When I do a check manifold both objects have 0 non-manifold edges but when I combine them it becomes non-manifold and when I try and fix this it covers the hole which will have the screw in it. How can I make this manifold without it covering the screw hole. This is to be 3D printed and my 3D printer doesn't like non-manifold objects.

The box with the threads inside of it has been tested to work well when printed with my 3D printer so I just want to continue to re-use it without adjusting it's scale.

PS: I checked for doubles and did merge by distance.

How I did the join:

  1. Select screw/nut or object that will make hole
  2. Select object that will get the hole
  3. Ctrl (-) (do a boolean difference)
  4. Apply modifier
  5. Select both objects and select Join



When I click Make Manifold it covers up part of the screw hole at the bottom.


I'm using Blender 2.90 Blend file is attached

  • $\begingroup$ You want the block containing the thread to project from the outside, as seen. Do you want it to project on the inside, too? Or do you want the inner profile to be flush, circular? $\endgroup$ Sep 2 '20 at 9:58
  • $\begingroup$ @RobinBetts The box with the thread needs to go on the inside to since it needs to be 3D printed. And that's the thread size that has been tested to print well with my printer. But I'm open to see what it looks like flush on the inside, would be interesting to see how the 3D printer handles it $\endgroup$
    – Rick T
    Sep 2 '20 at 12:36
  • $\begingroup$ Sorry for delayed reply.. As far as I have understood, you want to achieve bottom illustration , without flaws, and block projecting both inside and outside pp compression end. Correct me if false. The screw-thread itself is pathological, it's self-intersecting. Hard to repair / replace, since we would need to know your exact desired dimensions. $\endgroup$ Sep 2 '20 at 16:01
  • $\begingroup$ @RobinBetts yes the bottom illustration is what I'm trying to get. Using both manifold objects. When I do a difference and join them they become non-manifold. Not sure about the needed to know the dimensions question since the dimensions / objects are in the attached blend file. $\endgroup$
    – Rick T
    Sep 2 '20 at 16:19
  • 1
    $\begingroup$ This image tries to illustrate what's wrong with the thread. it gets past manifold checking (The final surface is watertight). But it does show up as intersecting faces.Hard to fix without knowing what you need the profile of the thread to be. We don't want screws not fitting. $\endgroup$ Sep 2 '20 at 17:11

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