4413Chapter_10_TOLERANCING

CH10: TOLERANCES AND FITS
Mechanical Engineering Department
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MECHANICAL DRAWINGMECHANICAL DRAWING
Chapter 10:Chapter 10:
TOLERANCES AND FITSTOLERANCES AND FITS

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Chapter Objectives
1. Describe the nominal size, tolerance, limits allowance …
2. Identify a clearance fit, interference fit, and transition fit.
3. Describe the basic hole and basic shaft systems.
4. Describe the classes of fit.
5. Draw geometric tolerancing symbols.
6. Specify geometric tolerances.

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INTRODUCTION
Objectives of specifying tolerances:
 Makes it possible that any two mating parts will fit together.
 Increased precession makes the part more expensive to manufacture.
 The degree of accuracy is according to functional requirements.
Tolerance:
 Is the difference between the maximum and minimum permitted size of a
part.
 Is the total amount a specific dimension is permitted to vary.

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TOLERANCES
Example:
Maximum Limit
Minimum Limit
Hole Minimum size
Maximum size
Tolerance = total amount of variation
31.78 – 31.75 = 0.03
Dimension could be 31.78 or 31.75
or any value between them
Shaft
Tolerance = 31.70 – 31.67 = 0.03
It is a decision of the designer to choose a suitable dimension with lower cost.
Note that the previous parts are interchangeable (mating parts).

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TOLERANCES
Example:

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DEFINITIONS
 Nominal size: the dimension to which a part to be made if it were possible to
manufacture to exact size.
 Tolerances: the total amount by which a given dimension may vary or the difference
between limits.
 Limits: the max. and min. sizes indicated by a tolerance dimensions.
For the previous ex.: limits for the hole = 1.251 and 1.250
limits for the shaft = 1.247 and 1.248
 Allowance: minimum clearance space.
clearance fit = (+ve) allowance interference fit = (-ve) allowance

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DEFINITIONS
 Manufacturing costs increases considerably as tolerances zones decrease.
Hence, general tolerances are often adopted, which allow reasonable limits
for every day applications. They may vary depending on the type of product
and degree of precision required.
 General tolerances:
Manufacturing process General tolerances (mm)
Drilling + 0.125
Turning – diameters ± 0.15
Grinding – surfaces ± 0.025

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Relating limits to fits
 Manufacturing limits are specified relative to the basic size and expressed
unilaterally or bilaterally according to the system of limits adopted.
 Basic size/limits: the nominal size to which tolerances must be added or
subtracted, and is equal for both mating parts.
 Unilateral limits: limits whose difference are from one side only of the
basic size.
 Bilateral limits: limits whose difference result in a tolerance zone on both
sides of the basic size.
Hole
Max. limit
Min. limit
Basic size
Bilateral
Hole
Max. limit
Min. limit
Basic size (could be any one)
Unilateral

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Basic hole and shaft systems
Basic hole system: the minimum hole is taken as the basic size. (Normally
standard drilling machines are used to generate the hole and the shaft is
easier to be machined to the desired diameter).
Basic shaft system: the maximum shaft is taken as the basic size (shaft cannot
be machined to size easily).

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Basic hole and shaft systems
Example:
Given basic size of 15mm, allowance of 0.02mm, hole machining has a precision
of ±0.01mm and shaft machining of ± 0.02mm. Determine the hole and shaft
limits for a basic hole fit.
Solution:
Basic hole fit  min. hole size = 15mm
Min
hole
Dia.
Max hole
Dia. = 15.02
Hole tolerance 0.02
15mm
Allowance = 0.02
Max
shaft
Dia.
14.98 Min shaft
Dia. = 14.94
Shaft tolerance 0.04

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FITS
Fits: relation between mating pairs before assembly:
Pairs: shaft/hole.
Fits purpose: assemble/disassemble of parts.
sliding a part relative to the other.
Types of fit:
 Clearance fit: always there is a
clearance between the max. limit of the
shaft and the min. limit of the hole.
 Interference fit: min. limit of the shaft is
always greater than the max. limit of the
hole.
 Transition fit: the fit could be either
clearance or interference. (small
interference and clearance)
ClearanceInterferenceTransition

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METRIC SYSTEM OF TOLERANCES AND FITS (ISO)
International tolerance grade: IT, uniform degree of accuracy provided to the basic size
Fine tolerance,
Precise work
Coarse tolerance

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SPECIFYING TOLERANCES
 General Tolerances Notes
"ALL TOLERANCES ±1 MM
UNLESS OTHERWISE NOTED.
ANGLES ±1 DEGREE."
 Limit Tolerances/Dimensions
The high limit is placed above
the low limit.
In single-line note form, the low limit
precedes the high limit separated by a
dash

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SPECIFYING TOLERANCES (General)
 Plus-or-Minus Tolerances
Unilateral tolerances
Bilateral Tolerances

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SPECIFYING TOLERANCES (Mating Parts)

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METRIC SYSTEM OF TOLERANCES AND FITS
Terms Related to Metric Limits and Fits

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METRIC SYSTEM OF TOLERANCES AND FITS (ISO SYMBOLS)
 Hole base system:
Hole: H (uppercase)
Shaft: (c, d, f, g, h), (k, n), (p, s, u) (lowercase)
 Shaft base system:
Hole: h (lowercase)
Shaft: (C, D, F, G, H), (K, N), (P, S, U) (uppercase)
Each letter has a meaning in
accordance with the class of fit.
As will be explained in the next
slides/tables

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CLASSES OF FITS (ISO SYMBOLS)
I) Clearance fits:
1) Loose running fit: H11/c11
Wide commercial tolerances. Precision is not important.
2) Free running: H9/d9
Accuracy is not important, or large temp. variation, or both conditions.
3) Close running: H8/f7
Running on accurate machines, accurate locations at moderate speeds
and journal pressure. Accurate location and minimum play are desired.
4) Sliding fit: H7/g6
For easy move and turn (no free motion), and accurate location.
5) Locational fit (precision clearance fit): H7/h6
Location of stationary parts (static applications).
I) Transition fits:
II) Interference fits:

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CLASSES OF FITS (Clearance fits)

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Example:
For free running clearance fit, find hole and shaft limits for a base size of 12mm.
Also provide the fit symbol in the preferred form.
Solution:
From the previous table for free running fit,
Hole limits: 12.000-12.043
Shaft limits: 11.907-11.950
Fit symbol: 12H9/d9

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CLASSES OF FITS (ISO SYMBOLS)
I) Clearance fits:
II) Transition fits:
Compromise between clearance and interference fits. For application
where accuracy of location is important (small clearance or interference
is permissible). Ex., gears and keys on shafts.
1) Location transition: H7/k6
2) Location transition: H7/n6 (more accurate location)
III) Interference fits:
1) Location interference: H7/p6
Slight interference (press fit). Parts to be assembled/disassembled with
accuracy without bore pressure. Ex: couplings, brass collars…
2) Medium drive: H7/s6
Ordinary steel parts, shrink fit on light sections, tightest fit for cast
iron.
3) Force fit: H7/u6
Parts to be joined with high stresses, shrink fit on medium sections.

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CLASSES OF FITS (Transition & Interference Fits)

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Example:
For a basic hole diameter of 40mm, H7p6, determine the class of the fit, the
limits for the hole and the shaft.
Solution:
From the previous table for free running fit,
Type of fit: locational interference
Hole limits: 40.000 – 40.025
Shaft limits: 40.026 - 40.042
40H7/p6

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CLASSES OF FITS (Summary)
Transition
fit

4413Chapter_10_TOLERANCING

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