Diploma Measurements and Metrology Important questions and answers
We are adding Diploma Measurements and metrology Important Questions and answers for N Scheme important Questions and answers for the Unit 1 – 14 Marks. Check the link below to download Diploma Measurements and metrology important questions and answers pdf & Diploma Measurements and metrology notes
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Measurements and Metrology
Unit – 1, Part 1 – Basic Concepts of Measurements
Important Questions and Answers – 14 marks
- Explain Briefly about the objectives of Engineering Metrology (7 marks)
The basic objectives of metrology are;
- To provide accuracy at minimum cost.
- Thorough evaluation of newly developed products, and to ensure that components are within the specified dimensions.
- To determine the process capabilities.
- To assess the measuring instrument capabilities and ensure that they are adequate for their specific measurements.
- To reduce the cost of inspection & rejections and rework.
- To standardize measuring methods.
- To maintain the accuracy of measurements through periodical calibration of the instruments. 8. To prepare designs for gauges and special inspection fixtures
- Explain the methods of Measurements
METHODS OF MEASUREMENTS
These are the methods of comparison used in measurement process. In precision measurement various methods of measurement are adopted depending upon the accuracy required and the amount of permissible error. The methods of measurement can be classified as:
- Direct method of measurement: This is a simple method of measurement, in which the value of the quantity to be measured is obtained directly without any calculations. For example, measurements by using scales, vernier callipers, micrometers, bevel protector etc. This method is most widely used in production. This method is not very accurate because it depends on human insensitiveness in making judgment.
- Indirect method of measurement:
In indirect method the value of quantity to be measured is obtained by measuring other quantities which are functionally related to the required value. E.g. Angle measurement by sine bar, measurement of screw pitch diameter by three wire method etc
It is aslo called fundamental method. The measurement and the value depends on other quantities.
In this method the value is compared with some know values or quantities. It is uded to determine the deviations E.g. Dial Indicators or other comparators
It is a differential method of measurement. In this method, very small difference are measured by coincidence of lines e.g. Vernier caliper
- Contact Method
The measuring tip of the instrument makes contact with the surface being measured e.g. Micrometer, Dial indicator.
- Briefly Explain the Various standards of Length
STANDARDS OF LENGTH
In practice, the accurate measurement must be made by comparison with a standard of known dimension and such a standard is called “Primary Standard”. The first accurate standard was made in England and was known as “Imperial Standard yard” which was followed by International Prototype meter” made in France. Since these two standards of length were made of metal alloys they are called „material length standards”.
INTERNATIONAL PROTOTYPE METER:
It is defined as the straight line distance, at 0oC, between the engraved lines of pure platinum-iridium alloy (90% platinum & 10% iridium) of 1020 mm total length and having a “tresca‟ cross section as shown in fig. The graduations are on the upper surface of the web which coincides with the neutral axis of the section.
The tresca cross section gives greater rigidity for the amount of material involved and is therefore economic in the use of an expensive metal. The platinum-iridium alloy is used because it is non oxidizable and retains good polished surface required for engraving good quality lines.
IMPERIAL STANDARD YARD:
An imperial standard yard, shown in fig, is a bronze (82% Cu, 13% tin, 5% Zinc) bar of 1 inch square section and 38 inches long. A round recess, 1 inch away from the two ends is cut at both ends upto the central or „neutral plane‟ of the bar. Further, a small round recess of (1/10) inch in diameter is made below the center. Two gold plugs of (1/10) inch diameter having engravings are inserted into these holes so that the lines (engravings) are in neutral plane. Yard is defined as the distance between the two central transverse lines of the gold plug at 620F. The purpose of keeping the gold plugs in line with the neutral axis is to ensure that the neutral axis remains unaffected due to bending, and to protect the gold plugs from accidental damage.
LIGHT (OPTICAL) WAVE LENGTH STANDARD:
Because of the problems of variation in length of material length standards, the possibility of using light as a basic unit to define primary standard has been considered. The wavelength of a selected radiation of light and is used as the basic unit of length. Since the wavelength is not a physical one, it need not be preserved & can be easily reproducible without considerable error.
- Mention the classification of Standards. Explain Line and End standard.
Standards Can be classified into
- Line standard
- End standard
LINE STANDARDS When the length being measured is expressed as the distance between two lines, then it is called “Line Standard”. Examples: Measuring scales, Imperial standard yard, International prototype meter, etc.
Characteristics of Line Standards:
- Scales can be accurately engraved but it is difficult to take the full advantage of this accuracy. Ex: A steel rule can be read to about ± 0.2 mm of true dimension.
- A scale is quick and easy to use over a wide range of measurements.
- The wear on the leading ends results in „under sizing‟.
- A scale does not possess a „built in‟ datum which would allow easy scale alignment with the axis of measurement, this again results in „under sizing‟.
- Scales are subjected to parallax effect, which is a source of both positive & negative reading errors‟.
- Scales are not convenient for close tolerance length measurements except in conjunction with microscopes.
When the length being measured is expressed as the distance between two parallel faces, then it is called „End standard‟. End standards can be made to a very high degree of accuracy. Ex: Slip gauges, Gap gauges, Ends of micrometer anvils, etc.
Characteristics of End Standards:
- End standards are highly accurate and are well suited for measurements of close tolerances as small as 0.0005 mm.
- They are time consuming in use and prove only one dimension at a time.
- End standards are subjected to wear on their measuring faces.
- End standards have a built in‟ datum, because their measuring faces are flat & parallel and can be positively located on a datum surface.
- They are not subjected to the parallax effect since their use depends on “feel”.
- Groups of blocks may be “wrung” together to build up any length. But faulty wringing leads to damage.
- The accuracy of both end & line standards are affected by temperature change.
- Explain the various need of inspection.
NEED OF INSPECTION:
The need of inspection can be summarized as:
- To ensure that the part, material or a component conforms to the established standard.
- To meet the interchangeability of manufacture.
- To maintain customer relation by ensuring that no faulty product reaches the customers
- Provide the means of finding out shortcomings in manufacture. The results of inspection are not only recorded but forwarded to the manufacturing department for taking necessary steps, so as to produce acceptable parts and reduce scrap.
- It also helps to purchase good quality of raw materials, tools, equipment which governs the quality of the finished products.
- It also helps to co-ordinate the functions of quality control, production, purchasing and other departments of the organization. To take decision on the defective parts i.e., to judge the possibility of making some of these parts acceptable after minor repairs
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