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Variables and sketch patterns! There's nuance to it.
Evan_Reese
Member Posts: 2,066 PRO
The recent update allows count variables to drive sketch patterns, which is really wonderful! Thanks, dev team!
This may seem like an obvious functionality, but there were actually some great reasons it hadn't been implemented from the start, and now that it has, there are some non-obvious implications that some of you may want to understand. Here's a quick explanation as I understand it.
Previously it hadn't been implemented because of some edge cases that would make models non-determenistic. That means that you might have two models with identical user inputs that could have different results, which would be a huge no-no for a tool like Onshape. This was a thing because of how flexible sketch patterns were designed to be. Sketch pattern instances can even be deleted, trimmed, or set to construction, which is great. However, if you delete your 4th instance, then take the count down to 3 and back to 4, it will come back. That could mean you'd have an extrude feature that would create 3 holes in the first case, and 4 in the second, even though everything is set up identically. That's not acceptable, so the previous solution was to not allow sketch patterns to use variables for the count. You can read more about it here: https://forum.onshape.com/discussion/comment/30219#Comment_30219
This new update had to get around that somehow, and here's how it works now:
- If you don't use a variable to drive it, it behaves exactly the same as before. modified/deleted instances will come back unmodified if you drop the count low enough to make the disappear, then raise it to bring them back. See the gif below. Notice the deleted one comes back and the dotted one becomes solid
- BUT if it is driven by a variable, the modifications stay in tact! That means an instance you delete will still be deleted when you roll the number down and back up. See the gif below.
- One implication here is that if you want to get an instance back but your pattern is driven with a variable, you need to remove it and add it back, like so.
Anyway, I was curious how they'd handle the determinism issue, so I did some testing. I hope this is helpful to someone.
This may seem like an obvious functionality, but there were actually some great reasons it hadn't been implemented from the start, and now that it has, there are some non-obvious implications that some of you may want to understand. Here's a quick explanation as I understand it.
Previously it hadn't been implemented because of some edge cases that would make models non-determenistic. That means that you might have two models with identical user inputs that could have different results, which would be a huge no-no for a tool like Onshape. This was a thing because of how flexible sketch patterns were designed to be. Sketch pattern instances can even be deleted, trimmed, or set to construction, which is great. However, if you delete your 4th instance, then take the count down to 3 and back to 4, it will come back. That could mean you'd have an extrude feature that would create 3 holes in the first case, and 4 in the second, even though everything is set up identically. That's not acceptable, so the previous solution was to not allow sketch patterns to use variables for the count. You can read more about it here: https://forum.onshape.com/discussion/comment/30219#Comment_30219
This new update had to get around that somehow, and here's how it works now:
- If you don't use a variable to drive it, it behaves exactly the same as before. modified/deleted instances will come back unmodified if you drop the count low enough to make the disappear, then raise it to bring them back. See the gif below. Notice the deleted one comes back and the dotted one becomes solid
- BUT if it is driven by a variable, the modifications stay in tact! That means an instance you delete will still be deleted when you roll the number down and back up. See the gif below.
- One implication here is that if you want to get an instance back but your pattern is driven with a variable, you need to remove it and add it back, like so.
Anyway, I was curious how they'd handle the determinism issue, so I did some testing. I hope this is helpful to someone.
Evan Reese / Principal and Industrial Designer with Ovyl
Website: ovyl.io
Website: ovyl.io
9
Comments
Nice tip!
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I had figured out part of this, but not the hack to get back deleted elements. It seems like if you do want to delete elements, toggling construction might be easier since it doesn't require going back and editing the count.