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Pattern

Piotr_PosmykPiotr_Posmyk Member Posts: 2
I have a question about the Linear pattern feature.
I found an interesting video on Youtube:
Could someone explain it:
Why the formed array is not symmetrical about the X-Y plane? In the sketch of Revolve feature there is symmetry with respect to X-axis, why this symmetry is not maintained in the sketches of Pattern's instances? The symmetry of the array with respect to X-Z is maintained.

Best Answers

  • mahirmahir Member, Developers Posts: 1,213 ✭✭✭✭✭
    edited August 1 Answer ✓
    I noticed that your sketch isn't constrained to be symmetric. However, fixing that doesn't fix the lack of XY symmetry. It seems that the patterned sketches don't respect the symmetric constraint placed on the upper and lower construction lines. However, if you remove the symmetry and instead replace it with a horizontal line connected to the arc midpoint, the pattern surfaces look as expected. It shouldn't matter, but it does ¯\_(ツ)_/¯ 

    https://cad.onshape.com/documents/222ca838b9324929223d7a54/w/c8c53e48d363f4a0cd1ac67a/e/d9765d036a451bf112e8cb38

  • NeilCookeNeilCooke Moderator, Onshape Employees Posts: 4,315
    Answer ✓
    Patterned sketches do not regenerate external refs - this is a limitation from my first “What’s New” post in Jan 2016. Ouch 
    Director, Technical Services, EMEAI

Answers

  • mahirmahir Member, Developers Posts: 1,213 ✭✭✭✭✭
    edited August 1 Answer ✓
    I noticed that your sketch isn't constrained to be symmetric. However, fixing that doesn't fix the lack of XY symmetry. It seems that the patterned sketches don't respect the symmetric constraint placed on the upper and lower construction lines. However, if you remove the symmetry and instead replace it with a horizontal line connected to the arc midpoint, the pattern surfaces look as expected. It shouldn't matter, but it does ¯\_(ツ)_/¯ 

    https://cad.onshape.com/documents/222ca838b9324929223d7a54/w/c8c53e48d363f4a0cd1ac67a/e/d9765d036a451bf112e8cb38

  • MichaelPascoeMichaelPascoe Member Posts: 876 PRO
    edited August 1
    @mahir Perhaps it is copying the sketch and breaking the external references. Then leaving the sketch where it was last positioned.


  • mahirmahir Member, Developers Posts: 1,213 ✭✭✭✭✭
    @MichaelPascoe That seems feasible if not ill advised on OS' part. Generally, it's considered a good thing to design sketches using reference features (planes, origin). Not sure why OS would purposely break those refs. At least knowing this allows one to create a seemingly well behaved result by keeping the sketch constraints self-contained.
  • MichaelPascoeMichaelPascoe Member Posts: 876 PRO
    edited August 1
    @mahir
    I used to only mirror with planes while in sketches. Now, I try to always use construction lines. Self contained sketches with the exception of a few constrained construction lines, are easier to fix when the references are broken. It also makes it easier to copy and paste sketches to a new document.
  • NeilCookeNeilCooke Moderator, Onshape Employees Posts: 4,315
    Answer ✓
    Patterned sketches do not regenerate external refs - this is a limitation from my first “What’s New” post in Jan 2016. Ouch 
    Director, Technical Services, EMEAI
  • ilya_baranilya_baran Onshape Employees, Developers, HDM Posts: 1,120
    I would call it a compromise, rather than a limitation.  If we were to keep external constraints when patterning sketches, a linear pattern of a sketch that has, say, an external constraint to a point (that's not being patterned) would not be able to create identical duplicates of the sketch (unless the pattern direction was normal to the sketch plane).  And since a large portion of sketches have some external constraints, and a large portion of the time, when patterning you just want identical duplicates, we felt it was better to drop those constraints than to try to cater to the more esoteric patterning situations.
    Ilya Baran \ VP, Architecture and FeatureScript \ Onshape Inc
  • S1monS1mon Member Posts: 884 PRO
    @ilya_baran
    This compromise makes a certain amount of sense for many situations, but it would be great if this was an option in patterning. I've had to do use much more complex sets of features which didn't have sketches in order to work around this. Much like we have the "apply per instance" option, it would be nice to have some way to make the sketches more independent.
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