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dimensions, tolerances and assemblies - Coggle Diagram
dimensions, tolerances and assemblies
Dimensions in drawing
How to dimension
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Extension line - Continue or extend from the surface of the object and establish the size of the dimension. They dii not touch the object lines and extend slightly past the dimension line.
Extension line offset - A gap between the object surface and the extension line, so it is not confused with the visible line of the model.
Dimension text - base dimension, usually the unis ( in mm ) is not indicated on every dimension text, but at t he bottom corner of the drawing.
Type of dimensioning
- Parallel Dimensioning - Consist 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 - Should only 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 arrows 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. Uses N x dimension
- Dimensioning of Radius - Shows a radius dimensioned with the centre of the radius located on the drawing and shows how to dimension radii which is 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 lo0cated by their centerlines.
- Dimensioning Angles - It is marked with a curved dimension line. and has the degree symbol.
Rules of Dimensioning
Dimensions should not be duplicated, avoid dimensioning to hidden lines and features, dimension lines, extension lines and object outlines should not cross or overlap.
Tolerance
Type of tolerance
- Bilateral tolerance: allowed in vary two directions from the specified dimension. Both direction 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.
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Importance of tolerance
If tolerance is not given, exact measurement of the parts must be made. Rejection rate will be very high and is not realistic.
The range of tolerance will translate into the manufacturing method needed, since different manufacturing method has different degree of tolerance.
Same parts can be made from different companies and they have to be interchangeable. Tolerance ensures good quality control for parts from different companies.
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