An introduction to Metric Limits and Fits An introduction to Metric Limits and Fits
Design and Manufacturing
A machine element, after design, requires to be manufactured to give it a shape of a product. Therefore, in addition to standard design practices like, selection of proper material, ensuring proper strength and dimension to guard against failure, a designer should have knowledge of basic manufacturing aspects
First and foremost is assigning proper size to a machine element from manufacturing view point. As for example, a shaft may be designed to diameter of, say, 40 mm. This means, the nominal diameter of the shaft is 40 mm, but the actual size will be slightly different, because it is impossible to manufacture a shaft of exactly 40 mm diameter, no matter what machine is used. In case the machine element is a mating part with another one, then dimensions of both the parts become important, because they dictate the nature of assembly. The allowable variation in size for the mating parts is called limits and the nature of assembly due to such variation in size is known as fits
Definitions
Terms used are illustrated in Fig. 1 and are defined as follows:
1. Basic size is the size to which limits or deviations are assigned and is the same for both members of a fit. It is measured in millimeters.
2. Deviation is the algebraic difference between a size and the corresponding basic size.
3. Upper deviation is the algebraic difference between the maximum limit and the corresponding basic size.
4. Lower deviation is the algebraic difference between the minimum limit and the corresponding basic size.
5. Fundamental deviation is either the upper or the lower deviation, depending on which is closest to the basic size.
6. Tolerance is the difference between the maximum and minimum size limits of a part.
7. International tolerance grade (IT) is a group of tolerances which have the same relative level of accuracy but which vary depending on the basic size.
8. Hole basis represents a system of fits corresponding to a basic hole size.
9. Shaft basis represents a system of fits corresponding to a basic shaft size.
Limits
Tolerance is the difference between maximum and minimum dimensions of a component, ie, between upper limit and lower limit. Depending on the type of application, the permissible variation of dimension is set as per available standard grades.
Tolerance is of two types, bilateral and unilateral. When tolerance is present on both sides of nominal size, it is termed as bilateral; unilateral has tolerance only on one side.
Allowance
It is the difference of dimension between two mating parts.
International Tolerance Grades
The variation in part size, also called the magnitude of the tolerance zone, is expressed in grade or IT numbers. Seven grade numbers are used for high-precision parts; these are
ITOl, ITO, ITl, IT2, IT3, IT4, IT5
The most commonly used grade numbers are IT6 through IT16, and these are based on the Renard R5 geometric series of numbers. For these, the basic equation is
(1)
where D is the geometric mean of the size range under consideration and is obtained from the formula
(2)
The ranges of basic sizes up to 1000 mm for use in this equation are shown in Table 1. For the first range, use Dmin = 1 mm in Eq. (2).
With D determined, tolerance grades IT5 through IT16 are found using Eq. (1) and Table 2. The grades ITOl to IT4 are computed using Table 3.
An introduction to Metric Limits and Fits
Design and Manufacturing
A machine element, after design, requires to be manufactured to give it a shape of a product. Therefore, in addition to standard design practices like, selection of proper material, ensuring proper strength and dimension to guard against failure, a designer should have knowledge of basic manufacturing aspects
First and foremost is assigning proper size to a machine element from manufacturing view point. As for example, a shaft may be designed to diameter of, say, 40 mm. This means, the nominal diameter of the shaft is 40 mm, but the actual size will be slightly different, because it is impossible to manufacture a shaft of exactly 40 mm diameter, no matter what machine is used. In case the machine element is a mating part with another one, then dimensions of both the parts become important, because they dictate the nature of assembly. The allowable variation in size for the mating parts is called limits and the nature of assembly due to such variation in size is known as fits
Definitions
Terms used are illustrated in Fig. 1 and are defined as follows:
1. Basic size is the size to which limits or deviations are assigned and is the same for both members of a fit. It is measured in millimeters.
2. Deviation is the algebraic difference between a size and the corresponding basic size.
3. Upper deviation is the algebraic difference between the maximum limit and the corresponding basic size.
4. Lower deviation is the algebraic difference between the minimum limit and the corresponding basic size.
5. Fundamental deviation is either the upper or the lower deviation, depending on which is closest to the basic size.
6. Tolerance is the difference between the maximum and minimum size limits of a part.
7. International tolerance grade (IT) is a group of tolerances which have the same relative level of accuracy but which vary depending on the basic size.
8. Hole basis represents a system of fits corresponding to a basic hole size.
9. Shaft basis represents a system of fits corresponding to a basic shaft size.
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| FIG 1 |
Limits
Tolerance is the difference between maximum and minimum dimensions of a component, ie, between upper limit and lower limit. Depending on the type of application, the permissible variation of dimension is set as per available standard grades.
Tolerance is of two types, bilateral and unilateral. When tolerance is present on both sides of nominal size, it is termed as bilateral; unilateral has tolerance only on one side.
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| FIG 2 |
Allowance
It is the difference of dimension between two mating parts.
International Tolerance Grades
The variation in part size, also called the magnitude of the tolerance zone, is expressed in grade or IT numbers. Seven grade numbers are used for high-precision parts; these are
ITOl, ITO, ITl, IT2, IT3, IT4, IT5
The most commonly used grade numbers are IT6 through IT16, and these are based on the Renard R5 geometric series of numbers. For these, the basic equation is
(1)
(2)With D determined, tolerance grades IT5 through IT16 are found using Eq. (1) and Table 2. The grades ITOl to IT4 are computed using Table 3.
By combining the IT grade number and the tolerance position letter, the tolerance symbol is established which identifies the actual maximum and minimum limits of the part. The toleranced sizes is thus defined by the basic size of the part followed by a symbol composed of a letter and a number
the schematic view of a standard limit and fit system. In this figure tolerance is denoted as IT and it has 18 grades; greater the number, more is the tolerance limit. The fundamental deviations for the hole are denoted by capital letters from A and ZC, having altogether 25 divisions. Similarly, the fundamental deviations for the shaft is denoted by small letters from a to zc.
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| FIG 3 |
The formula for the fundamental deviation for shafts is
(3)
Shafts designated j through zc have the lower deviation equal to the fundamental deviation. For these, the upper deviation is the sum of the IT grade and the fundamental deviation.
Hole Deviations. Holes designated A through H have a lower deviation equal to the negative of the upper deviation for shafts. Holes designated as J through ZC have an upper deviation equal to the negative of the lower deviation for shafts.
An exception to the rule occurs for a hole designated as N having an IT grade from 9 to 16 inclusive and a size over 3 mm. For these, the fundamental deviation is zero. A second exception occurs for holes J, K, M, and N up to grade IT8 inclusive and holes P through ZC up to grade 7 inclusive for sizes over 3 mm. For these, the upper deviation of the hole is equal to the negative of the lower deviation of the shaft plus the change in tolerance of that grade and the next finer grade. In equation form, this can be written
Upper deviation (hole) = -lower deviation (shaft) + IT (shaft) - IT (next finer shaft)
PREFERRED FITS
The nature of assembly of two mating parts is defined by three types of fit system, Clearance Fit, Transition Fit and Interference Fit.
Clearance Fit: In this type of fit, the shaft of largest possible diameter can be fitted easily in the hole of smallest possible diameter.
Interference Fit : In this type of fit, irrespective of tolerance grade there is always a overlapping of the matting parts.
Transition Fit: In this case, a clearance is present between the minimum dimension of the shaft and the minimum dimension of the hole. However, the fit is tight, if the shaft dimension is maximum and the hole dimension is minimum. Hence, transition fit have both the characteristics of clearance fit and interference fit.
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