This week, I'm designing a tool to remove the back cover of a mechanical watch. The specific watch is the HMT Rajat automatic watch model, which I picked up on eBay for $7 as a neat project.
The destination of this project - the reverse side of the watch case. I removed the watch band to make measurement easier.
This model is considered a "fake" by most HMT watch collectors, because the dial face is a replacement. The red dial faces are never original to the HMT Rajat model, but the modified red dial works fine either way for my purposes.
Another shot of the front of the watch. Very intriguing!
The reverse of the watch. According to what I've seen online, the rear case of the watch unscrews by gripping onto the six divots on the bottom shell.
It is likely that the print will need to have considerable structural strength in order to grip into these shallow divots without snapping.
To begin designing a 3D model for the caseback removal, I started measuring the dimensions of the rear of the watch.
Both the outer and inner measurements of the cone on back of the watch were crucial. In order to maximize the structural strength of the caseback remover, I needed to make sure it fit as snugly as possible with the rear of the watch.
Of course, the divot size was also a crucial measurement for creating the caseback remover. I also measured the distance between the divots.
Once I had enough of the measurements collected, it was time to create the caseback remover in OpenSCAD. I first compiled all of the data together and modeled the back of the watch, then subtracted the model of the back of the watch from my caseback remover's blank frame.
The blue color indicates a subtraction - in this case, the subtraction is that the back of the watch is being subtracted from the caseback remover's design.
Here is a bottom view of the caseback remover. I designed it with two 18mm holes so that I could use my fingers in the tool for more leverage when unscrewing the rear of the watch.
A top view. In this image, the size of the tool is evident - it's not too large or too small.
The six-pointed "screwdriver" part of the design is raised from the rest of the tool in order to avoid a collision with the curved rear of the watch that does not include the caseback.
Tune in next week for printing out the caseback remover and testing it on the watch!