ben-counterweight/README

Counterweight
=============

This project defines a counterweight to prevent the Ben NanoNote from
falling over, and  describes the process of making a wooden mold for
casting the counterweight using a lead alloy commonly used for
soldering.


Problem statement
-----------------

The weight distribution of the Ben NanoNote makes the device unstable
and often fall over when the display is opened. This can be remedied by
adding a counterweight near the front of the bottom shell.

Experiments have shown that a torque of about 2.5 mNm is sufficient to
balance the device with the display fully opened. A feeling of solid
stability is reached around 6.5 mNm.

The counterweight defined in this project is made of plumbing solder, a
Pb67Sn33 alloy with a density of about 10.0 g/ccm. The counterweight
has a nominal mass of 17.9 g and a nominal torque of 7.3 mNm. Due to
mold compression (which one could compensate for) and process
tolerances, the mass achieved in DIY casting ranges from about 14.5 g
to 17 g. This mass is slightly increased by the addition of protective
painting.

In experiments, the torque produced by these counterweights proved to
be sufficient to give the user the feeling that the Ben is solidly
standing on its feet.


Mechanical stacking
-------------------

From the bottom to the top, we have the following elements:

- Ben case, bottom shell
- a few drops of glue or silicone, to hold the counterweight in place
- the counterweight, covered by protective paint
- a few drops of glue or silicone, to keep the cover sheet in place
- a cover sheet of thin hard plastic, e.g., the type of plastic film
  used to make transparencies
- isolating tape, applied to tall components of the Ben's main PCB
- the Ben's main PCB


Protection
----------

The counterweight is covered by one or more layers of paint, to prevent
direct skin contact with the lead during handling. The paint may also
offer some amount of protection against electrical contact.

The counterweight is covered by a layer of hard plastic that isolates
from electrical contact and that also resists being punctured by pointy
components or solder joints of the main PCB.

Finally, all elements on the main PCB that are unusually tall are taped
over, to further reduce the risk of them working their way into the
counterweight. Right now, the only component where problems are
considered likely is the buzzer.


Workflow
--------

This is the workflow for generating the CAD model and making a mold for
gravity casting with a Roland Modela MDX-15 CNC mill.

- analyze geometry, e.g., by viewing ben-bottom-inside-500um
- define CAD model in cw.py
- generate in HeeksCAD with "import cw" (requires HeeksCNC)
- define Zig-Zag operation
- generate Python script and run it (takes a while)
- save NC file
- convert G-code to gnuplot, with cncmap/g2gp
- mount piece and determine geometry with millp
  (from http://svn.openmoko.org/developers/werner/cncmap)
- define conversion in "doit" script (to do: put in repository)
- coordinate transform and conversion to Roland's RML-1
  ./doit >job
- send job with cncmap/spool


Gravity casting
---------------

Gravity casting is an efficient process for producing small numbers of
counterweights. The mold is milled from a block of pinewood and has a
life expectancy of about 20-40 cycles.