PROBLEMS
3.8 SNAP GAUGES
Snap gauges are used to gauge the shaft to ascertain whether the dimensions of the shafts are well within the tolerance limits. These gauges like plug gauges are also manufactured using hardened and stabilized wear-resistant steel. The gauging surface is ground and lapped. Shafts of sizes ranging from 3 to 100 mm can conveniently be gauged using double-ended snap gauges, as shown in Fig. 3.34. For sizes over 100 and up to 250 mm, single-ended progressive-type snap gauges are used. A progressive snap gauge is schematically represented in Fig. 3.35.
MULTIPLE-CHOICE QUESTIONS
1. Identify the correct feature control frame from the following:
(a) f 0.01 (c) f 0.01 X XT (b) f 0.01 X (d) f 0.01
2. Which of the following does the symbol indicate?
(a) Profile of semicircle (c) Profile of line (b) Profile of surface (d) Profile of circle 3. Which of the following symbols indicates acceptable for each of the dimensions used to
define shape and form, and ensure satisfactory operation in service. The difference between the upper and lower limits is termed permissive tolerance.
• The components of a product are manufactured in one or more batches by different persons on different machines and at different locations and are then assembled at one place. To achieve this, mass production becomes inevitable, but at the same time, adherence to the specified limits of accuracy is also important. Manufacture of components under such conditions is called interchangeable manufacture.
• When identical components, manufactured by different operators, using different machine tools and under different environmental conditions, are assembled and replaced without any further modification during the assembly stage, without affecting the functioning of the component, it is known as interchangeable assembly.
• When the tolerance distribution is only on one side of the basic size, it is known as unilateral tolerance, and when it lies on either side of the basic size, it is called bilateral tolerance.
• Fit is a relationship that results between the two mating parts to be assembled, that is, a hole and shaft, with respect to the difference in their dimensions before assembly.
• Depending on the actual limits of the hole or shaft, three basic types of fits can be identified,:
clearance fit, interference fit, and transition fit.
• An allowance is the intentional difference between the MMLs, that is, LLH and HLS (minimum clearance or maximum interference) of the two mating parts.
• To obtain the desired class of fits, either the size
of the hole or the size of the shaft must vary.
Hole basis and shaft basis systems are used to represent the three basic types of fits.
• Fundamental deviation is the minimum diff- erence between the size of a component and its basic size. This is identical to the upper deviation for shafts and lower deviation for holes. It is the closest deviation to the basic size.
Fundamental tolerance is also called ‘grade of tolerance’. In the Indian Standard system, there are 18 grades represented by number symbols for both hole and shaft. Fundamental deviation and fundamental tolerance indicate the type and quality of fit, respectively.
• Limit gauges ensure that the components lie within the permissible limits, but do not determine the actual size or dimensions. Gauges are scaleless inspection tools, which are used to check the conformance to the limits of the parts along with their forms and the relative positions of the surfaces of the parts.
• The gauges required to check the dimensions of the components correspond to two sizes conforming to the maximum and minimum limits of the components. They are called GO gauges or NO GO or NOT GO gauges, which correspond, respectively, to the MML and LML of the component.
• According to Taylor’s principle, the GO gauge is designed to check maximum metal conditions.
It should also simultaneously check as many related dimensions such as roundness, size, and location, as possible. The NOT GO gauge is designed to check minimum metal conditions.
It should check only one dimension at a time.
Thus, a separate NOT GO gauge is required for each individual dimension.
LIMITS, FITS, AND TOLERANCES 77
circular runout?
(a) (c)
(b) (d)
4. Which of the following represents the type of fit for a hole and shaft pair, given that
+0.04 +0.060
hole = 50 +0.00 mm and shaft = 50 +0.041 mm?
(a) Clearance fit (c) Loose fit (b) Transition fit (d) Interference fit 5. How many grades of tolerances does the ISO
system of limits and fits specify?
