Chapter 4: Creating Simple Parts, Assemblies, and Drawings . . . . . . . .
Deciding where to start
The first feature that you create should be positioned relative to the Origin. Whether there is a cor-
ner of a rectangle that is coincident to the Origin, the rectangle is centered on the Origin, or
dimensions are used to stand the rectangle off from the Origin at some distance, you need to lock
the first feature to the Origin with every part you build.When working with a simple part, the entire part can usually be described as rectangular or cylin-
drical. In cases like these, it is easy to know where to start: you simply draw a rectangle or a circle,
respectively. On complex parts, it may not be obvious where to start, and the overall part cannot
be said to have any simple shape. In cases like these, it may be best to select the (or a) prominent
feature, mounting location, functional shape, or focus of the mechanism. For example, if you were
to design an automobile, what would you designate as the 0,0,0 Origin? The ground might be a
reasonable location as would the plane of the centers of the wheels. As long as everyone working
on the project agrees, many different reference points could work. With that in mind, it seems log-
ical to start the rectangular part by sketching a rectangle. Select the Top plane and sketch a rectan-
gle centered on the part Origin.Building in symmetry
Your next decision is about part symmetry. This part is not completely symmetrical: modeling a
quarter of it and mirroring the entire model twice is not going to be the most effective technique.
Instead, you should build the complete part around the Origin and mirror individual features as
appropriate. To start this type of symmetry, you need to sketch a rectangle centered on the Origin.
The centered rectangle is something that you will create frequently.Figure 4.2 shows a centerpoint rectangle that has been sketched with the centerpoint at the part
Origin. This creates symmetry in both directions. You can use additional construction geometry
and sketch relations to make the rectangle only symmetrical side to side.Tip
To make a rectangle work like a square, use an Equal sketch relation on two adjacent sides. This only requires a
single dimension to drive the size of the square. n
Beginning with the rectangle you sketched in the previous section, apply one horizontal dimension
by clicking the Smart Dimension tool on a single horizontal line, placing the horizontal dimension
(4.00 inches), by clicking a vertical line, placing the vertical dimension (6.00 inches). The sketch is
fully defined at this point because both the size and position of the rectangle have been established.
Best Practice
If you are dimensioning a horizontal line, the best way to do it is to simply select the line and place the dimen-
sion. Selecting the line endpoints can also work, but selecting the vertical lines on either side of the horizontal
lines is not as robust. The problem is that if you use this third method, deleting either of the vertical lines causes
the dimension to be deleted. In the first two dimensioning methods, dimensions are not deleted unless you
remove one of the endpoints, which requires deleting two lines: the horizontal line and one of the vertical lines. n