditions & due correction is needed to obtain results full from error lec# 5 & 6

17. SOURCES OF ERROR (CONT..) Controllable Errors can be due to: (cont..) 1.3. Stylus Pressure: Errors induced due to stylus pressure are also appreciable. When ever any component is measured under definite stylus pressure the deformation of the work piece surface & deflection of the work piece shape will occur. 1.4. Avoidable Errors: These errors include the errors due to parallax & the effect of misalignment of the work piece centres. Instrument location errors such as placing a thermometer in sunlight when attempting to measure air temperature also belong to this category of errors lec# 5 & 6

18. SOURCES OF ERROR (CONT..) 2. Random Errors: The random errors occur randomly the specific causes of such error can not be determined. The likely sources of this type of error are: • Small variation in the position of setting standards & work piece • Slight displacement of lever joints in the measuring instrument lec# 5 & 6

19. SOURCES OF ERROR (CONT..) • 2. Random Errors: (Cont..) • Transition fluctuation in the friction in measuring instrument • Operator errors in the reading scale & pointer type displays or in reading engraved scales positions lec# 5 & 6

20. SOURCES OF ERROR (CONT..) Conclusion: • Controllable errors are those which are repeated consistently with the repetition of the experiment, where as • Random errors are those which are accidental & whose magnitude & sin cannot be predicted from the knowledge of measuring system & conditions of measurement lec# 5 & 6

21. CLASSIFICATION OF MEASURING INSTRUMENTS: • A measuring instrument is any device that may be used to obtain dimensional or angular measurement • The function of measuring instrument is to sense or detect a parameter encountered in a scientific process or research, such as temperature, pressure, resistance, current, voltage, flow & motion etc • The Measuring instrument should have the capability of detecting any changes that occur in the measured parameter lec# 5 & 6

22. CLASSIFICATION OF MEASURING INSTRUMENTS: Classification according to use: • Linear Measurement: A. Direct Reading i. Rule ii. Combination set iii. Depth gauge iv. Vernier caliper v. Micrometer vi. Measuring Machine a. Mechanical b. Optical lec# 5 & 6

23. CLASSIFICATION OF MEASURING INSTRUMENTS: Classification according to use: B. Instruments for transferring measurements: i. Calipers & Dividers ii. Telescopic gauge 2. Angular Measurement i. Protractors ii. Sine bar iii. Combination set iv. Angle gauge block v. Dividing Head lec# 5 & 6

24. SELECTION OF INSTRUMENTS • The selection of measuring instrument depend upon its technical specifications eg: • Scale value, scale division, graduation range • Linearity, threshold value, Sensitivity • Accuracy, Precison, measuring force • Working capacity, free strokes, • Error of measuring surface, • Error of the straightness of the guide ways, etc lec# 5 & 6

25. CARE AND HANDLING OF EQUIPMENT/APPARATUS Do your self (go through the book: Egg Metrology & Instrumentation by: R.K Rajput, pg#26) lec# 5 & 6