(a) 12 (b) 08 (c) 18 (d) 25 6. The most appropriate reason for specifying
grades of tolerances in ISO system is (a) to improve accuracy of manufacture (b) for convenience
(c) for ease of manufacture of parts
(d) to help quality personnel accept more comp- onents
7. In a shaft basis system, the upper deviation of the size of shaft is
(a) 1 (c) not related to size (b) less than 0 (d) 0
8. In the hole and shaft pair designation of 40 H7/
d9, the numbers 7 and 9 indicate (a) nothing of importance (b) tolerance grade
(c) accuracy of manufacture (d) ease of assembly
9. NO GO gauges are designed (a) for maximum passability (b) for maximum impassability (c) without any specified conditions
(d) without attaching any importance to them 10. An allowance is provided
(a) to help the operator
(b) to aid in production (c) intentionally
(d) as permissive tolerance 11. MML corresponds to the
(a) higher limit of a hole and lower limit of the shaft
(b) lower limit of a hole and lower limit of the shaft
(c) higher limit of a hole and higher limit of the shaft
(d) lower limit of a hole and higher limit of the shaft
12. LML corresponds to the
(a) lower limit of a hole and higher limit of the shaft
(b) higher limit of a hole and lower limit of the shaft
(c) lower limit of a shaft and lower limit of the hole
(d) higher limit of a shaft and higher limit of the hole
13. Limit gauges are used to
(a) measure flatness of the component (b) measure exact size of the component
(c) check if the component dimension lies within permissible limits
(d) measure surface roughness of the component 14. According to Taylor’s principle, GO gauges are
designed to check
(a) maximum metal condition (b) minimum metal condition (c) both of these
(d) none of these
15. The relationship that results between the two mating parts before assembly is called
(a) tolerance (c) limit (b) allowance (d) fit
REVIEW QUESTIONS
1. Briefly explain the need to specify tolerance on components.
2. Define unilateral and bilateral tolerances. Give examples for each.
3. Explain why a unilateral tolerance system is
generally preferred over bilateral system.
4. Explain the terms interchangeable manufacture and interchangeable assembly.
5. With an example, briefly explain the selective assembly approach.
PROBLEMS
1. The tolerances for a hole and shaft assembly having a nominal size of 50 mm are as follows:
+0.021 −0.040
Hole = 40 +0.000 mm and shaft= 40 −0.075 mm Determine
(a) maximum and minimum clearances (b) tolerances on shaft and hole (c) allowance
(d) MML of hole and shaft (e) type of fit
2. A shaft is manufactured within the specified limits of 30.02 and 29.98 mm. Find the high
and low limits of the bush to give a maximum clearance of 0.10 mm and minimum clearance of 0.02 mm.
3. Calculate the limits, tolerances, and allowances on a 25 mm shaft and hole pair designated H7/g6 to get a precision fit. The fundamental tolerance is calculated by the following equation:
i = 0.4533 D + 0.001D The following data is given:
(a) Upper deviation of shaft = −2.5D0.34 (b) 25 mm falls in the diameter step of 18–30 mm (c) IT7 = 16i
