Dimension, Tolerances and Assembles

Dimension

Dimension Text

Base dimension, usually in mm unit.

Rules od Dimensioniong

Locate at the bottom corner of the drawing

Tolerance

(+ -) 0.2 is the tolerance.

Define how much the dimension is allowed to deviate

Dimension line

It has an arrowhead on each end

Extension line

continue or extend from the surface of the object

Establish the size of the dimension

They do not touch the object lines and extend slightly pass the dimension line

Extension line offset

A gap bewteen the object surface and the extension line so to not confused with the visible lines of the model

Type of dimensions

Parallel (Datum) Dimensioning

consists of several dimensioning originating from a common reference surface

Superimposed running dimensioning

Simplifies parallel dimensions in order to reduce the space used on the drawing

Chain Dimensioning

Can only be used if the function of the object would not be affected by the accumulation of the tolerances

Combined Dimensioning

Uses both chain and parallel dimensioning

Dimensioning small features

Placing arrow heads between the extension lines may make the dimension difficult to read

Arrow head can be placed outside the extension line instead

Dimensioning of Multiple Features

To dimension multiple same dimensions need used N x dimension

Dimension of radius

Radial dimensions start with capital R

shows a radius dimensioned with the centre of the radius located on the drawing

Shows how to dimension radii which do not need their centres locating

Dimension of Circle,Arc and Hole

A circle is dimensioned by its diameter

An arc is dimensioned by its radius

Holes are located by their centerlines

Dimensioning Angles

marked with a curve dimension line and has a degree symbol

Dimension might not be duplicated

Same dimemsion should not be repeated on different views, or on the same direction

Avoid dimensioning to hidden lines and features

Dimension lines, extension lines and object outlines should not cross or overlap

Tolerances

If tolerance is not given, exact measurement of the parts must be made

The range of tolerance will translate into the manufacturing method needed

manufacturing method has different degree of tolerance

Rejection rate will be very high and is not realistic

Same parts can be made from different companies and they have to be interchangeable

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Tolerance ensures good quality control for parts from different companies

Type of tolerances

Bilateral tolerance

allowed to vary in two directions from the specified dimension

Both directions need not vary by the same amount

Unilateral Tolerance

allowed to vary in one direction

Limit Tolerance

the upper and lower limits of the dimensions are specified

Calculating Tolerance

Tolerance= Upper limit- Lower Limit

Assembling Multiple Parts

In order to bring a product to life,very often,multiple parts and components are needed.

Clearance fit

consider a shaft which is to ineterface with a hole

need calculated Maximum shaft diameter, Minimum hole diameter

can see there will always be sufficient clearance for the shaft

Interferemce fit

It mean the minimum shaft is larger than the maximum hole

Application of clearance and interference fits

Clearance fit

there would still be an allowance for two parts to move, such as in a door hinge

Interference fits

It would be difficult to move or remove the two interfacing parts.

A lot of force is needed to remove the two parts

Assembly Modelling

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bilateral equal (1.5000 + - 0.002)

bilateral unequal (1.500 +0.005 -0.003)