Cut Sweep with composition curve 1

[triggerguard1]

I've been trying to run a cut sweep over an object that I've used a composite curve of the outline of it's geometry for the guide curve. Basically, what I've got is an outline of a part that is similiar to an S curve. I'm trying to cut a radius profile on it's surface. I continue to get an error that says," Sweep resulted in topologically invalid body."

What the hell does that mean? The help file has no such phrase listed.

The profile of the cut is in line with the composite curve, and it does not intersect itself.

This has been killing me for about a week. Any info would be greatly appreciated.

 

[meintsi]

Here's a shot at an explanation...(with no simple solution)

Typically there are two types of data that are stored for solid modeled objects...

Geometric data, which contain the basic shape definition parameters, and

Topological data, which includes the connectivity relationships among the geometric components.

The validity of the geometric data (in this case the boundry of the solid) is checked using Euler's formula which defines the necessary conditions to be a valid soild object.


From:

http://www.ncst.ernet.in/~gfaculty/lecture_notes/solid.pdf

A “Polyhedron” is a solid bounded by a set of polygons such
that two and only two polygons meet at an edge and it is
possible to traverse the polyhedron by crossing the edges and
moving from one face to the other.

A “Simple Polyhedron” is one that can be deformed into a
sphere. The relationship between vertices, edges and faces is
V - E + F = 2
which is known as Euler’s Formula.

For a general polyhedra, I.e. a polyhedra that have faces with
holes, the generalisation of the Euler’s formula holds
V - E + F - L = 2 (S - G)


It sounds like your cut-sweep is too complex for the SolidWorks algorithm to verify one of these relationships in order to be a valid solid object.



Remember...
"If you don't use your head,
your going to have to use your feet."

 

[triggerguard1]

The strange thing about it, is the fact that I've got it to work before on another part. I went back and checked out everything that I had done on the previous example, and it's all the same. This countour that is around the edge of the part is also closed. Do you think that may have something to do with it?

 

[macPT]

Have you created the 2 parts in the same version of SW and the same SP? If you check the first part, do you get a "No invalid edges/faces found" report?

Regards

 

[triggerguard1]

I've done a complete check on the contour, and came up with no problems. I also used the contour select tool without seeing any breaks in the contour and it still doesn't work. If I set my parameters to Keep Normal Constant, it works, but the contour does not stay consistent along the length of the profile, as it should, seeings how the profile does not follow the path.
The part is a triggerguard for a Winchester Model 70 rifle. The outside, and inside of the guard bow needs to have a 1.737 radius profile milled on it. I've got the system to work on another design of guard, which on the inside profile, it shows me the preview of the cut sweep, but on the outside, it does not, even though it actually makes the cut properly. I started off with importing a DXF file into the system, but later completely reconstructed the geometry in Solidworks. The really strange part about it is the fact that I've gotten it to work once, but even after completely copying the entire construction process of the first model, I still can't seem to get it to work. Not to mention, I've never heard of, or seen any example of a cut sweep or extrude, or for that matter any type of function that if it was working properly, it would not show you a preview. 99% of the time, if you don't get a preview, there's something wrong with the sketch, either self-intersecting, or open contours.

 

[meintsi]

I've been working on some pretty complex geometry lately and have seen loft previews fail when telling SW to actually build them. (SW2001+ SP3) ERROR: invaid geometry created, self-intersecting geometry, etc...

One way I've gotten around this is to use the same construction geometry as a surface loft. Most of the time it will build then. Then it is a fairly simple procedure to knit on some ends and create the solid.

Remember...
"If you don't use your head,
your going to have to use your feet."

 

[macPT]

In some cases you can also regenerate successfully an invalid geometry. My VAR did not believe it until he actually see one model I created, successfully regenerated, but with a strange apearence. When I performed a Tools\Check, SW then founds invalid edges/surfaces (but no error message during regeneration).

From my experience, I think meintsi is right. Modeling complex geometries using surfaces it's a lot easier and with better results than using solids. Maybe triggerguard1 could try this approach.

Regards

 

[Theophilus]

For excellent advice on creating complex surfaces, I recommend checking this:

http://www.prodesignsrv.com/Tutorials.html

See "Curvy Stuff 102" for tough surface/solid techniques.

Also, there is:

http://www.zxys.com/swparts/

by Paul Salvador, which is a collection of tricky parts made in SolidWorks. You can download models, then step through the process to see how things are made.


Jeff Mowry
DesignHaus Industrial Design

http://www.designhaus-i-d.com