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)