Help for Snap-fit

Cylindrical parts are normally joined by screw threads, or sometimes bayonet locks.

Parts that are thin enough to participate in a snap fit will not have a great moment capacity, i.e. a substantial cantilever is contraindicated.

Blow molding does not normally produce the sort of tolerances you need for snap fits, with the possible exception of cases where the parison is injection molded, and may support relatively tight tolerances.

Of course I am speaking in gross generalities, because you have provided essentially zero information about what you are actually trying to do.


Mike Halloran
Pembroke Pines, FL, USA

Thank you,
These following files are what I was talking about, for current design is normal cantilever snap fit plus lugs but I know it doesn't work properly. I wanted to make it screw thread but my boss doesn't like that and he wants something like snap joints.
I would be appreciated if you share with me your ideas about this.

The parts you have provided look a little like the two halves of a smoke detector; a flat circle with a short cylindrical flange, and a cone with two cylindrical flanges. It's not clear how the second part attaches to the first, and there's no indication of how large a moment the joint has to withstand.

Smoke detector housings sometimes use an interrupted multiple start thread, often with a snap lock feature at one end of the ramp, or a radial snap fit with axial slits in one or both flanges so they can be disassembled with either a twist or a vigorous yank, respectively.

Screw threads can be difficult to mold complete, because the core has to unscrew from the part somehow when the mold opens. However, an interrupted thread, or three or four interrupted threads, can be easier to mold, because the male thread can be formed directly in a simple two-piece mold, and the female thread can be molded with radial slides for ejection.

Two axial flanges, as you have sort of shown, given a traditional snap fit, can be difficult both to assemble and to disassemble, because of the large hoop stresses induced at assembly and disassembly. Axial slits/notches in one or both flanges greatly reduce the force required because the snap fits then induce only bending in what amount to crowned tabs instead of expanding the entire hoop.

To get a feel for what you face, you might prototype the assembly by machining a snap fit flange in two round plates of Delrin, then attempting to assemble them, then cutting radial slots in one of the flanges with a razor saw. The more slots you add, the easier it gets.

Failing the budget to do that, find some topologically similar joints in consumer products and do some comparative anatomy.



Mike Halloran
Pembroke Pines, FL, USA

Thanks a lot

Hi,
I have an idea and wanted to share it with you.
What if I design some bosses (3 or 4) and instead of screws I design a male part such as pen and a hole like pen lid, what do you think? does it work?

A single boss and complementary mating feature can be made to snap together, but snap fits are very difficult to make reliable or consistent at pen-like diameters.

Adding one, two, or three identical features makes the problem worse because you are adding location uncertainty to the size uncertainty. GD&T texts should explain how multiple features interfere with each other.


We here cannot ascertain 'how it works', since we have no idea what you are trying to make, or how it's intended to work, or what loads it must withstand.



Mike Halloran
Pembroke Pines, FL, USA

Thanks for responding me.
I forgot to say what it is! It is a wall clock shell and the force is force of a hand to lock and unlock these two parts.

Many resin suppliers have nice design guides. The one I've attached is just one example. Also explore "Design for Assembly". Good luck.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.