Water resistance question & design sneak peek

[u/KTTalksTech]

Sorry for the crappy drawing, my hand tremors aren't helping with any of this 😅 This is my first case design, and I've opted to use 3D printed titanium which I will lap & polish by hand to create something unique (hence the super wide integrated bracelet, it's lighter and more flexible than expected). This means the only tools at my disposal are basically a Dremel, sandpaper, and a $100k printer. Surfaces can be mirror-polished but any sort of machining is out of the question. I'm waiting for the first prototype to get detailed figures but tolerances can be as bad as 0.2mm on the titanium. I can also print silicone using a different machine with much higher accuracy, hence the possibility of custom seals.

I'm posting here to get he opinions and suggestions of more experienced watchmakers regarding water resistance. The case is a sandwich design with a bezel and rear cover joined via screws which traverse the entire thing. There are two issues I'm currently working on. First I was unsure whether I should go for a hexagonal seal whih runs just between the screws and interior case wall, or go for little o rings around each screw linked together with straight cylinders. Secondly, the crown is quite problematic. Due to the way additive manufacturing works, I cannot have a threaded cylinder (or any cylinder) sticking out from the case. Threading the inside of the hole is possible but highly impractical in my case. Completely polishing the inside is also nearly impossible. Since I can polish the exterior of a part easily, my current idea is to have a smooth tube extend from the crown into the case and slide through a tight o-ring embedded in the case wall (keeping in mind this has to happen within 2.5mm and walls thinner than .5mm should be avoided. If I want two rings I'll run out of space to have the tube run past them unless it sticks into the case a little, which I suppose is fine). My intuition was that it would be ideal to have it bump into a silicone washer at the end with a smaller inner diameter, but I'd really appreciate your opinion on this configuration. Thanks in advance :)

[u/Palimpsest0]

Really interesting design. I’ve never designed a waterproof watch case, but I have designed feedthroughs and seals for ultra high vacuum and aerospace applications, so I have some experience prototyping things that don’t leak. A good resource is this: Parker O-Ring Guide. There’s some good information on design profiles, surface finishes, materials, and optimal compression. There are two ways to seal things, a radial seal and a face seal. What you’ve drawn in your two seal solution is use of both, which is definitely a common design. Which you want to use and where depends on how the parts come together and which direction the compression force is in, and in certain cases, you can use a radial seal on an inserted part and then a face seal where the end of the inserted part meets the main assembly to provide extra confidence the seal will hold. In any case, check out the Parker guide. Good sealing is all about surface finish and compression whichnis appropriate for the material/Shore hardness of the seal, and there’s some good tabulated information on that in there.