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Dimensions and Tolerances - Coggle Diagram
Dimensions and Tolerances
Dimension text is the base dimension. Usually the units (in mm) is not indicated on every dimension text, but at the bottom corner of the drawing.
±0.2 is the tolerance. It defines how much the dimension is allowed to deviate.
The dimension line has an arrowhead on each end.
Extension lines continue or extend from the surface of the object and establish the size of the dimension. They do not touch the object lines and extend slightly past the dimension line.
Extension line offset is a gap between the object surface and the extension line, so it is not confused with the visible lines of the model
Types of Dimensioning
Parallel Dimensioning
(Datum Dimensioning)
Superimposed Running Dimensioning
Chain Dimensioning
Combined Dimensioning
Dimensioning Small Features
Dimensioning of Multiple Features
Dimensioning of Radius
Dimensioning of Circle, Arc, and Hole
Dimensioning Angles
Rules of Dimensioning. Generally, dimensions and drawings must be neat.
Dimensions should not be duplicated, i.e. same dimension should not be repeated on different views, or the same information need not be given in two different ways
Avoid dimensioning to hidden lines and features
Dimension lines, extension lines and object outlines should not cross or overlap
The Importance of Tolerance. In practice, parts cannot be manufactured exactly to the dimensions stated in the drawing, due to manufacturing variations. Tolerances are included with dimensions in engineering drawings to take into account these manufacturing limitations.
The 3 types of tolerance are Bilateral Tolerance, Unilateral Tolerance, and Limit 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.
Calculating tolerance. Tolerance is defined as the difference between the upper limit of a dimension and the lower limit of the dimension. Its formula is given by: Tolerance = Upper limit – Lower Limit
Application of Clearance and Interference Fits. In clearance fit, there would still be an allowance for two parts to move, such as in a door hinge. In interference fit, it would be difficult to move (or remove) the two interfacing parts. Usually, a lot of force is needed to remove the two parts. Examples are shafts that are fitted through bearings, so that the shaft does not slip.