To trim or boolean Difference a solid?

level 1 rubber ducky task… shows line and trim command in use to cut head.

Having been told to avoid use of trim on surfaces if at all possible…
if I have a basic solid and wish to carve chunks out of it, should I draw shape, then extrude then boolean difference with extrusion, or use Boolean split ( which can create as well as remove I find, so I get my solid appear twice, an extra delete required ) or use split (someone on www using split to chop solids), or is it in fact ok to draw a curve or line and reach for trim command ?

should trim be for non solids only ?

a) a solid
b. a surface ?

what is best practice, dos and donts ?

is Level 1 and 2 likely to say what tools are bad ?


In level 1 you get to build both screwdriver and a hammer- but no advice is given as to which, if either, of these tools is bad. It turns out the screwdriver is really bad for sinking nails, and the hammer is useless for driving screws.


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Hi Pascal,

Are you saying Trim and a curve or line is ok for solids, or that level 1 and 2 wont say when not to use certain commands ?
In the forums certain commands are flagged upas ‘best avoided’, being told to read Level1 and 2 I wondered if such advice would come from there.
I just wish to know what is best practice for cutting parts out of solids ?


I’d use Wirecut if the part cutout needs to be a solid as well. It’s all very subjective to what your goals are for the geometry though and I think users most often give opinions on which commands to use based on personal experience. In my view there is no substitute for this when learning any new vocation.

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Hi Steve- I was actually saying two things- one, that tools are tools and at some point it’s your job to use the right tool to accomplish what you want… No, it is not always obvious, you do need to understand and think about what tool to use in what conditions, which leads me to my second point- Look, I and many others more patient than me have been answering your questions here on this forum, and on the old newsgroup, trying to explain how surfaces work, how Rhino works etc etc. for years. Years… So to have you turn up and ask what’s in the Level 1 training material is just a little… what’s the word… I can’t think of it just now but you get the idea…



Quiz time Steve1! What’s the difference between a solid and a surface?


wirecut better than Boolean split or same, are both going to create the same amount of points along a curve ?

I was cutting out and deleting parts from solids.One can learn from ones mistakes, if one knows why it caused problems later on, I prefer to know beforehand if at all possible. Those here know from experience what works and what is best avoided and why.


It wasnt ‘whats in’, I was after ‘policy’,
I dont remember from my first work through of level1 seeing advice on avoid this tool etc, I only receive that in the forum, so if I know policy on such second time read through, and see one tool in use, and know of two that will do the job, do I assume the other is not suited for that task in that example, i,e is it policy to mention only one of two, or would both be mentioned if both were equally suited . Just trying to suss what to read into what I will come across second time around.

I am only going by posts here saying dont use this or that. JoinEdge being one I recall strongly being advised against for fixing naked edges. Another being minimal use of trim command on surfaces. I will be looking out for things in a different way and in a different light on my revisit.

skyg…at the moment I say a solid is surfaces joined with no leaks. Also formed from solid tools. solids can have booleans performed on them. Booleans fail if there is a leak, also on other things like fine slivers. Surfaces are zero thick, if joined but not forming an enclosed leakproof shape they are polysurfaces. No doubt I am totally wrong…ask me again after Levels 1and 2 and more I have here to look through…which I can start on AFTER I finish this 1.5 yr HELL job. all of a week away, if I can get there.


I think they’re the same but wirecut doesn’t require you to make one of the solids.

You’re correct actually, the only difference between a solid and a surface is that a solid is closed/watertight. So, since a solid is made up of surfaces that are joined together, there is no difference between using trim on a solid, and trim on a surface. Sometimes trims are unavoidable. Over time you learn how to do them well when they are needed.

  1. “Solids can have booleans performed on them.” Yes, that is technically correct but it shouldn’t be understood as “ONLY solids can have booleans performed on them”. You can even boolean curves.

  2. “Booleans fail if there is a leak”. You don’t pass on this one. Booleans only fail if the intersection between objects is not complete. Most often, this will mean that the intersection between two objects should be a closed curve but in some cases even that is not necessary. Think of a single plane on the XY plane and another single plane standing vertically on the first one with the bottom edge crossing the entire surface from the middle of one edge to the middle of the opposing edge. You can perfectly well boolean these. The result depends on the direction of the surfaces - something that you don’t have to worry about with solids.


oh ? so a naked edge on or a naked point on one of the two being unioned can still see union occur ?

I also looked at Rhino wiki and it said leaks fail booleans. a bit confused now.

I have never needed to join such as your example, two planar surfaces forming a letter T, but that is interesting and now logged mentally…

just tried it and ctrl j sees nothing happen, so try boolean union to weld them together, and one half of the T vanishes ?

oh ??

did I interpret your object correctly ? what went wrong ?

attached 3md.
Boolean union two surfaces at edge.3dm (54.6 KB)

No it doesn’t. It only says open intersection curves can make them fail.

Hi, I was just about to edit that out, before someone read it :smile:
you are right, maybe I did my own wiki as having fixed naked edges I get booleans to work, so maybe in so doing I also made all intersections closed loops, sheer fluke etc.

if Booleans fail for reasons of unclosed curves, running intersect would be a good indicator of what is to come, failure wise.


That is correct.
BUT: the curves do not necessarily have to be closed! Which brings us to this:

You interpreted my object correctly, yes. And nothing went wrong. (I haven’t looked at your file but it sounds good from your description).

I suppose I should play that game show thingy a bit further and ask you to explain why nothing went wrong but I’m not in the mood. As long as the intersection between the objects - a single line in this case - goes from an edge and ends up at an edge of the object that is cut, the boolean will work. The boolean commands are just automated trim and join processes.

Really HTH.


Maybe someone could look at the 3dm and see why, when they go edge to edge, (well they snapped at the edges) one half vanished.


Steve, that means you don’t understand! It is working correctly.

Hi wim

oh right…I thought to union two surfaces would weld them, into a T, not lose one side.


“T” structures are not allowed in Rhino…

…go get a bucket of sparks ? :smile:

I’m not totally sure what sparks is but if it is anything like it sounds like in Norwegian you are still trying to make the T-structure, in spite of what has been written in this thread.

A boolean splits and throws away what’s on the wrong side. with wrong being defined by the operation (difference, union, or intersect) and the direction of the surfaces. The T becomes an L.