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ME 1110 – Engineering Practice 1
Engineering Drawing and Design - Lecture 7
Fits and Tolerances
Prof Ahmed Kovacevic
School of Engineering and Mathematical Sciences
Room C130, Phone: 8780, E-Mail: a.kovacevic@city.ac.uk
www.staff.city.ac.uk/~ra600/intro.htm
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Objectives for today
• To learn about fits and tolerances
• To learn how to define tolerance in order for
parts to function correctly
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Example detailed drawing
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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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Tolerance Declaration
Tolerance can be expressed in different ways:
1. Direct tolerancing method (size)
» Limits specifying the allowed variation in each dimension
(length, width, height, diameter, etc.) are given on the
drawing
2. General tolerance note
» Notes like “ALL DIMENSIONS HELD TO ±0.05”
3. 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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Direct Tolerancing method
(1) Limit Dimensioning
(2) Deviation tolerancing
unilateral bilateral
equal
unequal
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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
Symmetric
3 Min Limit
Upper and
lower limit
Hole/shaft
tolerance
Deviation
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BS4500: ISO Units
A system of 18 grades and tolerances related
to part size ranges (i.e. basic size range)
These standard tolerances are related to the
zero line by a letter code i.e. deviation
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Tolerance and Fits - terminology
6 4
4 ZERO LINE
6 5
5
2shaft 3shaft 1 3hole 2hole
1 Basic Size - From which limits are fixed
Deviation - Difference - The basic and the another size
2 Max Limit - Of size permissible
To differentiate
3 Min Limit - Of size permissible between holes and
4 Upper deviation - 1-2 shafts, upper and
lower case letters are
5 Lower deviation - 1-3 used
6 Tolerance
9
- 2-3 or 4-5 H – Holes; h - Shafts
Ahmed Kovacevic, City University London
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Fitting Two Parts
Fit: Clearance or Interference
Tolerance of B
Part B
Tolerance of A
Part A
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Tolerance Terminology – continue
6 4
4 ZERO LINE
6 5
5
2shaft 3shaft 1 3hole 2hole
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
11 (upper limit – lower limit Ahmed
or 2-3 or City
Kovacevic, 4-5)
University London
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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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Shaft and Hole Fits
Clearance Transition Interference
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Shaft and Hole Fits
Clearance Transition Interference
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Preferred Hole Basis System of Fits
Standardised by BS4500: ISO Units and Fits
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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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Example
ISO Tolerance
Grades
Fundamental
Deviations
Homework
Max Clearance Cmax=0.050
Min Clearance Cmin=0.009
Allowance = Cmin
+0.025 0 40.025 40.000 -0.009 -0.025 39.991 39.975
System19Tol: TS =0.041 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
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