offsetsurface

[Mr. Jaap van der Heide 0]

Hi all,

 

I am trying to figure out how to create a solid (meshable) using a nurbs surface (either imported from Polycad/yachtlines as an IGES surface or created using the "initial B-spline surface for hull design sketch) and the offsetsurface command. Any thoughts on that, bacause I am not able to figure out how to do that in a way where I can manipulate the hull shape later.

Is it possible to use the perimeter edges of both for a ruled surface (how should that be done?) and then convert the closed volume to a solid?

Or even just close the volume with said ruled surface and then mesh the volume defined by the B-spline surface, the offset surface and a ruled surface connecting the first two?

 

Regards,

 

Jaap

[Jörg 29]

Hi Jaap,

 

Probably it is easier if you simply upload an example. For a single surface, creating an offset surface is not a big deal, as you know. However, using the offset surface functionality for a patch work can be tricky, often there are gaps in the resulting surfaces (depending on the input surfaces).

 

Have you tried our your suggestion (e.g. using the boundary curves to create ruled surfaces)? At least, this sounds reasonable - with the limiting issues that I mentioned above.

 

Cheers

Joerg

[Mr. Bram1 Kerkhofs1 0]

Hi Jaap

 

As I understand your situation: you have an arbitrary surface, and you want to make it into a solid with a certain thickness.

 

If the innerside of the surface didn't matter, I would use the "Extrude edges to plane" function of the breps.

since it is a arbitrary surface you probably are best off with the offset surface, or just use an image surface and translate it, depending on what kind of edge you want. The offset surface will result in edges with a straight angle, the image surface will result in edges which go in the direction of the translation. The remaining part is to get those surfaces connected.

 

In any case I would advise to use breps for the connecting part. Add the two surfaces (original - offset, or original - image) to the sources and add the operation "Close planar holes" or "Close non-planar holes". This normally does the trick. You can If these commands don't work, try the "faces from planar curves" and use the edges of the surfaces.

If this doesn't help because of reasons, you can always use the ruled surface. But bear in mind that the orientation of your edges need to be correct: use the edge or the ReversedEdge. If this isn't making your edge as you want you can use 'fillet surface'. And of course if you want complete freedom: make a metasurface out of it.

 

In order to learn the command to get the edges of existing surface or breps, I would advise to get working with featureDefinitions. You can use these commands also in the input screens of the objects in the object tree. Select the two surfaces from here above and right hand click, select 'create feature def". Open your feature definition. get on a new line, type the name of a thing that you created in there (for the surfaces above, probably one is called "s1") add a dot "." and press "ctrl + space" and see all the options available. This is how I discovered what all is possible.

 

side note:If the brep doesn't seem to be responding to your commands, or the system crashes, it has to do that you made a circular reference: you are defining things based on the thing which is being defined. This sometimes happens when you refer to edges of the brep which you are currently defining. Just make a new brep and add the first one as a source. Then do the operation.

 

If your brep only has green edges, it means it is closed and you can export it right away as an stl. you can use the Display options: Chord height, angle tolerance and 3D max length in order to control the tesselation and accuracy of the model. Don't forget to put the "face display" on "face triangles", it shows you the triangles :). you can also use the brep as an input for the solid.

In order to modify with complete freedom your nurbs-surface I have the following 3-part strategy (I'm not claiming it's the best, but it works):
Part 1:
Make a feature definition: with following input: nurbs surface points of sampling in each direction.
Let the feature definition do the following: sample some points (let's say 10 in each direction, so a 100 in total), check with a new nurbs surface, with these points as input, if it approximates your new surface sufficiently. if so that is where part 1 ends. This is your base line.

Part 2:

create an object of this feature definition and look at that beauty. right click on it and select 'detach'. This will take a while, but it is making all your points accessible for adaptation from the interface.

Part3:
Create a new nurbs-surface with the points as an input. Since you can still adapt the points as you want the new surface will follow.

 

The above gives you complete freedom of the shape, if your adaptations are always the same or less 'intuitive' as here above you can automate it all in the feature definition. If meant by "adapting the surface" as in: put a new in, you just need to make this an input argument of your feature definition :).

 

Hope this helps you.

 

Kind regards

Bram

 

[Jörg 29]

Good input, Bram!

 

Learning from auto-generated feature code is a quick way to find out how to define things. For breps, there is also another syntax available which I have attached in a small example from my colleague. With this, things are much easier to read and to understand if you use this type of syntax. Just FYI :-)

BrepScripting_example.fdb

[Mr. Bram1 Kerkhofs1 0]

Hi Joerg

 

I didn't know you could make different instances of the Brep-operations. looks nice! thanks!

What is the result of the 'type provider' in the 'attributes' menu of the feature definition? All my feature definitions have the generic mark in front and these have the Breps mark.

[Jörg 29]

Hi Bram,

 

Check out this very short video tutorial which shows the effect of the type provider. Basically, the feature is then interpreted as this type, i.e. you can use the feature object in the same way like the type you have chosen. Example: take the type from a resulting curve (e.g. type FCurve) and you can enter this feature object anywhere in CAESES where a curve is expected. The main purpose of this: Convenience, and a clearer object tree structure (not only a generic feature icon).