Problem finding center point of arc after boolean

Rhino Rhino for Windows
1

hopefully the .3dm shows the gist of my question:

cPoint_Q.3dm (271.1 KB)

  • is this a tolerance issue? (this file is the template/settings i generally draw in)
  • is it something to do with the commands i’ve used?

basically, i’m just asking what i need to do to avoid having this happen… it’s not entirely practical for me to keep the original cutting curve with every copy of the board (of which each gets a custom miter/bevel/length etc)

thanks for any insight.

2

i just tried setting to inches (which i’m meaning to do to my template anyway but…)
still at .001 absolute tolerance.

didn’t seem to change anything as far as this is concerned though… the edge curve after the boolean is still elliptical.

3

Instead of using an curve extrusion to cut through the box, use the Arc as splitting curve on both sides. In that case the edge is set to the arc used to Split, not an elliptic arc.

Of course, now it is more work. you need to split the other side too, and Sweep2 the curved surface. But, you end up with a true Arc as the edge curve.

In this picture I duplicated the curved edges and show the What result.

NOTE (added later)
When using Sweep2, the rails should be the short, straight edges, and the contours should be the arcs. That way, also the sweeped surface ends up with true arcs as edge curves.

4

@jeff_hammond: try this…

Turn off extrusions before extruding your arc curve. When you extrude that, your surface will be a sum surface (defined by the arc and a line). BooleanSplit the box. The edge curve will be a true arc and have one center point.

The duplicated edge of your original Boolean split - the one that reports as an elliptical arc - will simplify to an arc, but unfortunately, you cannot “simplify” polysurface edges themselves.

So, chalk another one up for non-transparence of extrusion objects.

–Mitch

2 Likes
5

@menno
thanks. there are a few other ways to draw it accurate… i was more wondering why this way wasn’t working as i would expect it to.

@Helvetosaur
well- that finally settles my question of “should i use extrusion objects?”… i didn’t know they could bring inaccuracies into the equation like this.
thanks for pointing this out :thumbsup:

6

Well, they shouldn’t… Hence my remark about “transparency” - you shouldn’t be able notice the difference in functions like this. @pascal IMO there is a conversion/translation error there that needs to be fixed there…

–Mitch

7

in that case, yes, please fix this :smile:

the model I’m working on currently has hundreds of extruded solids which are boxy in nature and I assume performance is benefitting by having those as extrusion objects. maybe a fourth of those will then be modified or drawn in a way similar to the original post.

but needing to switch between polysurfaces and extrusions throughout modeling depending on which type of operation you’re doing sounds a bit impractical and confusing to manage. I’d rather just keep extrusion objects on at all times but because of this error I’ve come across, I’m going to have to leave them off at all times (for the foreseeable future at least).
thanks for looking into it

8

I see that- thanks, I’ll see what if we can fix that. WireCut with the arc seems to do the right thing.

Hmmm - fooling around a bit more, I can copy the extruded curve and box and BD at a different location and get an arc on the edge. If I move or copy the arc and box and extrude the arc and BD at a different location, I get an elliptical arc…

-Pascal