Model the backplate to closeup back of watch body, to insulate the electronics, and to accomodate wrist straps.
Scale and OffsetFactoring the gap between faceplate and backplate is a special case. Rather than selecting the same tolerance factor (around 0.5mm) that I determined for other fit places, used the Offset tool (rather than scale) shrink the faceplate by 1mm for a slightly looser fit.
In CAD tools, Scale shrinks the object across all three dimensions (though many tools permit scaling each axis separately). Scale was a good choice for making the button caps smaller all the way around.
Offset takes a curve and extends it (or reduces it) by a user assigned factor. Offset was a better choice for the backplate because scaling would cause the two prongs to move closer to each other, so the backplate would not have fit.
Planning for MaterialsWhen you are creating a 3D model that you want to share as files for people to download to print on their own desktop printers, it is important to consider sourcing considerations for people who might want to try your project. I made sure that my project would fit straps of 20mm and 3/4", both of which are pretty common sizes you could find anywhere in the world. Because I wanted this project to be easy to get parts for, I made sure to leave the slits on the backplate a little looser and spread out so that people can try things such as neoprene or even woven bands.
Accomodate Range of Wrist SizesLikewise, I wanted to make sure that anyone who wants to print this watch can wear it, hang it, stick it in a pocket -- anything they'd like to do with it. So I worked out a logic of slits on the backplate that works for narrow wrists (thread the strap out of the innermost slits on either side so that the watch hugs tightly to a slighter wrist) or even go really maximum and route the strap out of sides of the watch so you can mount it on your thigh or backpack.
Printing Backplate TestsI printed the backplate a number of times to try to figure out a good height that would suit a range of results depending on TIMESQUARE owners' soldering skils -- the height I chose is also similar to 1/16mm acrylic for those who want to laser cut this part.
The important thing with this part is what we already went over above -- you need to keep the scale the same and use offset/inset commands in your design tool to grow or shrink your backplate to marry it to the other 3D printed parts you designed.