Trying tio get lofts to work- help needed

Hello,

I have been trying to get a solid loft to work that uses four section
sketches and about a dozen guide curves, but have been finding it
problematic. All the section sketches are closed and all the guide
curves intersect the section sketches.

The feature I am trying to loft is symmetrical so there are several
mirrored guide curve pairs.

I am finding that getting a loft to work in SW cn be quirky and is a
bit of a hit n' miss kind of process that can lead you up the garden
path.

eg. You select all your four section sketches and arrange them in the
right order. Sometimes SW shows a succesful loft preview without even
selecting any guide curves. So you now select your guide curves and as
you do, SW can jump from showing you a successful loft preview to not
showing any and giving you the "Loft operation failed to complete"
error. This can occur as you select/deselct each guide curve.

It seems like only some combinations of guide curves result in a
successful loft. Strangely, I have found that there are cases where if
I select both guide curves of a mirrored set, SW will fail to complete
the loft, whereas if I just select one, it will successfully complete
the loft, albiet not in a way I want it to.

To try and avoid this, I have even tried lofting just one half of the
feature, using one half of the sections skecthes and just one of the
guide curves from each mirrored pair. Even here I run into problems
with the same error "Loft operation failed to complete".

These light blue connector dots/strings that appear are also a bit of a
mystery. I don't understand how SW decides where to place them (by
default they always seemt to be very wrong) or how many it decides to
use, or even which blue conenctoir dot string it decides to show by
default (right clicking on teh dot can "Show All"). Sometimes these
conenctor dots can be dragged freely along a line segement but other
times they can only be shifted from discrete positions at each vertice
in a profile sketch, which is frustrating in itself, especially when
adjacent section sketches don't share the same number of vertices. I
don't understand whats runnign the show there.

What advice can you give on getting these lofts to work the way you
want them to?

Are there a set of firesafe rules for creating profiles and guide
curves to ensure a successful loft? eg. symetrical parts should always
have a vertice on the sketch coincident with the plane symmetry and a
guide curve running along it, or if you use enough guide curves, the
loft will eventually work etc

Regards

Bullman

 

Search for Curvy Stuff 101 on this newsgroup.

 

Unfortunately that tutorial is out of date. It was written before loft
connectors, and SWx re-wrote guide curves to overcome issues about
guides that were mentioned in that tutorial. I think that at SWx World
2007 we might skip on the advanced stuff and instead do a 'loft boot
camp' to bring all of that stuff back to current.

To the question at hand -
The first step, before adding any guides, it to get your sections
connecting cleanly. Try to pick them in order FROM THE SCREEN, picking
as close as you can to vertexes on each profile that you want to
connect (for instance, lower left corner to lower left corner to lower
left corner to...)
Then RMB and 'show all connectors' this will show how the loft is
connecting between your profiles. Hoepfully, all the connectors are
roughly 'parallel' to each other... if not, there is a lot of stuff
that can be happening (segment count in each profile should be similar,
for instance, but you can edit the connectors to correct this without
ahving to redo your profiles) , but it is hard to go into it without
seeing the sample.

Finally, add guides only as needed - I advocate trying to set up lofts
so they naturally creat the shape you want without resorting to guides
(this is covered in Curvy stuff 101, and is still valid). I mostly use
guides where the loft needs to be in contact with an existing model
edge, but others have success using their guides for much more.

It is so hard to help with what little info we have - I hope this helps
a little.

 

First, listen to what Ed and Paul said. Second, if you're going to be doing
this type of work much, go through all of Ed's tutorials at
http://www.dimontegroup.com/. My own small contribution, probably learned
from Ed and Paul, is to suggest that less guide curves is usually better; a
dozen sounds like way too many. Sometimes zero is the right number. Perhaps
you need to loft the other direction, with a dozen profiles and four guide
curves. It's also usually a good idea, if at all possible, to have the same
number of segments in all of your profiles. Spacing the added split points
on the profiles that need them can be very important to getting the right
appearance.

Symmetry is often a problem. Usually you want to make the complete part, as
you are trying to do, as making half and mirroring can leave a less than
ideal joint at the center plane. But sometimes you're better off to make
"skirt" reference surface off of the center line and loft half of the part
with tangency to the skirt, then mirror the part.

Jerry Steiger
Tripod Data Systems
"take the garbage out, dear"