Lesson 5:
In the next three pages we are going to learn some new rules that will allow us to dimension more complex figures than the ones seen in 1st of ESO. If you don’t remember the basic dimensioning rules you should have a look at that page before.
Dimensioning of rounded parts.
A rounded part can be a circumference or a part of it, what is known as an arc. A particular case of an arc is the semicircumference, which is a 180º arc. In the images below you can see (from left to right): a circumference, a 210ºarc, a semicircumference and a 60º arc:
When dimensioning rounded parts two rules must be considered:
- If it is a circumference or an arc of more that 180 degrees, the diameter will be dimensioned.
- If, on the contrary, it is a semicircumference or an arc of less than 180 degrees the radius will be dimensioned.
Therefore, the previous arcs should be dimensioned like this:
You must be careful that:
- The arrows of the dimension lines end exactly in the circumference or arc.
- Dimension lines that indicate a diameter must cross the center of the arc and those which indicate a radius must start just in its center.
- Diameter and radius dimension lines should never be drawn over the dotted lines that mark the center of the arc or circumference.
The following dimensionings are wrong for some of the reasons explained before:
If you need to dimension several concentric circles dimension lines should be evenly distributed:
Now we are going to see several examples of how figures with rounded parts must be dimensioned, as well as some typical mistakes. Realize that dimension lines must always indicate the location of the center of the arcs:
