Introduction to Basic Manufacturing Processes and ... - always yours

26.3.7 Fundamental Deviation
Inspection and Quality Control 469
It is the one of the deviations, which is conventionally chosen to define the position of
tolerance zone in relation to the zero line. The deviation of the tolerance band on shaft or
hole away from the basic size is called the fundamental deviation. It is shown in Fig. 26.3.
The zero line shown is the line of zero deviation and represents the basic size. A zero line
is a straight line to which the deviations are referred. For conventions, the zero line is drawn
horizontally. The positive deviations are shown above and the negative deviations are shown
below it.
Tolerance zone
Tolerance
Zero line
Fundamental deviation
Fig. 26.3 Fundamental deviation
26.4 FITS
26.4.1 Allowance
In a specific type of fit, the difference between hole size and shaft size is called allowance.
Allowance is used to explain the difference between clearance fit and interference fit. Positive
allowance specifies the clearance fit whereas negative allowance in a fit specifies the interference
or force fit.
The relationship existing between two parts, shaft and hole, which are to be assembled,
with respect to the difference in their sizes before assembly is called a fit. When the parts
are assembled into sub-assembly units and sub assembly units are assembled into full assembly,
the mating surfaces of different components are joined together for proper functional
requirement. One of them may fit into the other in form of joint or fit. The fit may be with
suitable degree of tightness and freedom for required relative movement between mating
parts for specific functional requirements of the fit. The classification of fits is given in
Fig.26.4.
Fits
Clearance fit Transition fit Interference fit
Sliding
fit
Running
fit
Light keying
fit
Push
fit
force fit
or
Shrink fit
Press fit
or
Driving fit
Fig. 26.4 General tree classification of fits
Components are assembled to perform a specific function. The characteristic of the
assembly is determined by the fit. Fit is the general term used to signify the relative degree
of tightness or looseness of assembled parts, which decides the relative movement between
mating parts. Particular type of fit results from the difference in the sizes of mating parts.
Fig. 26.5 shows the various types of fits. Two parts can fit each other in three ways, viz.