Manufacturing Engineering – Limit, Fits & Tolerances

by / Thursday, 15 June 2017 / Published in Blog


Limits, fits and tolerances are the very important for designing and manufacturing. As we know accuracy does not exist anywhere, the only thing we can do is to try to approach the true value as close as possible. So, to provide a little flexibility to a manufacturer the designer makes use of tolerances and fits, which in turn determine the kind of assembly produced.

Explanation
Every production process involves a combination of 3 elements, i.e. man, material and machine. It is not possible to make any part precisely to a given dimension. Even if by chance the part is made exact it is possible to measure it accurately enough to prove it. If attempts are made to achieve perfect size the cost of production will increase tremendously. Therefore, some permissible variation has to be allowed.

Limits:
The dimension of a manufactured part can only be made to lie between max and min limits.

Tolerance:
The permissible variation in size or dimension is tolerance. The difference between the upper limit and lower limit of a dimension represents the margin for variation and is called as tolerance zone. Also it is the amount by which the amount is allowed to go away from accuracy without causing any functional trouble when assembled.

System of Writing Tolerance:
1. Unilateral Tolerance:
Dimension of a part is allowed to vary only on one side of the basic size.

2. Bilateral Tolerance:
The dimension of a part is allowed to vary on both the sides of basic size.

Terminologies:
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Shaft: It not only refers to the diameter of circular shaft but also to any external dimension of a component.

Hole: it not only refers to diameter of circular hole but also to any internal dimension of a component.

Basic Size: it is the standard size of a part which refers to which the limits of variations of a size are determined.

Actual Size: dimension as measured on manufactured part.

Zero Line: the straight line drawn to represent the basic size.

Deviation: The algebraic difference between the actual size and corresponding basic size.

Upper Deviation: The algebraic difference between max limit of size and corresponding basic size.

Lower Deviation: The algebraic difference between min limit of size and corresponding basic size.

Fundamental Deviation: it is that one of the 2 deviations either upper or lower which is the nearest to the zero line for either hole or a shaft. For hole FD is denoted by capital letters A, B, C….ZA, ZB, ZC, and for shaft by small letters a, b, c….za, zb, zc.

Tolerance Grade: is an indication of degree of accuracy of manufacture and is designated by IT followed by a number. IT stands for international tolerance grade.

Fit: it the degree of tightness or looseness between 2 mating parts.

1. Clearance Fit: It is obtained when shaft is always smaller than hole.

2. Interference Fit: the minimum permissible diameter of the shaft is always greater than the maximum allowable diameter of hole. Elastic strains are developed on the mating surfaces during the process of assembly and prevent relative measurement of the mating part.

3. Transition Fit: it lies mid-way between clearance and interference fit.

System of Obtaining Different Types Of Fit:

1. Hole Basis System: In this system the size of hole is kept constant. The lower deviation of the hole is zero. The high limit of hole and the 2 limits of size for shaft are then varied to obtain desired type of fit.

2. Shaft Basis System: In this system the size of shaft is kept constant. The upper deviation of the shaft is zero. The low limit of shaft and the 2 limits of size for hole are then varied to obtain desired type of fit.

Allowance: It is the term given to the minimum clearance called positive allowance or maximum interference called negative clearance. Also it is the clearance or interference which gives the tightest possible fit between the mating parts.
The Indian standard consists of 18 grades of fundamental tolerances or the grades of accuracy for manufacture and 25 types of FD. The fundamentals tolerances are designated as IT01, IT0, IT1, IT2, IT3…….IT16.
And fundamental tolerances for holes are designated by
A,B,C,D,……..ZA,ZB,ZC.
Similarly funadamental tolerances for shaft are designated by
A,b,c…..za,zb,zc.
Innumerable fits ranging from external clearance to those of external interference can be obtained by a suitable combination of FD and tolerance. Each of 25 holes have the choice of 18 tolerances. The numerical values of standard tolerances are determined in terms of standard tolerance unit, “i” in microns.
i=0.45 3D+0.001 D Microns
The diameter steps specified by IS-919 are-
1-3
3-6
6-10
10-14
14-18
18-24
24-30
30-40.etc
The values of tolerances for different grades are given below-
IT5 = 7i
IT6 = 10i
IT7 = 16i
IT8 = 25i
IT9 = 40i
IT10 = 64i
IT11 = 100i
IT12 = 160i
IT13 = 250i
IT14 = 400i
IT15 = 640i
IT16 = 1000I
Designation:
50H7f8
Where 50- Basic size
H- FD for hole
7- IT7 = 16i
f- FD for shaft
8- IT8 = 25i

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