Completing a V0.16 Assembly with V0.17 PartDesign

[chrisb]

3. I do not yet see the advantage of datum planes since you can attach sketches in exactly the same manner. If you attach a datum plane to an edge,e.g. concentric to a circle, and then modify the body the datum plane loses its attachment in my experience.

I see the following advantage:
1. A datum plane is a plane, so while a sketch line can change due to toponaming, a datum plane, preferably a well attached one will not move.

The proper attachment is the trick. If you control the plane by a face which you would have used for attaching a Sketch anyway then nothing is gained. But if you can control the Datum Plane by something else, e.g. the principal planes or a basic master sketch, then the model becomes much more stable.

Its kind of funny that I just started a topic concerning Datum Planes where I had hoped for lots of input. But as I could have expected: one answer from Normand was enough.

[abdullah]

The proper attachment is the trick. If you control the plane by a face which you would have used for attaching a Sketch anyway then nothing is gained. But if you can control the Datum Plane by something else, e.g. the principal planes or a basic master sketch, then the model becomes much more stable.

I agree.

However, while you generally attach to faces of the last feature (I honestly do not know if it is possible to attach further up in the hierarchy, but it would lead to a nightmare of dependencies), with planes you can attach them to the feature closest to the starting feature having that face. This minimises the probability of having the plane broken by changes in the N-1 feature between the first, where the plane is attached and feature N where you would attach it, shouldn't you use a plane.

Of course using sketches or reference planes to attach the datum planes is the most robust you can get, but using planes on "early" faces can be very robust and depending on the part it may be more convenient/practical.

I also think that models using reference planes consistently are easier to repair if broken for two reasons:
a) because it is more obvious to detect where the model has broken.
b) because it often involves "just" fixing the plane attachment to where it should be.