Dimensions, Tolerances, Assemblies
Dimensions in Drawings
Dimensions text
Base dimension: usually the units not included on every dimension text, but at the bottom of the drawing.
Tolerance
Defines how much the dimension is allowed to deviate
Extension line
Continues or extend from the surface of the object and establishes the size of the dimension. They do not touch the object lines and extend the slightly past the dimension lines.
Extension line offset
A gap between the object surface and the extension line, so it is not confused with the visible lines of the model.
Parallel dimensioning (datum dimensioning)
Consists of several dimensions originating from a common reference surface (also known as the datum)
Superimposed Running Dimensions
Simplifies parallel dimensions in order to reduce the space used on a drawing
Chain Dimensioning
Should 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 very difficult to read. Arrow heads can be placed outside the extension lines instead
Dimensioning of multiple features
To dimension multiple same dimensions, use N x Dimension.
Circle, Arc and Hole
A circle is dimensioned by its diameter, An arc is dimensioned by its radius, Holes are located by their centerlines.
Tolerance
Bilateral Tolerance
Unilateral Tolerance
Allowed to vary in two directions from the specified dimension. Both directions need not vary by the same amount
Allowed to vary in only one direction
Limit Tolerance
The upper and lower limits of the dimension are specified.
Formula
Tolerance = Upper Limit - Lower Limit
Clearance Fit
Still allowance for two parts to move
Interference Fit
Difficult to move (or remove) the two interfacing parts. Usually, a lot of force is needed to remove the two parts.
Assembly Modelling
Rigid Joint
Fixes two components to one another; it provides no degrees of freedom
Revolute Joint
Has a single rotational degree of freedom, much like a hinge.