stenapan wrote: ↑
Tue Oct 27, 2020 7:02 pm
As far as I understand, it is recommended to base more or less on datum planes. These datum planes should usually be detached.
I think it is important that I reply for the sake of anyone else who might be miss lead by that statement, it is completely wrong on both counts.
Datum planes have some specific use cases but they are something you would be likely to not need to use very often at all, they are for special use cases definitely not for regular use as something to attach a sketch to. In the majority of cases you just attach your sketch or other object to whatever you need directly, and hence there is no need for a datum plane.
Your confusion, I think, comes from the very good advice to attach your sketches to a plane of the "axis system of the Body" to make a more robust model that is not subject to the "Topological naming limitation (which is not a bug, just common sense)".
So for example you should typically attach a sketch to (for example) the XY plane of the Body (meaning the XY plane of the axis system of the Body, not some random added datum plane) rather than (for example) a face of a previous Pad in that same Body.
freedman wrote: ↑
Tue Oct 27, 2020 11:34 pm
You probably got the idea to detach planes from some of my posts, that's the way I work. Ya! not in the wiki.
Why on earth would you be doing that? Detaching things is a special use case that may occasionally be appropriate in some very special circumstances, it's not something that you should be doing regularly at all.
For the record, "Topological naming limitation" is not a bug, it is just obvious common sense. Think of this obvious simple example. If you create a simple sketch containing a rectangle, then attach something to one of its geometric objects like "edge4" and everything then works as expected, great, quick, powerful, convenient...but just not robust. Because now imagine that you decide to go back and edit that sketch and now change the rectangle into a triangle. Now there is no "edge4"... so obliviously FreeCAD is going to chuck a wobbly, have a fit, have a mental, go ballistic, chuck a spazz attack, and not have any idea what to do with the instruction 'attach to "edg4"' when there is no "edge4". Like a teenage girl who can't find a stupid shirt that is the correct matching colour for her stupid pants ...total melt down!!!
Sometimes using direct references to objects constitution geometry is a good idea, just not most times. For example 1) So for some quick and convenient or even temporary jobs, you may not need the model to be robust or not care if your model has issues if modified in the wrong way. Then the speed and convenience will be a worth while trade off against robustness. 2) If you are modifying an imported STEP or other solid that by definition can not be directly edited hence its constituent geometry will never be renamed, hence you can safely directly attach something to its constituent geometry.
However as a general rule for most of your normal everyday modelling, you should create robust models by avoiding the topological naming limitation in any CAD application that works on similar principles to FreeCAD.