chapter 7Types of comparators-04.08.2024 - 1.pptx

Comparators

1. CHAPTER 7 CO M P AR A T ORS Presentation Prepared by Prof. Naman M. Dave Assistant Prof. (Mechanical Dept.) Gandhinagar Institute of Technology

2. I N T RODUC TI ON A comparator is a precision instrument used for comparing the dimensions of a workpiece with a working standard or master setting which represents the basic size. A comparator w o r k s o n r e l a t i ve m ea s u r e m e n t s , i t d oe s not m e a s u re t h e actual dimension but indicates how much it differs form the basic dimension . The indicated difference is normally.

3. I N T RODUC TI ON

4. In case of mass production , the identical component parts are produced on a very large scale . It is very difficult, expensive and may not be feasible to measure the dimensions of all components with use of vernier caliper, micrometer etc. To check dimensional variation from the standard or basic dimension of part. Comparing manufactured part to the master (standard) part . The comparator requires little skill for the operator , eliminates human element for taking measurement and gives quick and highly consistent results . Why comparator is required ?

5. Characteristics of good Comparator The instrument must be robust design and construction so it can withstand the effect of ordinary uses without affecting its measuring accuracy. The instrument should be very sensitive and withstand a reasonable ill usage without permanent harm . The comparator design must be such that it can be used for wide range of operations . The instrument working should not be affected by environment change in temperature, pressure and humidity.

6. Characteristics of good Comparator D i s p l a y o r i nd i c a ti n g s y s t e m s hou l d d i s p l a y results in least possible time . S en s i n g o r c on t ac t p o i n t s ho u l d b e h a rd enough to minimize wear effects . It m u st h a v e h i g h m a gn if i ca ti o n i n o r d e r t o measure or detect very small change in input. It should be free from backlash and unwanted friction .

7. Characteristics of good Comparator Robust design and construction : The design and construction of the comparator should be robust so that it can withstand the effects of ordinary uses without affecting its measuring accuracy. Linear characteristics of scale : Recording or measuring scale should be linear and uniform (straight line characteristic) and its indications should be clear. High magnification : The magnification of the comparator should be such that smallest deviation in size of component can be read . Quick in results : The indicating system should be such that the readings are obtained in least possible time . Versatility : Instruments should be designed that it can be used for wide range of measurements . Minimum wear of contact point : The measuring plunger should have hardened steel contact or diamond to minimize wear effects. Further the contact pressure should be low and uniform.

8. Characteristics of good Comparator Free from oscillations : The pointer should come rapidly , to rest and should be free from oscillations. Free from back lash : System should be free from back lash and unnecessary friction . Quick insertion of workpiece : Means should be provided for lifting the plunger for quick insertion of work. A d j u s t a b le T a b le : T h e t a b le o f t h e i n s tr u m en t s h o u l d, preferably, be adjustable in a vertical sense. Compensation from temperature effects : The indicator should be provided with maximum compensation for temperature effects. Means to prevent damage : Suitable means should be provided for preventing damage of the instrument in the event of the plunger moving through a greater distance than that corresponding to the range of its measuring scale.

9. Applications of comparators : The comparators are used to check the parts in mass production at a very fast rate . They are used to inspect newly purchased gauges . They are worked as a laboratory standards from which working or inspection gauges are set and correlated. They are used as final inspection gauges in selective assembly of parts where parts are graded in three groups depending upon their tolerance. They are used for checking parts received form outside sources.

10. Applications of comparators : The various ways in which comparators can be used are : Laboratory Standards : Comparators are used as laboratory standards from which working or inspection gauges are set and correlated. Working Gauges : They are also used as working gauges to prevent work spoilage and to maintain required tolerance at all important stages of manufacture. Final Inspection Gauges : Comparators may be used as final inspection gauges where selective assembly, of production parts is necessary. Receiving Inspection Gauges : As receiving inspection gauges comparators are used for checking parts received from outside sources. For checking newly purchased gauges : The use of comparators enables the checking of the parts Components in mass production at a very fast rate.

11. Applications of comparators :

12. Classification of Comparators :

13. M ec h a n is m s Rack and pinion Cam and gear train Lever with toothed gear . Compound levers Twisted taut strip Lever combined

14. Dial Indicator The dial indicator or dial gauge amplifies the length or displacement and translate it into rotational motion of pointer over a circular scale. It is used for pointer over a circular scale.

15. Dial Indicator

16. Dial Indicator in use

17. Dial Indicator (l) It can be used to compare two heights or distances between narrow limits. It is used for determining errors in Applications geometrical forms ( ovality, out of roundness, taper etc.) It is used for testing the alignment, roundness and parallelism of work piece. The dial indicators are most common instrument for quality control and inspection work

18. Dial Indicator A pp lic a t i o n s

19. Dial Indicator

20. Johansson Mikrokator This comparator works on principle that when a tension is applied to the twisted strip, this causes the strip to untwist resulting in rotation of pointer which attached at center of twisted strip. The two halves of the strip from the center are twisted in opposite directions so that any tension or pull on the strip will cause the center to rotate.

21. Johansson Mikrokator One end of the strip is fixed to the adjustable cantilever and other end is attached to bell crank lever which pivoted at the frame of comparator, and their other end contacts with top of the plunger. Plunger moves upward due to contact with the workpiece, the bell crank lever pulls the twisted strip, and hence the pointer which is attached at center of twisted strip rotates. The rotation of pointer is proportional to the change in length of strip and plunger movement.

