Added problem statement and described more details of the process.

README

@@ -1,3 +1,36 @@
+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
 -------------------
 
@@ -7,8 +40,8 @@ From the bottom to the top, we have the following elements:
 - 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
+- a cover sheet of thin hard plastic, e.g., the type of plastic film
-  make transparencies
+  used to make transparencies
 - isolating tape, applied to tall components of the Ben's main PCB
 - the Ben's main PCB
 
@@ -33,9 +66,12 @@ 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"
+- generate in HeeksCAD with "import cw" (requires HeeksCNC)
 - define Zig-Zag operation
 - generate Python script and run it (takes a while)
 - save NC file
@@ -43,5 +79,14 @@ Workflow
 - 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)
-- ./doit >job
+- 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.