topic Re: How does AutoCAD join solids in AutoCAD Forum

How does AutoCAD join solids

rie24RYR — Fri, 13 Jan 2023 17:00:31 GMT

Hello,

I noticed strange results in calculating volume of solids. I have a model with approximately 500 solids, created from extrusion of rectangles. The solids are combined in several groups (UNION command). Some of the solids are joined beforehand. When I manually sum their separate volumes (Mass Properties), the total volume is 515.506. When I use UNION to join group 1 and group 2, I get the message “Null solid created – deleted“. When I join group 2 and group 3, their volume is 341.588. When I join group 1 to “group 2 + group 3” solid, the total volume is 499.237. If I join group 1 + group 2 + group 3 simultaneously, the total volume is 527.495.

So, which one is the right answer and why is there a difference?

Re: How does AutoCAD join solids

Washingtonn — Fri, 13 Jan 2023 17:28:52 GMT

Check the union results are identical by performing the following:

Encase each of the resulting solids completely in its own separate 3D cube solid (one much larger than the part solid).

Use the subtract command to subtract the part solid from the cube solid.

Create another 3D cube smaller than the original 3D cube but larger than the part solid - position it to fit completely within the original 3D cube.

Use the subtract command to subtract the original 3d cube from the smaller 3d cube.

this will result in 3 identical copies of the original part.

Re: How does AutoCAD join solids

rie24RYR — Fri, 13 Jan 2023 18:09:50 GMT

I tried your method several times, but every time I get the message "Null solid created - deleted".

Re: How does AutoCAD join solids

Washingtonn — Fri, 13 Jan 2023 19:27:32 GMT

Your model is 4.7 million drawing units away from the origin - make a copy and move it to the origin to minimize calculation rounding errors.

Is there a need to set such a thick line weight?

Have you investigated the model for issues? You do have a problem at the location shown below. Everything unions together no problem except that one solid which causes an error number 84028

Re: How does AutoCAD join solids

rie24RYR — Fri, 13 Jan 2023 20:44:12 GMT

Thank you, Washingtonn, for your quick replies and support!

The model represents a double concrete retaining wall in real geodetic coordinate system (that is why it is so far away from the origin). All the drawing files in this project are in the same coordinate system and I easily copy&paste elements between the drawings to original coordinates XYZ. The wall is 0.25 (0.275) m thick, the foundation’s cross section is 2.00x0.30 m.

I moved the model to coordinates 0,0,0, as you suggested. It behaves the same way. The errors and numbers are the same (volume 513.631 vs 513.632, negligible difference of 0.001). I ran the AUDIT command, it says “Total errors found 0”. I ran the PURGE command, but 2 of the unused layers were not deleted.

When I deleted the history of all the solids and tried to join the solids, I got the error “The Boolean operation on solids failed. Inconsistent face-body relationships” (before deleting the history, the solids joined without problems - joining group 1 + group 2 + group 3 simultaneously). This error was caused by the element that you showed.

How did you find this error number 84028 and its location? I read about this kind of error here:

https://knowledge.autodesk.com/support/autocad/troubleshooting/caas/sfdcarticles/sfdcarticles/Trying-to-union-3D-Objects-produces-an-error.html

but it did not help.

I deleted the solid that you showed me (volume 0.180), joined the rest of the solids and got the volume of 513.634 (relatively no difference to 513.631 – the 0.180 are not added). Obviously there was an error in this specific solid, but it is a mystery why – it is just one of the 67 copies of a simple solid.

If there are no more problematic solids, it looks like the real volume is 513.634+0.180=513.814.