Engineering Metrology.pdf
- 1. Page 1 of 5 CENTRAL INSTITUTE OF PLASTICS ENGINEERING & TECHNOLOGY GUINDY, Chennai-600 032 Question Bank COURSE: DPMT SEMESTER: III SUBJECT: Engineering Metrology Section – B (2 mark – questions) 1. Define Accuracy. Ans: It can be defined as “the closeness of the measured value with the true value”. Accuracy is the degree to which the measured value agrees with the true value of the measured quantity. 2. What is Error and what are the sources of error. Ans: Error may be defined as the quantity which is equal to the difference between true value and the value found by measurement (indicated value). Sources of Error: 1. Defect in instruments. 2. Manual adjustment of an instrument. 3. Manufacturing defect in instrument. 3. List the Objectives of Metrology. Ans: It plays important role in every field of inspection. It can be used to achieve standardization. It can be used to find out the sources of errors. 4. What is Line Standard? Ans: Line standard is the standard in which distance is measured between two lines or two surfaces. It is quick and easy. It is not suitable for close tolerance length measurement. Parallax error can occur in measurement. 5. Define the measuring instrument? Ans: measuring devices transform the measured quantity or a related quantity into an indication or information. 6. What is the process of selection of Measuring Instruments? Ans: Range of measurement carried by the instrument. Sensitivity of the Instrument. 7. Define End Standard. Ans: When length is expressed as distance between the two flat parallel surfaces then it is called as End Standard. 8. What are Angular Measuring Instruments? Ans: The angle measuring instruments are: 1. Sine Bars 2. Sine Centre’s 3. Auto-Collimator 4. Bevel Protractor 5. Taper plug. 9. Write about the Slip gauges? Ans: Slip gauges or gauge blocks may be used as reference standards for transferring the dimension of the unit of length from the primary standard to gauge blocks of lower accuracy and for verification and graduation of measuring apparatus. 10. What is sine bar? Ans: The slip gauges can be used for measurement of angles with the help of a simple piece of apparatus known as sine bar. It is a hardened steel beam mounted on two hardened equal diameter cylinders at known distance. 11. Define Auto Collimator. Ans: It is an optical instrument used for measuring small angular differences. It provides a very sensitive and accurate approach for small angular measurements. It consists of three main parts viz, micrometer microscope, lighting unit (source of light) and collimating lens.
- 2. Page 2 of 5 12. How the interchangeability plays a vital role in metrology? Ans: An interchangeable component is one which can be substituted for similar part manufactured to the same drawing. Interchangeability is possible when certain standards are strictly followed in different manufacturing plants. 13. What is Nominal Size? Ans: It is used for the purpose of identification of a component. It is the size by which it is specified in a drawing as a matter of convenience. It is never used in precision measurements. 14. What is Basic Size? Ans: It is the standard size of a component with reference to which all the limits of variation of size are found. It is same for hole and corresponding shaft. 15. Define Limit? Ans: Limit refers to the two extreme permissible values between which the actual size of a component is present. It can be classified as maximum limit (upper limit) and minimum limit (lower limit) of size. 16. Define Tolerance. Ans: It is the algebraic difference between the maximum limit and minimum limit of a hole or a shaft. It can also be defined as the difference between the upper deviation and lower deviation and it has an absolute value without sign. 17. What is Fit? What are the different types of fits? Ans: When two parts are to be assembled, the relation resulting from the difference between their sizes before assembly is called as fit. It is the degree of tightness or looseness between the two mating parts to perform a particular function. The fits may be classified as i) Clearance fit ii) Interference fit iii) Transition fit. 18. What is Hole basis system? Ans: In this system, the hole size is kept constant and the shaft diameter is varied to get the different types of fits. The hole has constant high and low limits for all fits of the same accuracy and grade. It is denoted by ‘H’. 19. What is Shaft basis system? Ans: In this system, the shaft size is kept constant and the hole size is varied to get the different types of fits. In this case, the design size of a shaft whose upper deviation is zero is assumed as basic size. It is denoted by ‘h’. 20. What is the difference between Gauges and Instruments? Ans: Gauges: These are inspection tools without any scale arrangements. It does not give actual readings of a component. Gauges have limited application. Instruments: These are measuring tools with calibrated scale arrangements. It gives actual readings of a component. Instruments have wide range of applications. 21. What type of gauges can be used to measure or test the surface of a machined work piece? Ans: Plug gauges and Snap gauges are used to measure the test surface of a machined work piece. 22. What is comparator? What are the applications of comparators? Ans: The comparator is a precision type of instrument which enables a comparison between the part being measured and a length standard. These are used for inspecting newly purchased gauges. It can be used as a laboratory standard from which working gauges can be set by comparison. 23. Define CMM and its types. Ans: CMM is Co-ordinate Measuring Machines are used for post process inspection, where the work piece is removed from the machine tool and taken to CMM. Types are: i) Cantilever type ii) Bridge type iii) Column type iv) Gantry type.
