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Old 4th January 2018, 06:24 PM   #1
InternalCakeEngine
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Default Subtraction problem; leftover triangles.

I'm trying to build some objects to 3D print. The shape I want is a hollow tube with a protruding lip, with longitudinal sections removed at 90 degree positions so that it has enough flex to clip into a hole.

So, I create either a rectangle or a polygon, offset it from the origin and revolve it through 360 degrees to make my hollow tube. I then create a primitive cuboid wider than the tube and position it such that it intersects with the tube on opposite sides. I then cut and paste this cuboid and rotate the copy though 90 degrees. I can then subtract the the cuboids from the tube to leave the shape I want.

It looks like it almost works but I get leftover triangles at odd positions on the surface of the removed volume. I've attached a screenshot.

Am I doing something wrong? I've made sure all my surfaces are single sided and that the normals are pointing the right way. I have a license for version 7 but I'm using a trial of version 8 at the moment.

Looking through the forums I see that there are quite a few similar issues with subtraction and the advice is generally to construct the object differently. I'm happy to do this but can't work out how to achieve the result I'm looking for by other means. Suggestions appreciated.

Thanks.

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Old 5th January 2018, 05:38 AM   #2
Andy
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Default Re: Subtraction problem; leftover triangles.

It's best to keep the number of boolean operation to the minimum so merge the cuboids together and just do a single subtract (you may be doing this already).

You could try scaling the whole model up e.g. press "200%" a couple of times before doing the subtraction, then scaling down after. That may increase the precision of the cuts.

If you can't get it working, post the model here and I'll have a look.
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Old 15th January 2018, 05:07 PM   #3
InternalCakeEngine
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Default Re: Subtraction problem; leftover triangles.

Sorry for the delay - I've been designing (with AC3D) and printing some other bits.

I've tried reproducing the problem but have so far failed. It's worked a couple of times, left holes a couple more (which the fill-holes menu option can't seem to fix). I tried scaling the model up 2000% times before doing the subtract and now one of my CPUs is locked at 100%, the fan is running at full tilt and the screen won't redraw :-(

As an observation, I do need to merge the four cuboids before doing the subtract because if I try one at a time, the first subtract leaves the tube in a state where it has open edges which can't be filled. I guess the lesson is to use subtracts as infrequently as possible and to check the results carefully before carrying on working.
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Old 15th January 2018, 06:49 PM   #4
InternalCakeEngine
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Default Re: Subtraction problem; leftover triangles.

So eventually it said in a popup:

"Exception occurred in command boolean
Last Error (not necessarily the problem): (null)
Please report this to Invis."
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Old 16th January 2018, 12:42 PM   #5
Andy
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Default Re: Subtraction problem; leftover triangles.

Unfortunately the boolean code is extremely complicated and can fail for various reasons.

If you have the model file that caused this, I can have a look.
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