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I am designing a watch case and would like to know how to make an annular snap fit for the backplate. The material used is stainless steel. I could find references for Metal-plastic snap fit but I need both parts to be metal. I have seen a similar design in a Withings watch.

PFA image link: https://www.wareable.com/media/images/2015/01/img-6044-1421770737-9RIG-full-width-inline.jpg

For plastics I used : http://fab.cba.mit.edu/classes/S62.12/people/vernelle.noel/Plastic_Snap_fit_design.pdf

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  • $\begingroup$ A first thought is for either internal or external, make one part thin enough to flex. $\endgroup$
    – Solar Mike
    Mar 5 '18 at 8:50
  • $\begingroup$ @SolarMike I agree it needs to be thin. I was looking for reference documentation to make the design. $\endgroup$ Mar 7 '18 at 8:08
  • $\begingroup$ You asked "how" - not for documentation - I gave you a possibility... I won't do your work or research... $\endgroup$
    – Solar Mike
    Mar 7 '18 at 8:37
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An annular snap fit using inflexible material such as stainless steel (or aluminum!) will require a yield point in the ring.

Allowing that any ring you can design will be a non-zero thickness cross section of a cylinder, the resulting item will resist compression (internal snap component) or will resist stretching (external snap component) and require excessive force to assemble.

Such assembly will likely damage one or both components. Disassembly will likely damage one or both components.

Consider to provide a slot in one or both components, allowing for compression of the inside portion or expansion of the outside component.

A common implementation of such a design is called a snap ring or circlip. This is a C-shaped piece of metal designed to engage a groove in another part. The metal is often a stamped out (cut/sheared) ring which will distort in the plane of the sheet when "stretched" open to engage the groove.

You would therefore have to allow for such distortion in your design by providing for such flex, although I am uncertain of the direction of flex if your C-shape uses cylindrical cross section.

The image below comes from a commerical web site for this type of product:

snaprings

Some of the rings have a proportionally large gap, while others are designed to close completely but allow for spread during operation.

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  • $\begingroup$ And yet an awful lot of watches use metal-to-metal snap fits. My guess is that they're relatively soft version of stainless, and further the snap fit is not an environmental seal -- there's usually a gasket hiding in there. $\endgroup$ Mar 5 '18 at 16:23
  • $\begingroup$ @CarlWitthoft, The metal snap fit is not for environmental sealing. They all keep a Viton ( or similar) O-ring inside it. The purpose is to open the back plate and place it back in with ease. Mainly to avoid screws. $\endgroup$ Mar 7 '18 at 8:05

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