3D Model default features

[rburgess-2]

I hope someone can answer this. Are default datum planes/coordinate system required? I was taught waaay back when, that 0,0,0, (origin), defination was paramount. Coordinate system more-so than planes. Any feed back welcome.

[tbraxton]

It is possible to create a start part with not features. An empty start part will not let you sketch anything so it is not useful for modeling with features. You need a minimum set of references to establish a sketch plane so yes some datum features need to be present before you can create most features.

If you import data (STEP model) into an empty part it will use the csys reference info in the STEP file as a placement reference. Creo defines an origin even if there is no csys feature in the model otherwise it would not be able to place the import geometry.

Explicit definition of an origin in a cartesian frame (Csys) is arbitrary so depending on your design intent it may or may not be critical. In most environments your start part for modeling should include a minimum of three orthogonal planes and a Csys at the intersection of the planes.

[StephenW]

Creo is a funny beast. At least on Creo 4, you can start an empty part (no planes or datums) and then you can Extrude or revolve a feature with no references. I didn't really know this was possible until I just tested it.

But if you have a coordinate systems as a default, you CAN'T do anything unless you add default planes (as best as I can tell).

You could conceivably work in an environment with no datums planes. I think its a bad idea but I have been proven wrong SO MANY TIMES in the past!!!

Giving a user a starting point (usually 0,0,0) in a model is likely a good idea in any application I can think of.

[tbraxton]

I can confirm that extrude and revolve are possible without datums present in the model in Creo 4. There may be other features possible as well.  Sketched curve is not available for me. You can see that Creo is using an internal origin reference to place the extrude or revolve once you complete the feature.

Suppress the extrusion and then create a default csys and planes. Then resume the extrusion (which is not dependent on these datums) and you can re-order it to the end of the tree. Note that the proximal plane used to create the extrusion is aligned with the default Csys and the sketch plane is in the XY plane.

I am using legacy start models created in previous releases when making these observations and I can say that this may have an influence on how Creo behaves. So if you see something different that would be the first thing to compare.

[StephenW]

Yes, the default sketch is definitely always on the x-y plane.

Interestingly, when you extrude and start that sketch, you can "pan" around in the empty model, but that first sketch click you make becomes the default origin of the model. I tried to fool it by sketching something random at first, then sketching another item and deleting the first sketch but first click on the used sketch became the origin. Even a spline uses that first click origin but orientation is a mystery so far.

[kdirth]

The default coordinate system is always at 0,0,0 of the model.  This gives you something to reference for creating the model.  Three datum planes through the default coordinate system would be the next features to create in the model as references for modeling.

[KenFarley]

I'll likely be branded as a heretic, but my default for parts and assemblies is not to create a coordinate system and three planes. I only create planes, and especially coordinate systems, if I will need them for assembly or other geometry creation. Often, if I do need them I'll add them further down in the model tree. I am restricted as to what the first feature can be, but 99 times out of a hundred I'm creating a extrude or revolve, so it works out fine.

When working on complex assemblies, though, like aircraft, ships, laptops, etc. it is generally absolutely required that a coordinate system at the very least is included in every part or assembly for positioning in the overall product space. Especially if working on something that is also being worked on by dozens or hundreds of other folks.

But, for my projects, if there is no need for those datums, I see no reason to have them. They can become obstructive if I'm manipulating an assembly of hundreds of parts and I *need* to constrain something with a datum plane but there are then 3 X (hundreds) of other datums polluting the screen. And a shorter model tree is a better model tree, I always say...

[rburgess-2]

To clarify, here's what I'm dealing with

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[Chris3]

I would highly suggest that you use start parts. It is extremely helpful to have default datums, layers, etc in your parts. ModelCheck and help fix parts to some degree but it is always easier to have standards defined up front. If you need help defining your standards you could try this book: https://www.amazon.com/gp/product/B08X7BSPGV

[rburgess-2 · Accepted solution]

As with all things Creo, there is more than (1) way to skin that cat... So.....

  What I'm gleaning from the responses is part/assembly creation without default datum features is possible, but not "industry standard" and subject to personal/Company preference... Start parts/assemblies should contain personal/Company standard feaures and be used as default except when circumstances dictate otherwise.

Thanx all for your input!