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Accuracy of a sketch setting? / Solids "blending" on their own!

Question asked by Devin Weston on Sep 18, 2017
Latest reply on Sep 20, 2017 by Paul Salvador

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Ok, so my coworker was trying to make an extrude off of another planar surface by offsetting the whole sketch (a rectrangle with fillet corners) by 0.0001". The preview shows what you would expect (a shelf of .0001"), but when the model rebuils, SW decided to just merge it into one! So, really there is a tiny taper, like a loft. Very weird! Is there a setting for this "tolerance" of the model itself? I tried cranking the line/model quality all the way up (the tessalation, as it were), but it has no bearing on this. It's like the program is a making a decision to "blend" the stepped surface into a ramp.

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Devin Weston Sep 18, 2017 4:35 PM
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OK, so check this out. It's definitely the program making some strange decisions when the "merge result" is selected for the 2nd extrude. I made the feature 2 ways: 1 with "merge result" on, and one with "merge result" off, and doing a Combine to make 1 body....and I get 2 results! The first way gives a funky, blended result, and the 2nd way draws the geometry as it should be:

With "Merge Result" turned on:
With "Merge Result" turned off, then use Body Combine:
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Dan Pihlaja Sep 18, 2017 5:31 PM
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Way back when I was using SDRC I-DEAS, when we wanted to change something by a very small number, we had to "trick" the software in it.

For example....if we wanted to modify a diameter by .001", then if we just did the .001" directly, none of the prints NOR would the model actually update.

The way we would trick it was to first modify it by .5", then bring it back .499".

What happens when you do it this way?
1) first create the offset of, say, .25"
2) Create the feature using the offset of .25"
3) Then go back and modify the offset to bring it back down to .0001"?
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- Devin Weston @ Dan Pihlaja on Sep 19, 2017 4:01 PM
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  Hi Dan... Nope. Did not work!
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Solid Air Sep 18, 2017 9:19 PM
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Can you post your part?
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- Devin Weston @ Solid Air on Sep 19, 2017 3:59 PM
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  I didn't save, but I'll try to re-do it and upload when I get a chance.
  
  It's literally 2 features though. Just try any extrude using a .0001" distance on an Offset Entities and I'm sure you'll get the same result! (Unless there's some setting hiding I don't know about...)
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Devin Weston Sep 19, 2017 4:03 PM
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Here's the file if anyone wants to open it

Dropbox - Stepfail.SLDPRT
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- Solid Air @ Devin Weston on Sep 19, 2017 9:04 PM
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  Looks to me to be an optical contortion. If I analyze the top plane to the top surface, the result is parallel
  If I analyze the two top surfaces, the result is parallel and 0.0001 normal distance (so internally SolidWorks has it right).
  But graphically, that is another story.
  If I cut away the first feature and analyze the corner radius on both sides, the arc lengths are the same but the cut side is not recognized as a radius.
  If I cut away the second feature and analyze the corner radius on both side, the arc lengths are not the same (0.000118 off).
  What does this tell us? Nothing really.
  However if I create a drawing view, the step is shown correctly (minus the vertical "split" line).
  So it looks like its a graphics issue with SolidWorks having a hard time representing the stepped surfaces in the model.
  I personally would not worry about it since it analyzes correctly and the drawing is correct too.
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  - Devin Weston @ Solid Air on Sep 20, 2017 12:33 PM
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    Thanks, Solid Air. That was a good idea, making a drawing. I tried the same and you're right...the 2 are indeed parallel! And a dimension added shows .0001". Very strange.
    
    I wonder why it displays it fine when you do the separate cut + merge, but when the "merge results" auto command is executed in the extrude, it looks wrong.
    
    As another side note, if I do this operation as a cut instead of an extrude, with the same .0001" offset entities, it looks fine as well. Weird.
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Steven Mills Sep 20, 2017 1:53 PM
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Just to point out the obvious here - but don't select 'merge result' when making the new feature. You'll get a new solid body in your model. Unless a later feature extrudes across both bodies of course.

As for the visual things - check your settings in the performance section. Adjusting the level of detail slider to more detail should reduce oddities like that.
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- Devin Weston @ Steven Mills on Sep 20, 2017 4:13 PM
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  Right, that's what I proved in my first post. But, the "merge result" option in the 2nd extrude SHOULD yield the exact same result as unchecking "merge result" and then doing a Boolean operation to combine the 2 solids. But...it doesn't. Why? That is the question.
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  - Paul Salvador @ Devin Weston on Sep 20, 2017 4:56 PM
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    It's a good question!
    I don't know the answer but would guess the solver for the merge (auto stitch boundary within .00039in?) is running a different code (loose tolerance) than combine (and it autostitches within 0.00000039in)?
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Accuracy of a sketch setting? / Solids "blending" on their own!

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