symetry with lofts

I try to avoid splines. SW's spline mojo isn't quite where it should be.

If there aren't too many defining points, I find it is better to use an ellipse or parabola to get what I want. Ellipses are especially handy for defining fillets and scallops.

Overall, surfaces generated with more analytic geometry tend to hold up better for many things, including holding symmetry. There is less approximation and more true calculation of geometry.

The nature of lofts is that they are approximated surfaces. The more precision you can build into their structure by using analytic curves instead of splines, the better.

Also, try using sweeps with guide curves instead of lofts. Use a pierce constraint to pin your section to the guide curves.

Lee,

Generally, when using splines (SW2005 is better) and lofts for symmetry, as you have found out, the best way is to focus on one half of the shape and applying a loft that is normal to the mirror plane or tangent to a face from which you are mirroring. Or, you could have xsec's which help the loft run normal or tangent to the plane or face (apply tangency) but it normally it's not likely to be good enough (well, maybe but that's subjective to the want/need or if the loft is not changing direction much)?

If the loft is closed and crossing over the mirror plane and you trim it at the mirror plane, then mirror it,.. your chances of having the trimmed/mirrored surfaces being tangent to eachother are possible if the loft is simple but usually not likely.

Since there is no "make tangent" function to force the adjacent boundary be C2 or better, you're sol. So, if you prefer to trim you're all in one go closed loft and mirror it, you'll have to workaround the inaccuracy of the seam by either trimming back the seam and blending it using a fillet or loft using C2.

Otherwise, depending on the geometry, doing a sweep should give you something which could do it in one go but that also is dependent on your spline section and gc's being setup properly.

Also, turn on your curvature combs to check that the open or close spline comb is symmetric or if the spline sections are normal to the mirror plane, that they have minimal disturbance and normalcy at the mirror plane (reset the spline).

Good luck.

..

Lee, It is just not easy....

What I normally do is to define a guide curve on the mirror plane. Then the other profiles defining the shape pierce this. Getting the curvature comb looking "correct" for symmetry is critical and fussy. I select the control arrow on the end point that intersects the mirror plane, and constrain that to horizontal (or vertical) which helps control the spline. It is harder to describe how the curvature comb should look, but in principle you should image what the mirrored comb would look like and ensure that that intersection has a continuous character. And no, this is not ideal - there should be the ability to define implied curvature as in Alias.

The next critical step is to ensure that when you define the loft, that you select the guide curve on the mirror plane and define the condition to be "normal to curve". If you do not do that, you have little chance of having a decent mirror line.

I have also been asking for implied curvature / symmetry planes as that would simplify the whole process for these types of products. Certainly send it in as a feature request!

Daniel

There are several things you can do to help your symmetrical design. I haven't found a single formula which tells you exactly what to do in all situations.

1) the first thing you shouldn't do is avoid splines. Lines and arcs give you K-cars. Splines give you Shelbys. Granted splines make it easier to make bad geometry, so you just have to know what you're doing. Unfortunately there is no replacement for competence.

2) Sometimes I model all the faces which do not cross the symmetry plane on one side of the part, mirror those faces, and then create the faces that go across the symmetry plane (using loft or fill). This is probably the best way to ensure good geometry across the symmetry plane.

3) Remember that lofts and sweeps can go in two different directions. If you can't get where you want to go in the first try, turn the problem 90 degrees and see if you can use GCs as Profiles and vice versa.

4) Tangency often is not enough. When you need curvature continuity across the symmetry plane, then you either have to loft across the plane or put a loft profile on the plane and use the curvature end condition. Still, the curvature end condition requires you have a face on the other side of the symm plane with the correct curvature, so you're kind of stuck. This is why I usually use 2) above. The Fill surface, Sweep and Loft GC will not allow you to specify a c2 boundary.

5) It's usually a problem that you can't mirror curves, so what I do is use the curve to create some type of surface which can be mirrored, and then use the edge of the mirrored surface as if it were a mirrored curve.

matt

"Lee Bazalgette - Factory" wrote in news: snipped-for-privacy@nnrp-t71-03.news.uk.clara.net:

yeah, most of that (and the other suggestions) I've been trying.

Wouldn't mirror>curve be useful???? I end up doubling on loads of stuff - reacreating lines etc. I like your suggestion about creating a surface and mirroring that - but it's still lots of stuff for a simple thing!

Also, when you specify symetrical condition for a spline point it doesn't maintain the symetry for the handles, and that would be useful too! (or at least dont think it does, but I'll check!)

Thanks everyone!

Someone brought up curvature continuity across symmetry plane. One thing to remember is that is a surface edge is coincident to the symmetry plane and the surface is normal (perpendicular) to the symmetry plane along that edge, curvature continuity is implicit when that surface is mirrored.

I know it's not elegant to have mirrored surfaces with a line down the symmetry plane, but sometimes it's the only way.

"Lee Bazalgette - Factory" wrote in news: snipped-for-privacy@nnrp-t71-03.news.uk.clara.net:

Yes, Mirror>Curve would be very useful, as would move, copy and rotate curve, but since it doesn't exist, it's not too helpful for you at present.

Another way I mirror something like a projected curve is to make a derived sketch of one of the sketches, and flip that using Modify Sketch, then create another projected curve.

A mirrored 2D spline in SW05 should mirror the end point handles tangency direction and weight. If you're using SW04 then it won't.

Splines in SW05 have improved a lot, but there is certainly still some room for improvement.

matt