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Turbo Talk => Tips and Tricks => Topic started by: jmwright1058 on November 08, 2017, 11:13:35 AM

Title: Using Rail Sweep
Post by: jmwright1058 on November 08, 2017, 11:13:35 AM
I'm using the Rail Sweep tool with some success, however, I'm not getting repeatable results.
I'm having trouble understanding the constraints.

I'm getting the error "The resultant object of ACIS Solid type is selfintersecting or invalid. The correct object can't be created. ACIS message: Top and bottom profiles of sweep would intersect."
What's confusing to me is that I play around with the Reference Point of the profile, the path and the orientation of the path and eventually I get it to produce what I want.
However, this process is painfully slow. If I knew why the error is being reported, and how to avoid it, I would be much happier.
This idea of the profile intersecting itself as it follows the path makes sense to me, It just seems very arbitrary why it sometimes succeeds, even though I have not altered the profile and only slightly altered the path.
(jmwright1058 - Note - I modified the previous sentence...I previously said 'the path intersecting itself makes sense to me")

My profile is a moderately simple polyline. My path (the simplest one that fails) consists of a straight line connected to an arc that traverses 90 degrees (1/4th of a circle).
Title: Re: Using Rail Sweep
Post by: murray dickinson on November 08, 2017, 08:48:21 PM
It's not the path intersecting itself, it's that the orientation of the profile to the curvature of the path means that the end caps of the sweep will overlap.  Complete guesswork without your example, but that's usually why *ACIS* throws it up.
Leave a gap in the profile and see how it turns out.  Open sweeps can self-intersect.   
Title: Re: Using Rail Sweep
Post by: jmwright1058 on November 09, 2017, 07:33:10 AM
Thanks Murray,

I will try your idea if I need to.

I think now, one cause I need to eliminate first is to not violate a basic requirement of the sweep tools, which is to always start the path 'touching' (in contact with) the profile. I was getting away with this on many sweeps, so I didn't believe it was necessary, however, a video created by Paul Tracy indicates this is required.
Since I began adhering to this rule I've been much more successful.
Title: Re: Using Rail Sweep
Post by: Jeffin90620 on November 10, 2017, 04:35:35 AM
It's not always the ends of the Rail Sweep that cause the problem.  When it happens to me, it is most often because the item being swept encounters a corner in the path that results in a portion of the object being folded back upon itself.

My solution is to change the Reference Point for the swept object such that all of the object is on the "outside" of the curves.  If that is not feasible, then I split the path and sweep portions of it (to be 3D Added later).

Title: Re: Using Rail Sweep
Post by: jmwright1058 on November 10, 2017, 08:29:08 AM
Yes, Jeffin90620,

This is a good solution; that is, breaking the sweep into multiple paths, doing 2 sweeps, then connecting the 3D solids. This enables you to change the reference point between sweeps, since the reference point has direct bearing on whether the profile folds back on itself.

A good example is a path (in a single plane) that is first straight for a distance, then turns 90 degrees to the right (then straight for a distance), then turns 90 degrees to the left (then straight for a distance). Depending on where the start of path is placed on the profile, either the first 90 degree turn will cause the profile to fold onto itself, or the 2nd 90 degree turn will fold on itself.

My results were really wacko when I didn't connect the path to the profile. There was no rhyme or reason why it failed one time and succeeded the next.