- 3. Page 3 of 5 24. Write about the Tool Makers Microscope? Ans: Tool Makers Microscope is a versatile optical measuring machine. It is a direct reading type measuring instrument and mainly used in inspection of gauges, tools, screw threads, jigs, fixtures, dies, etc. It is used for external and internal length measurement, curvatures, angles etc. 25. List the different types of comparator. Ans: i) Mechanical Comparator ii) Optical Comparator iii) Pneumatic Comparator iv) Electrical Comparator v) Mechanical Optical Comparator. 26. Define Bilateral Tolerance. Ans: When one limit of size is on one side of zero line and other limit of size is on other side of zero line then the specified tolerance is called as bilateral tolerance. For example: 30 +_0.02 27. List the classification of standards depending upon the importance and accuracy required for the work. Ans: 1. Primary Standards. 2. Secondary Standards. 3. Tertiary Standards. 4. Working standards. 28. Define repeatability. Ans: It is the ability of the measuring instrument to repeat the same results when measurements are carried out: by same observer With the same instrument. Under the same conditions. Without any change in location. Without change in the method of measurement. 29. Write about the clearance fit. Ans: If the dimension of shaft is less than the dimension of hole even in the case of maximum material condition of hole & shaft, it is called clearance fit. 30. What is interchangeability? Ans: Manufacture of machine tools, automobiles, IC engines, aircraft etc requires thousands of components which are identical. In such large scale production each male component should fit with corresponding female component without interchanging the parts present in a lot of identical items. 31. Define Taylor’s principle of gauge design. Ans: Limit gauges are designed based on Taylor’s principle. It states that (i) Go gauges should be designed to check the maximum material limit while the No-Go gauges should be designed to check the minimum material limit of shaft/hole. 32. Write about the Interferometer. Ans: Interferometers are optical instrument used for measuring flatness and determining the length of slip gauges. They are based upon the interference principle and employ wavelength of light as their measuring units. 33. What is surface roughness and what are the sources of it? Ans: Surface roughness is nothing but repetitive or random deviation from the nominal surface. surface. The irregularities on the surface are in the form of succession of hills and valleys varying in height and spacing. These irregularities are named as: surface roughness, finish, texture & quality. 34. List the roughness values along with the manufacturing process. Ans: . S.No. Manufacturing Process Ra value in microns 1. Lapping 0.012 – 0.16 2. Super finishing 0.016 – 0.32 3. Honing 0.025 – 0.04 4. Polishing 0.04 – 0.16
- 4. Page 4 of 5 35. List the roughness symbols along with the roughness grade symbol. Ans: 36. Define the zero line. Ans: The line corresponding to basic size is called as zero line. This has zero deviation. The positive deviations are above the zero line and negative deviations are below the zero line. 37. What is the need of tolerance? Ans: It is never possible to make a component of actual given size and no method is available for accurate measurement of that size. It is difficult to produce a component of exact size due to variations in material properties of product and production machines themselves have some inherent accuracy in them and human error. 38. Define Sine Centre. Ans: Sine centre is a sine bar with block holding centre’s which can be adjusted and rigidly clamped in any position. It is mainly used for inspection of conical components which are centered at each end. The alignment accuracy of the centres ensures that the correct line of measurement is made along the work piece. 