Adding rivets to an object for 3D printing I have an object which is going to be 3D printed to which I would like to add lot of small rivets, and to do this without losing its manifoldness.  The way I've decided to do this (stupid it may be) is to place spheres in the appropriate place so that a portion of sphere is sticking out, the spheres being separate objects to the thing they are "riveting":





However, keeping them as separate objects is kinda risky (one false move...) and so I'd prefer to incorporate them into the main object.  But I need to do this without destroying my manifoldness. I've tried a Boolean Union but that leaves stuff hanging over on the inside of my object, which makes it non-manifold.  Is there a way to achieve my aim, or maybe an entirely different rivet strategy which I should adopt?
You could model this, if you're starting with  a topology something like the  one illustrated here:

With Pivot set to 'Individual Origins'


CtrlR add three edge-loops
AltP poke the two middle face-rings
Select a couple of consecutive rivet-vertices (not necessarily just one apart)  on the central loop - use CtrlShiftNumpad+ to select the rest, and then CtrlShiftB bevel those vertices, in 'Offset' mode
Space .. look up the 'Circle' operator, and use it on your newly created faces. (They should still be selected)
perform whichever series of insets, extrudes, and scales create the kind of rivets you want.




I don't know whether you're sub-d modelling, but this is the result after putting on a level of subdivision.



You could use the Loop Tools add-on > 'Space' option at the vertex stage, in various ways, if you want to improve the distribution of rivets. 

Although the result is not all quads (before subdivision), it's in the flat, for printing, and is not expected to deform, so that's probably OK?