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LESSON 6 - Coggle Diagram
LESSON 6
Dimension in drawing
This is the base dimension units are mm, is not indicated on every dimension text but at the bottom corner of the drawing
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3.Tolerance
+/-0.2 is the tolerance, it define how much the dimension is allowed to deviate
Extension line
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
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
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8.dimensioning of circle, arc and hole
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Rules of Dimensioning
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every feature must be dimensioned once. 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
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Dimension lines, extension lines and object outlines should not cross or overlap and dimension should be placed outside the part
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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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Types of Tolerance
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Bilateral Tolerance: allowed to vary in two directions from the specified dimension. Both directions need not vary by the same amount.
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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.
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From here, we can see that there will always be sufficient clearance for the shaft to interface with the hole. In such a scenario, we will have a clearance fit.
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In the below image, the minimum shaft diameter is 49.8, which is larger than the maximum hole diameter of 49.5. Hence, this would be an interference fit.