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# Thread: Void Issue - Can't Keep Elements Joined When Flexing

1. ## Void Issue - Can't Keep Elements Joined When Flexing

Hey all,

Running into an issue with a void not cutting a revolve at very specific values and am wondering if anyone can tell me why? I've knocked up a quick example family that demonstrates the issue. I've created a rather crude tripod, and I would like to be able to control the thickness and depth of the legs. The legs are a solid revolve created in elevation, with a void extrusion drawn in plan to remove the parts I don't want. Everything appears to function correctly, except when I flex the "leg thickness" and "leg depth" parameters i get the "Can't keep elements joined" error.

I'm thinking there must be some sort of geometric/mathematical reason for this?

With the legs at a width of 25mm, the following occurs when I adjust the depth:
1-19mm it works fine, 20mm it breaks, 21mm it works fine, 22mm it breaks, 23mm+ it works fine

However, if I adjust the width of the legs to 30mm, then I get a whole new set of values that will and won't work:
0-42mm works fine, 43-56mm it breaks, 57mm+ it works fine

Obviously I can just avoid the combinations that don't seem to work through trial and error, but I would love to know what's causing the issue?

2. Originally Posted by LeChumpOfStultz
...I'm thinking there must be some sort of geometric/mathematical reason for this? ...
Obviously I can just avoid the combinations that don't seem to work through trial and error, but I would love to know what's causing the issue?
What causes the issue is the strategy of using a void, and hoping that all the cuts made by a void in all possible combinations of values is going to work. It does not. As soon as there is a combination of values that causes a void to be too close to an edge of the solid being cut (and we don't know how much exactly it is to be too close), the family will break with the "can't keep elements joined" error.

In summary, avoid voids as much as possible.

I suggest to use a simpler strategy, without using any voids. Like this: Family 1 is the leg, with parameters and trigonometry. Load Family 1 into a Family 2, and create 3 instances of Family 1 and arrange them in plan view as a tripod. Then tie parameters From family 1 to parameters of Family 2. The tripod will work all the time.

3. Originally Posted by Alfredo Medina
As soon as there is a combination of values that causes a void to be too close to an edge of the solid being cut (and we don't know how much exactly it is to be too close), the family will break with the "can't keep elements joined" error.
Just seems odd that it can seemingly go from functioning to breaking, back to functioning with no real obvious correlation between the dimensions and likelihood of breaking. When unjoining the elements to try and identify any close edges that might be causing issues there really don't appear to be any. In any case, I'll just chalk it up to voids being temperamental and avoid them where possible.

Thanks for the reply, I'll go the nesting route.

4. Originally Posted by LeChumpOfStultz
...Thanks for the reply, I'll go the nesting route.
Yes, that is a better approach. The other approach of subtracting the 3-part-cylinder from the pyramid was interesting, but it does not work well. I wish there was a more precise explanation, but as you have seen, it is difficult to detect a pattern that causes the errors.

5. Solution with nested families instead of voids:

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