Fits, Geometrical...

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

../../../../My%20Webs/Design%20web/PageME1105.htm

mailto:[email protected]


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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


../../Design web/ME1105/Tutorials/CAD-1/Example.htm

../../Design web/ME1105/Tutorials/CAD-1/Example.htm


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Example detailed drawing



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Fitting Two Parts

Part A

Tolerance of A Part B

Tolerance of B

Fit: Clearance or Interference



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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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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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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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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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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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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 Interference



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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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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

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


ITGrades.jpg

Deviations.jpg


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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

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



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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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


Fits, Geometrical...

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