ENB215 Lecture Fundamentals of Mechanical Design
Fits and Tolerances
Exact dimensions of parts cannot be achieved due to the nature of manufacturing processes. The Deviations of
dimensions can occur and these deviations must not be so large that the part is useless. Designer specifies upper and
lower limits to the dimensions.
Tolerance is the difference between the maximum and minimum size limits of a part.
Too tight tolerance -> High manufacturing cost.
Too large tolerance -> part may not function as expected.
Nominal Size: It is the designation used for general identification and is usually expressed in common fractions.
Basic size of Basic Dimension: It is the theoretical size from which limits of size are derived by the application of
allowances and tolerances.
Actual Size: The measured size of the finished part.
Allowance: Is the minimum clearance space (or maximum interference) intended between the maximum material
condition (MCM) of mating parts.
Basic Systems for Fit Specification
Basic Hole System (BHS) Basic Shaft System (BSS)
Minimum hole diameter is taken as the basis, lower Maximum shaft diameter is taken as the basis. Upper
deviation for the hole is equal to zero.max is deviation for the shaft is equal to zero. mins prescribed
prescribed according to the specific tolerance. according to the specific tolerance.
Bits between mating parts
Fit is the general term used to signify the range of tightness or looseness that may result from the application of a
specific combination of allowances and tolerances in mating parts. There are three types of fits between parts:
1. Clearance fit: an internal member fits in an external member (as a shaft in a hole) and always leaves a space
or clearance between the parts.
2. Interference Fit: The internal member is larger than the external member such that there is always an
interference of material.
3. Transition fit: May result in either a clearance or interference condition.
Clearance Fit Interference Fit Transition Fit
1 Week 4 Tuesday, 27 August 2013 ENB215 Lecture Fundamentals of Mechanical Design
Fits and possible application
Specifications of Tolerances
The high limit is placed about the low limit. In single-line
lot form, the low limit precedes the high limit seperated
by a dash
2 Week 4 Tuesday, 27 August 2013 ENB215 Lecture Fundamentals of Mechanical Design
Terms Related to Metric Limits and Fits
An engineering component may be cast, forged, drawn, welded or stamped etc. All the surfaces may not have
functional requirements and need not be equally finished. Some surfaces (owing to their own functional
requirements) need additional machining that needs to be recorded on the drawing.
The geometrical characteristics of a surface include:
2. Surface waviness
The surface roughness is evaluated by the height, R andtmean roughness index R of the macro. Surface roughness
number represents the average departure of the surface from perfection over a prescribed sampling length. Surface
roughness number (R ) ia expressed in microns. The measurements are usually made along a ling, running at right
angle to the general direction of tool marks on the surface.
Actual Profile, A f Reference profile, R f Datum Profile, D f
It is the profile of the actual surface It is the profile to which the It is the profile, parallel to the
obtained by finishing operation. irregularities of the surface is referred reference profile. It passes
to. It passes through the highest point of through the lowest point B of the
the actual profile. actual profile.
Mean Profile, M Peak to valley height, R Mean roughness Index, R
f t a
It is the profile, within the sampling It is the distance from the datum It is the arithmetic mean of the
length chosen (L) such that sum of profile to the reference profile absolute value of the highest H i
the material-filled areas enclosed between the actual and mean
above it by the actual profile is equal profile.
to the sum of the material void area