4.1 LIMITS AND FITSTwo extreme permissible sizes of a part between which the actual size is contained are called limits. The relationship existing between two parts which are to be assembled with respect to the difference on their sizes before assembly is called a fit.ToleranceTolerance is defined as the total permissible variation of a size. It is the difference between maximum limit and minimum limit of size.

4.2 FITSWhen two parts are to be assembled the relation resulting from the difference between their sizes before assembly is called a fit. The fit signifies the range of tightness or looseness which may result from the application of a specific combination of allowances and tolerances in the design of mating parts.4.2.1 Types of FitsThere are three general types of fit between the mating parts1. Clearance fit A clearance fit is one having limits of size so prescribed that a clearance always results when mating parts are assembled.2. Interference fit An interference fit is one having limits of size so prescribed that an interference always results when mating parts are assembled.3. Transition fit A transition fit is one having limits of size so prescribed that either a clearance or an interference may always result when mating parts are assembled.The three types of fits are shown in Fig. 4.1 The disposition of tolerance zones for the three classes of fit are shown in Fig. 4.2.

4.3 TERMINOLOGYThe terminology used in fits and tolerances is shown in Fig. 4.3 The importantterms are

Basic size It is the exact theoretical size arrived at by design. It is also called nominal size.Actual sizeThe size of a part as may be found by measurement.Maximum limit of sizeThe greater of the two limits of size.Minimum limit of sizeThe smaller of the two limits of size.AllowanceIt is an intentional difference between maximum material limits of mating parts. It is a minimum clearance or maximum interference between mating parts.DeviationThe algebraic difference between a size (actual, maximum, etc.) andthe corresponding basic size.Actual deviationThe algebraic difference between the actual size and the corresponding basic size.Upper deviationThe algebraic difference between the maximum limit of size and the corresponding basic size.Upper deviation of hole = ES (&art Superior)Upper deviation of shaft esLower deviationThe algebraic difference between the minimum limit of size and the corresponding basic size.Lower deviation of hole = El (Ecart Inferior)Lower deviation of shaft = eiUpper deviation Lower deviation + ToleranceZero lineIt is the line of zero deviation and represents the basic size.Tolerance zoneIt is the zone bounded by the two limits of size of the parts and defined by its magnitude, i.e. tolerance and by its position in relation to the zero line.Fundamental deviationThat one of the two deviations which is conveniently chosen to define the position of the tolerance zone in relation to zero line, as shown in fig. 4.4.

Basic shaft A shaft whose upper deviation is zero.Basic hole A hole whose, lower deviation of zero.Clearance It is the positive difference between the hole size and the shaft size.Maximum clearanceThe positive difference between the maximum size of a hole and the minimum size of a shaft.Minimum clearanceThe positive difference between the minimum size of a hole and the maximum size of a shaft.

4.4 STANDARD TOLERANCESThere are 18 standard grades of tolerances as specified by BIS with designations ITOI, ITO and IT to IT 16.

The standard tolerances for the various grades are given in Table 4.1 and tolerance grades for various manufacturing processes in Table 4.2Table 4.1 Standard tolerances.

Table 4.2 Tolerance grade in various manufacturing processes.

4.5 HOLE BASIS AND SHAFT BASIS FOR FITS1. Hole basis systemIn this system, the different clearances and interferences are obtained in associating various shafts with a single hole, whose lower deviation is zero.2. Shaft basis system In this system, the different clearances and interferences are obtained in associating various holes with a single shaft, whose upper deviation is zero.

4.6 SELECTION OF FITSHole basis system is the most commonly used system because due to the fixed character of hole production tools, it is difficult to produce holes with odd sizes. Commonly used types of fits are given in Table 4.3. Shafts a to h produce clearance fit, j to n transition fit, and p onwards interference fit with hole.

4.7 DIMENSIONING OF TOLERANCES -RULES1. The upper deviation should be written above the lower deviation value irrespective of whether it is a shaft Or ahole (Fig. 4.5 (a)).2. Both deviations are expressed to the same number of decimal places, except in the cases where the deviationin one direction is nil (Fig. 4.5 (b)).3. Tolerances should be applied either to individual dimensions or by a general note, assigning uniform orgraded tolerances (Fig. 4.5 (c)).

4. The use of general tolerance not greatly simplifies the drawing and saves much labour in its preparation.On the drawing, the limits on a dimension can be specified in two ways, i.e. (i) unilateral, and (n) bilateral.In unilateral tolerance system, the variation in size is permitted in one direction

4.8 LIMIT GAUGESTwo sets of limit gauges are necessary for checking the size of various parts. Thereare two gauges : Go limit gauge, and Not Go limit gauge.1. Go LimitThe Go limit applied to that of the two limits of size corresponds to the maximum material condition, i.e. (1) an upper limit of a shaft, and (ii) thelower limit of a hole.This is checked by the Go gauge.2. Not Go LimitThe Not Go limit applied to that of the two limits of sizecorresponds to the minimum material condition, i.e. (1) lower limit of a shaft,and (ii) the upper limit of a hole. This is checked by the Not Go gauge.

