Newbie question - how to make a solid icosahedron?

phillip_wright

Hi all,

I'm just learning CAD. I have a pretty simple problem, but it seems to be beyond me in OnShape.

I'm want to create a solid icosahedron. I've tried a few different approaches, but I feel like everything I do gets messy and tangled up.

I've seen some tutorials for SketchUp and Solidworks using 3d sketching approaches, but that doesn't seem to be how things are done in OnShape. I guess I'm looking for a solution based on 2d sketches and extrusion? (again, new to CAD so not really sure).

Thanks in advance.

andrew_troup

You can extrude a solid from a pentagon, then add draft to all five faces in a single feature, and end up with a five-facet pyramid (provided the extrude distance exceeds the height of the finished pyramid, it will terminate neatly at a point)

A pair of these would give you the "top" and "bottom" - you could create the latter from the former by mirroring about the mid-height plane, then rotating one of the pyramids 180 degrees about its axis.

(ON EDIT: You will need to sketch a line on a vertical plane, through the origin, to use as this axis)

Then it would 'just' be a matter of filling in the lattice between them with equilateral triangles, half 'point up' and the the other half 'point down'. Not quite as easy as it sounds, given the inability (at present) to sketch in 3D, because (as far as I can see) it would require providing a plane for each facet. You could probably model the facets as surfaces, and just provide two of the 'lattice' surfaces, then use a circular pattern to make the other eight.

ON EDIT:
Onshape cannot yet create a planar surface from a sketch, so this is not strictly possible, but you could thicken or extrude each of an adjacent pair of triangular sketches from the "lattice" as far as  (or slightly past) the central axis, and then use these to create a circular pattern as previously suggested. A final boolean addition would tidy up, and create a single solid part.

andrew_troup

You can extrude a solid from a pentagon, then add draft to all five faces in a single feature, and end up with a five-facet pyramid (provided the extrude distance exceeds the height of the finished pyramid, it will terminate neatly at a point)

A pair of these would give you the "top" and "bottom" - you could create the latter from the former by mirroring about the mid-height plane, then rotating one of the pyramids 180 degrees about its axis.

(ON EDIT: You will need to sketch a line on a vertical plane, through the origin, to use as this axis)

Then it would 'just' be a matter of filling in the lattice between them with equilateral triangles, half 'point up' and the the other half 'point down'. Not quite as easy as it sounds, given the inability (at present) to sketch in 3D, because (as far as I can see) it would require providing a plane for each facet. You could probably model the facets as surfaces, and just provide two of the 'lattice' surfaces, then use a circular pattern to make the other eight.

ON EDIT:
Onshape cannot yet create a planar surface from a sketch, so this is not strictly possible, but you could thicken or extrude each of an adjacent pair of triangular sketches from the "lattice" as far as  (or slightly past) the central axis, and then use these to create a circular pattern as previously suggested. A final boolean addition would tidy up, and create a single solid part.

phillip_wright

Great - thanks Andrew.

I ended up using "loft" to raise the pentagon to a point (because I'm able to define the height of the apex point in terms of the side-length of the pentagon). I hope that's an appropriate use of loft.

A like the idea of the extruding the triangles to the interior and then boolean-ing them to neaten it up, as well as using circular rotate - awesome.

I'm glad you mentioned things like "Onshape cannot yet create a planar surface from a sketch", and confirming that 3d sketching is not currently possible. Saves me from worrying about whether I'm missing something.

Thanks again,
Phil

andrew_troup

@phillip_wright - Nice enhancement using loft. My method relied on knowing the angle analogous to the pitch of a roof - I infer that must be a constant for all regular icosahedrons (although I have no idea what it is) ... but it's nicer to come up with methods which derive all their variables from sketch geometry, and loft is a great way to do that (and neater than my kludge, to boot)

andrew_troup

Onshape does not (on the 'face' of it) have a way of creating a plane which contains a specified line and point, which is needed for constructing the 'lattice' triangles, but (except to someone used to having that option), it's probably obvious that the user can easily pick the endpoints of the line, in order to instead use the "three points" option for the plane.