Department of Mechanical and Aeronautical Engineering

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MGC110

Assembly Drawings: TolerancesAfr: Toleransies

Department of Mechanical and Aeronautical Engineering

Tolerances

• Surface Tolerances

Measure of roughness or degree of finish a surface must adhere

to during the manufacturing proses

• Limits and fits

The amount of allowable variation on a dimension or a surface

of machined parts

• Geometrical tolerances

Controls the shape and positional distribution of a component

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Department of Mechanical and Aeronautical Engineering

Surface Tolerances

Surface texture is indicated by the following symbols in a detailed drawing:

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Department of Mechanical and Aeronautical Engineering

Surface Tolerances

The following detail can be indicated on a machining symbol

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Machining process specification

Sampling length (mm)

Surface

texture (µm)

Direction of

machining marks

Machining

allowance (mm)

Department of Mechanical and Aeronautical Engineering

Surface Tolerances

Machining symbols must be placed:

� Information is read from LEFT to RIGHT or BOTTOM to TOP.

� Symbols either placed on the surface of on an extension line pointing to a surface

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Department of Mechanical and Aeronautical Engineering

Surface Tolerance

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Department of Mechanical and Aeronautical Engineering

Limits and fits

• All manufactured parts are subject to manufacturing

tolerances.

• For example: a 10mm shaft may measure 9.98mm or

even 10.03 in diameter, this difference in actual size

may be described by a tolerance.

• One might be tempted to think that an outside

diameter of 10.03 could be considered equal to

10mm, which in most cases would be true, except

when this 10.03mm shaft needs to go into a

10.00mm hole, then there is a problem, that is why

tolerances are important

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Department of Mechanical and Aeronautical Engineering

Limits and fits

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Department of Mechanical and Aeronautical Engineering

Limits and fits

Categories of fits:

1. Clearance fits permit relative

freedom of motion between a

shaft and a hole (radially and

axially)

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Department of Mechanical and Aeronautical Engineering

Limits and fits

Categories of fits:

2. Transition fit

3. Interference fit

Interference and

transition fits

secure a certain

amount of tightness

between parts

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Department of Mechanical and Aeronautical Engineering

Limits and fits

ISO standards for fits between components specify a fit

as follows:

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25 (H7 – g6 )

Base or zero

line dimension

H refers to the hole and

numerical value of the

width of the tolerance

band

g refers to the shaft

and the numerical

values of the width of

the tolerance band

Department of Mechanical and Aeronautical Engineering

Limits and fits

Example:

a) Calculate the tolerances for a shaft and a hole with

ISO fit standard 25(H7-g6)

b) Show the application on a drawing

STEP 1: Use the table in your textbook to determine the

tolerances for the given fit standard.

� Diameter of 25 falls in the section 18-30 (red block)

� H7 – g6 in the green block

� Where they cross you find the answer (Remember ‘g’

for shaft and ‘H’ for hole)

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Department of Mechanical and Aeronautical Engineering

Limits and fits

Department of Mechanical and Aeronautical Engineering

Limits and fits

STEP 2: For the shaft the deviation from the zero line

for a base diameter of 25mm with g6 clearance fit is

shown as:

-7 and -20. This means:

25.000 – 0.007 = 24.993

25.000 – 0.020 = 24.980

*NOTE: Values from the table in µm

STEP 3: For the hole the deviation from the zero line for

a base diameter of 25mm with H7 clearance fit is shown

as: +0 and + 21. This means:

25.000 + 0.000 = 25.000

25.000 + 0.021 = 25.021

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Department of Mechanical and Aeronautical Engineering

Limits and fits

If the whole is manufactured to the tolerances obtained

in Step 2 we will get the correct clearance fit.

b)

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Department of Mechanical and Aeronautical Engineering

Geometrical tolerances

Geometrical tolerances are tolerances that control the

shape and positional distribution of a component.

Geometrical tolerances add to manufacturing costs and

should only be specified when:

a) the shape of the component is detrimental to the

function of a component (i.e. the shaft of a roller

press)

b) the degree of accuracy obtained by normal

manufacturing techniques is not sufficient.

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Department of Mechanical and Aeronautical Engineering

Geometrical tolerances

Single feature geometrical tolerances specify form or

shape of the component

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Department of Mechanical and Aeronautical Engineering

Geometrical tolerances

Related feature relate the attitude or position of two

features relative to each other

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Department of Mechanical and Aeronautical Engineering

Geometrical tolerances

Application of geometrical tolerances

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Department of Mechanical and Aeronautical Engineering

Geometric tolerances

Positioning of geometric tolerances

NB!! Arrows presented here are not the same as the

ones you have to draw by hand.

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Department of Mechanical and Aeronautical Engineering

Geometric tolerances

Reference feature

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Department of Mechanical and Aeronautical Engineering

Geometric tolerances

Indicating a reference plane

OR

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Department of Mechanical and Aeronautical Engineering

Single feature geometric tolerances

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Department of Mechanical and Aeronautical Engineering

Single feature geometric tolerances

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Department of Mechanical and Aeronautical Engineering

Related features geometric tolerances

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Department of Mechanical and Aeronautical Engineering

Related features geometric tolerances

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Department of Mechanical and Aeronautical Engineering

Position of tolerances

Normal dimension tolerances

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Department of Mechanical and Aeronautical Engineering

Position of tolerances

Geometric tolerances of position

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