6. Distinguish between tolerance and allowance.
7. Explain the following terms:
(a) Limits
(b) Fundamental deviation (c) Fundamental tolerance
8. Define fit and with the help of neat sketches, explain the different types of fits.
9. What are the essential conditions to obtain clearance and interference fits?
10. Differentiate between hole basis and shaft basis systems.
11. Explain the effect of work tolerance on manu- facturing cost.
12. Explain the terms local interchangeability and universal interchangeability.
13. Explain why hole basis system is generally preferred.
14. With a neat sketch, discuss compound tolerance.
15. Define the following terms:
(a) Basic size (b) Zero line (c) Tolerance zone
(d) International tolerance grade (e) Tolerance class
(f) Upper and lower deviations
16. What do you mean by accumulation of tolerances? Explain how it can be overcome.
17. Give a detailed classification of fits.
18. Discuss geometric tolerances.
19. Explain the different types of geometric tole- rances and symbolically represent them.
20. Briefly explain the principle of limit gauging.
21. List the essential considerations made in the selection of materials for gauges.
22. Write a note on gauge materials.
23. Distinguish between a measuring instrument and a gauge.
24. State and explain Taylor’s principle of gauge design.
25. Describe why a GO gauge should be of full form.
26. Classify gauges.
27. List the different points that have to be consi- dered in the design of gauges.
28. List the guidelines followed in the British system.
29. Explain the different systems used to specify tolerances on gauges.
30. How are plain plug gauges designated?
31. Explain the term ‘gauge maker’s tolerance’.
32. Discuss why wear allowance should be provided to gauges.
33. With a neat sketch, explain double-ended plug gauge.
34. Give a sketch of and explain a snap gauge.
35. Describe a progressive plug gauge with a neat diagram.
36. With the help of neat sketches, discuss metal limits for hole and shaft gauging.
37. Explain why special attention should be given to GO gauges compared to NOT GO gauges during the design of gauges.
LIMITS, FITS, AND TOLERANCES 79
ANSWERS
Multiple-choice Questions
1. (b) 2. (c) 3. (b) 4. (d) 5. (c) 6. (a) 7. (d) 8. (b) 9. (a) 10. (c) 11. (d) 12. (b) 13. (c) 14. (a) 15. (d)
Problems
1. (a) Maximum clearance = 0.096 mm, Minimum clearance = 0.040 mm (b) Tolerance on hole = 0.021 mm,
Tolerance on shaft = 0.035 mm (c) Allowance = 0.040 mm
(d) Maximum metal limit of hole = 40.00 mm, Maximum metal limit of shaft = 39.96 mm (e) Clearance fit
+ 0.08
2. Hole = 30 + 0.04 mm 3. Hole tolerance = 0.021 mm,
Shaft tolerance = 0.0132 mm,
Maximum clearance = 0.0412 mm, Minimum clearance = 0.007 mm, Allowance = 0.007 mm,
+ 0.021 − 0.0070
Hole = 25 + 0.000 mm, Shaft= 25 − 0.0202 mm 4. i = 1.316 microns, Hole tolerance = 0.021 mm,
Fundamental deviation = − 0.0200 mm, Shaft tolerance = 0.033 mm,
+ 0.021 − 0.020
Hole = 30 + 0.000 mm and Shaft= 30 − 0.053 mm,
+ 0.00231
Limits of GO Plug gauge = 30 + 0.00021 mm, + 0.0231
Limits of NOT GO Plug gauge = 30 + 0.0210 mm, − 0.02033 Limits of GO Snap gauge = 30 − 0.02363 mm,
− 0.0530
Limits of NOT GO Snap gauge = 30 − 0.0563 mm 5. i = 1.571 microns, Hole tolerance = 0.025 mm, Fundamental deviation = − 0.080 mm,
Shaft tolerance = 0.063 mm,
+ 0.025 − 0.080
Hole = 50 + 0.000 mm, Shaft= 50 − 0.143 mm, + 0.00275
Limits of GO Plug gauge = 40 + 0.00025 mm, + 0.0275
Limits of NOT GO Plug gauge = 50 + 0.0250 mm,
− 0.08063
Limits of GO Snap gauge = 50 − 0.08693 mm, − 0.1430
Limits of NOT GO Snap gauge = 50 − 0.1493 mm (d) IT6 = 10i
(e) Wear allowance = 10% of gauge tolerance In addition, determine the maximum and
minimum clearance.
4. Design the general type of GO and NO GO gauge for components having 30 H7/f8 fit.
Given that
(a) i = 0.4533 D + 0.001D
(b) upper deviation of ‘f’ shaft = −5.5D0.41 (c) 30 mm falls in the diameter step of
18–30 mm (d) IT7 = 16i (e) IT8 = 25i
(f) wear allowance = 10% of gauge tolerance 5. Design a general type of GO and NO GO
gauge for components having 50 H7/d9 fit.
The fundamental tolerance is calculated by the following equation:
i = 0.4533 D + 0.001D The following data is given:
(a) Upper deviation of shaft = −16D0.44
(b) 50 mm falls in the diameter step of 30–50 mm
(c) IT7 = 16i (d) IT9 = 40i
(e) Wear allowance = 10% of gauge tolerance