Cannot extrude face as and "add" to part.

nelson_arblaster

Watch the video below on my explanation and problem.  I can extrude in some instances and "add" to the existing part.  Then in some situations I cannot, and I must choose "new part" to finalize the extrude.  Any help you can give would be great.  This could be a user error on my part as I am new to CAD in Onshape.  I teach middle school students the basics in CAD so my level expertise is not advanced.

https://youtu.be/-9Pg7sFQEVU

alan_baljeu

Yes this is called non-manifold topology and is not allowed.  What you have here is two separate subvolumes that share a common edge.  The simple rule in modeling is every edge must be attached to two faces, not 4 as would be in this case. 

The reason is this rule drastically simplifies the math of computing solids.  The other reason is the shape you just modeled is impossible to manufacture.  You can't have a sharp edge meet another sharp edge precisely.  Either there will end up being a gap (in which case model with a gap) or there will be a thickness (in which case model with an overlap included).

Decide which you want, and to make it happen you modify the sketch to put a non-sharp corner on one or both of the rectangles, or extend one of the rectangles to overlap the other shape.

john_mcclary

Nice explanation Alan

Here is a look at one of the work around's he is talking about,  it's only .005" you can probably make it much smaller if you need.
I assume you are  3D printing these and these are smaller than the tolerance of your machine right?

nelson_arblaster

Thanks for your help.  We are not 3D printing, rather my students will be creating these out of .75" wooden cubes.  It was just frustrating that we could not create the shape as one solid piece when it looks so simple.  If you were to make this part from scratch, how would you do it to avoid workarounds that may be very confusing to a 12 year old?  Thanks. 

john_mcclary

Well, the problem is a limitation with a computer trying to draw an infinitely tiny solid where the two ends meet. In reality we know it will just crack or break through. But a computer can't reconcile that without a bunch of logic to know when it's close enough to ignore.

So, for a 12 year old. they should be able to understand two edges coming together like that is a problem, once shown an example.

Otherwise just have them assemble a bunch of perfect cubes in an assembly, and create sub-assemblies for each 'part'