Naked Edges on Fillets and Chamfers in Rhino

[spincaster]

I sometimes design parts that will be converted to stl file and 3D printed. The 3dm model needs to be "water tight" with no naked edges before exportng as a stl file.

I'm designing a part with holes in it. The edges of the hole need to be chamfered. When I chamfer the surfaces I end up with naked edges at the chamfers. Repairing them works in some instances but some can't be repaired.

Am I doing something wrong that causes this? Are there any tricks that will avoid the naked edges?

Any insight or advice would be appreciated!

[chmillman]

Well, it would help to see an example. The naked edges are in your STL or in the Rhino file? There are a number of things that could cause naked edges including poor geometry or inappropriate tolerances. Usually they can be fixed by more accurate modeling or doing some manual cleanup. But it's difficult to know what's wrong without seeing a hunk of the actual model. --ch

[spincaster]

Attached is a rendering of the flywheel. The model was developed by drawing a 1/2 cross section of the flywheel and revolving 360deg. Then the four large holes were made by placing 4 cylinders and doing a boolean difference. So far so good, no naked edges. I do a chamfer on one of the edges and end up with naked edges.

I solved the problem by undoing the chamfer and making some cones which were subtracted from the flywheel. That being said, this isn't always possible and won't work with a fillet at all.

I really need to master fillets and chamfers because I routinely model parts that replicate castings with fillets and chamfers.

What do you mean by tollerances/gemetry?

Jack

[chmillman]

That part looks simple enough that you shouldn't be having any problems. Question - are you using Rhino 3 by any chance? In V3 chamfers weren't joined to the main body, you had to join them after... Otherwise, again, can you post the model (not an image) in its pre-chamfered state?

As far as "mastering" fillets and chamfers in Rhino, it's a matter of some experience. There are cases you will know (after awhile) that an "automatic" fillet is not possible with the FilletEdge tool, and you will have to build it manually with the FilletSurface tool, then trim things out and join them manually. I can't really give you many guidelines except these:

1) You can't "wrap" a fillet of a larger or equal radius around another fillet. Always do the larger fillets first, then the smaller. If you need a corner where a number of fillets of equal radius come together (like the corner of a cube), you need to do them all at once.

2) Fillets can give you problems if they cross multiple surface boundaries, especially if the surface are non-tangent by a small angle.

3) Make sure there is enough room for the fillet, trying to put a fillet into an area where it won't fit will fail.

Have a look here for more help/ideas:

http://en.wiki.mcneel.com/default.as...Filleting.html

--ch

[svenakela]

I have problems with chamfers and fillets as well. I'm evaluating Rhino at the moment, I have surfaces/edges twisted in all directions and I haven't got it running easily with making edges smooth. The last edge I did I had to make a radius curve as a sweep along the edge and the trim the surfaces to fit the new radius sweep.

[chmillman]

As I said, if there is a model we can usually figure out why the fillets fail, or why the geometry isn't like you think it should be. Sometimes it's just your approach to modeling an object, sometimes things can get pretty complex in free-form land. One of the best places to go is the main Rhino newsgroup (suggest you use the nntp news inteface) -
news://news.rhino3d.com/rhino - or attach the relevant part of what you're doing here. --ch

[margni74]

I've got a similar issue I think. I extruded some text and want to chamfer the edges. It looked like the chamfers are there, but hidden underneath the top cap.

I keep fumbling around trying to erase the lines to expose the chamfer, but Im getting nowhere fast..

[chmillman]

The chamfer fails because there is at least one point where Rhino is unable to determine the intersection of the chamfer surfaces and the top/side surfaces. Rhino will make the chamfer surfaces as best it can, but it cannot trim the letters with them, so it inserts the chamfers without cutting the originals. That's what you are seeing.

Now, as to why it can't make the chamfers... Typefaces are notorius for having bad generating curves. They are often full of kinks. On top of that, if you try to fit a chamfer/fillet around a corner that it is too tight, it will fold over on itslef, and that will cause the trim to fail. In short, trying to chamfer type with a chamfer that is relatively significant relative to the type size is very failure prone. If you temporarily hide the letter and just look at the generated chamfer surface closely, you will most likely catch the trouble spot(s). they will be either gaps or folds in the chamfer surface. You can then try to fix the letter curves and re-extrude the letter....

--ch

[chmillman]

Attached is an example file...

On the single letter, the chamfer failed. I fixed the curve first - this entails checking out the areas of non-tangency and fixing those, I actually did some rebuilding of the curves as well - then I re-extruded and chamfered. Also make sure you explode the letter curves before you extrude, then extrude all at once. Rhino likes it better that way. --ch

[margni74]

Attached is an example file...

On the single letter, the chamfer failed. I fixed the curve first - this entails checking out the areas of non-tangency and fixing those, I actually did some rebuilding of the curves as well - then I re-extruded and chamfered. Also make sure you explode the letter curves before you extrude, then extrude all at once. Rhino likes it better that way. --ch

Thanks, I guess I need to play with it alot more and work with some simpler shapes, fonts. I can definately see where rhino gets confused where two lines intersect.

How do you go about rebuilding the curves? delete the original and draw your own approximation?

Why would I need to explode the letter curves?

I basically extruded the whole thing, I think I may have joined all the lines together....

[chmillman]

Typefaces are composed of many single span Bezier splines (usually degree 2) because they are usually designed in Illustrator type programs. Since they are designed for printing and not for manufacturing, there are almost always slight errors in tangency between the segments that compose the letter - for printing it doesn't matter at all...

Rhino is particularly sensitive to small errors in tangency when creating fillets/chamfers. Given a large angle it knows to split the fillet/chamfer at that point and make an angle, but when the angle is low (like 1-5&#176 it often fails, because it doesn't find the correct split point.

So, you can almost always make type chamfers succeed if you go and fix the non-tangencies in the curves first. You can leave the bezier splines if you know how to fix the tangency with those, but sometimes it's easier to select a series containing tangent errors and use Rebuild to convert them to a single degree 3 spline. You need to play with the # of control points to find a minimum distortion caused by the rebuild. What this basically means is you need to fix the type letter by letter...

The thing about exploding the curves before extruding: Rhino has another particular habit of extruding joined tangent curves as a single internal composite surface - this is perfectly legal, but it can wreak havoc later with things like fillets and also meshing. You cannot turn this behavior off, so it is best to get in the habit of exploding all curves before extruding... It makes solids that have a much better chance of getting accepted by downstream programs and will even work better inside Rhino. --ch

[Imacman]

Hello... I'm just strating with Rhino 3D... I've done a few tutorals basics that were included... and they went fine... but today when I started playing around with it... the HISTORY button keeps turniong off and I can not use the undo command it's making it impossible to learn this program. I must be missing something.. I am using the Mac version... Last night the undo command worked gret.. but today nadda... anyone have any idea what i'm doing wrong?

thanks

Imacman