Beginner's Questions on Proper .STL Design

Hello, Folks, my first post here:
I am a modelmaker with decades of experience in manual techniques who now wants to make the leap to 3D printing for a model I've been contracted to make for the New York State Bureau of Historic Sites. I have 16 years of experience in 3D modeling but am doing my first .stl files. At this point I have two fundamental questions that I need advice on.

First - overhangs. I understand the premise and am trying to divide my models to minimize overhangs, but here't the question - let's say I have a vertical surface with a shallow horizontal overhang. I'm doing architectural models - so imagine a groove between two siding boards, or a groove between stones, or the top of a door or window frame. Let's say these features are only a half a millimeter deep - possibly less. Is there some maximum/minimum depth below which overhangs will print properly? If so, what values are safe? Does it depend upon print resolution?

Second - is there a need to model in units of the print resolution - in all three axes? For example, let's say I've got a printer with a 100 micron z resolution. I model a feature that's 197 microns tall - or 178, or 220, you get the picture. What happens? Does the slicing software average things out to the printer's resolution, or will the print fail?

Thanks for whatever feedback you can give me!

Are my questions too hard for you guys? ;)

For your first question, regarding shallow overhangs, it is really printer dependent. Some printers are capable of handling overhangs upwards of 60 degrees without issues while others may struggle. Print resolution plays a factor in that as well. And as you mentioned if the depth of the overhang is shallow enough it may not matter at all. To play it safe with averages, most FDM/FFF printers now print between 100-200 micron (.100mm-.200mm) so a .5mm deep or less horizontal overhang of around 45 degrees or so should be capable on just about any printer without support needed. As the designer, it is nice that you are concerned about this, many are not. Quite often, people with the printers are left to figure out if a design is printable or not and how and where to add supports to the model.

For your second question, it isn't entirely critical to model at the resolution of the printer IF your models are more artistic/sculptural/static. Quite often we are used to converting units back and forth between mm and inches as well so that is not a big problem. However, IF your models are more functional/mechanical or have close tolerances or moving parts, then yes, you need to model at scale. Otherwise when the model gets scaled up or down those tolerances are ruined and the model no longer works as designed.

I am just inferring from the information in your post, but it sounds like your design will be more of a historical building type model, in which case you don't really have to worry about designing at any certain resolution. In fact, with models like that, quite often Makers like to print them bigger and smaller anyway. The Eiffel Tower has been printed in every size you can imagine. :)

Thank you, Boozekashi, that is very helpful. Yes you're right, I'll be doing historical architectural models. Most of what I will have to model is rectilinear. My concern is whether I can do things like shallow horizontal grooves (like grooves to represent clapboard siding, or grooves between stones in stone walls) where the top of the groove is at 90 degrees but only maybe 300 microns deep.
I think I'm going to try to minimize the issue by aligning thins so that highly detailed surfaces print horizontally. But I know there will be instances where that can't happen 100%.

You're also right that my models will not be mechanical. However - I'm modeling in the final 1/160th scale, converting inches to metric, and where I can, I might as well try to keep things in sensible 100 micron units to minimize the averaging the machine will have to do. I did do my first piece at an arbitrary Imperial scale and convert it but from here out I think I'll work in the finished scale.

Yes, I've seen many versions of those Eiffel Towers - Amazing! Thanks again for your feedback.

No problem, good luck, sounds like you are well on your way already. The kind of very shallow horizontal overhangs you are talking about can be handled by most printers that have any decent bridging capability. 300 micron with another layer right below it to build on is less than a typical layer width so it should not present much of a problem at all.

BoozeKashi - a friend of mine was good enough to print out some samples for me to look at, at 200, 160 and 100 micron Z resolutions, that have some of the kinds of shallow, unsupported horizontal insets I need to do. Better than I would have guessed. I'm feeling much more optimistic about this now.
Thanks again for your input!

it's surprising how thick a line or groove you can get away with.
One thing to bear in mind is that you need active cooling on the print area. This cools and sets the plastic much faster than leaving it to cool by itself. So you get cleaner over and underhangs and better bridging and raised detail.

One way of dealing with raised features on vertical prints is to line the bottom edges with asmall column. I'll often stick a 1mm column along an edge. it's very hard to see with the human eye, but even at .2mm layer height it gives the printer 2-3 layers to build out (I should say half a column). At higher resolutions you can get away with smaller diameter columns.

There are a number of little design tricks you can use to reduce supports and improve the final printed model.