Ch9 Tolerance

Ch.9 Tolerancing
Objective: Learn how to present tolerance, types
of tolerance presentation, fit types and
Why tolerance is so important in manufacturing
Types of tolerance presentation
Important terms you want to be familiar with
Fit types
Basic hole vs. basic shaft system
Concept of geometric dimensioning and tolerancing
Tolerancing – why is it needed?
Tolerance = the total amount a dimension may
vary and is the difference between the maximum
and minimum limits (e.g. 4.650±0.003)
Tolerances are used to control the variation
existing on all manufactured parts.
Toleranced dimensions control the amount
of variation on each part of an assembly.
Tolerances allow for interchangeable parts.
Parts made by different companies
functionally “fit” although they may not
perfectly fit.
Tolerance Presentation
Direct limits, or as tolerance values applied
directly to a dimension
geometric tolerances
Notes referring to specific conditions
A general tolerance note in the title block
All decimal dimensions to
be held to ±.002”
Note: A tolerance added to a dimension always
supersedes the general tolerance, even if the
added tolerance is larger than the general
Direct limits vs. tolerance values
Direct limits: ASME
preferred method 
the max and min sizes
are specified as part
of the dimension.
Plus and minus
dimensions: the base
size is given, followed
by a plus/minus sign
and the tolerance value
 Unilateral and
bilateral tolerance.
Important terms
Nominal size = a
dimension used to describe
the general size
Basic size = the
theoretical size used as a
starting point for the
application of tolerances
Actual size = the
measured size of the
finished part after
Limits = the max and min
sizes shown by the
toleranced dimension
Allowance = the min
clearance or maximum
interference between parts,
or the tightest fit between
two mating parts
Important terms (cont)
Tolerance = the total
allowable variance in a
Max material condition
(MMC) = the condition of a
part when it contains the
greatest amount of material
Least material condition
(LMC) = the condition of a
part when it contains the
least amount of material
Piece tolerance = the
difference between the
upper and lower limits of a
single part
System tolerance = the
sum of all the piece
Fit types
Type 1: Clearance fit = Occurs when two toleranced mating
parts will always leave a space or clearance when assembled
Type 2: Interference fit = Occurs when two toleranced
mating parts will always interfere when assembled
Fit types (cont)
Type 3: Transition fit = Occurs when two toleranced mating
parts are sometimes and interference fit and sometimes
clearance fit when assembled.
Fit type determination
The loosest fit is
the difference
between the
smallest feature A
and the largest
feature B.
The tightest fit is
the difference
between the largest
feature A and the
smallest feature B.
About “functional
dimensioning 
When dimensioning a
part, identify the
functional features
first. Functional
features are those
that come in contact
with other parts,
especially moving
parts. Dimension and
tolerance these
features first, then do
all the remaining
Tolerance stack-up
The additive rule for
tolerance is that
tolerances taken in
the same direction
from one point of
reference are
additive. The corollary
is that tolerances to
the same point taken
from different
directions become
edges were
used in this
tolerance stack-up
In this method,
tolerance stack-up
eliminated by using
the same reference
Standard precision fits
Running and sliding fits (RC) = the loosest of the fit
classes. Good for moving parts. There is always clearance
between the shaft and the hole.
Clearance locational fits (LC) = Tighter than the RC
class fits, but the shaft and hole may be the same size
(line-to-line fit)
Transition locational fits (LT) = the transition between
LC and LN fits.
Interference locational fits (LN) = Pure interference
fits, where the shaft is always considered larger than the
hole. Used to transmit torque, e.g. securing a pulley or
bearing to a shaft, even if there is a twinting force.
Basic size = the exact theoretical size
from which the limits of mating parts are
assigned when tolerancing.
Basic hole system = the smallest hole is assigned the basic diameter
from which the tolerance and allowance are applied. (more popular than
the basic shaft system because of ease to control the hole size)
Basic shaft system = the largest diameter of the shaft is assigned the
basic diameter from which all tolerances are applied.
Applying tolerances for a clearance fit
using the basic hole system
Geometric dimensioning and tolerancing (GDT)
Size tolerances alone are sometimes not enough to meet
the design needs of a part. Relationships between features
may also need to be controlled. In such cases, notes can
be added to the drawing defining these relationships. And
these extra notes can be placed by geometric
dimensioning. The GDT symbols specify concise
requirements for features.
Feature control box
Examples of feature control frames