4.9 MACHINING SYMBOLSDuring the manufacture of a machine, some surfaces of a component are to be machined, which are required to be indicated in the drawing. This will enable the pattern maker to provide machiinWallowance on that surface. Similarly, the grade of surface finish is required to be indicated on the surface to enable the machinist to carry out the job accordingly. Thus, on production drawings it is necessary to indicate the surfaces tobe machined or finished by certain specific symbols.The machining symbol is indicated to the left of the system as shown in Fig. 4.6.The value of allowance is expressed in mm.

The basic symbol used for indication of surface roughness consists of two legs of unequal length inclined at 600 to the line representing the surface under consideration, as shown in fig. 4.7. It may only be used alone when the meaning is expressed by a note.

The following guidelines may be used while specifying the machining symbols:1. When the surface is produced by any method, it is indicated as shown in Fig.4.8 (a).2. When the removal of material by machining is required, a bar is added to the basic symbol, as shown in Fig. 4.8 (b).

3. Whenever the removal of material is not permitted In a circle is added to the basic symbol, as shown n Fig. 4.8V(c).4. When some special surface characteristics are to be indicated (say a milled surface), a line added to the longer leg of the basic symbol, as shown in Fig. 4.8 (44.9.1 Indication of Surface RoughnessThe roughness values Ra (urn) are given in Table 4.4Table 4.4 Surface roughness values, Ra (a m)

The value defining the roughness value Ra in micron and roughness grade symbols are given on production drawings as shown in Fig. 4.9.

1. When it is necessary to specify the maximum and minimum limits of the surface roughness, both the values or grades should be given as shown in Fig. 4J0.

Fig. 4.10 Invocation of the maximum and minimum limits of surface roughness.2. If it is necessary to indicate the sampling length, it is shown adjacent to the symbol (see Fig. 4.11 (a))3. If it is necessary to control direction of lay or the direction of the predominant surface patterns, it is indicated by a corresponding symbol added to the surfaceroughness symbol (see Fig. 4.11 (b))

4. Whenever, it necessary to specify the value of machining allowance, it is indicated in the left of the symbol (see Fig. 4.11 (c)). This value is generally expressed in millimetres.(Refer table 2-5 for direction of lay)Thus, combining the above points, we can establish that the specification of surfaceRoughness should be placed relative to the symbol as shown in Fig. 4.11 (dWhere, a = Roughness value Ra in micrometersor Roughness grade symbol NI to N12b = Production method, treatment or coating to be usedc = Sampling lengthd = Direction of laye = Manufacturing allowance -f = Other roughness value in bracket

5. If it is necessary to define surface roughness both before and after treatment should be explained in a suitable note or in accordance with Fig. 4.12.

4.10 METHODS OF PLACING MACHINING SYMBOLS ON ORTHOGRAPHIC VIEWSThe machining symbol should be placed on orthographic views in such a way thatalong with the inscription, they may be read from the bottom or the right hand side ofthe view as shown in Fig. 4.13. If required, the symbol may be connected to the surface by a leader line terminating in an arrow. The symbol or the arrow should point from outside the material of the component either to the line representing the surface or to an extension of it.

In accordance with the general principles of dimensioning, the symbol should be used once for a given surface and, if possible on the view which carries the dimension defining the size of position of the surface (see Fig. 4.14).If the same roughness is required on all the surfaces of a part, it is specified:(a) Either by a note near a view of the part, near the title block or in the space devoted to general notes (see Fig. 4.15).(b) Following the part number on the drawing (see Fig. 4.16)

If the same surface roughness is required on the majority of the surfaces of a part, it is specified as above with the addition of:(a) The notation except where otherwise stated (see Fig. 4.17).(b) A basic symbol (in brackets) without any other indication (see Fig. 4.18)(c) The symbol or symbols (in brackets) of the special surface roughness or roughness (see Fig. 4.19).The symbols for the surface roughness which are exceptions to the general symbols are indicated on the corresponding surfaces. Surfaces (see Fig. 4.20 and 4.21)

Fig.4.21 Surface flnshes obtained by different machinirg processes.Sometimes machining symbols as given in Fig. 4.22 are used to indicate surfaceroughness followed in industries.

Example 4.1 How the tolerances are specified and indicated on drawings?Solution:Refer to art. 4.7.

Example 4.2 What do you mean by basic size?Solution :Refer to art. 4.3.

Example 4.3 Give various types of fits with symbols.Solution: Refer to art. 4.2.1.

Example 4.4 Give the classification of fits with symbols.Solution :Refer to art. 4.2.1.

Example 4.5 Draw machining symbol for surface removal.Solution:Refer to art. 4.9.

Example 4.6 What do you mean by transition fit?Solution :Refer to art. 4.2.1.

Example 4.7 How will you indicate machining allowance ?Solution:Refer to art. 4.9.

Example 4.8 What is the maximum material condition?Solution :Refer to art. 4.8.

Example 4.9 Define the following terms:(i) Tolerance(ii) Limits and FitsSolution :Refer to art. 4.1.

Example 4.10 Sketch a figure illustrating the following terms:(i) Upper and lower deviation(ii) Basic size and zero lineSolution :Refer to art. 4.3.

Example 4.11 Explain the following terms:(i) Fundamental deviationSolution: P2fer to art. 4.3.(ii) Go and Not Go LimitsSolution:Refer to art. 4.8.

Example 4.12 Dimensioned drawing with tolerances.Solution: The parts of a knuckle joint are shown in Fig. 4.23 (a). The dimensions of various parts with tolerances are given in Fig. 4.23 (b).