2. INTRODUCTION
• No two parts can be produced with identical measurements by any
manufacturing process.
• In any production process, regardless of how well it is designed or how
carefully it is maintained, a certain amount of natural variability will always
exist.
• A manufacturing process essentially comprises five m’s—
man, machine, materials, money, and management.
• Variations in any of the first three elements induce a change in the
manufacturing process or manufactured part.
• All the three elements are subjected to natural and characteristic
variations.
3. • Natural variations (NV) are random in nature and are the cumulative
effect of many small, essentially uncontrollable causes.
• Characteristic variations (CV) can be attributed to assignable causes
that can easily be identified and controlled.
• Usually, variability arises from improperly adjusted machines,
operator error, tool wear, and/or defective raw materials.
• It is therefore impossible to produce a part to an exact size or basic
size and some variations, known as Tolerances, need to be allowed.
• Tolerance is the total amount a dimension may vary and is the
difference between the upper (maximum) and lower (minimum)
limits.
• The permissible level of tolerance depends on the functional
requirements, which cannot be compromised.
4. • It can only be made to lie between two limits, upper (maximum) and
lower (minimum).
• Limits: The maximum and minimum permissible sizes within which
the actual machined size lies are called limits.
• Maximum limit: Maximum permissible size for a given basic size.
From the fig, the maximum limit is ϕ30+0.035 = ϕ30.035 mm.
• Minimum limit: Minimum permissible size for a given basic size. From
the fig, the minimum limit is ϕ30-0.215 = ϕ29.785 mm.
• Tolerance: Is the amount of variation permitted to a basic size.
• It is the difference between the maximum and minimum limits of a
basic size.
From the fig., the tolerance is = ϕ30.035- ϕ29.785 = 0.25 mm.
5.
6. • Deviation: It is the difference between the limit sizes (either
maximum or minimum) and the corresponding basic size.
✓ Upper Deviation: Algebraic difference between the maximum limit
and the basic size = ϕ30.035-ϕ30 = 0.035 mm.
✓ Lower Deviation: Algebraic difference between the minimum limit
and the basic size = ϕ29.785-ϕ30 = -0.215 mm.
✓ Actual Deviation: Algebraic difference between the actual measured
size and the basic size = ϕ29.925-ϕ30 = -0.075 mm.
7. Classification of Tolerance
• Tolerance can be classified under the following categories:
1. Unilateral tolerance
2. Bilateral tolerance
3. Compound tolerance
4. Geometric tolerance
8. Unilateral Tolerance
• When the tolerance distribution is only on one side of the basic size,
it is known as unilateral tolerance.
• In other words, tolerance limits lie wholly on one side of the basic
size, either above or below it.
• Unilateral tolerance is employed when precision fits are required
during assembly.
9. Bilateral Tolerance
• When the tolerance distribution lies on either side of the basic size, it
is known as bilateral tolerance.
• In other words, the dimension of the part is allowed to vary on both
sides of the basic size but may not be necessarily equally disposed
about it.
• This system is generally preferred in mass production where the
machine is set for the basic size.
10. Compound Tolerance
• When tolerance is determined by established tolerances on more
than one dimension, it is known as compound tolerance.
• For example, tolerance for the dimension R is determined by the
combined effects of tolerance on 40 mm dimension, on 60o, and on
20 mm dimension.
• The tolerance obtained for dimension R is known as compound
tolerance. In practice, compound tolerance should be avoided as far
as possible.
11. FITS
• Manufactured parts are required to mate with one another during
assembly.
• The relationship between the two mating parts that are to be
assembled, that is, the hole and the shaft, with respect to the
difference in their dimensions before assembly is called a fit.
• An ideal fit is required for proper functioning of the mating parts.
• Three basic types of fits can be identified, depending on the actual
limits of the hole or shaft:
1. Clearance fit
2. Interference fit
3. Transition fit
12. Clearance fit: The largest permissible diameter of the shaft is smaller
than the diameter of the smallest hole. This type of fit always provides
clearance. Small clearances are provided for a precise fit that can easily
be assembled without the assistance of tools or force. When relative
motions are required, large clearances can be provided, for example, a
shaft rotating in a bush. In case of clearance fit, the difference between
the sizes is always positive.
13. Interference fit: The minimum permissible diameter of the shaft
exceeds the maximum allowable diameter of the hole. To assemble the
parts with interference, tools, heating or cooling may be required.
When two mating parts are assembled with an interference fit, it will
be an almost permanent assembly. In an interference fit, the difference
between the sizes is always negative. Interference fits are used when
accurate location is of importance and also where such location relative
to another part is critical.
14. Transition fit: The combination of maximum diameter of the shaft and
minimum diameter of the hole results in an interference fit, while that
of minimum diameter of the shaft and maximum diameter of the hole
yields a clearance fit. Since the tolerance zones overlap, this type of fit
may sometimes provide clearance and sometimes interference. Precise
assembly may be obtained with the assistance of tools, for example,
dowel pins may be required in tooling to locate parts.
15. • In a clearance fit;
Minimum clearance = low limit of the hole (LLH) - high limit of the shaft (HLS)
• In transition fit;
Maximum clearance = high limit of the hole (HLH)- low limit of the shaft (LLS)
In an interference fit;
Minimum interference = HLH - LLS
Thus, in order to find out the type of fit, one needs to determine HLH − LLS
and LLH − HLS. If both the differences are positive, the fit obtained is a
clearance fit, and if negative, it is an interference fit. If one difference is
positive and the other is negative, then it is a transition fit.
16. Allowance:
• An allowance is the intentional difference between the maximum
material limits, that is, LLH and HLS (minimum clearance or maximum
interference) of the two mating parts.
• It is the prescribed difference between the dimensions of the mating
parts to obtain the desired type of fit.
• Allowance may be positive or negative. Positive allowance indicates a
clearance fit, and an interference fit is indicated by a negative
allowance.
• Allowance = LLH − HLS
17.
18. Hole Basis and Shaft Basis Systems
• To obtain the desired class of fits, either the size of the hole or the
size of the shaft must vary.
• Two types of systems are used to represent the three basic types of
fits, namely clearance, interference, and transition fits. They are (a)
hole basis system and (b) shaft basis system.
• Although both systems are the same, hole basis system is generally
preferred in view of the functional properties.
19. Hole Basis System
• In this system, the size of the hole is kept constant and the shaft size
is varied to give various types of fits. In a hole basis system, the
fundamental deviation or lower deviation of the hole is zero, that is,
the lower limit of the hole is the same as the basic size. The two limits
of the shaft and the higher dimension of the hole are then varied to
obtain the desired type of fit. a. CF, b.TF, c. IF
20. • This type of system is widely adopted in industries, as it is easier to
manufacture shafts of varying sizes to the required tolerances.
• Standard size drills or reamers can be used to obtain a variety of fits
by varying only the shaft limits, which leads to greater economy of
production.
• The shaft can be accurately produced to the required size by standard
manufacturing processes.
21. Shaft Basis System
• The system in which the size of the shaft is kept constant and the hole
size is varied to obtain various types of fits is referred to as shaft basis
system. In this system, the fundamental deviation or the upper
deviation of the shaft is zero, that is, the HLH equals the basic size.
The desired class of fits is obtained by varying both limits of the hole.
• A. CF, b.TF, c.IF
