From d497d6740f8f173167abfd82cb6d92e11e69b624 Mon Sep 17 00:00:00 2001 From: Werner Almesberger Date: Sat, 7 Aug 2010 18:40:55 -0300 Subject: [PATCH] Added problem statement and described more details of the process. --- README | 53 +++++++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 49 insertions(+), 4 deletions(-) diff --git a/README b/README index a039a06..26b1735 100644 --- a/README +++ b/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 - make transparencies +- 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 @@ -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.