DEPARTMENT OF MECHANICAL ENGINEERING 7TH SEMESTER COURSE CODE – BME 420 LECTURE NOTES ON SUB : METROLOGY, QUALITY CONTROL AND RELIABILITY
SYLLABUS Module- I Introduction: 1. Need of inspection, sources of errors, basic types of errors precision and accuracy. Method of estimating accuracy and precision , standard and their evolutions. (4) 2. simple measurement tools: Rules, calipers, height gauges, micrometers, depth gauge dial indicator, slip gauges, sine bar.(4) Module-II 3. Limit, fits and tolerance and gauge design: Basic concepts of limit fits and tolerance interchangeability and selective assembly, ISO system of tolerance, Taylor’s principle of gauge design, Gauge design- basic design rules for plug and ring gauges. (6) 4. Interferometers: Types of light sources and interferometers, Types of scale and grading, optical flats.(4) Module-III 5. Screw thread measurement : Standard thread profiles, effective diameter, measurement of effective diameter by 2 wires and 3 wires methods. Best wire size (5) 6. Surface roughness: Source of surface irregularities in manufacturing. Roughness and waviness RMC and CLA values measurement of surface roughness using Taylor Hobson’s Talysurf. (5) Module- IV 7. Statistical quality control: Frequency distribution, process capability variables and attributes control chart (X & R chart) for variables, control chart for attributes (p,np and C chart) OC curve single and double sampling plan.(7) 8. Reliability: Definition, relationship of reliability with maintainability and availability, failure data analysis- bath tub curve, system reliability, reliability improvement.(5)
BME 420: METROLOGY QUALITY CONTROL & RELIABILITY MODULE-I -CHAPTER-1 -INTRODUCTION Definition of Metrology? Ans: Science of measurement/pure science of measurement Engineering metrology (Defn):- Measurement of dimension: length thickness , diameter, taper angle flatness straightness profiles and others. Ex: slideway for machine tool( lathe) it must have specific dimension angle and flatness for its desired function. Inspection (Defn) checking the dimension of other defects of a part which has already being produced. New of inspection 1. To ensure that the part material or a component conforms to the established standard. For dimensional control as per specification. 2. To meet the interchangeability of manufacture. 3. To control the performance of man/mk/process. 4. It helps in the process of quality control. 5. It protects the customers in accepting family products. 6. It helps in mass production of assembled part. 7. It helps to assemble various parts produce at different station/place. 8. It provides the means of finding out shortcoming in manufacture. Sources of errors 1. Calibration error Each measures instrument should be calibrated with a standard one at certain time interval (may be once in a year once in every 6 months) If the above procedure is not followed the instrument may give erroneous result, it is called calibration errors. 2. Environmental error These errors are due to surrounding in pressure temperature and humidity. Internationally agree standard value of temperature pressure are : (i) Temperature= 20 c (ii) Pressure = 760 mm of Hg + 10 mm of Hg vapour pressure. If the ambient condition various from the above standard valves the measured value will be erromeous. 3. Contact pressure/ stylus pressure Errors are also introduced due to pressure exerted at stylus. It is more prominent in case of soft work piece. Ideally the stylus should touch the top surface of w/p. due to stylus pressure both deformation & deflection of w/p take place. This type of errors are also induced when the force applied on the anvils of micrometer varies.
4. Error due to supports The elastic deformation/ deflection of a long measuring bar due to position of support cause error in measurement. So G.B Airy found out the position of supports to give minimum error. Two support conditions are: (i) for minimum deflection(fig 1.3) (ii) for zero slope at ends (fig 1.4) 5. Error due to alignment Abbe’s alignment principle should be followed to avoid error due to alignment. According to this principle the axis of measurement should coincide with measuring instruments. If D= True dia L= Apparent length = micrometer arivil dia Then D= L cos - sin Error= L-D= L L L cos dsign Fig 1.6 sin & cosine error both 6.Parallax error occur when line of vision is not directly in line with measuring scale PA= parallax error 7.Error due to dust Presence of dust in the atmosphere change reading in the order of fraction of micron. When high accuracy in measurement is required dust should be cleaned by clean chamois.