Lesson 6 Dimensions, Tolerance and Assemblies
Dimensions
Base dimension text in mm
Tolerance. It defines how much the dimension is allowed to deviate
Dimension Line/Arrows on each end
Extension Line that extend from the surface of the object and establish the size of the dimension
Extension Line Offset, a gap between object surface and extension line to avoid confusion
Type of Dimensioning
Parallel Dimensioning or Datum Dimensioning
Consists of several dimensions originating from a common reference surface.
Superimposed Running Dimensioning
Simplifies parallel dimensions in order to reduce the space used on a drawing
Chain Dimensioning
Only use 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
Arrow heads and dimension text should be placed outside the extension lines if the dimension is too small to avoid having difficulty when reading.
Dimensioning of Multiple Features
To dimension multiple same dimensions we use N x Dimension
Dimensioning of Radius
Radial dimensions use R
It shows a radius dimensioned with the centre of the radius located on the drawing
Also shows how to dimension radii which do not need their centres locating
Dimensioning 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
Drawn or Mark with a curved dimension line and has the degree symbol
Importance of Tolerance
Tolerances are included with dimensions in engineering drawings to take into account these manufacturing limitations
Parts cannot be manufactured exactly to the dimensions stated in the drawing, due to manufacturing variations
Types of Tolerance
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 only one direction
Limit Tolerance: the upper and lower limits of the dimension are specified
Formula
Tolerance = Upper limit - Lower limit
Types of joints
Rigid
Revolute
Slider
Cylindrical
Pin Slot
Planar
Ball