39. What is the Angle Gauge? Ans: Angle gauges are wedge shaped block of hardened and stabilized steel. They can be used as standard for angle measurement. The measuring faces of angle gauges are lapped and polished to high degree of accuracy and flatness. These are 76 mm long and 16 mm wide. 40. Write about the Auto Collimator. Ans: It is an optical instrument used for measuring small angular differences. It provides a very sensitive and accurate approach for small angular measurements. It is a telescope used for collimating other instruments. It converts the light rays into a parallel beam of light. 41. Write about the Dial Indicator? Ans: It is the most commonly used mechanical comparator. It consists of solid base with perfectly flat surface and column carries a bracket on which spindle and indicator are mounted. The linear movement of the plunger is magnified with the help of gear train into the rotation of pointer on the scale. 42. List the various LASER techniques used in metrology for measurements? Ans: Following are some LASER techniques used in metrology for measurements: i) Scanning laser Gauge ii) Photo diode array imaging iii) Diffraction pattern techniques iv) Laser triangulation sensors v) Two frequency laser interferometer vi) Laser scanning gauge S.No. Ra value in microns Roughness grade symbols 1. 50 ~ 2. 12.5 – 25.0 ▼ 3. 1.6 – 6.3 ▼▼ 4. 0.2 – 0.8 ▼▼▼ 5. 0.025 – 0.1 ▼▼▼▼
- 5. Page 5 of 5 43. What are the different grades of slip gauges? Ans: Slip gauges are divided in five grades, they are as mentioned below: Grade II: These grades are used in workshop for rough checks and in ordinary inspection work. Grade I: These are used for precise work such as measurement of tools, sine bar etc. Grade 0: These are also known as inspection grades used in tool room for inspection. 44. Define the Calibration. Ans: It is the process of checking the accuracy of the measuring instruments or the dimensions and tolerances of a gauge by comparing it with same or standard instrument/gauge which has been certified as per the standards. 45. What is Error and types of error? Ans: Error may be defined as the quantity which is equal to the difference between true value and the value found by measurement (indicated value). Mathematically, Error = True value – Indicated value. 1) Systematic Error (controllable error) 2) Random error. 46. Define the Uncertainty of Measurement. Ans: Instruments always give slightly different value of a measured quantity with repeated instruments. Uncertainty of measurement is that part of the result of measurement which states the range of values within which the true value is estimated to lie. It denotes the degree of accuracy with which the measurement has been made. 47. Write about the wringing of slip gauges. Ans: The accuracy of measurement depends on the phenomenon of wringing. The slip gauges are wrung together by hand through a combined sliding and rising motion. The gap between 2 wrung slip gauges is only of the order of 0.00635 microns which is negligible. 48. Write the difference between tolerance and allowance. Ans: Tolerance: It is the permissible variation in dimension of a part either a hole (or a shaft). It is also the difference between higher and lower limits of dimensions of a part. Allowance: It is the prescribed difference between the dimensions of two mating parts (hole and shaft). It is the difference between the maximum shaft and minimum hole size. 49. Define about the Optical flats. Ans: Optical flat works on the principle of light interference. It is made of any transparent material usually glass or quartz with 2 highly polished parallel flat surfaces. The yellow orange light eradicated by helium gas is most satisfactorily for use with optical flat. For greater accuracy of optical flat it must be used in areas where the temperature is constant. 50. List the advantages and disadvantages of limit gauges. Ans: Advantages: It can be used in mass production at relatively low cost. It is a faster inspection method which leads to increase in productivity. It confirms the interchangeability of the component so that they are easy to assemble. Disadvantages: It only indicates that whether the component is in specified limit or not. It does not give any information about its actual dimensions. While checking wear phenomenon may introduced and with the limit gauges it is difficult to check component with finer tolerance. *********************