TOLERANCES: Nearly impossible to make the part to the exact ...
Design of TOLERANCES handout 10 1 TOLERANCES - Introduction Nearly impossible to make the part to the exact dimension by any means of manufacturing approach tolerances of the dimension. - Dimension 30 (mm) wont be made exactly as 30 (mm) Fig. 1 (a) (b)
30 - It may be made as 30.10 (mm) or 30.05 (mm). 30 - maximum may be 30.10 (mm) handout 10 2 Introducti on (a) 30.01 (shaft)
- situations for assembly of (a) and (b)? (b) 30.005 (hole) (a) and (b) are impossible to be assembled without any special treatment. (a) 30.00 (shaft) (b) 30.20 (hole) (a) and (b) are assembled with a possibility of poor function of the system (see Figure 2) handout 10 3 Introducti on
Figure 2 L L . handout 10 4 Introduction In summary, designers need to specify tolerances for (a) Parts manufacturing: interchangeable (b) System functions: with low cost
Specify a tighter tolerance only if necessary, because the tighter the tolerance, the higher the cost. handout 10 5 Introduction Objectives of the lecture: o To learn principles behind the international standard (e.g., ISO) for determining tolerances. o
To learn the procedure to determine tolerances based on the standard by making use of the standard. handout 10 6 Basic Concept Definition of tolerance: Tolerance is the total amount a specific dimension is permitted to vary, which is the difference between the maximum and the minimum dimensions. Tolerance is always a positive number ! handout 10 7
Three types of fits Basic Concept (a) 1.247 - 1.248 (b) 1.250-1.251 o o shaft hole Clearance fit
(a) 1.2513-1.2519 shaft (b) 1.2500-1.2506 hole Interference fit (a) 1.2503-1.2509 shaft (b) 1.2500-1.2506 hole Transition fit Tolerance refers to a single part, while fit refers to a connection of two parts. he tolerances of the two connected parts contribute handout 10 8 to a fit between the two connected parts.
Standard Suppose tolerance range of a shaft is 0.1 and the nominal dimension is 30. There are various possibilities of allocation of this tolerance: (1) max: 30.10, min: 30.00; (2) max: 30.00, min: 29.90; (3) max: 30.05, min: 29.95; (4) max: 30.15, min: 30.05, etc. These different allocation of tolerances will have different implication to fit to the hole. Min 30 30 30 Max
30 30 handout 10 9 Standard Basic idea of the standard: Basic hole rule Basic shaft rule To give restriction on these possibilities. o
Nominal dimension is a positive integer with the last letter 0 or 5. o Limit (max or min) dimension is the same as the nominal dimension. For instance, the low limit (min) dimension for a hole is 30, which is the nominal dimension; the high limit (max) dimension for a shaft is 30. o Tolerance can only be a certain groups, the same as the nominal dimension with 0 or 5 as the last letter. handout 10 10
Basic hole versus basic shaft concept Basic size = Nominal size making the nominal dimension as one of the dimension limit (max or min) Tolerance is developed along this direction (b) Basic Hole Rule (a) 30 Tolerance is
developed along this direction handout 10 30 Basic Shaft Rule 11 Basic Hole System Purpose: take a hole as a reference to determine the shaft limit. The minimal hole size as the basic size Reason: in some applications, the hole can be made more precise (Reamers, Broachers,
Gages), while the machining of the shaft varies. handout 10 12 Basic Shaft System Purpose & reason: in some applications, the shaft could be better made as a reference to determine the hole limit. Different fits with the same shaft Take the shaft as a reference The maximal shaft size as the basic size handout 10
13 Example Basic size =0.5 0.502 0.500 0.498 0.495 0.505 0.502 Basic hole system 0.500
0.499 Basic shaft system handout 10 14 Basic size (preferred) Dimensions are initially determined by designers from a point of view of
function. From the view point of function, the length of a bar may be like 39.6 From a point of view of manufacturing, 39.6 is not a convenient figure, and therefore needs to be rounded up (say, 40) (see figure 1) handout 10 15 Figure 1 o ISO takes ANSI in this case
handout 10 16 To determine the tolerance D DT handout 10 17 (a) Reference line for the shaft 30 Fundamental deviation
block. Its location is measured with reference to the basic size (b) 30 Tolerance: determined by location of the fundamental deviation block and thickness of the block handout 10 18 o Fundamental deviation (FD) Deviation closest to the basic size or the location of the FD block.
