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Dimensions, Tolerances and Assemblies - Coggle Diagram
Dimensions, Tolerances and Assemblies
Dimensions
How to Dimension?
- Dimension Text
- Base dimension
-units (in mm) not indicated on every dimension text, but at the bottom corner of the drawing
- Tolerance
- +- 0.2 is the tolerance
- defines how much dimension is allowed to deviate
3.Dimension Line
- It has an arrowhead on each line
4.Extension Line
- Extention 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
- 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)
- Consist of several dimensions originating from a reference surface (also known as datum)
- Superimposed Running Dimensioning
- 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 take the dimension very difficult to read
- Arrow heads can be places outside the extension lines
- Dimensioning of multiple features
- To dimension multiple same dimensions, use N x Dimension
- Dimensioning of Radius
- Radial dimensions starts with capital 'R'
- Dimensioning of Circle, Arc, and Hole
- A circle is dimensioned by it's diameter
- An arc is dimensioned by its radius
- Holes are located by the centerlines
- Dimensioning Angles
- Marked with a curve dimension line and has the degree symbol
Rules of Dimensioning
- Dimensions should not be duplicated, i.e same dimension should not be repeated on different views, or 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
Tolerances
- 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.
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
Calculating Tolerance
- Tolerance is defined as difference between upper limit of a dimension and the lower limit of dimension
- Formula : 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.
-Max. shift diameter is smaller than the minimum hole diameter.
-Difference between the two diameters is the amount of clearance
- 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.
-Min. shift diameter is larger than the max. hole diameter
-Difference between the two diameters is the amount of interfearance
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