Tolleranze dimensionali, gradi di tolleranza, posizione scostamenti, accoppiamenti raccomandati ISO — Transcript

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1. 0:00Good morning and welcome to Meccanica
2. 0:02Live. In today's video lesson, we will
3. 0:05talk about tolerances, specifically
4. 0:07dimensional tolerances and fits. Intro
5. 0:11music. Every part requires a technical
6. 0:41drawing to be manufactured, and
7. 0:43consequently, a nominal dimension or
8. 0:45size. However, when manufacturing
9. 0:49mechanical parts, we must face reality
10. 0:52and verify the actual dimensions of the
11. 0:55pieces created. Therefore, during
12. 0:59machining operations, tolerances define
13. 1:02the maximum allowable error for the
14. 1:04actual dimension compared to the
15. 1:07nominal dimension on the technical
16. 1:09drawing. So, during mechanical
17. 1:14processing, tolerances define the
18. 1:17maximum allowable error on the actual
19. 1:19dimension of a part relative to the
20. 1:22nominal dimension of the drawing. This
21. 1:28maximum allowable error can be greater
22. 1:31than the indicated nominal dimension,
23. 1:35in which case we speak of an upper
24. 1:38deviation and upper tolerance limit, or
25. 1:41lower than the nominal dimension. In
26. 1:46that case, we speak of a lower
27. 1:49deviation and lower tolerance limit.
28. 1:58The difference between the upper
29. 2:00tolerance limit and the lower tolerance
30. 2:04limit determines the tolerance field
31. 2:07allowed for that operation. Let us now
32. 2:10consider the upper and lower deviations
33. 2:12. If the upper deviation and the lower
34. 2:16deviation have the same absolute value,
35. 2:19we speak of symmetric tolerances. If,
36. 2:23instead, the upper and lower deviations
37. 2:25have different absolute values, we
38. 2:27speak of asymmetric tolerances. If even
39. 2:31just one of the upper or lower
40. 2:33deviations is equal to zero, we speak
41. 2:35of unilateral tolerances. Let us now
42. 2:39consider the classic shaft-hole fit
43. 2:42before assembly, where the two mating
44. 2:44elements have well-defined diameters.
45. 2:48If the diameter of the hole is greater
46. 2:50than the diameter of the shaft, we
47. 2:52speak of clearance. Conversely, if the
48. 2:55diameter of the shaft is greater than
49. 2:57the diameter of the hole, we speak of
50. 3:00interference. When performing various
51. 3:05mechanical operations, therefore, one
52. 3:08must consider if the shafts and holes
53. 3:11will form fits, and what type? The
54. 3:16possible types of fits are clearance,
55. 3:19transition, or interference. To
56. 3:23facilitate this operation, tolerance
57. 3:25systems have been established. The most
58. 3:29widely used of these tolerance systems
59. 3:32is the ISO system, whose acronym you
60. 3:35see here. The width of the tolerance
61. 3:44field is defined by the tolerance grade
62. 3:46. In the ISO system, there are 20
63. 3:50tolerance grades, ranging from IT00,
64. 3:53IT0, and IT1 up to IT18. The higher the
65. 3:59number following the IT indication of
66. 4:02the tolerance grade, the wider the
67. 4:04tolerance field will be, and
68. 4:06consequently, the coarser the machining
69. 4:08will be. Conversely, the lower the
70. 4:13number following the IT indication of
71. 4:16the tolerance grade, the narrower the
72. 4:22tolerance field will be, and
73. 4:23consequently, the more precise the
74. 4:25machining will be. The position of the
75. 4:30tolerance field relative to the nominal
76. 4:33dimension is defined in the ISO system
77. 4:37by uppercase letters, one or more
78. 4:40uppercase letters for holes and
79. 4:43lowercase letters for shafts.
80. 4:47Specifically, from the uppercase letter
81. 4:53A to ZC for holes, and from lowercase a
82. 4:58to zc for shafts. To reduce the
83. 5:03possible combinations of shaft-hole
84. 5:05fits, the ISO system suggests keeping
85. 5:08the position of either the shaft or the
86. 5:11hole fixed and varying the other
87. 5:14element. Therefore, there are two types
88. 5:18of systems: the base-hole system and
89. 5:20the base-shaft system. In the base-hole
90. 5:26system, holes have only one possible
91. 5:29position, uppercase H, which
92. 5:31practically means that the minimum
93. 5:34limit size of the hole coincides with
94. 5:36the nominal size, meaning the lower
95. 5:39deviation for the hole is equal to 0.
96. 5:46In the base-shaft system, however,
97. 5:49shafts have only one possible position,
98. 5:52the lowercase h position, which
99. 5:54practically means that the maximum
100. 5:57limit size for the shaft coincides with
101. 6:00the nominal size, meaning the upper
102. 6:03deviation for the shaft is equal to 0.
103. 6:12Let us now review the recommended fits
104. 6:15according to the ISO system. Well, if
105. 6:27you enjoyed the video, I kindly invite
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108. 6:35bell for future videos. Greetings, and
109. 6:38see you at the next video lesson on
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