Negative part subtraction, or how to retain cavities?

laird_broadfield

1. Novice user; novice CADder.  2. Actually thought I had solved my problem and was just looking for improved methods -- until I put together a document for sharing, and realized I haven't quite solved all of my main problem.

I'm trying to create a reusable mating/self-aligning feature, principally for 3D printing use.  So, I created a truncated, rounded pyramid:

I extended the bottom a bit, 

to accomodate a hollow for a 2mmx6mm neodymium magnet:

That's the male side of the part.

I then duplicated the top of the male part, 7% larger, to be the female cavity, and added a cylinder cavity for this side's magnet, and connected the two with a tiny pillar, so they'd stay one part and be easy to manipulate, derive, etc.  My plan is to subtract this part from my component body


Tiny pillar: 


And there's my first question: Is there a better way to do the keeping-together other than with the tiny pillar?

So, as I mentioned at the top, I thought I was complete at that point.  However, when I unioned/subtracted with my demo cubes, my female part did the right thing:



but on the male part I lost the sunken portion of the magnet cavity.  I get why it happened; the gray cube occupied that space, and it got kept.



...and I suppose I could leave the male pyramid solid, and bring in another part (magnet cavity) and subtract that -- but that seems inelegant compared to having male pyramid and male-pyramid-magnet-cavity all in one.

Ideas?

Document here: https://cad.onshape.com/documents/976b3e05d7cfba5fa841c3a3/w/9a4ba8750dbc29b3c09dcd3f

Narayan_K

@ laird_broadfield, Instead of making male part solid and subtracting other part using solid part.
you can directly subtract other body with male part.by doing this you can find one more part in feature tree(material in the cavity). So you can delete that part using delete part option.

 

chris_8

I don't know a better way to keep parts together than the tiny pillar.  I've used similar tiny bars for the same purpose.

For your union/subtract question though:  It seems an easy way would be to first subtract your small part from the block, and keep the tool.  That would leave a new small part floating inside the cavity of your tool.  So delete that new small part.    Then use union between the original tool and the block

Narayan_K

@ laird_broadfield, Instead of making male part solid and subtracting other part using solid part.
you can directly subtract other body with male part.by doing this you can find one more part in feature tree(material in the cavity). So you can delete that part using delete part option.

 

chris_8

I don't know a better way to keep parts together than the tiny pillar.  I've used similar tiny bars for the same purpose.

For your union/subtract question though:  It seems an easy way would be to first subtract your small part from the block, and keep the tool.  That would leave a new small part floating inside the cavity of your tool.  So delete that new small part.    Then use union between the original tool and the block

laird_broadfield

Got it; thanks to both Narayan_K and chris_8.

(Incidentally, if anyone is thinking of reusing these, I printed these on a benchtop FDM printer last evening, and at 1:1 scale, I need a little more than the current clearance.  Also, the magnet void needs a little more room as well.  Might be better to increase the clearance on the female, also increase the taper on the pyramid, and move the magnet out down to the base part.  It'd be nice to have it all-in-one, but the pyramid has to be pretty bulky to accomodate the magnet.)