One of the things that you want to do with NURBS modeling is to use 2D profile curves and boolean operations to form much of your model. As opposed to building the 3D wireframe and then trying to fill that in like you would do in poly modeling.

Check out here for some tips for people coming from a poly modeling background:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4865.2

- Michael

Thanks Michael for the first tip. Now i understand.
For the second question i have attached the file

Hi Toha, so the issue for the second question is that one of the detail objecs is not a closed solid. When you select the upper one the object type in the upper-right corner of the screen shows "Joined srf" instead of "Solid".

That means it has what are called "Naked edges" in it which are edges that only belong to one surface instead of being joined between 2 surfaces like it needs to be to be a closed solid.

You can see the problem area by selecting naked edges. In v4 you can do this by the "Details..." dialog which will list 3 naked edges. If you click on that label you can select them to see which area they are in:



You can also select them by setting up a keyboard shortcut as described here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6051.2

Booleans work by intersecting the objects and then using which solid volume a piece is contained inside of to decide which things to keep and which to discard, if the object is not a closed solid it can't do that properly.

Somehow part of the trimming boundary on one of the faces has gotten sliced up into a small fragment, I've attached a version where I deleted the faces around the problem area and reconstructed it and this version should now give you a boolean ok.

- Michael

Thank you very much! Now it works correctly.

Best regards,
Toha

İs there easiest way to remove this edge which does not affect on surface?

Hi Toha, to remove an edge you would need the 2 surfaces on either side to be just one larger surface instead. That can be difficult to do after you've done booleans and such to an object. It's easier to do it earlier on when the surfaces are initially constructed. At that time you would want to focus on the curve structure that you used to construct the surface and make sure you don't have multiple segments in the curve.

Why do you want to remove the edge?

- Michael

it is not a big deal, but i wonder to know why it creates an additional edge when fillet it?

i marked fillet edges

Hi Toha, can you please post your model file so I can take a look at it?

- Michael

here is

Hi Toha, I think it's because these surfaces are not totally smooth to each other:



When surfaces meet up at a sharp edge, then the fillets constructed between them do not touch each other directly. So it's building the fillets and then putting in a blended corner patch between them, that's the small piece.

It looks like the surfaces are smooth over on that side but maybe not on the other. That's one of the problems with trying to construct models by building the 3D wireframe and then filling in pieces directly to the wireframe. It's very easy for the filled in surface to not be fully smooth with each other.

Your surfaces there have control points like this:


What you would want to see for best geometry quality is something like this:



You'll get this type of structure if your base shape is an extended block and then you use 2D profile curves to trim away pieces to form the outline.

This makes a simpler "underlying surface" that has trimmed away areas. One surface made like this will be smooth throughout but if you go in and try to fill in a 3D wireframe that won't be the case.

- Michael

i cut this part with a curve which was detached from another surface. I tried rebuild detached curve but it still had many points on it since i want to keep the shape. As i understand this is not a good idea to use detached curve for cutting. Am i right? Instead of i need to create a new 2d curve?

You can use Construct / Curves / Iso Curve for have directly same number of points...

Hi Toha, yes a curve that was created as a result of intersections between surfaces can be pretty dense. But the thing that is most undesirable about the above surfaces is not that the control points are dense, it's that the surface shape is being affected by forcing it to hug the boundaries directly.

It's pretty subtle in this case but look at when I draw 2 lines connecting CVs of the triangular surface:



Then if I move the upper line down so it is on top of the lower one you can see they are not going in exactly the same direction:



It's a small amount but there's a slight twisting happening throughout that surface.

If the edges trace out the same silhouette from the side view, you would have better surface quality if the surface was a straight extrusion and the triangular border comes from trim curves on the surface rather than the surface itself hugging the border. That's what I'm trying to show by this:


Surfaces like that which are simple extended extrusions with trim boundaries on them will be generated when you do a boolean on a solid object cutting it with a 2D profile curve.

If you instead set up the model by building the 3D wireframe for it and then filling in areas using freeform surface commands between the edges you won't get trimmed surfaces, you'll get surfaces that hug directly to the edges. They can have a precise surface shaping in some cases like if the edges are the same exact shape or mirror images, but if they are of different lengths then the result will not be quite the same as an extrusion that has trims on it.

The main thing is that if you have extended shapes and use booleans with 2D curves as the cutting objects you will get the precise simple trimmed surfaces from that method automatically.

So when it's possible to do so it is better to use booleans to construct the model, especially using 2D curves as the cutting objects. This is where the NURBS modeling strategy is quite different from poly modeling.

- Michael