handout 10a1 How to determine the tolerance for a given dimension? Input or pre-condition is: 1.design (dimension) 2.manufacturing technology (e.g., casting)

handout 10a2 Standard (ISO, etc.): limits a freedom of choices but promotes the exchange of parts manufactured with - different approaches - different equipment - different workers - in different cultural and societal situations To assist in the determination of tolerance Standard Unified approach to determine the tolerance

handout 10a3 Standard Different countries and regions together to develop  Concepts  Rules  Systems

handout 10a4 Standard Basic idea: To give restriction on the possibility of the dimension and then its tolerance 30 (a) (b)  Nominal dimension is positive integer with the last letter 0 or 5.  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. Example: Tolerance range is with the nominal dimension of 30. Restrict to 30 to only.

handout 10a5 Basic hole versus basic shaft concept: making the nominal dimension as one of the dimension limit

handout 10a6 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 10a7 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 Basic size =0.5 Basic hole system Basic shaft system Example

handout 10b9 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 10b10 Figure 1

handout 10b11 Figure 1 To determine the tolerance

handout 10b12 Tolerance: composed of location of the fundamental deviation block and thickness of the block 30 (a) (b) Reference line for the shaft Fundamental deviation block. Its location is measured with reference to the basic size

handout 10b13 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: IT0, IT1, IT2,.... IT16 Small Deviationlarge Deviation large tolerance is given to large grade

handout 10b14 Figure 2 IT grades are further associated with manufacturing processes

handout 10b15 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 10b16 IT 1IT 5Basic size Tolerance with respect to size group and IT group

handout 10b H G J h k  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)

handout 10b18 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 10b19 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 10b20 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 10b21 Figure 3 Complete set of FDs for hole and shaft

handout 10b22 A combination of location of FD and IT grade Basic size 40 H7 Tolerance zone

handout 10b23 Hole  minimum hole size as basic diameter  denoted by Capital letter (say, H) Basic sizeFundamental Deviation 40 H8 IT grade Tolerance zone ISO Method to Determine Tolerance

handout 10b24 Shaft  maximum shaft size as basic diameter  denoted by small letters (say, h) Basic sizeFundamental Deviation 40 h7 IT grade Tolerance zone

handout 10b25 Figure 4. Preferred fit

handout 10b26  The tolerance for a part (A) is also constrained by the fit of the part with the other part (B).

handout 10b27 Preferred fit: Product Function determines Fit. For instance, two parts need to have relative motion, so we require therefore clearance fit.

handout 10b28 Tolerance Location of the FD block Thickness of the FD block  Basic size  IT grade (required accuracy)  Basic hole and basic shaft  Fit Design and manufacturing

handout 10b29 Procedure for determining the tolerance 1.Basic size selection 2.Determine the preferred fit 3.International Grade 4.Determine tolerances

handout 10b30 Example:  Basic hole system  Running of accurate machines  Basic diameter, say 39 Step 2: Go to Figure 4, H8/f7 Step 1: go to Figure 1, the closed size to 39 is 40.

handout 10b31 Step 3: Go to Table 1a, we will find that under the size 40, and column H8 H8f7Fit (max clearance) (min clearance)

handout 10b32 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 10b33

handout 10b34 More Examples Given:basic hole system locational transitional fit basic diameter =57 mm Figure 1 -> 60 Figure 4 -> H7/k6 Table 1:HoleShaft

handout 10b35 HoleShaft Max interference Max clearance Tolerance

handout 10b36 Representation on the drawing 60H ( ) k ( )

handout 10b37 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).