Placing on face sometimes changes orientation  

Im afraid you will need to save your project (file>save project as) and then take the resulting .3mf file, zip it up and attach it to a post here for anyone to have any chance of working out whats going on.  Could be a bug or it could be the errors in your model (note the yellow warning triangle in the model tree list).

Auto-center parts enabled?

Any chance you have auto-center parts enabled in preferences?

Here is the file.

The caution is due to "errors" that were a result of having several pieces in the model (to make editing easier), but I had used Netfab to repair those in the original case.  I didn't do that to take the pictures, but the change in orientation changes in either case.

Thank you very much for the file.  On initial inspection I'd say its looking like a possible bug in the place to face function as there's really no need for it to rotate z axis too.

However its easy to work around.  Once you have flipped it over change the co-ordinate system in the tree view from global to local.  You will see that its changed Y 180 degrees and Z 73.27.  If you zero out the z angle it goes back to the 'proper' orientation.

RE: Placing on face sometimes changes orientation

@dik-harrison

The big "flat" bottom isn't really flat....

Rotate 90° in Y, or X and you will see this

edit: No caution in the file I downloaded

To late for edit....

Checked in Fusion 360, the part IS uneven, differs 0,003° to 0,009° when I measured between a few triangles.
I guess PS take the "biggest" flat area of triangles it could find and align it accordingly, maybe, I think....

Thanks to all...

I'll see if I can fix the model.

I don't think that is necessary on This particular part, it looks good in Prusaslicer and will probably print just fine.
As long as you don't try to print it the way I rotated it  😀 

The reason for the strange alignment was due to uneven flat face.

Out of curiosity, which program do you use

SketchUp Pro

I use SketchUp Pro 2018 (can't afford their subscription as a hobbyist user) mainly because I have used it from back when it first came out.  I've tried Fusion 360 and like it, but I'm much faster with SU.  Guess I need to stop basing my models on STLs of something close and draw from scratch so that these slight inconsistencies introduced from the STL won't be a problem.

Unfortunately I can't help you with SU.

Haven't used it for ages so I don't know how SU deals with STL's

RE: Placing on face sometimes changes orientation

I loved the idea of the solution to this issue suggested by @neophyl, but no matter what I do I can't seem to get my World or Object Coordinates to indicate how many degrees the program has auto-rotated so I can get it back to square with the build plate. I tried, oh did I try.

However, I did find a super easy work around, at least for my current model. Rather than using the F key to set the model on the upward facing side, I simply punch 180 into the X rotation box on the right hand side in the Object Manipulation screen and Bob's your uncle, the part is flat on the previously upward facing side and remains locked to square with the build plate.

In the past I've always just used the Rotate option on screen left/the Big Blue Arrow and eyeballed the rotation because who really cars if it's off a degree, but recently I was doing a finished face down model where the infill line layout mattered to me so after 3 years needed to figure this out.

RE: Placing on face sometimes changes orientation

In the past I've always just used the Rotate option on screen left/the Big Blue Arrow and eyeballed the rotation because who really cars if it's off a degree, but recently I was doing a finished face down model where the infill line layout mattered to me so after 3 years needed to figure this out.

If you rotate the model using the menu icon from the left then there are special marks on the rotation, if you move mouse pointer on it, then the rotation will snap to it, so it is easy to rotate it at certain angles - internal lines are every 45 degrees, and the outside is every 5 degrees.

video:

https://storage.googleapis.com/prusa3d-content-prod-14e8-wordpress-forum-prod/2025/08/f6503058-2025-08-01-08-42-39.mp4

RE: Placing on face sometimes changes orientation

Yes sir, you are correct. When you are looking for a snap-to of full degrees that feature is very helpful.

But in the case presented by the OP where the slicer "F" function rotates the model an unknown amount of degrees and we are trying to get back to square with the build plate, that feature is of no help unfortunately because the random rotation is never 5 or 10 degrees....it's more like 9.8746 or whatever.

Posted by: @_kaszpir_

In the past I've always just used the Rotate option on screen left/the Big Blue Arrow and eyeballed the rotation because who really cars if it's off a degree, but recently I was doing a finished face down model where the infill line layout mattered to me so after 3 years needed to figure this out.

If you rotate the model using the menu icon from the left then there are special marks on the rotation, if you move mouse pointer on it, then the rotation will snap to it, so it is easy to rotate it at certain angles - internal lines are every 45 degrees, and the outside is every 5 degrees.

video:

https://storage.googleapis.com/prusa3d-content-prod-14e8-wordpress-forum-prod/2025/08/f6503058-2025-08-01-08-42-39.mp4