22. Johansson Mikrokator

23. Reed type Mechanical Comparator A fixed block is rigidly fastened to the gauge head case and floating block B which carries measuring spindle end connected horizontally t o b l ock A by r eeds C. Vertical reeds D a attached to each block with upper ends joined together. Beyond this joint extends a pointer. Linear motion of spindle moves the free block B vertically causing the vertical reed on the floating block to slide past the vertical reed on the fixed block. Since vertical reeds are joined at their upper end instead of slipping the movement causes both reeds swing through an arc. The amount of reeds swing or pointer rotation is proportional to the distance the floating block B moves with the spindle.

24. S I G M A Com p ara t or The sigma comparator works on leverage principle in which high magnification is obtained entirely by mechanical means.

25. Mechanical measurement and metrology Prof. Naman M. Dave S IG M A C o mp ara t or

26. Mechanical measurement and metrology Prof. Naman M. Dave S IG M A C o mp ara t or

27. Mechanical Comparators Advantages : (l) They are cheaper compared to other types of comparators. They do not require any external electrical energy. They are normally robust, compact and easy to handle. Normally, they have a linear scale. Mechanical comparators are suitable for ordinary workshop conditions and being portable can be issued from a store. Disadvantages : Due to more moving parts, the friction is more and subsequently less accuracy obtained. The accuracy of this comparator is considerably reduced due to backlash. Less sensitive due to inertia of moving parts. Errors due to parallax is possible as moving pointer moves over a fixed scale. The range of instrument is limited as pointer moves over a fixed scale They have a less magnification compared to optical comparators.

28. Working principle of Optical Comparator

29. Mechanical-Optical Comparator Mechanical optical comparator consists of plunger which is attached with lever and a mirror on which the light strikes from the source. The light coming form source is directed on the calibrated scale. The angle of tilting min or is changed by movement of plunger.

30. Mechanical-Optical Comparator

31. Mechanical-Optical Comparator

32. Mechanical-Optical Comparator

33. Advantages of Mechanical-Optical Comparators : They have a less number of mechanical moving parts, hence less friction and inertia effect, and gives higher accuracy The scale can be made to move past a datum line and thus have high range and no palallax. They have high magnification and hence suitable for precision measurements. The scale in the optical comparators' illuminated and hence enables readings to be taken irrespective of room Lighting conditions. Disadvantages Mechanical-Optical Comparators : They require the light source The comparator is usually large and expensive. When the scale is projected on a screen, then it is necessary to use these instrument in a dark room in order to take reading easily. Instrument is inconvenient for continuous use because the scale is to be viewed through eyepiece. Due to high magnification, heat generated from transformers and lamp may cause the setting to drift.

34. Electrical Comparators

35. Electrical Comparators

36. Electrical Comparators Advantages of electrical and electronic comparators Few number of moving parts : The electric and electronic comparators have few number of moving parts, and there is less friction and wear. High magnification : It has a wide range of magnification. Not sensitive to vibrations : The mechanism carrying the pointer is very light and not sensitive to vibrations. Easy to set up and operate. Less error due to sliding friction : operation of the instrument on AC supply reduces sliding friction errors. 5. The instrument is small and compact. 7. The indicating instrument need not be placed close to the measuring unit Disadvantages Fluctuation in the voltage or frequency of the electric supply rnay affect the results Heating of coils in the measuring unit may cause zero drift and alter the calibration. When measuring unit is remote from the indicating unit, reliability is lower. Cost is generally more than mechanical comparator. If only a fixed scale is used with a moving pointer than with htgh magnification a very small range is obtained.

37. Pneumatic Comparator When air passes through the narrow constant space: between nozzle and the workpiece the pressure of air remains constant. If the space between nozzle and workpiece changes, it causes change in the pressure of the air. This change in pressure is utilized to measure the deviation of the dimension being measured from the standard dimension.

38. Pneumatic Comparator The pneumatic comparator is normally used to determine the roundness of the job. The system designed in order to supply air at constant pressure to the measuring jets. If the pressure of the air supplied is higher than the predefined pressure, some air will bubble out from the bottom of the dip tube and air moving to control orifice will be at predefined constant pressure. The constant pressure air then passes through control orifice and escape from the rneasuring jets when there is no restriction provided by workpiece to the escape of air, the level of water in the manometer tube will same as level of water in cylinder.

39. Pneumatic Comparator But when restriction is provided at the measunirg jets, resulting increases the back pressure and level of water in the manometer fall down which vary the height of water in manometer tube, a calibrated scale being set beside this tube. In other words the restriction to the escape of air depends upon the variations in the dimensions to be measured.

40. Pneumatic Comparator Advantages of pneumatic Comparators It is possible to obtain high degree of magnification (30,000 : 1) or more coupled with good stability and readability. The gauging member does not come in contact with the part to be measured and hence practicality no wear takes place on gauging member, 3, It has few number of moving parts and in some cases none. Thus the accuracy obtainable is more due to absence of friction and less inertia. 4. Measuring pressure is very small and the jet of air helps in cleaning the dust if any, from the part to be measured- S. The indicating instrument can be remote from the measuring unit. It is very suitable for measuring diameter of holes whose the diameter is small compared with the length. It is probably the best method to determine the ovality and taperness of circular holes. Single or number of dimensions can be inspected simultaneously. Disadvantages 1. Limited range of measurement is available with these comparators 2 It gives low speed of response compared with electrical magnification system. 3. It requires elaborate auxilary equipment such as accurate pressure regulator. 4.The scale is generally not uniform. When indicating device is the glass tube, then high magnification is necessary in order to avoid the meniscus errors. The apparatus is not easily portable. 7 Different gauging heads are required for different dimensions.