Two closed solid polysurfaces wont join

Hey Guys,

I am currently working on a project where I need to model a thread that will be 3D-printed. After slicing my Model it turned out that the extruding piece of the thread, consisting of two polysurfaces (magenta), didnt join properly. After trying to join the two surfaces for one hour, i still havent made any progress.

Can someone offer any advice on how to join these two elements? Help is much appreciated!

thread_wip.3dm (3.4 MB)

Can’t you Boolean them ?

if there are no intersection between the 2 solids it’ll not join

You can not Join “Closed” objects.
If your polysurfaces intersect each other, then a Boolean Union should do the job.
The Boolean intersects the two solids, trims them using the intersection curves, deletes the appropriate bits based on which Boolean command you ran, and Joins the results.

Thank you for the instructions! I will try solving the problem this way!


I have now tried the “Boolean Union” command to join the two pieces together. Unfortunatly the Boolean Union command doesnt work, because the “Boolean union failed”. I have also tried to join the two surfaces by first duplication them, than “Boolean difference” them, and putting the two results back together. This, unfortunatly, also does not work. The problem seems to be that although they are labeled as “closed” surfaces , they clearly are not closed (no closed inner volume (see 3ds file).

If you have any other suggestions why this could be, or how to explain this difference in between appearance (non closed) and catogorization (closed solid), I would be happy to hear them.

Thank you for your time!

(and sorry for my bad english, i am not a native speaker)

The manual fix is just as I described.

  1. Use the Intersect command between the two solids.
  2. Inspect the resulting curves and manually fix them as needed.
  3. Use the fixed curves to Split or Trim the solids.
  4. Delete the interior surfaces as needed.
  5. Join the results.

This is the process described in the Frequently Asked Questions article:

Any luck?



Thankyou for your the step by step guide. although splitting or trimming the objects by using the Intersecting curve wouldnt work, exploding them and deleting one of the two resulting “trimmed surfaces” + joining the results together solved my problem!

Thanks again for your patience with my question, have a good evening!

A good rule of thumb is that the objects that shall be booleaned need to intersect more than the file tolerance. (Preferably more)

And keep in mind that for 3D printing they don’t need to be one part as two intersecting solids will be fused into one.

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this is a pro tip… especially if you are doing a client job and have a client that likes to make tons of changes…NOT booleaning parts together is much more efficient if you have to edit stuff later. Work in logical solid “chunks” and just smoosh them together to print. (overlapping of course.)

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