Anelok: refining the wedge

[Werner Almesberger]

Sorry for the slow reply. Got sidetracked by lots of things. Some of
them are good news, though. More about that soon.

Bas Wijnen wrote:
> The same can be achieved by trying several times and
> tweaking the settings (for example, changing the size of the model a bit
> so that the result is what you want instead of what you ask for).

Yeah, CNC has that "what you get is what you get" property, too :)

> Hmm, if you get that much then my estimate of near zero deflection may
> be overly optimistic.  Acryllic may be slightly more flexible than PLA,
> but I don't think the difference is that big.

Oh, that's a very useful data point: if PLA is a little more rigid
than acrylic, then I can design for acrylic and PLA should just work.

> I don't have a lot of experience with it.  undercuts are generally hard
> to print, unless they are small.

Everything in that case is small ;-)


Meanwhile, I've been making a few doodles of some yet unresolved case
details:

http://downloads.qi-hardware.com/people/werner/anelok/tmp/me2/bat.pdf

This one is about the battery cover. The idea is that the cover snaps
on the main case (in the X+ direction of the side view). I've drawn
the snapping mechanism on the top side. One problem here is that this
has a relatively narrow structure between (plexi)glass and the cover.

I also drew a variant without that structure on the right, but I fear
that having removable battery cover and glass meet directly would
produce an untidy border.

On the left side is a view of one of the battery terminals: the wall
is 1.5 mm but there is room for a ring-shaped low wall around the
battery contact, so the whole structure should be reasonably stiff.


http://downloads.qi-hardware.com/people/werner/anelok/tmp/me2/bat2.pdf

The next try: I let the cover slide into the main case, which moves
the visible edge of the cover further away from the glass. I had to
increase the wall thickness of the battery cover to 2 mm in order to
let the case not get extremely thin anywhere. It still has a number
of thin areas, though.


The above two designs were just concerned with the battery area. The
main case would then consist of two parts, one for the top and one
for the bottom. Now, what if we integrate the battery cover into this
and then just have two parts ?

One difficulty there is that any snapping mechanism that goes on the
battery side would have to fit into the wall. That hardly seems
possible without significantly growing the case. Then it occurred to
me that the mechanism could be on the inner side of the barrel i.e.,
near the main PCB. From there, it wasn't a big leap to move to an
even simpler mechanism using screws:

http://downloads.qi-hardware.com/people/werner/anelok/tmp/me2/bat3.pdf

The case is split "horizontally" (*) into a top and a bottom part.
The side walls are part of the top wall but could also be divided
between the two (e.g., to make it easier to make openings).

(*) For simplicity, I just assume top and bottom to be parallel. I
    hope the moderate inclination they have in reality won't throw
    off my guesstimates by too much.

The bottom part would slide into the top on the thin side and be
held by one or multiple "tongues" that can rotate a little. Then one
would close the case and fix it with screws inserted from the side.

This is how a screw would travel into the case:

- first, it passes a large opening in the outer wall. This opening
  will contain the screw head.

- next, it passes through a narrower opening in a cylinder that
  protrudes from the wall. This hole is narrow enough to stop the
  head from entering but lets the screw's thread move freely.

- finally, it enters a structure from the bottom part that contains
  the matching thread.

This has a number of interesting features:

- it seems to be quite simple,

- screws are generally considered child-proof,

- processes that don't allow accurate production of the small
  openings for the screw can be fixed by just drilling these holes,

- mechanical loads are mainly perpendicular to the axis of the screw
  and thus produce minimum stress on the thread,

- if all else fails (or during prototyping), one can just use a wire
  instead of a screw.

A number of variations of the theme are also possible, e.g.,

- top and bottom roles could be swapped,

- the thread could be in the section next to the screw's head instead
  of the one near its tip,

- one could even replace the threaded cylinder with a nut, thus
  eliminating the risk of an over-tightened screw stripping the
  thread from the plastic.

Issues:

- the cosmetic overlap on the thick end has thin walls. That may be
  a problem for 3D printing (where one could also just leave the cut
  flat), but injection molding shouldn't have a problem with this.

- if play exists somewhere, one can't compensate for it by tightening
  the screws a little more,

- it leaves the thick end side where top and bottom meet without
  direct support. Not sure if this is a big issue, though, given the
  general smallness of the case.

- the mechanics on the thin side may be more complex than shown,
  since the movements there not only have to allow the bottom to
  slide in place, but it must also not interfere with the battery,
  if it takes part in any openings for PCB-mounted connectors, these
  must have enough tolerance to allow the closing, and line where
  top and bottom meet at the thin end should not have a gap.

So far for the drafts. Now I have to turn all this into a proper 3D
model ...

- Werner