How to determine the tolerance for a given dimension? Input or pre-condition is: 1. design (dimension) 2. manufacturing technology (e.g., casting) handout 10a 1 Standard Standard (ISO, etc.): limits a freedom of choices but promotes the exchange of parts manufactured with - different approaches Unified approach to determine the tolerance - different equipment - different workers - in different cultural and societal situations To assist in the determination of tolerance handout 10a 2 Standard Different countries and regions together to develop Concepts Rules Systems handout 10a 3 Standard Example: Tolerance range is 0.0.5 with the nominal dimension of 30. Restrict to 30 to 30+0.05 only. Basic idea: To give restriction on the possibility of the dimension and then its tolerance Nominal dimension is positive integer with the last letter 0 or 5. (a) (b) 30 30 Limit dimension is the same as the nominal dimension. For instance, the low limit dimension for a hole is 30 which is the nominal dimension. Tolerance can only be a certain groups. handout 10a 4 Basic hole versus basic shaft concept: making the nominal dimension as one of the dimension limit handout 10a 5 Basic Hole System Purpose: take a hole as a reference to determine the shaft limit given allowance and tolerances. 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 10a 6 Basic Shaft System Reason: in some applications, the shaft could be better made as a reference Different fits with the same shaft The maximal shaft size as the basic size handout 10a 7 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 10a 8 Dimensions are initially determined by designers Basic size (preferred) 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 10b 9 Figure 1 handout 10b 10 To1 determine the tolerance Figure D DT handout 10b 11 Reference line for the shaft (a) 30 Fundamental deviation block. Its location is measured with reference to the basic size (b) 30 Tolerance: composed of location of the fundamental deviation block and thickness of the block handout 10b 12 Fundamental deviation (FD) Deviation closest to the basic size or the location of the FD block International Tolerance Grade (IT) FD can vary in an infinite number of possible numbers. To restrict FD a finite number of possibilities, we group FDs into 16 as follows (IT1, IT2, …, IT16, the number means grade: Small Deviation large Deviation IT0, IT1, IT2, .... IT16 large tolerance is given to large grade handout 10b 13 Figure 2 IT grades are further associated with manufacturing processes handout 10b 14 Basic size group 1. Group basic sizes into groups. The tolerance is the same to any dimension in the groups. 2. Large basic size gets large tolerance. handout 10b 15 Tolerance with respect to size group and IT group Basic size 10-18 18-30 IT 1 IT 5 0.0012 0.008 0.0015 0.009 handout 10b 16 FD (block in the following diagram) is located with respect to basic size (in total there are 27 FDs) A Different location is given a name (letter) 50.005 50.030 G H k J h 50 49.05 handout 10b 17 H: a special location of FD, and this location makes the minimum diameter of the hole is the basic size of the hole (basic hole system) handout 10b 18 Shaft h: a special location of FD, and this location makes the maximum diameter of the shaft is the basic size of the shaft (basic shaft system) handout 10b 19 Basic shaft system with the indication of three types of fits Basic hole system with the indication of three types of fits Basic shaft: the maximum diameter is the basic or nominal diameter. Basic hole: the minimum diameter is the basic or nominal diameter. handout 10b 20 Figure 3 Complete set of FDs for hole and shaft handout 10b 21 A combination of location of FD and IT grade Basic size 40 H7 Tolerance zone handout 10b 22 ISO Method to Determine Tolerance Hole minimum hole size as basic diameter denoted by Capital letter (say, H) Basic size Fundamental Deviation 40 H8 IT grade Tolerance zone handout 10b 23 Shaft maximum shaft size as basic diameter denoted by small letters (say, h) Basic size Fundamental Deviation 40 h7 IT grade Tolerance zone handout 10b 24 Figure 4. Preferred fit handout 10b 25 The tolerance for a part (A) is also constrained by the fit of the part with the other part (B). handout 10b 26 Preferred fit: Product Function determines Fit. For instance, two parts need to have relative motion, so we require therefore clearance fit. handout 10b 27 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 10b 28 Procedure for determining the tolerance 1. 2. 3. 4. Basic size selection Determine the preferred fit International Grade Determine tolerances handout 10b 29 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 10b 30 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 10b 31 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 10b 32 handout 10b 33 More Examples Given: basic hole system locational transitional fit basic diameter =57 mm Figure 1 -> 60 Figure 4 -> H7/k6 Table 1: Hole Shaft 60.030 60.021 60.000 60.002 handout 10b 34 Shaft 60.021 60.002 -0.021 0.028 Max interference Tolerance Tolerance Hole 60.030 60.000 Max clearance 0.019 0.030 handout 10b 35 Representation on the drawing 60H7 ( 60.030 60.000 ) 60.021 60.002 60k6 handout 10b ( 60.021 60.002 ) 36 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), (d) find the tolerance with the basic basic size and tolerance zone code (e.g., H7). handout 10b 37