Lodestone Latch Box (Patreon)

Hi, hi, hi, wonderful people!

Well, this was a complicated one!  It's a snap closure box that uses magnets as a spring to keep the lid securely shut until released via a sliding switch!  

There were two main problems to solve in getting this one to come together: how would the slidey switch actually be assembled, and what kind of mechanism would allow the lid to be locked in place firmly without opening when being pulled upwards, and still be printable in the orientation I needed it.

I'm going straight into too-much-detail rambling mode here, but you can always just skip straight to the assembly instruction images instead!

The Switch

There are really two things that the switch needed to do, and they do kind of sound like the same thing.  Firstly, it needs to be a sprung mechanism within the box that can be pushed out of the way by the pin and hold the latch mechanism that keeps the pin in place.  Secondly, it needs to provide a way to move that whole mechanism from the outside.

While the solution that I've ended up with probably seems like an obvious one, there were a bunch of other options that probably would have worked just as well, such as having a separate switch that bolted through the front of the box, but the way this thing ended up worked quite nicely and is also easy to print reliably.  Basically, there's an inner block that fits into a void in the box body and an outer block that's the switch, and they're connected over the top of the box body wall.

The block on the inside can slide freely from side to side.  Well, it would be able to, except... magnets!

The Magnets

Yes, the magnets!  There are two pairs of magnets involved, one attracting and one repelling, and both serving to drive the switch to one side and resist moving the other way.  The switch interior block has two magnets, and either end of the void in the box body likewise has matching magnets.  The orientation of all these is important, and is shown in the attached assembly instruction images.  Basically, though, we just want the switch to be pushed so that the notched bit is pushed into where the lid pin will go.

The Latch Mechanism

Now, this is the cool bit!  The latching system actually needs to do three things:

1) push the switch sideways when the lid pin first moves downwards

2) lock the lid so that it can't be pulled upwards

3) pull the lid downwards once past the lock point.

It's appropriate, then, that the motion of the pin and switch goes through three distinct phases, addressing each of those needs.  When the pin first encounters the switch block, oblique planes on each engage, which forces the switch sideways against the force of the magnets, letting the lid drop further and storing the energy that will drive the next two parts of the motion.  There is a threshold point at which those planes slip past each other, and the switch is able to snap back, engaging locking planes on either side.  There's still magnetic motion happening, though, and the switch moves past that lock point and yet another set of angled planes engage, this time driving the pin downwards into the switch void until it bottoms out.

At that point, it's possible to pull the lid up just a little, which moves the switch just a little, but only until the locking planes engage again.  The locking planes are oriented such that putting force against them will not move the switch; the switch itself needs to be moved out of the way before the lid can move upwards.

The magic of magnets! :)

It's worth mentioning that those planes are all very small features!  From memory, I think the locking plane engagement is all over in about two millimetres of sideways shifting.  Much of the prototype refinement involved very small changes to parts, in the order of tenths of a millimetre.  It would be fun to make something like this at a much larger scale, though the magnets would need to be correspondingly more powerful, too :)

Magnets Required

You'll need four 6x3mm cylindrical magnets for this one, and they really aren't optional this time, since things won't really work without them!

Print Description

This is an assembly in which the tolerances and dimensions do affect the operation of the mechanisms, so make sure your first layer is nice and neat and that there aren't any printing issues that might impact dimensional accuracy! 

Print Dimensions

The main box body with lid open for printing occupies 145mm x 88mm on the print bed and is 48mm tall.

Supports Needed?

Not at all!  Designed for straightforward printing!

Scalability

This one probably isn't the best to scale up or down, as the latching mechanism will be affected and the magnets won't fit :P

Print Orientation

The box body prints open, with the lid against the print bed.  The switch and switch plate print right-way-up, while the bolts can print standing either way, but are probably most reliably printed head-down.

File Location

You'll find this one at at 532 Lodestone Latch Box

Link to dropbox post: https://www.patreon.com/posts/31697592

Further Thoughts

Oh, I almost forgot to mention that the switch also features a mechanism to open the lid slightly and make the box generally easier to open.  It's only a small convenience feature, and it would certainly be more fun if it sprung open wildly.  I did in fact try adding a repelling magnets in the hinges to achieve such a thing, but the results were a bit underwhelming and didn't justify the added complexity!

Happy assembling! :D

xoxo

Sven.