I have been thinking about building my own watch for a while. The things that always kept me from starting the project were two main problems: Size and battery life. You can easily take a small LCD screen, plug it into an Arduino nano and call it a watch. But it will be difficult to make it look decent. Also, unless you have your display turned off most of the time, battery life will be limited to a few hours.
The first step was finding components that solved these problems. The display usually consumes a lot of energy. A few commercial watches - like the original Pebble - solve this by using an e-paper-like display, that only needs energy for updating the screen. I came across the Sharp Memory LCD LS013B4DN04, which is a 96x96 pixel screen with basically no static power consumption. They cost about 20€. Adafruit even has a rather expensive breakout board (https://www.adafruit.com/product/1393) for them. I ended up buying the much cheaper Texas Instruments breakout board from Farnell (farnell.com/texas-instruments/430boost-sharp96/sharp-lcd-boosterpack-msp430-launchpad/dp/2429519).
As I wanted to keep the project Arduino-compatible, I chose an Atmega328p as a microcontroller. It has a 100nA sleep mode, so I only had to find a way to wake it up when necessary. For this, the Atmega has two external programmable Interrupts. I ended up connecting one to a real time clock, which triggers once per minute. Now basically the only thing consuming power is the RTC.
I chose a coin cell as a battery. A rechargeable Li-ion cell would require a charging circuit and a plug.
For all of my projects in the past I used perfboards. To size everything down, I wanted to create my first ever custom PCB. But before that, I wanted to get a grip on how the watch should look. Therefor I took the components I already knew and arranged them in Onshape.
I got a thickness of around 7mm including the lcd, pcb, Atmega and a cr2025 coin cell.
Next, I wanted to see how big the watch would have to be and what shape it would fit in. I really like the Pebble time with its rounded corners. So this was a great design to start with. Pebble even has a Github page with their CAD-files (https://github.com/pebble/pebble-3d).
The sharp display almost fits the pebble time, but because of the lower bezel it would not be centered. I decided I would stick with a slightly offset screen instead of making the whole watch larger.
For designing the body of the watch I switched to Fusion 360. Mainly because a project of this size is not comfortable to work with online, especially with a slow internet connection.
To make the body printable I split it in half. The bezel of the screen, which acts as a protector and design element is one part. The other is the main body, which is big enough to fit the electronics. What I ended up with is basically a copy of the Pebble Time, although I modeled it from the ground up. I tried to avoid 90° angles, to make for an organic feel. To hide the cut between the two parts, I included a curved seam.