o International Tolerance Grade (IT) FD can vary in an infinite number of possible numbers. To restrict FD to a finite number of possibilities, we group FDs into 16 groups or grades as follows (IT1, IT2, , IT16): Small Deviation IT0, IT1, IT2, .... large tolerance is given to largehandout grade10 large Deviation IT16 19
Figure 2 IT grades are further associated with manufacturing processes handout 10 20 Basic size group 1. Group basic sizes into groups. The tolerance is the same to all dimensions in the groups. 2. Large basic size gets large tolerance. handout 10
21 Tolerance with respect to size group and IT group Basic size IT 1 IT 5 10-18 0.0012 0.008 18-30
0.0015 0.009 handout 10 22 FD (block in the following diagram) is located with respect to basic size (in total there are 27 FDs) Different location is given a name (letter) 50.005
Hole G H J h 50.030 k Shaft 50
49.05 handout 10 23 H: a special location of FD, and this location makes the minimum diameter of the hole be the basic size of the hole (basic hole system) Hole handout 10 24 Shaft
h: a special location of FD, and this location makes the maximum diameter of the shaft be the basic size of the shaft (basic shaft system) handout 10 25 Basic hole system with the indication of three types of fits Basic shaft system with the indication of three types of fits Hole
handout 10 Shaft 26 Figure 3 Complete set of FDs for hole and shaft handout 10 27
Location of FD IT grade Basic size 40 H7 Tolerance zone Tolerance zone = FD handout 10 28 ISO Method to Determine Tolerance
Hole minimum hole size as basic diameter denoted by capital letter (say, H) Basic size Location of Fundamental Deviation 40 H8 IT grade Tolerance zone
handout 10 29 Shaft maximum shaft size as basic diameter denoted by small letters (say, h) Basic size location of Fundamental Deviation 40 h7 IT grade
Tolerance zone handout 10 30 Figure 4. Preferred fit ISO Method to Determine Fit Either basic hole (H) or basic shaft (h) handout 10 31
The tolerance for a part (A) is also constrained by the fit of the part with the other part (B). handout 10 32 Preferred fit: Product function determines Fit. For instance, two parts need to have relative motion, so we require therefore clearance fit. handout 10 33
Tolerance Thickness of the FD block Location of the FD block Basic size IT grade (required accuracy) Basic hole and basic shaft
Fit Design and manufacturing handout 10 34 Procedure for determining the tolerance 1. 2. 3. 4. Basic size selection Determine the preferred fit International Grade
Determine tolerances handout 10 35 Example: Basic hole system Running of accurate machines Basic diameter, say 39 Step 1: go to Figure 1, the closed size to 39 is 40. Step 2: Go to Figure 4,
H8/f7 handout 10 36 Step 3: Go to Table 1a, we will find that under the size 40, and column H8 H8 f7 Fit 40.039
39.975 0.089 (max clearance) 40.000 39.950 0.025 (min clearance) handout 10 37 The following figures and table are used:
Figure 1: Get a preferred size as well as IT grade Figure 4: Get a preferred fit Table 1a: Get tolerance handout 10 38 handout 10
39 handout 10 40 handout 10 41 More Examples Given: basic hole system locational transitional fit basic diameter =57 mm Figure 1 -> 60
0.028 Max interference Tolerance Tolerance Hole 60.030 60.000 Max clearance 0.019 0.030
handout 10 43 Representation on the drawing ( 60.030 60H7 60.000 60.021 60.002 60k6 ( 60.021
60.002 ) handout 10 44 ) Summary (expected to know): 1.Tolerance is about one part but it has effect on the fit of two parts. 2.It is the fit that makes sense for the quality of machine running. 3.Tolerance is determined by the constraints of (a) machine running condition, (2) manufacturing technology, (c) design to meet the function. 4.Standard is to assist in determination of the tolerance fast and to facilitate the part exchange.
5.Standard consists of several tables and charts. 6.Procedure: (a) decide basic hole or basic shat, (b) decide the basic dimension, (c) decide the fit (which contains the IT grade), handout 10 45 (d) find the tolerance with the basic size and tolerance zone code (e.g., H7).
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