Sweep along spline leads to local 180°-twist

Martin Vogel

Whenever I try to sweep a profile along a spline in BricsCAD 15 or 16 running on Windows or Linux there occurs a geometric error exactly in the middle of the spline: the profile flips around by 180 degrees. When I open the same drawing files in AutoCAD the sweeping works as expected.

Torsten Moses

Seems best to send us a support request (bug report) here ... looks like a bug :-)
Please also attach necessary source + path entity, with a short description of your procedure, so that we can reproduce ...

many thanks & many greetings !

Jason Bourhill

A potential workaround could be to use EXTRUDE instead. This doesn't seem to suffer from issue seen with sweep.

The attached LISP uses extrude to mimic sweep. Essentially it aligns the profile(s) to be swept for you, then extrudes along the selected path(s).

Regards,

Jason Bourhill

CAD Concepts

MySweep-2016-04-28.zip

Rakesh Rao

Hello Jason & others,

I have encountered problems with SWEEP myself and have already raised SR 68380 with Bricsys. It appears that too many vertices in the swept path polyline can cause problems with sweep even if the sweep profile does not self-intersect.

I have not tried the EXTRUDE alternative that you suggested.

On my tests, AutoCAD seems to handle this better

Ideally, I would love to see an accommodating version of SWEEP that may fuse a few faces here and there and create a smooth visual SWEEP even if there are too many vertices or potential self-intersecting faces. I am not sure if Bricsys wants to do something like this but for conceptual modeling where accuracy is not very important, this is a useful feature.

Best Regards
Rakesh Rao
Smarter .dwg CAD

Jason Bourhill

Attach a test drawing with 2 pairs of swept profiles.

1st pair (blue) is done in BricsCAD. Using the SWEPT command you get unexpected twisting. Using the LISP routine I attached earlier that uses EXTRUDE it works as expected.

2nd pair is done in AutoCAD using the same method. Here I get the same result for both, and a match with EXTRUDE option used in BricsCAD. Interestingly in AutoCAD I had to rotate the profile 180 deg to get it to SWEEP in the manner I wanted, did it upside down otherwise.

Regards,

Jason Bourhill

CAD Concepts

Sweep.dwg

Sweep-2016-04-28_17-59-20.png

Richard Webb

 Try this sequence. It seems counter-intuitive but it does seem to straighten out the swept path:

: SWEEP

Entities in set: 1

Select sweep path or Alignment/Base point/Twist: twist

Enter twist angle or allow banking for a non-planar sweep path : bank

Select sweep path or Alignment/Base point/Twist: 

Answering "bank" to the twist angle prompt seems to be the magic. That also seems to be "sticky" in that, at least within the current BricsCAD session, it continues to produce smooth swept objects.

Martin Vogel

It seems that the error does not occur when the spline is planar, eg in the x-y-plane.

It does not occur, too, after the spline has been converted into a polyline (via pedit).

The "bank magic" does not work in the German version, there seems to be another strangeness.

First, let's look at the twist:

[code]_spline

250,200,10

50,0,10

200,250,30

0,0,30

_rectang

10,10

60,20

_sweep

10,10

250,200,10

[/code]

Richard Webb found the way to avoid the flipping:

[code]

_spline

250,200,10

50,0,10

200,250,30

0,0,30

_rectang

10,10

60,20

_sweep

10,10

_twist

_bank

250,200,10

[/code]

The German version works in a similar way:

[code]

Spline

250,200,10

50,0,10

200,250,30

0,0,30

Rechteck

10,10

60,20

Sweep

10,10

dreh

neig

250,200,10

[/code]

Alas, the required token "neig" is neither documented nor used in the GUI-menu. The menu delivers only the letter "N" which does not work. Neither works the usage of the complete word "Neigung" …

Martin Vogel

Obviously I misinterpreted the button [insert code] of the forum software.

You can copy the code lines from this text file:

Sweep and Twist.txt