Comparators

Comparators

1. Matoshri College of Engineering and Research Center Nashik Department of Mechanical Engineering Subject:- Metrology And Quality Control Class:-TE Mechanical Unit: II Comparators, Thread and Gear Metrology, Surface Roughness Measurement(Session 1) Semester:-V 2020-21 By Prof R.S.Patil (Assistant Professor MCOERC )

2. Syllabus Unit –II Comparators, Thread and Gear Metrology, Surface Roughness Measurement Comparators: Mechanical, Pneumatic, Optical, Electrical (LVDT). Measurement of Thread form: Thread form errors, Measurement of Minor, Major and Effective diameter (Three Wire Method), Flank angle and Pitch, Floating Carriage Micrometer (Numerical). Gear Metrology: Errors in Spur Gear form, Gear tooth Vernier , Constant chord, Base tangent (Numerical), Gear Rolling Tester. Profile Projector, Tool maker’s microscope and their applications Surface Roughness Measurement: Introduction to Surface texture, Parameters for measuring surface roughness, Surface roughness measuring instrument: TalySurf . Matoshri College of Engineering and Resarch Center Nashik

3. CONTENT Comparators Mechanical Pneumatic Optical Electrical (LVDT). Matoshri College of Engineering and Resarch Center Nashik

4. COMPARATORS A comparator is a precision instrument employed to compare the dimension of a given component with a working standard (usually slip gauges). It thus does not measure the actual dimension but indicate how much it differs from the basic dimension. In mass production identical component parts are produced on a very large scale. Micrometer and Vernier caliper are not more feasible because skill involve and time required to measure the dimension . In such case comparator is more suitable for the measurement . Matoshri College of Engineering and Resarch Center Nashik

5. Classification: 1. Mechanical Comparator : It works on gears pinions, linkages, levers, springs etc. 2 . Pneumatic Comparator : Pneumatic comparator works by using high pressure air, valves , back pressure etc. 3. Optical Comparator : Optical comparator works by using lens, mirrors, light source etc. 4. Electrical Comparator : Works by using step up, step down transformers. 5. Electronic Comparator : It works by using amplifier, digital signal 6. Combined Comparator : The combination of any two of the above types can give the best result. Matoshri College of Engineering and Resarch Center Nashik

6. MECHANICAL COMPARATORS Some of the comparator which belong to this class are: Dial indicator Reed type comparator Sigma Comparator Johnson Mikrokrator Matoshri College of Engineering and Resarch Center Nashik

7. Dial Indicator: It operates on the principle, that a very slight upward pressure on the spindle at the contact point is multiplied through a system of gears and levers. It is indicated on the face of the dial by a dial finger. Dial indicators basically consists of a body with a round graduated dial and a contact point connected with a spiral or gear train so that hand on the dial face indicates the amount of movement of the contact point. The indicator is set to zero by the use of slip gauges representing the basic size of part. Matoshri College of Engineering and Resarch Center Nashik

8. Dial Indicator: (different uses) Matoshri College of Engineering and Resarch Center Nashik

9. Applications: Comparing two heights or distances between narrow limits. To determine the errors in geometrical form such as ovality , roundness and taper. For taking accurate measurement of deformation such as intension and compression. To check the alignment of lathe centers by using suitable accurate bar between the centers. To check trueness of milling machine arbours and to check the parallelism of shaper arm with table surface or vice.   Matoshri College of Engineering and Resarch Center Nashik

10. Johansson Microkator : Principle: It works on the principle of a Button spring, spinning on a loop of string like in the case of Children’s toys. It employs a twisted metal strip. Any pull on the strip causes the centre of the strip to rotate. A very light pointer made of glass tube is attached to the centre of the twisted metal strip. The measuring plunger is on the slit washer and transmits its motion through the bell crank lever to the twisted metal strip. The other end of the twisted metal strip is fastened to the cantilever strip. The overhanging length of the cantilever strip can be varied to adjust the magnification of the instrument. The longer the length of the cantilever, the more it will deflect under the pull of the twisted metal strip and less rotation of the pointer is obtained.

11. Reed Type Mechanical Comparator The reed mechanism is frictionless device for magnifying small motions of spindle. It consists of a fixed block A which is rigidly fastened to the gauge head case. The floating block B carries the gauging spindle and is connected horizontally to the fixed block by reeds C. A vertical reed is attached to each block with upper ends joined together, as shown in the figure . Beyond this joint extends a pointer or target. A linear motion of the spindle moves the free block vertically causing the vertical reed to slide. This movement causes the pointer or target to swing through an arc over the scale proportional to the distance traveled by the spindle and of course very much magnified. Comparators of this type may have sensitivities of the order of 0.25 micron per scale division. Matoshri College of Engineering and Resarch Center Nashik

12. Mechanical - Optical Comparator

13. Principle: In mechanical optical comparator, small variation in the plunger movement is magnified first by mechanical system and then by optical system Matoshri College of Engineering and Resarch Center Nashik

