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Chapter 27 in the continuing saga -- "Will not 3D Subtract"
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* June 03, 2009, 09:19:01 AM
My current problem has to do with 3D subtraction failing. The attached HereIsAnotherOne.tcw is the current problem child. I've already tried all of the old tricks, i.e., cutting and pasting the offending parts to new file, etc., without success. I even disassembled the polyline to it's individual components and tried three separate extrusions, also without success. I examined all of the options or settings or properties of both the sweep extrusion and door panel and saw nothing amiss.

Every attempt to 3D subtract the sweep extrusion from the door panel resulted in the following error messages:

  • Operation unsuccessful
  • The resultant ACIS Solid is degenerated. The boolean operation is discarded

I would appreciate it very much if someone can tell me:

  • Why the subtraction keeps failing, and
  • What I can do to make it work.

After much experimentation with sweeping extrusions, I believe I have that problem solved. For those who followed the Works in one file but not the other thread, the problem seems to has something to do with the algorithm that TurboCAD uses to define the sweep path. Apparently TurboCAD is confused by contiguous and/or adjacent lines that belong to the same layer. The attached file Extrusion-SECRETS.TCW was my experimentation file.


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* June 03, 2009, 10:21:19 AM
#1
The problem is the upper part of the door panel -- the yellow part in the attached screenshot. Slice it away and you'll be able to subtract the sweep extrusion from what's left of the door panel (the cyan part). I realize this leaves unanswered the question of how to trim the yellow part.

Henry H

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* June 03, 2009, 11:10:53 AM
#2
The problem is the upper part of the door panel -- the yellow part in the attached screenshot. Slice it away and you'll be able to subtract the sweep extrusion from what's left of the door panel (the cyan part). I realize this leaves unanswered the question of how to trim the yellow part.

Well aint that a frustration fulfilled.

The yellow part in your screen shot was formed by a 3D subtraction at the same time as the rest of the top of the door panel. The door panel started as a simple 3D box 13 1/2" x 17 7/8" x 3/4" in size. A sweep extrusion from the identical shaped profile was used to modify the top edge of the panel and the panel in the drawing is the result. I don't have a clue why that should either cause the top edge to show up as a different color or cause a problem.
« Last Edit: June 03, 2009, 02:02:13 PM by photowriters »

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* June 03, 2009, 12:04:10 PM
#3
Well . . . . .

It's been several years, but I recall a related error in an earlier version of TurboCAD where the error message was something about a divide by zero error. I think it was either in version 8 or 10.5. If you will fiddle with the attached Solution4AnotherOne.tcw, that you will find that each of the sweep extrusions will subtract normally.

All I did was to displace the polyline by 0.001" in the plus Y direction. I suspect that there is a logic or algorithmic error in the 3D addition and subtraction routines in the ACIS engine that has existed ever since the first time 3D addition and subtraction capabilities were added to TurboCAD. Apparently the programming/design team at IMSI solved the earlier divide by zero error by just changing the code so that the program would not blow up rather than by changing the algorithmic logic.

Whether I am correct or not that IMSI opted for the a quick fix rather than correcting the faulty reasoning in the original algorithm, it is a serious BUG in a program that positions itself in the market place as a competitor to AutoCAD.



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* June 03, 2009, 07:35:11 PM
#4
I'd be more inclined to blame how easily irrationality creeps in, especially with blends (up pops pi) or adding or subtracting solids that might have been created with something as innocuous as a /3 snap point.  It's awfully difficult to imagine EVERY circumstance that'll need to reconcile irrationality with floating point calculation, and to write error collection and correction routines that do so.  I use another CAD program that also uses ACIS, and it'll do things like refuse to scale an object at certain ratios because it's associative with a construction feature and the scale ratio has an irrational result.  I'm only aware of it because an error message pops up, but the error is mine, not the program's, although I could blame it for not thinking ahead of me, because I have to deal with it some way.  I'm always reminded of Bill Gates announcing the first Windows and telling the world how intuitive GUIs are.  I was surprised to find out how much my intuition wasn't like Bill's.....

Care to save as V15 and post?  More heads thinking about it couldn't hurt..... 
« Last Edit: June 03, 2009, 08:07:00 PM by murray dickinson »

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