# How to place objects on top of another object and join their geometry

I have 2 objects. I want to put the dome right on top of the cylindrical ring.

How do I do this so it's pixel perfect? I need to do this for 3D Printing purposes.

Later I plan to join them together the two objects. I'm not sure how to do this either so their geometry connects?

I have tried snapping but it doesn't seem to do anything even when I try to snap on faces. I'm not sure what I'm doing wrong so any help is appreciated!

I should note the two objects have diameters and thickness that are identical.

It looks as though your objects are aligned. In that case:

• Set your Snap to 'Active' and 'Vertex'
• Put the dome into Edit Mode, select an accessible rim vertex, and hit A to select the rest. The rim vertex should still be active.
• GZ move the dome down and snap the active vertex to the corresponding one on the ring.

(If the objects are not aligned on the Global Z, create a Custom Orientation from one of the rim faces, and use the Z from that.)

Now you have your object-rims exactly coincident.

Later, you want to join the objects into a single manifold mesh.

• In Object Mode, CtrlJ join the objects.
• Edit Mode, select all vertices,M > By Distance merge the coincident vertices.

This will leave you with a ring of internal faces. You can either:

• In Edit Mode, select none and then Header > Select menu > All By Trait > Interior Faces and X > Delete Faces
• Or do it by hand: H hide a face next to the rim to give you access, Alt - select the internal ring of faces, and again, delete them, before AltH unhiding your hatch.

I can't be sure of the source of the mismatch between the dome and the ring, but it might be this:

This is the result of modifier - Solidifying a hemisphere. Note the inner circumference is lower than the outer.. not in the plane of the equator. Solidify is using the average face-normals of the ring to the immediate north, but there is no counterbalancing ring of faces to the south. If you solidify while the sphere is intact, and cut it after applying, this doesn't happen, or you could scale the rim to 0 in Z about one of the outer vertices, after applying the modifier.

In general, for modelling that isn't ultra-precision, I would turn up the merge distance to the minimum that merges the vertices I want to, bearing in mind I can restrict the merge to selected vertices. This is the usual case for me, thinking like a carpenter rather than an engineer, taking the 'measurement' of a length or angle, simply by copying an existing part, risking the propagation of small errors, to get a match.

For precision modelling, (where perhaps you should be in CAD anyway,) I would keep the merge distance to an absolute minimum. Unmerged vertices will then alert me to an error in measurement, somewhere.

• This worked beautifully! Very simple answer and got the job done. Commented Jul 1, 2020 at 1:12
• My only issue is for some reason this method overlaps the parts a bit after the snap, not sure why it's doing that? That is to say the top dome goes into the ring ever so slightly when you zoom in a bit. It's not perfectly seamless. When I try deleting the interior faces it deletes the ring which I don't want. Commented Jul 1, 2020 at 1:22
• The gap is on the inside of the dome between the ring part not the outside. Outside looks perfect! Commented Jul 1, 2020 at 1:28
• Also what setting should you use for the merge by distance? I noticed if I increased this the overlap issue seems to resolve itself but creates some weird geometry a bit in some places. Commented Jul 1, 2020 at 1:32
• @JosephAstrahan I've got to say 'Hello!' to use enough characters :) .. See edit. Commented Jul 1, 2020 at 8:47

I've been doing this quite a bit lately, but BEWARE, I'm very much a newbie myself, so hopefully you'll get better answers from others. To join that dome to the cylinder here's what I would do.

1. I would fist make a duplicate of both of the two pieces (using shift-D) on each, and then I'd hide the original pieces. This is so that I can always just delete the copies, and start over with the original pieces if something gets fouled up. Don't bother renaming the copies yet.
2. One mistake you really don't want to make is to join two pieces like the one in your example that will have faces touching once you join them! Look at the bottom of the dome. That face on the bottom of the dome, once you join the two pieces, will occupy precisely the same 3 dimensional space as the top face of the cylinder. That will confuse the dickens out of blender. So...
3. With the copy of the dome, delete the faces that comprise the bottom surface of it. (Delete them, don't dissolve them). Then do the same thing with the faces on the top surface of the cylinder copy.
4. In the window on the lower right in your example you appear to be in the modifier tab. Switch to the scene tab (the one that looks like a cone with a tiny sphere next to it). If you working in metric, switch to millimeters, if you're working in feet, switch to inches. This will give you finer dimensions to work with. (since you seem to be having trouble with snapping I'll try to minimize using it, but you'll have to learn how to use snapping sooner or later. Watch a bunch of YouTube videos on snapping, trust me snapping is simply too useful to continue to try to do without. That said I'll try to use snapping as little as possible.)
5. Engage the measure tool. That's the bottom icon in you're main window that looks like the letter L with tiny lines on it. Go into wire-frame mode, and make sure the x-ray function is off. Click and hold near where you want to begin the measurement, drag to near where you want to end the measurement (kind of like pulling out a tape measure). Now here's where you'll have to use snapping. In you're example you appear to be set to snap to faces, switch to snapping to vertex. Look for a corner on the bottom of the dome that is directly above a corner on the cylinder. (any corner will have a vertex to snap to) Click and hold on one end of the measuring tool you made. Drag it a short distance in any direction, and while still holding the mouse button down hold the control button on your keyboard down (this engages the snapping action). Still holding the mouse button and control key down, move the end of the measuring tool to the corner where you want the measurement to start and when it gets close to that corner's vertex it will snap to that vertex, and you can let go. You now have one end of the measurement tool anchored to the fist corner. Use exactly the same procedure to snap the other end of the measure tool to a corner directly below the first corner. Check using the front view, and then a side view to make certain that the measurement tool is vertical and not at some angle that will throw off you measurement. Now you have a precise measurement of how far you have to move either the cylinder, or the dome in the Z direction to get them to meet. Write it down! I assume you know how to move them once you know how far to move them. Once you've completed the move you can now join the two.
6. Once you've joined them, go into edit mode, select all (I use the hotkey A to do that), then go into Mesh, Clean-up, Merge by Distance and set the distance to more than the default, but much less than the shortest distance between any vertices. (your diameter is 42mm, so set the distance in the merge to say 5mm. If lines start jumping all over the place it means your merge distance is too high. Keep decreasing the merge distance until things merging that you don't want to merge stop doing it. Keep an eye on the little blue boxed note the pops up at the bottom of the screen, it will tell you how many vertices where merged. If you keep seeing 0 vertices merged, that's not good. Think about it. The bottom of the dome and the top of the cylinder form circles that are essentially identical. Which means when they are joined you're now going to have 2 vertices at each corner making up those circles. That's going to give you bad geometry. So keep increasing the merge distance until the little blue pop-up window shows that you've merged at least a dozen or so vertices.

Decrease the merge distance whenever it deforms your geometry, Increase the merge distance whenever you get a value of 0 vertices merged.

Many times, like when you use booleans, you will sometimes get 0 vertices merged, and it's ok, because there weren't any vertices on top of each other that need to be merged. But it this particular case you know for a fact that there are going to be vertices on top of each other because you just finished joining the circles at the bottom of the dome to the circles making up the top of the cylinder. So there has to be some overlapping geometry there.

You can keep changing the merge distance until you click on something. If you do a merge that fouls things up, there's always Control-Z to undo it.

Once you've finished, and the two copies are now one piece, it's finally a good time to rename your new object.

Hope this helps, Mike.

• Thanks for your answer :), the answer below worked best in my situation but it's good to know these tricks also when perhaps alignment isn't perfect. So thank you! I think this will help a lot of other users. Commented Jul 1, 2020 at 1:13