CH10: TOLERANCES AND FITS Mechanical Engineering Department MECHANICAL DRAWING 1 MECHANICAL DRAWING...
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Transcript of CH10: TOLERANCES AND FITS Mechanical Engineering Department MECHANICAL DRAWING 1 MECHANICAL DRAWING...
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
1
MECHANICAL DRAWINGMECHANICAL DRAWING
Chapter 10:Chapter 10: TOLERANCES AND FITS TOLERANCES AND FITS
MECHANICAL DRAWINGMECHANICAL DRAWING
Chapter 10:Chapter 10: TOLERANCES AND FITS TOLERANCES AND FITS
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
2
Chapter ObjectivesChapter 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.
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
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INTRODUCTION 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.
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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TOLERANCES 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).
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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TOLERANCES TOLERANCES
Example:
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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DEFINITIONS 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
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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DEFINITIONS 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
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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Relating limits to fits 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. limitMin. limit
Basic sizeB
ilate
ral
Hole
Max. limitMin. limit
Basic size (could be any one)
Uni
late
ral
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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Basic hole and shaft systems 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).
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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Basic hole and shaft systems 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
MinholeDia.
Max holeDia. = 15.02
Hole tolerance 0.02
15mm
Allowance = 0.02
MaxshaftDia.
14.98 Min shaftDia. = 14.94
Shaft tolerance 0.04
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
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FITS 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)
Cle
aran
ce
Inte
rfer
ence
T
rans
ition
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
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METRIC SYSTEM OF TOLERANCES AND FITS (ISO)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
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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SPECIFYING TOLERANCESSPECIFYING 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
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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SPECIFYING TOLERANCES (General) SPECIFYING TOLERANCES (General)
Plus-or-Minus Tolerances
Unilateral tolerances
Bilateral Tolerances
CH10: TOLERANCES AND FITS
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MECHANICAL DRAWING
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SPECIFYING TOLERANCES (Mating Parts)SPECIFYING TOLERANCES (Mating Parts)
CH10: TOLERANCES AND FITS
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METRIC SYSTEM OF TOLERANCES AND FITS METRIC SYSTEM OF TOLERANCES AND FITS
Terms Related to Metric Limits and Fits
CH10: TOLERANCES AND FITS
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METRIC SYSTEM OF TOLERANCES AND FITS (ISO SYMBOLS)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
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
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CLASSES OF FITS (ISO SYMBOLS)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).
II) Transition fits:
III) Interference fits:
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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CLASSES OF FITS (Clearance fits)CLASSES OF FITS (Clearance fits)
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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CLASSES OF FITS (Clearance fits)CLASSES OF FITS (Clearance fits)
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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CLASSES OF FITS (Clearance fits)CLASSES OF FITS (Clearance fits)
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
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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CLASSES OF FITS (ISO SYMBOLS)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.
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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CLASSES OF FITS (Transition & Interference Fits)CLASSES OF FITS (Transition & Interference Fits)
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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CLASSES OF FITS (Transition & Interference Fits)CLASSES OF FITS (Transition & Interference Fits)
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
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CLASSES OF FITS (Transition & Interference Fits)CLASSES OF FITS (Transition & Interference Fits)
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
CH10: TOLERANCES AND FITS
Mechanical Engineering Department
MECHANICAL DRAWING
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CLASSES OF FITS (Summary)CLASSES OF FITS (Summary)
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