Twisting Sweeps

http://www.cadtutor.net/forum/showthread.php?t=33648&highlight=section+properties

In this thread a user posted his situation about why his sweep was not working correctly. In short, I too have run into this similar problem when trying to extrude or sweep shapes along a path. When I'm using a round pipe there is no problem, but sometimes when I use rectangles, squares, anything besides a concentric circle, the sweep ends up twisting along the path much like this guy was getting. I read the complete post and still don't seem to understand why a sweep or extrude takes of twisting in the first place and how does one force a sweep or extrude to stay in plane?

This is a very wide open question. Difficult to answer without specific geometry.

You forgot to zip and attach what you have so far with your profile and path for the Sweep.

When I had this problem I found I had to change the twist variable.

After selecting the object to sweep the command line will show:

Select object:-or Alignment/base point/scale/twist

Type t and it will show what twist angle is set to. Change this to get desired result then select path.

Hope this helps.

Here is an example of what I don't seem to be grasping in the extrude and sweep operations. This is a handrail for a spiral staircase. When I use a concentric pipe for the rail all is well. But when I use a rectangle or other shape the sweep comannd ends up giving me less than what I was looking for. The handrail ends up rotating and twisting along the path instead of remaning in the same vertical orientation.

I made the path for the rail (the magenta Polyline) by simply drawing a line of the desired height from the end of one of the treads. Then I copied this line to the same point along each tread. I drew a 3dpolyline by using the top end of each of these lines as a point to draw the polyline to. Then I inserted the handrail profile and tried to sweep. The results are shown on the attached drawing file.

As per Rob-GB's post, I checked the twist property and it was set to 0.

Any tips on how I could force the profile to stay on the correct path would be appreciated.

sweep-problem.dwg

Just had a play with it and adjusted sweep's twist to 100. This is what I believe the angle of end points are. Starting at bottom of stair. The twist is to allow for the natural curvature of the handrail as it rises: ie in order to keep top part facing toward the ceiling. Hope it helps.

sweep-problem[1a].dwg

Rob-GB, can you tell me how you arrived at the 100 figure? I don't seem to follow that one.

Hi Bill, When I measured the angle between the ends of the handrail it came up as 100. Having tried again I am not getting the same results!

So on to plan "B".

The latest dim that I got was 60 degrees but when I set twist to this it was clearly not enough. I then clicked on handrail sweep and altered the twist angle via the Properties pallette. It took some trial and error, however, 120 degrees seems to give the best result. There is an easier way, I am sure, but it escapes me at the moment. This screen shot may help with the my text above.

Bill,

Simple solution to your problem, use the 'helix' command instead of 3d polylines or splines when creating the sweep path for your handrail. I have found the helix command very useful for spiral and helix stairs.

Found it difficult to master though.

sweep-problemTJJ.dwg

After beating my head against my monitor, I tried the Twist option. Various angles didn't help, then I entered 0 for the angle. Worked just fine. Seriously, AutoCAD??!!

Tonyi is right. If you make your handrail using a HELIX and the the SWEEP, the problem is solved. The reason why it doesn't work with your 3D-Polyline is a geometric problem, as it appears to me, similar to my last posting. The point is, that basically, you are trying to connect skew lines with planar faces, which cannot work. The top of your handrail is a so-called "screw surface" (I can only translate this directly from German). My guess is that the SWEEP command compensates for your horizontal rail profil by cumulating the aberrance via the seen distortion from stair to stair.

Here's something useful to play with:

  [font=&quot]; Mehrfaches Rotieren mit Z-Abstand[/font]
 
 [font=&quot](DEFUN C:SPIRAL ()[/font]
 [font=&quot]            (SETQ OBJEKTE (SSGET))[/font]
 [font=&quot]            (SETQ MPUNKT (GETPOINT "\nDrehpunkt :"))[/font]
 [font=&quot]            (SETQ WINKEL (GETREAL "\nDrehwinkel :"))[/font]
 [font=&quot]            (SETQ ABSTAND (GETREAL "\Z-Abstand :"))[/font]
 [font=&quot]            (SETQ ANZAHL (GETINT "\nWie oft soll kopiert werden? :"))[/font]
 [font=&quot](COMMAND "_OSNAP" "")[/font]
 [font=&quot]            (SETQ X 0)[/font]
 [font=&quot]            (WHILE (< X  ANZAHL)[/font]
 [font=&quot]                        (COMMAND "_COPY" "_SI" OBJEKTE MPUNKT MPUNKT)[/font]
 [font=&quot]                        (COMMAND "_ROTATE" "_SI" OBJEKTE MPUNKT WINKEL)[/font]
 [font=&quot]                        (COMMAND "_MOVE" "_SI" OBJEKTE "0,0,0" (LIST 0.0 0.0 ABSTAND))[/font]
 [font=&quot]            (SETQ X (+ X 1))[/font]
 [font=&quot]            )[/font]
 [font=&quot]            (COMMAND "_OSNAP" "_END,_INT,_MID,_CEN,_INS,_PER")[/font]
 [font=&quot]            (COMMAND "_REDRAW")[/font]
 [font=&quot]            (SETQ OBJEKTE NIL)[/font]
 [font=&quot])[/font]
 

Multiple Rotation with Z offset - Rotation Center - Rotation Angle - Z-Distance - Number of Copies.

Here's the result.

Spiral and Helix.dwg