TechDraw Section Examples

Introduction
The TechDraw workbench made a big step forward regarding the generation of section views. To not overload the reference pages this page's intention is to give examples and supply proper naming for the performed tasks.

I'm doing my best to find the correct terms, but since I'm not a native speaker it's your turn to fix my mistakes if you find one.

Sections
Sections or section views are used to look inside an object to show details that are invisible or hard to recognise from views with hidden lines enabled.

Usually there is a projection group showing a simple object from at least 2 directions for a complete graphical description; complex objects require more views to describe each detail, the hardly visible and the oddly positioned ones.

One of the existing views will be used as a base view where you draw a section line to define how to derive a section view. That is the way for manually drafting and how it is done by most CAD software. FreeCAD does not work like that yet!

Sections the FreeCAD way
It is not yet (version 1.0) implemented to draw a section line in a 2D view and transfer information such as position and angle back into 3D space.

Instead FreeCAD provides a Set View Direction widget in the Task panel plus either a Section Plane Location widget for simple sections or an Object Selection widget and a Projection Strategy within the Section Parameters for complex sections. (see Simple section and Complex section)

Example Object
This object has no use at all except to describe the different kinds of section representations.
 * (BTW, I rather use the word object than part to avoid confusion with a Std_Part.svg Std Part)



Simple sections
Simple sections use a single section plane to cut through the whole object. Any section view requires a base view ( property) to position a section plane when using Insert Section View for simple sections. The vertical axis of the section plane is always the normal of the base view and the horizontal axis of the section plane is parallel to the section line. Usually the section view is oriented with its horizontal axis also parallel to the section line. The angle between section line and the base view's horizontal axis is controlled by widgets of the Set View Direction area:



The View Direction as Angle combo-box allows to set an arbitrary angle and the four buttons are used to cut the object at preset angles perpendicular to the horizontal and vertical view axes:

90°, 270°,  180°,  0°

Horizontal section
Section A-A (section up)





Section B-B (section down)





Vertical section
Section C-C (section left)





Section D-D (section right)





Arbitrary section
Section E-E (section at an arbitrary angle)





By default the section plane goes through the global origin (3D) which means the section line goes through the origin of the view. To get an offset section line (and plane respectively) we can change the values within the Section Plane Location widget.



Auxiliary view
FreeCAD is lacking a specific tool to derive auxiliary views from base views, but Insert Section View can handle that, too:

Using Section E-E from above and changing the mentioned values to X = and Y =  the section line/plane doesn't cut the object anymore and the section view turns into an auxiliary view. (Be careful when you change the values. Large steps tend to crash FreeCAD!)

The label was edited manually. The section line and one arrow have to be hidden in followig steps since one single arrow is enough to properly define an auxiliary view.

Single Section
If there is only one section view on a drawing and it is plain to see that the object is cut along a center line, the section line including the arrows and the view title may be omitted.



Internal section
A section view may be integrated into the base view. This case doesn't need arrows and title as well.



Complex sections
A complex section such as an aligned section or an offset section uses more than one plane to cut an object.

The TechDraw ComplexSection tool requires a base view (DataBase View property) to place several connected section planes to cut through the whole object; these are defined by a 3D polyline. (This tool could handle curves and curved section faces, but curved sections are a rather unusual use case)

The vertical axes of the section planes are always parallel to the normal of the base view. Their horizontal axes are derived from the related segments of the 3D polyline. The orientation of the Section view depends on one of the 3D polyline's segments and is influenced by the widgets in the Set View Direction area of the Edit Complex Sections dialog:



This tool provides 3 options in the Projection Strategy combo-box to handle the section line segments:
 * Offset (default), only segments perpendicular to the viewing direction are displayed
 * Aligned, all cutting segments are displayed in true length
 * NoParallel, all segments are projected along the same viewing direction. Depending on the angle between segment and viewing direction the projections are shorter than the cut area so that parallel sections are reduced to a line.

Offset section
An offset section starts with a base view plus a 3D polyline, a sketch in this case.

The view direction angle has to be set to a matching value or it might happen that no section view will be created or that the result will be other than expected.



Aligned section
An aligned section also starts with a base view plus another 3D polyline.

The view direction angle has to be guessed and set to a nearly matching value or the result could be other than expected.



This his how a bad guess result looks like:



Auxiliary view
The TechDraw ComplexSection tool can, like the  Insert Section View tool, create auxiliary views from base views:

An auxiliary view also starts with a base view plus a single 3D line placed outside the object.

The label was edited manually. The view direction (direction of the arrows) has to be extracted from the 3D line manually and you have to find the matching "View Direction as Angle" value manually. The section line and one arrow have to be hidden in followig steps since one single arrow is enough to properly define an auxiliary view.
 * There is a lot to do by hand, but it is a working solution, at least.

NoParallel section
A NoParallel section is a mixture of aligned and offset sections.

The arrow direction should have been horizontal, but in this case the tool did not work when the value of "View Direction as Angle" was set to. And so the sketch was rotated by 5° and said angle was set to, too.

Comparison NoParallel vs. Offset and Aligned


To be honest, if the value of "View Direction as Angle" of the aligned section was set to 5° exactly the result is faulty. When a section is activated for editing, said value is set to odd 5,14°, and it is confirmed with the check button then the expected result will be displayed.



Complex one line sections
One aspect that hasn't been focused on is that the length (width) of the section depends on the arrow positions i.e. the section line length. In contrast to simple sections where we have no influence on the section line length, we can control this length through the length of the used 3D line, but the results differ from Offset section to NoParallel section:



Half section
A view showing a symmetric object cut on one side of a center line and uncut on the other. The depth is usually defined by another center line.