Engineering Drawings:
Linear Fits and Tolerances
A short series of lectures on Engineering Drawing as Part of ENGG1960
By Paul Briozzo
Linear Fits and Tolerances
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Caters for imperfect manufacturing processes
Size within functional limits
Variation in value of a dimension
Controls the manufacturing process
Controls variation between mating parts
Allows the interchange of parts
The Three Methods of
Tolerancing
1. LIMITS OF SIZE:
20.4
20.2
2. UNILATERAL
0
20.4 ‐ 0.2
OR
+ 0.2
20.2 0
2. BILATERAL:
20.3 + 0.1
Dimensions in mm
Which type of tolerancing has been
used in this example ?
An example of chain dimensioning
An example of chain dimensioning
and progressive dimensioning
Some Important Definitions
When Discussing Shaft and Hole
Sizes and Limits
Some Definitions
HOLE: A hole is defined as the member which houses or fits the shaft
NOMINAL SIZE: This the size by which an item is designated as a matter of
convenience
BASIC SIZE: This is the size from which the limits of size are dereived by the
application of the upper and lower deviations. It is usually equal to the nominal
size.
LIMITS of SIZE: These are the extremes of size which are allowed for a toleranced
dimension.
Two limits are possible:
1. Max Allowable Size = “upper limit of size”
2. Minimum Allowable Size = “lower limit of size”
Maximum Material Limit (MML): The maximum (upper) limit of size for an
external feature (shaft), the minimum (lower) limit of size for an internal feature
(hole)
Least Material Limit (LML): The minimum (lower) limit of size for an external
feature (shaft), the maximum (upper) limit of size for an internal feature (hole)
Comparisons between the three
classes of fit using the unilateral
hole basis system