14. Mechanical - Optical Comparator(Cont.) In mechanical optical comparator small displacement of the measuring plunger are amplified first by mechanical system consisting of pivoted lavers. The amplified mechanical movement is further amplified by a simple optical system involving the projection of an image. Mechanical system causes a plane reflector to tilt about an axis and the image of an index is projected on a scale on the inner surface of a ground glass screen. Optical magnification provides high degree of measuring precision . Matoshri College of Engineering and Resarch Center Nashik

15. Mechanical - Optical Comparator(Cont.) In this system, Mechanical amplification = x2 / x1 Optical amplification = (x4 / x3 ) * 2 It is multiplied by 2, because if mirror is tilted by an angle δθ , then image will be tilted by 2 x δθ . Thus over magnification of this system = 2 (x2 / x1) *(x4 / x3) Advantages of Optical Comparators:- 1. It has small number of moving parts and hence a higher accuracy. 2. In the optical comparators, the scale can be made to move past a datum line and thus have high range and no parallax errors. 3. It has very high magnification. 4. Optical lever is weightless. Matoshri College of Engineering and Resarch Center Nashik

16. Sigma Comparator The plunger is attached to a bar which is supported between the bending plates at the top and bottom portion as shown in Figure The bar is restricted to move in the vertical direction. A knife edge is fixed to the bar. The knife edge is attached to the sapphire plate which is attached to the moving block. edge extorts a force on the moving block through sapphire plate. Moving block is attached to the fixed block with the help of crossed strips as shown in Figure . When the force is applied on the moving block, it will give an angular deflection. A Y-arm which is attached to the moving block transmits the rotary motion to the driving drum of radius r. This deflects the pointer and then the reading is noted . Matoshri College of Engineering and Resarch Center Nashik

17. Advantages of Mechanical Comparator 1. They do not require any external source of energy. 2. These are cheaper and portable. 3. These are of robust construction and compact design. 4. The simple linear scales are easy to read. 5. These are unaffected by variations due to external source of energy such air, electricity etc. Matoshri College of Engineering and Resarch Center Nashik

18. Disadvantages: Range is limited as the pointer moves over a fixed scale. Pointer scale system used can cause parallax error. There are number of moving parts which create problems due to friction, and ultimately the accuracy is less. The instrument may become sensitive to vibration due to high inertia. Matoshri College of Engineering and Resarch Center Nashik

19. Pneumatic Comparators ( Solex Gauge): Principle: It works on the principle of pressure difference generated by the air flow. Air is supplied at constant pressure through the orifice and the air escapes in the form of jets through a restricted space which exerts a back pressure. The variation in the back pressure is then used to find the dimensions of a component. Matoshri College of Engineering and Resarch Center Nashik

20. Pneumatic Comparators(cont.) Working: As shown in Figure the air is compressed in the compressor at high pressure which is equal to Water head H. The excess air escapes in the form of bubbles. Then the metric amount of air is passed through the orifice at the constant pressure. Due to restricted area, at A1 position, the back pressure is generated by the head of water displaced in the manometer tube. To determine the roundness of the job, the job is rotated along the jet axis, if no variation in the pressure reading is obtained then we can say that the job is perfectly circular at position A1. Then the same procedure is repeated at various positions A2, A3, A4, position and variation in the pressure reading is found out. Also the diameter is measured at position A1 corresponding to the portion against two jets and diameter is also measured at various position along the length of the bore

21. Pneumatic Comparators(cont.) Metrological Advantages of Pneumatic Comparators Reasons for Pneumatic Inspection of Small Holes 1. Pneumatic gaging elements can be very small. This permits precision measurement of smaller inside diameters that is possible by other means of measurement. 2. Deeper holes can be inspected because depth has little effect on ease of measurement. 3. Greater accuracy is obtained because rocking and centralizing are unimportant. 4. Nominal gaging force permits thin-walled and yielding materials to be measured. 5. Absence of metal-to-metal contact protects finely finished surfaces. Matoshri College of Engineering and Resarch Center Nashik

22. Disadvantages of Pneumatic Comparators i)They require elaborate auxiliary equipment such as accurate pressure regulation. ii) The scale is generally not uniform. iii) When indicating device is the glass tube, then high magnification is necessary in order to avoid the meniscus errors. iv) The apparatus is not easily portable and is rather elaborate for many industrial applications. v) Different gauging heads are required for different dimensions. Matoshri College of Engineering and Resarch Center Nashik

23. Electrical Comparator Principle of working of Linear variable differential Transformer(LVDT) It work on principle of mutual inductance which state that when core is inserted in a magnetic field generated by primary winding then the voltage induced in secondary winding is proportional to core position Matoshri College of Engineering and Resarch Center Nashik

24. LVDT working When core is centralize at the middle position of two secondary windings then the voltage introduced in each secondary coil will be same and out of phase When the plunger move on the surface to be checked then the core which is connected to plunger will start to move from its mean position The differential voltage appears across secondary in series which is further calibrated in term of linear movement of the core()plunger Matoshri College of Engineering and Resarch Center Nashik

25. Assignment Q1) Explain working principle of Mechanical comparator(any one) Q 2) State different types of comparators, explain sigma comparator in detail. Q3) Principle of working of Linear variable differential Transformer (LVDT) Any Question [email protected] Matoshri College of Engineering and Resarch Center Nashik