Dimensions, Tolerances and Assemblies
Dimensions
Types of Dimensions:
- Parallel 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
Parallel Dimensioning
It consists of several dimensions that originates back to the common reference point.
Superimposed Running Dimensioning
Helps to simplify parallel dimensioning to reduce space used on a drawing.
Chain Dimensioning
It should only be used when the object would not be affected by by the accumulation of tolerances
Combined Dimensioning
It is used in both chain and parallel dimensioning
Dimensioning small features
Arrow heads are placed outside the extension lines, instead of between the extension lines, for easier reading
Dimensioning of multiple features
N x Dimension is used to dimension multiple same dimensions
Dimensioning of Radius
Radial dimensions start with a capital 'R'
It can be shown as a radius dimensioned with the center of the radius located on the drawing, or dimensioning radii without their centers 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
It is a curved dimension line marked on the drawing with a 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
Why are they important?
- Rejection rate will be low and realistic when estimated measurement of parts is made due to manufacturing variations
- The range of tolerance will translate into manufacturing methods needed due to different manufacturing methods with different degrees of tolerance
- Some parts can be made from different companies and they need to be interchangeable. Tolerance ensures good quality control of parts from different companies
- Tolerance ensures that mating parts of an assembly interface and fit well.
Types of Tolerance:
- Bilateral Tolerance
- Unilateral Tolerance
- Limit Tolerance
Bilateral Tolerance
Allowed to vary in two different directions from a specified dimension. Both dimensions need not vary from the same amount.
Unilateral Tolerance
Allowed to vary in only 1 direction.
Limit Tolerance
The upper and lower limits of the dimension are specified
Tolerance formula
Tolerance = Upper limit - Lower limit
Assemblies
What is an Assembly Modelling?
It is when multiple parts are brought together to define a large and complex product representation.
Two types of fits:
- Clearance fit
- Interference fit
Interference fit
Happens when it is difficult to fit the two interfacing parts. When one is in this kind of situation, a lot of force is needed to remove the two parts.
Clearance fit
It happens when there is still allowance for two parts to move
How does it work?
It begins by bringing in the base component because of its central role. Other components that are brought in needs to be oriented and located to other components in the assembly. Assembly joints are used between elements of components.