Some Snap-Fits For Thought

While laser cutters, desktop mills, and 3D printers might be wonderful tools for rapid prototyping, it’s best to have a strong understanding on a few techniques to really “digitize” those sheets of Delrin and rolls of PLA into something meaningful. In a nutshell, we need to know how to cut-or-squirt parts that fit together.

[Yoav] has a few tips for HDPE. The first technique is a clip-on, clip-off feature meant for repeated use. The second joins two parts with a joint that can’t be removed except by removing a dowel pin, or other press-fit shaft that holds them together. The last technique is similar to the first, except it embeds the deforming geometry directly into the mating surfaces.

If you’re interested in some detailed design guides and a few equations, have a look at the Bayer Guide and DuPont Design Guide; both provide a detailed set of geometric techniques and information about their associated stresses and deflections.

Finally, if you’re looking for a triumph of snap-fit design, have a look at [Jonathan Ward’s] MTM Snap–a snap-fit desktop milling machine and the direct predecessor to the modern-day Othermill.

Thanks for the tip, [uminded]!

15 thoughts on “Some Snap-Fits For Thought

    1. While some of these have tabs that can be pressed to remove the fastener, I wonder if you could create a fastener with a natural resonant frequency, so that you could make it pop out by applying a tuned vibration?

  1. How to design with tiny things that break off easily. The whole world is held on by two tiny plastic tits that hold it in place. When one breaks off it’s going to spin out of control. Battery doors, lids, covers on small electronics. Many not child proof.

      1. That’s true but someone somewhere who is paid a shitload more than this guy *is* designing things that will break off *because” he (or she… whatever) didn’t design within the limitations of the materials. Happens all the fricking time.

        The wrong people are earning too much for doing the wrong thing.

    1. The design flaws are in parts that are intended to be disassembled and fail during that event. Many plastics contain plasticizers that allow the parts to deflect without breaking; those plasticizers evaporate leaving the remains very brittle. Some, like nylon, depend on moisture to be tough – put them in a situation with high temps and no water and they rapidly become brittle. Other times they are designed to engage with no thought to adding tapers to allow disengagement. Or they engage sheetmetal that hasn’t been deburred (that’s you GM) and the burrs gouge into the plastic and mechanically lock. And, my favorite, a large number are used in parallel requiring simultaneous disengagement to avoid overstressing individual connections, but there’s no way to release them all. Another problem is the extra expense of adding stress lowering radii to molds to prevent creating stress-multipliers, like sharp inside corners.

      Then there are parts of non-serviceable items that need some minor repair, but the housings are destroyed getting to the internals. Not really a design flaw but an irritant none the less.

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