Full Version: can't subtract extrusion solid from another solid

From: pierrehansen [#1]
 11 Oct
To: ALL

Hello,
I'm a beginner and I need help. File is attached.

I do this:

1) I draw a rectangle of whatever shape.
2) I extrude that rectangle to get a solid (Question: Do I have to first make a planar (Construct->Planar) out of my rectangle to get a "real" solid?) Is there a difference if I use that function?
3) I draw another rectangle that is smaller in size that sits on the same z height.
4) I extrude "Rectangle 2 solid" into "Rectangle 1 solid" but with a smaller depth (z).
5) "Rectangle 2 solid" is now completey wrapped into "Rectangle 1 solid".
6) Now I want to subtract "Rectangle 2 solid" from "Rectangle 1 solid" with Boolean -> Diff

Problem: (whatever I do, tried over an hour to understand the behaviour)

I can't select only "Rectangle 1 solid". Always both rectangles are selected.
I absolutley can't select only "Rectangle 2 solid". Wherever I hover.

An help would be (very) much appreciated!

Thank you very much!

Best wishes,
Pierre

Attachments:
test-rectangle.3dm

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From: Michael Gibson [#2]
 11 Oct
To: ALL

Hi Pierre, it looks like your problem is your extrusions are not actually solids, they are open at the top and bottom. The main way this would happen is if you unchecked the "Cap ends" option in the Extrude command:



By default "Cap ends" should be checked - maybe you were experimenting with the options and turned it off?


> (Question: Do I have to first make a planar (Construct->Planar) out of my rectangle to get a "real" solid?)

Nope, it's not necessary to use Construct > Planar first before doing the extrusion, you can generate a solid directly from the rectangle as long as "Cap ends" is turned on.

So I'd recommend deleting everything there except your 2 rectangles, doing the extrusions again making sure "Cap ends" is checked, and then do the boolean. One thing to note is that it can indeed be a little tricky to select the inner solid when doing the boolean because it's blocked from clicks by the larger object. One way to do it is to use the Edit > Hide button while you are in the boolean command and it's asking you to select objects to subtract. That will then hide the outer object so it's out of the way and you can see the inner one to click on it more easily. Another way you can select objects contained within others is to use window select - that's where you click the mouse down in empty space and drag out a selection window. To do that you'd probably want to hide or delete your generator curves so they won't be captured in the window selection either, if you end up selecting one of those curves as the cutting object it will cut a hole all the way through.

Hope that makes sense, please let me know if you're still stuck.

- Michael

Image Attachments:
extrude_cap_ends.jpg 

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From: pierrehansen [#3]
 12 Oct
To: ALL

Hello Michael,
thanks for your reply! That's much appreciated.

The Hide Button helps a lot.

Quote: "Another way you can select objects contained within others is to use window select - that's where you click the mouse down in empty space and drag out a selection window."

But this way I can never only select the part that is inside the other. If I start dragging in empty space it (of course) also selects the outer part.

Thank you,

Pierre

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From: Michael Gibson [#4]
 12 Oct
To: ALL

Hi Pierre,

re:
> If I start dragging in empty space it (of course) also selects the outer part.

But that won't be the case when you're doing a boolean difference because the outer part will have been already selected previously and won't be targeted for selection again at this point.

There's a mechanism in MoI called "selection lock" which is used to screen out objects that were previously selected to avoid this type of problem.

- Michael

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From: pierrehansen [#5]
 12 Oct
To: ALL

Ahhh!
Enlightening!
Thanks!

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From: Karsten (KMRQUS) [#6]
 12 Oct
To: ALL

Hello Pierre,

and there is also a difference in the direction of the window selection: Dragging a window from Left to right selects everything in the box - From right to left: everything in the box and also everything is crossed by the box.

Have a nice day
Karsten

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From: Mike (MGG942) [#7]
 12 Oct
To: ALL

A technique that I use is to name the parts, then I can select from the object pane. Doesn't matter whether the part is visible on screen or not.

Mike.

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