Constraining using default datums...

[jelston]

I read an article somewhere that said in short everything should be dimensioned from default datums. Is this a practical idea?

 

[Mindripper]

For the first model feature in the part, this is certainly true. But successive features need not be constrained exclusivley to the three planes and coordinate system - and probably shouldn't be. However, it is certainly a good design practice to use them when it doesn't compromise your design intent.


Remember that these first four references will never change throughout the history of the part, so they are completely stable: a regeneration failure simply cannot be associated with these features, so their use will give you fewer possible causes to chase down in the event of a failed feature. But failing to directly associate a feature with it's relevant relations can make editing the model a nightmare, and can result in regernation failures. Use relations to the relevant parent features to ensure smooth failure-free modifications to the model: that's what design intent is all about.

 

[dr_gallup]

I generally make a couple of secondary datums offset from the default datums which control overall size, mating constraints, etc. I use these datums to align sections etc. This way all the geometry in a part can be deleted or redefined and the assembly references are still valid. Very robust. However, many secondary features are not dimensioned from these primary and secondary datums because they depend on other geometry for their dimensioning scheme. We ONLY use shown (part) dimensions in drawings with very limited exceptions. So it is important to use the proper dimensioning scheme in your part to be able to show what you need on the drawing.

In general, you want to try to have as flat a model structure as possible (also called horizontal modeling) so that when you need to redefine something you don't loose whole gobs of geometry. Limit your references to secondary geometry whenever possible and make those rounds & chamfers last.