PROBLEMS
Grade 00 This set is kept in the standards room and is used for inspection/calibration of high precision only. It is also used to check the accuracy of the workshop and grade 1 slip gauges
6.7 PNEUMATIC COMPARATORS
6.7.4 Applications of Pneumatic Comparators
Pneumatic gauging is one of the widely used methods for inspection of holes. While it comprises relatively simple elements such as air filters, glass columns, manometer tubes, and bourdon tubes, the inspection can be carried out with an accuracy up to 1 µm. The gauging elements can be adapted to measure nearly any feature of the hole, including diameter, roundness, squareness, and straightness. Figure 6.25 illustrates the use of a single-jet nozzle, which can be used to carry out a variety of inspections.
The gauging element in pneumatic metrology can be classified into three types: type 1, type 2, and type 3. In type 1, the hole being measured is the exit nozzle of the gauging element. This (as illustrated in Fig. 6.26a) is only suitable for inside measurement and is used when the cross- sectional area is to be controlled rather than the shape. Typical applications include inspection of automobile cylinder bores, nozzle of carburettor, etc.
The gauging element in type 2 is illustrated in Fig. 6.26(b). In this case, an air jet not in contact with the part is the gauging element. The rate of flow of air depends on the cross- sectional area of the nozzle and the clearance between the nozzle and the part features. In other words, it is basically an air jet placed close to the part.
COMPARATORS 163
In type 3, the air jet is mechanically actuated by contact with the part. This is more suited for attribute inspection (GO and NO GO type). It is compact and can replace an LVDT.
It incorporates an air valve that changes the air flow in proportion to the linear change. This is often used interchangeably with an electronic gauge head.
The pneumatic gauging head may have one or more measuring orifices. Accordingly, a gauging head with a single orifice results in the indicator needle moving to either the positive or the negative side, depending on the variation in gap between the orifice and the work part. However, two opposing orifices in the measuring head can provide differential measurement. The clearance with respect to both the orifices will get added up, resulting in an equivalent gap. By rotating the measuring head, characteristics, for example, out-of-roundness can be reliably measured. Figure 6.27 illustrates four types of gauging heads with one, two, three, and four measuring orifices. Table 6.2 lists the typical applications of each.
Table 6.2 Applications of multiple-orifice gauging heads
Type No. of orifices Applications
A 1 Checking concentricity, location, squareness, flatness, straightness, length, and depth B 2 Checking inside diameter, out-of-roundness, bell-mouth, and taper
C 3 Checking triangular out-of-roundness (lobbing) D 4 Furnishing average diameter readings in a single setting
Stand
Workpiece Gauging head
Fig. 6.26 Types of pneumatic gauging elements (a) Type 1 (b) Type 2
(a) (b)
Fig. 6.27 Types of gauging heads Fig. 6.25 Use of a single-jet nozzle for inspection
Height gauge
Depth gauge
Plate straightness
Tube straightness
Squareness
Outside diameter
A
C
B
D
Pneumatic comparators are always preferred for the inspection of small holes (<15 mm).
Precision up to 10 µm is obtained easily. Pneumatic comparators are preferred for larger holes as well, because they provide a number of desirable features such as high amplification, excellent readability, non-contact operation, and absence of mechanical parts, among others. However, pneumatic comparators suffer from three limitations: short measurement range, sensitivity to surface finish of work parts, and the need for expensive gauging elements and masters that offset the low instrument cost.
A QUICK OVERVIEW
• A comparator works on the principle of relative measurement. It gives only dimensional differences in relation to a basic dimension or master setting.
• In direct measurement, precision is dependent on the least count of the scale and the means for reading it. In comparison measurement, it is dependent on the least count of the standard and the means for comparing.
• Comparators are classified as mechanical comparator, mechanical–optical comparator, electrical and electronic comparator, pneumatic comparator, and other types such as projection comparators and multi-check comparators.
• Optical comparators provide a high degree of precision in measurements due to reduction of moving members and better wear-resistance qualities.
• Electronic comparators are in widespread use because they give responses instantaneously and it is convenient to amplify the input. An electronic comparator in particular can achieve an exceptionally high magnification of the order of 105:1 quite easily.
• Electrical comparators generally depend on a Wheatstone bridge circuit for measurement.
Displacement of the sensing element, a plunger, results in an armature connected to one of the arms of the bridge circuit to cause imbalance in the circuit. This imbalance is registered as an output by a galvanometer, which is calibrated to read in units of linear movement of the plunger.
• An LVDT directly converts mechanical displacement into a proportional electrical voltage. This is unlike an electrical strain gauge, which requires the assistance of some form of an elastic member.
• The Sigma electronic comparator is extremely popular in inspection processes because of advantages such as ease of usage, high degree of accuracy and repeatability, provision for setting tolerances during inspection, and ease of integration into a computer-integrated manufacturing system.
• A pneumatic comparator is best suited for inspecting multiple dimensions of a part in a single setting ranging from 0.5 to 1000 mm. It is also amenable to on-line inspection of parts on board a machine tool or equipment. Another significant advantage of a pneumatic gauge is that it is relatively free from operator error.
