Boolean cut fails after 3D-Offset padding

[PepperSoris]

Hello,
does anyone know, why the boolean cut in the attached example fails?
I want to make a cage for a simple M4 nut. I have the model for the M4 nut and want to subtract it from a simple solid block, but with a little clearance margin.
So I used the Part->3D-Offset to make a little 0,5 mm offset (with the option for straigt intersecting faces).
The padded nut looks fine, but when I do the boolean cut (select first the Block1 and then the Offset-Nut), the cut fails. (Without an explicit error message.)

What is really strange: if I delete the 3D-Offset, and do the boolean cut with just the plain M4 nut, the cut succedes!
How can this be fixed to work with the offset, because I need the little wiggle room to fit the two parts together afterwards. I know, I could simply draw a scetch of the hex nut (or use a projection), but the workflow with the padding and boolean cut just seems the most elegant way.

Thank you in advance for any hints!

freecad bug - boolean cut.png (15.66 KiB) Viewed 447 times

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OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.17.13541 (Git)
Build type: Release
Branch: releases/FreeCAD-0-17
Hash: 9948ee4f1570df9216862a79705afb367b2c6ffb
Python version: 2.7.14
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 7.2.0
Locale: German/Germany (de_DE)

[chrisb]

Therror vanishes if you recompute the whole document. However, here remain some Z-fighting like artefacts in the display.

[PepperSoris]

Dear chrisb,
If I right-click the "Cut" item and then "Mark to recompute" nothing changes. There error icon is still there. Same behaviour when I select the document root and right-click "mark to recompute". How did you "recompute the whole document" ? Is this a menu item somewhere?

If I hide all the other items except the cut, I have an empty screen (because the "cut" operation failed).

I just found out, that if instead of the Part->Boolen->Cut operation, I use the Part->Boolean->Boolean... operation it fill fail for all methods with the error message "Performing intersection on non-solids is not possible".

So it seems the 3D-Offset generated a non-solid object. But why? It looks like a pretty decent solid to me.

Freecad - boolen cut 2.png (33.51 KiB) Viewed 436 times

[Chris_G]

So it seems the 3D-Offset generated a non-solid object. But why? It looks like a pretty decent solid to me.

Your M4 Nut object is a solid encapsulated inside a compound.
I think that's why the 3D offset didn't generate a solid.

[PepperSoris]

Your M4 Nut object is a solid encapsulated inside a compound.

How do you know that? I can't see it anywhere in the object tree. To me, it just looks like a simple solid (Icon: blue box).

I converted the shell that is generated by Part->"3D-Offset" to a solid by Part->"Convert to solid".

Now the boolean cut works! Problem solved.

Lessons learned: the cut Operation should throw an error like "Object not a solid. Please make it a solid first..."
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freecad boolen cut solved.png (15.3 KiB) Viewed 430 times

[Chris_G]

How do you know that? I can't see it anywhere in the object tree. To me, it just looks like a simple solid (Icon: blue box).

By doing a Part -> Check Geometry on the object :

check_geom.jpg (28.86 KiB) Viewed 423 times

[freecad-heini-1]

An easy hack, use the sketcher in PartDesign and draw a regular hexagon polygone , circle in the center.
Then create a pocket.

[chrisb]

Good to see you got it sorted.

For the sake of completeness: Recompute is done in two steps:
1) Select the item or the whole document for recompute, that's what you had done.
2) Click the recompute icon with the two blue circular arrows or hit Ctrl+R (Macos: Cmd+R)

[PepperSoris]

Should I file a bug reports for this?

1. Part->Boolean->Cut should show error message if a non-solid (Shell, etc.) is selected. (at least in the console window)
   Bug report: issue #3694
2. Part->3D-Offset should warn if resulting object is not a solid (or add option to try to make it one)
   Bug Report: issue #3703