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Ahmed Kovacevic, City University London
Design web
Fits, Geometrical Tolerances
Prof Ahmed Kovacevic
Lecture 7
School of Engineering and Mathematical Sciences
Room CG07, Phone: 8780, E-Mail: [email protected]
www.city-design.tk www.staff.city.ac.uk/~ra600/intro.htm
Engineering Drawing and Design ME 1110 – Engineering Practice 1
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Ahmed Kovacevic, City University London
Design web
Objectives
• To learn how to effectively select
tolerance for parts so that these
function correctly i.e. to have
appropriate fits with parts in connection.
• To learn about geometric tolerancing
and surface finishing issues.
SHOW THE EXAMPLE DRAWINGS
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Ahmed Kovacevic, City University London
Design web
Example detailed drawing
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Ahmed Kovacevic, City University London
Design web
Fitting Two Parts
Part A
Tolerance of A Part B
Tolerance of B
Fit: Clearance or Interference
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Ahmed Kovacevic, City University London
Design web
Tolerancing
• Definition: » Allowance for specific variation in the size and
geometry of a part
• Why is tolerancing necessary? » It is impossible to manufacture a part to an exact size
or geometry
» Since variation from the drawing is inevitable the acceptable degree of variation must be specified
» Large variation may affect the functionality of the part
» Small variation will effect the cost of the part requires precise manufacturing
requires inspection and the rejection of parts
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Ahmed Kovacevic, City University London
Design web
How is tolerance specified?
• Size
» Limits specifying the allowed variation in
each dimension (length, width, height,
diameter, etc.) are given on the drawing
• Geometry
» Geometric Tolerancing
– Allows for specification of tolerance for the
geometry of a part separate from its size
– GDT (Geometric Dimensioning and
Tolerancing) uses special symbols to control
different geometric features of a part
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Ahmed Kovacevic, City University London
Design web
Tolerance grades
18 tolerance grades related to part size ranges
Standard tolerances related to zero line by deviation
Standardised by BS4500: ISO Units and Fits
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Ahmed Kovacevic, City University London
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Tolerancing - Terminology
1 Basic Size:- From which limits are fixed
Deviation:- Difference - between the
basic and allowed size
2 Max Limit
3 Min Limit
Upper and
lower limit
Symmetric
Hole/shaft
tolerance
Deviation
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Ahmed Kovacevic, City University London
Design web
Tolerance Terminology – continue
ZERO LINE 4
5
3shaft 2shaf
t
6
4
5
3hol
e
2hole 1
6
1 Nominal Size – a general size, common fraction
Basic Size – theoretical size from which limits are fixed
Actual Size – measured size
2,3 Limits – maximum and minimum permissible sizes
4,5 Deviation – max. and min. difference from a nominal
size (1-2 or 1-3)
6 Tolerance – total allowable variance in dimensions
(upper limit – lower limit or 2-3 or 4-5)
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Ahmed Kovacevic, City University London
Design web
Fits
• Range of tightness between two mating parts
• Types of fit
» Clearance fits
– provides clearance between two mating parts.
» Interference fit
– results in interference between mating parts
» Transition fits
– results in neither of the above
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Design web
Shaft and Hole Fits Clearance Interference
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Ahmed Kovacevic, City University London
Design web
Shaft and Hole Fits Transition
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Preferred Hole Basis System of Fits
Standardised by BS4500: ISO Units and Fits
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Ahmed Kovacevic, City University London
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Basic Hole System or Hole Basis
Definition of the "Basic Hole System":
» The "minimum size" of the hole is equal to
the "basic size" of the fit
Example:
If the nominal size of a fit is 10 mm,
then the minimum size of the hole in the
system will be 10mm
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Fundamental deviations for shafts
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Fit Calculations
Clearance = Hole – Shaft
Cmax = Hmax–Smin
Cmin = Hmin –Smax
If:
» Both Cmax and Cmin >0 - Clearance fit
» Both Cmax and Cmin <0 - Interference fit
» Cmax > 0 and Cmin < 0 - Transition fit
Allowance = Hmin – Smax = Cmin
System tolerance: TS = Cmax - Cmin = Σ Ti
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Ahmed Kovacevic, City University London
Design web
Example
ISO Tolerance
Grades
Fundamental
Deviations
0 +0.025 40.025 40.000 -0.009 -0.025 39.991 39.975
Max Clearance
Min Clearance
Allowance
System Tol:
Cmax=0.050
Cmin=0.009
= Cmin
TS =0.041
Homework
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Design web
Datums – Plane surface or axis
– Designated in order that some other feature(s) may relate to it
– Datums are drawn as shown in the picture
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Geometrical Tolerances
– Geometric tolerance of
a feature
(point, line, axis, surface)
specifies the
tolerance zone in which
the feature is required to
contain.
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Notation Supplementary symbols
Tolerance frame variations
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Tolerance examples
Flatness
Straightness
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Tolerance examples
Roundness
Form
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Tolerance examples
Angularity
Symmetry
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Tolerance examples
Parallelism
Squareness
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Tolerance examples
Position
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Tolerance examples
Position
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Tolerance examples
Concentricity
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Tolerance examples Cilindricity
Maximum material condition
Profile tolerance
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Surface Roughness
Average deviation about the mean line
measured
Surface Roughness Measured by value
a O
A
1
A2
A3 An
Mean Line
L
+Y
-Y
RA
La
o
n
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Ahmed Kovacevic, City University London
Design web Surface texture quality
(mm) 0.025 0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 12.5 25 50
(minch) 1 2 4 8 16 32 63 125 250 500 1000 2000
N-Grade N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 N11 N12
Finish Ground Finishes Smooth Turned Medium Turned Rough Machined
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Manufacture methods and
roughness values
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Ahmed Kovacevic, City University London
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Exercise DrE-5
• Groups of 5. Each group has
one assembly with several parts.
• Measure parts in the assembly
together.
• Each member of the group will
have to do his/her own part.
• Make sketch - drawing with all
required dimensions, tolerances
and surface finish notes.
• This sketch has to be approved
and as such used as the basis for
CAD-1 exercise.
• 2 week exercise