Splitting a solid only part way through

benn_banks

Im trying to plit the lower paddle off this part without slicing all the way up the part.
I tried splitting with a sketch but i cant select a sketch when splitting. 
Whats the best way to remove just the lower portion without affecting anything higher on the part??

_Ðave_

I would draw a sketch with a thin slot on the face and do an extrude/remove.
_Dave_

øyvind_kaurstad

Draw a line on a sketch in a plane perpendicular to where you want the split, and extrude a suitably sized surface from that, and then use the extruded surface as the split tool.

jakeramsley

As øyvind_kaurstad said, create a surface and use that.  Surfaces are bounded.  Planes are infinite which will split through more of the part.

philip_thomas

It is not possible to create an infinitely thin 'split' part of the way through the part AND (in the test i ran) a surface used to split a part that extends only part of the way through, is treated as infinite and split the part entirely.
_Dave_ is actually correct, create a sketch defining the width of the slot and cut it to the desired depth.

kevin_o_toole_1

@benn_banks
Phillip is right that a planar surface will split infinitely, just like a construction plane.

To work around this, you can split with a surface that's only partially planar to get the behavior you want.

e.g.:

øyvind_kaurstad

Useful info, but quite unexpected (so much that I didn't actually test it when I wrote the reply) that Onshape splits all the way though even if the surface is bounded. That cannot be the desired behaviour, so I gather that's a bug?

philip_thomas

Kevin's trick of a 'not completely planar surface' is pretty cool.
Note that this is valid because it still produces manifold topology (in this case two bodies).
The OP asked to make an infinitely thin slit - this is non-manifold topology (a valid solid may not have more than one face meeting an edge and no two faces may be coincident - there is a more technical definition, but this will do for now).
The original ask is for a non-manifold body and Onshape won't let you do this - moreover, its non manufacturable.  i am not chastising anyone, this is just an interesting discussion. Any of the methods espoused here are valid.

øyvind_kaurstad

@philip_thomas I'd argue that Kevin's trick should not be necessary. The surface he drew should be valid for splitting even without the half circle at one end (to not make it not completely planar).

If a surface is bounded, then why would it (attempt to) split anything outside it's bounds. This surface should not split the upper part of this solid, but it does:

benn_banks

Awesome!! thanks guys!! that worked

øyvind_kaurstad

I'd still like to have some feedback from Onshape as to whether this really is the intended behaviour? While the proposed workaround gets the job done, it is really difficult to see why it is necessary in the first place. In this regard, Onshape clearly treats a bounded planar surface as if it is unbounded (i.e. as it it were a plane).

philip_thomas

øyvind_kaurstad said:

I'd still like to have some feedback from Onshape as to whether this really is the intended behaviour? While the proposed workaround gets the job done, it is really difficult to see why it is necessary in the first place. In this regard, Onshape clearly treats a bounded planar surface as if it is unbounded (i.e. as it it were a plane).

Oyvind - I checked with the VP of UX and this is the intended behaviour. You (as is everyone), are free to submit an enhancement request to change the behaviour so that users can vote on allocating resources to work on this.

kevin_o_toole_1

@øyvind_kaurstad:

You're right that the current behavior feels inconsistent. I believe it was a conscious UX decision, the reason being, once we start allowing people to split with faces, rather than just surfaces, a planar extension is the natural thing to do, e.g.:



The reason we don't extend non-planar surfaces is more a mathematical problem than a UX one  

This specific case has been filed as a bug, since you're right that we don't have a good (i.e. discoverable) way of solving it, or other cases like it. As with all things in UX, the solution here will be a matter of balancing a number of different considerations, and the exact solution could be changing the defaults, or adding explicit options, or something else entirely.

øyvind_kaurstad

@philip_thomas Thanks for checking. However, it still escapes me how on earth the devlopers could even think that this was a good way to do it. If you want to split infinitely, then it would be easier to just split by a plane, which by definition is unbounded. Are there any other cases where a bounded surface acts as if it is unbounded?