MULTIPLE-CHOICE QUESTIONS
1. In comparison measurement, precision depends
on .
(a) least count of the comparator
(b) least count of the standard
(c) least count of the scale of the instrument (d) all of these
COMPARATORS 165
2. In case of a comparator, measurement is done by (a) displacement method
(b) interchange method (c) direct method (d) Parkinson method
3. A balanced dial in a dial gauge has
(a) graduations in both metric and British systems
(b) graduations starting from zero and extending to the end of the recommended range (c) graduations marked both ways of zero (d) graduations in logarithmic scale
4. How is the precision of a dial comparator determined?
(a) By comparison of the readings with slip gauges
(b) By dispersal of a series of readings (c) From the manufacturer’s specifications (d) From the distance between dial graduations 5. contact point is most preferred in
dial gauges, since it presents point contact to the mating surface irrespective of whether it is flat or cylindrical.
(a) Spherical (b) Flat (c) Tapered (d) Button
6. The basic principle of the Johansson mikrokator is based on
(a) Johansson movement (b) Abbey movement (c) Abraham movement (d) Abramson movement
7. With respect to the Johansson mikrokator, which of the following statements is true?
(a) Magnification varies inversely with the number of turns and width of the metal strip.
(b) Magnification varies directly with the number of turns and width of the metal strip.
(c) The more the number of turns of the strip, the higher the magnification.
(d) The thicker the strip, the higher the magnification.
8. In a Sigma mechanical comparator, magni- fication is obtained
(a) in a single stage
(b) in two stages (c) in three stages
(d) depending on the manufacturer’s instruction 9. Double reflection of light using a pair of mirrors
is a unique feature of (a) an autocollimator (b) a clinometer
(c) a Zeiss ultra-optimeter (d) an optical projector
10. Which of the following comparators is best suited for inspection of small gears and screws?
(a) Autocollimator (b) Profile projector (c) Johansson mikrokator (d) Zeiss ultra-optimeter
11. Which of the following comparators is non- contact type?
(a) Johansson mikrokator
(b) Mechanical optical comparator (c) Sigma comparator
(d) LVDT
12. An LVDT works on the principle of (a) mutual inductance
(b) mutual capacitance (c) mutual resistance (d) magnetic induction
13. Which of the following gauges is relatively free from operator’s error?
(a) Sigma mechanical comparator (b) Zeiss ultra-optimeter
(c) Pneumatic gauge (d) All of these
14. Which of the following comparators can give amplification of up to 50,000?
(a) LVDT
(b) Solex pneumatic gauge (c) Dial gauge
(d) Sigma electronic comparator
15. How many orifices will you recommend for pneumatic gauging of triangular out-of- roundness (lobbing)?
(a) One (b) Two (c) Three (d) Four
REVIEW QUESTIONS
1. Compare comparison measurement with direct measurement.
2. Define a comparator. Discuss the functional requirements of a comparator.
3. Give the classification of comparators.
4. With the help of a neat sketch, explain the functional parts of a dial indicator.
5. Explain the working mechanism of a dial indicator.
6. Write a note on the various contact points used in a dial indicator.
7. What are the guidelines for the proper use of dial indicators?
8. Explain the factors that influence amplification in a Johansson mikrokator.
9. With the help of a neat sketch, explain the working principle of a Sigma mechanical comparator.
10. Explain the optical system in a mechanical optical comparator. What are its advantages when compared to a mechanical comparator?
11. Explain the function of a Zeiss ultra-optimeter.
12. Discuss the various metrological applications of an optical projector.
13. What is an LVDT? Explain its working principle.
Discuss the characteristic curve of an LVDT with a sketch.
14. What are the major advantages of electronic comparators that have made them the first
choice in inspection metrology?
15. Explain the working principle of a Sigma electronic comparator.
16. Discuss the functional and metrological features of pneumatic comparators.
17. Explain how the relationship between the clearance (between gauge head and work part) and air flow rate enables measurement in a free- flow-type pneumatic gauge.
18. A pneumatic gauge is relatively free from operator error. How do you justify this statement?
19. Explain the working principle of a pneumatic back pressure gauge. Discuss the relevance of the characteristic curve in measurement.
20. With the help of a figure, explain the working principle of a Solex pneumatic gauge.
21. Discuss the major applications of pneumatic gauges.
22. The operation of instruments requires energy.
What is the source of energy for dial indicators, pneumatic comparators, and electrical comparators?
23. What is the most important way in which an electronic comparator differs from a mechanical comparator?
24. What is a balanced bridge? What is its significance in electrical comparators?
25. What are the limitations of pneumatic metrology?
Answers to Multiple-choice Questions
1. (b) 2. (b) 3. (c) 4. (b) 5. (a) 6. (d) 7. (a) 8. (b) 9. (c) 10. (b) 11. (d) 12. (a) 13. (c) 14. (b) 15. (c)