anelok: the Y-Box paint job, part 4: alternatives

Werner Almesberger werner at almesberger.net
Sun Nov 17 10:05:13 EST 2013


I have several ideas for solving the problem of internal paint
scratching:

1) the easiest approach would be to avoid mechanical stress on the
   paint by masking areas where case parts meet. That would create
   a bright band since light is reflected on the two walls before
   being absorbed by the paint on the inside of the inner wall, but
   may not look too bad. Here's one of the cases with the paint
   scratched off in one corner:

   http://downloads.qi-hardware.com/people/werner/anelok/tmp/ybox-alt-scratch.jpg

   One issue is that the PCB is also held by the case and thus
   rubs against the inner wall. This could perhaps be solved by
   adding a gap and constraining horizontal PCB movement via some
   structures away from the wall, e.g., a pair of columns, similar
   to the LED "pipe".

2) a similar approach would be to add a gap between the case parts
   and holding them together with screws. One problem with screws
   is that making precise holes for them would require a two-sided
   milling process, which exceeds what my machine can do.

3) applying the paint on the outside. This would largely protect it
   from internal scratching but it would be exposed to scratching
   from the outside and also the "object inside acrylic" effect
   would be lost.

   I tried this on a set of parts with bad geometry (the case
   wouldn't close properly when the PCB is inside.) First, the
   bottom half:

   http://downloads.qi-hardware.com/people/werner/anelok/tmp/ybox-alt-out-back.jpg

   The paint is a bit bumpy. This is what happens if you get it
   right:

   http://downloads.qi-hardware.com/people/werner/anelok/tmp/ybox-alt-out-front.jpg

   Perfect reflections - good enough to defeat the camers's
   auto-focus (it even did it two times in a row.) Not sure why
   the two are so different. Maybe I didn't mix the paint well
   enough - I'm still new to all this.

   This is what the case looks like with the drop of masking fluid
   I used for the LED window removed. The damage in the upper front
   edge comes from difficulties removing the part from the workpiece,
   again caused by bad CNC settings.

   This is what it looks like next to a case painted from the
   inside.

   http://downloads.qi-hardware.com/people/werner/anelok/tmp/ybox-alt-out-compare.jpg

   I'd say the result looks quite acceptable but less fancy than the
   other one. It also seems to attract dirt a lot more.

4) a mixed solution: paint the top surface from the inside (so that
   the LED/OLED window looks nice) and paint everything else on the
   outside. This means that I need good masking at the edge of the
   top surface:

   http://downloads.qi-hardware.com/people/werner/anelok/tmp/ybox-alt-mixed-mask.jpg

   The result:

   http://downloads.qi-hardware.com/people/werner/anelok/tmp/ybox-alt-mixed-result.jpg

   The masking worked rather well and the result looks promising.
   I'd be a bit worried about paint easily getting rubbed off at the
   edge.

I think I'll aim for approach 1 for now. This means that I'll have to
add a few holes in the PCBs, make the cases a little wider, and put
some columns into them.

- Werner



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