CAD anomaly

[Peter_T1958]

Some years or so ago I had the chance to purchase the CorelCad programme very cheaply. I was in the mistaken belief, that I would be a small step from CorelDraw to CorelCad. It wasn't so, the more that there was no handbook included in delivery.
In the past two years I had to learn it autodidactically and – much to my astonishment – I archieved first successes rather soon and so I could design most parts form my current project with ease.
Now, my projects have become more and more demanding, and recently I am confronted with a problem I can't find any solution. I thought, I will ask all the 3D professionals here in my familiar surroundings, although I do not think, anybody uses CorelCad too.

What's the problem:
Whenever I am saving a drawing with roundings or bevels on my harddisk, there are more or less distinctive anomalies when I reopen them. This is also the case, when I saved them as STL files.
Here an example (bevore and after saving).


May be someone has an idea what it might be and could explaine me whats wrong here in simple words (remember, I am a CAD novice ;-)

Thanks in advance
Peter

[EZnKY]

Look for multiple identical objects occupying the same space.  I'm not a Corel user, but none of the modeling programs like duplicate objects in the same location.

[finescalerr]

Now that Eric has written that, I have enough confidence to agree and add that it looks as though some planes in your drawing may be slightly twisted. In SketchUp I sometimes encountered similar issues and it was always because I had made a tiny alignment error somewhere. I'm sure somebody with more knowledge than I can explain it more clearly and correctly. -- Russ

[Peter_T1958]

Thank you Eric and Russ

May be it helps, when I show here my approach:

1  Construction of the base in revolving the cross section




2  Construction of the « cone » in the same manner :




3   Rounding off the corners (0.75/0.50/0.25mm)




4  I aligned the «cone» inserted it in the base and finally subtracted it.




Ok, and finally it looks crumpled like this:

[WP Rayner]

Peter, it looks to be a boolean subtraction function issue. Boolean functions are the creation of a new form through the addition, subtraction or blending of separate forms. I am not at all familiar with Corelcad but boolean functions in any CAD program can be problematic, especially when filleted edges are involved. Your "cone" form is actually quite a complex shape with all the different radii. As an experiment, try removing the radii along the edges, the filleted edges, then try your subtraction... see if that gives you a cleaner result. If so, then fillet those edges on the final form. As a general rule, whenever I create a complex part in CAD, filleting the edges is always the final step in the process, after completing all the boolean functions.

[EZnKY]

I agree completely now that you've outlined your sequence.  Subtractions work best with as simple a form as possible.  And fillet last.

[Peter_T1958]

Hi
I wouldn't have thought to receive a reaction at all! Thank you very much for your support. I followed your advice in the hope of having understood everything right.

Alas, the result is about the same ☹.  Hmm, but I am rather sure you were right! The problem lies at the different radii...

[WP Rayner]

Hmmm.... try another experiment. Use your existing subtraction form (without the fillets) and try a subtraction on the end of a simple circular column shape, basically a primitive. If that works, it will tell us that the subtraction form is ok and CorelCad can handle it. If not, it will tell us that either the subtraction form is the problem or that CorelCad is unable to process the subtraction properly for some reason or you're pushing its abilities. If it's successful, then try it on a simple revolved form to test whether CorelCad has the ability to process the function properly on a revolved component.  I notice you've added more complexity to the main component; gear teeth, bolt heads and so on. It is always best to perform complex subtractions on the simplest forms possible, so perhaps you will need to attempt this particular subtraction operation earlier in the process. The sequence of performing complex functions is important for proper results and often, you have to experiment with different sequences to achieve the desired result.

[EZnKY]

The other strategy I've used in the past is to do all of the subtractions, fillets, etc. on a segment of the overall circular form, and then do a polar array of the resultant to get the full form.  The goal is the same as everyone has suggested - trying to keep the forms as simple as possible when doing complex functions like additions, subtractions, etc.

[Peter_T1958]

I really appreciate your help – thank you very much!!! What I did last night: I created a simple revolved object. It has the maximum dimensions of the later shape. Later I would substract all the unnecessary parts.
From this form I substracted a «core» without roundings. These roundings were made after the substaction on the main object.

The result ...  the same problem.  Arrrgh!!!



Just annother idea:
Might be the problem that I have to round some edges of the «core» with different radii?

[Bernhard]

Try adding the ribs instead of the subtraction method. And do it in such a way that they overlap with the base body. For example, by extending them to the outer diameter. Then they should automatically boolean with the main body. At least that's the case with Onshape, which I work with.

Bernhard

[Peter_T1958]

Hi Bernhard
Thanks for your input. Inserting the ribs was my first attempt. And it was a succes -at least partly.

Those ribs are part of a cast core and have to blend in smoothly in every direction. There are a lot of rounded edges, some of them with small, others with rather large radii. Here the ,,rib adding methode" reached its limits. Working with smaller radii and omitting some of the edges worked more or less. But this does not fully comply with the original!

What I overlooked up to now completely is the item with overlapping the base body. I always placed the ribs on top of the base... 
I will give it a try...

[WP Rayner]

Woohoo... well done Peter! At least you're getting closer to the desired result. Bernhard had the right idea. An additive approach can sometimes work better than a subtractive one as it generally uses less application and processing resources. You've also determined that Corelcad's subtractive boolean function is somewhat limited, useful knowledge for any future projects.

[Peter_T1958]

Things become more and more mysterious. Some month ago I draw this submarine replacement sail for my brother. It was uploded and printed by Shapeways without problems.
Now I wanted to add some slight improvements and opened the file. See what happend...

Here the stl. file uploaded at Shapeways:




And here the same as I found it tomorrow on my harddisk (bulge marked with an arrow):



Might a virus launch such an anomaly?

Confused, Peter

[WP Rayner]

Curious... Have you updated Corelcad or added a plug-in of some sort to it, or made any substantive changes to your computer after completing the original drawing?