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1Counterweight
2=============
3
4This project defines a counterweight to prevent the Ben NanoNote from
5falling over, and describes the process of making a wooden mold for
6casting the counterweight using a lead alloy commonly used for
7soldering.
8
9
10Problem statement
11-----------------
12
13The weight distribution of the Ben NanoNote makes the device unstable
14and often fall over when the display is opened. This can be remedied by
15adding a counterweight near the front of the bottom shell.
16
17Experiments have shown that a torque of about 2.5 mNm is sufficient to
18balance the device with the display fully opened. A feeling of solid
19stability is reached around 6.5 mNm.
20
21The counterweight defined in this project is made of plumbing solder, a
22Pb67Sn33 alloy with a density of about 10.0 g/ccm. The counterweight
23has a nominal mass of 17.9 g and a nominal torque of 7.3 mNm. Due to
24mold compression (which one could compensate for) and process
25tolerances, the mass achieved in DIY casting ranges from about 14.5 g
26to 17 g. This mass is slightly increased by the addition of protective
27painting.
28
29In experiments, the torque produced by these counterweights proved to
30be sufficient to give the user the feeling that the Ben is solidly
31standing on its feet.
32
33
34Mechanical stacking
35-------------------
36
37From the bottom to the top, we have the following elements:
38
39- Ben case, bottom shell
40- a few drops of glue or silicone, to hold the counterweight in place
41- the counterweight, covered by protective paint
42- a few drops of glue or silicone, to keep the cover sheet in place
43- a cover sheet of thin hard plastic, e.g., the type of plastic film
44  used to make transparencies
45- isolating tape, applied to tall components of the Ben's main PCB
46- the Ben's main PCB
47
48
49Dangers and protection
50----------------------
51
52The counterweight contains lead, which is toxic and also conducts
53electricity. While the health risk caused by handling the counterweight
54is very low compared to other lead sources, it's still a good idea to
55prevent accidental exposure. While there is normally an air gap between
56the PCB and the counterweight, they may touch if the countereight is
57improperly installed, if the PCB gets bent, or if the counterweight
58comes loose for some reason. Electrical contact can cause the Ben to
59malfunction and may even result in permanent damage.
60
61The counterweight is covered by one or more layers of paint, to prevent
62direct skin contact with the lead during handling. The paint may also
63offer some amount of protection against electrical contact.
64
65A layer of hard plastic is placed on top of the counterweight, to
66isolate it from electrical contact. The plastic also resists being
67punctured by pointy components or solder joints of the main PCB.
68
69Finally, all elements on the main PCB that are unusually tall are taped
70over, to further reduce the risk of them working their way into the
71counterweight. Right now, the only component where problems are
72considered likely is the buzzer.
73
74
75Workflow
76--------
77
78This is the workflow for generating the CAD model and making a mold for
79gravity casting with a Roland Modela MDX-15 CNC mill.
80
81- analyze geometry, e.g., by viewing ben-bottom-inside-500um
82- define CAD model in cw.py
83- generate in HeeksCAD with "import cw" (requires HeeksCNC and
84  HeeksPython)
85- define Zig-Zag operation (*)
86- generate Python script and run it (takes a while, about 10-20
87  minutes for the Python script on my Q6600, plus 20-100 minutes for
88  HeeksCAD to read the data back)
89- save NC file, using the name specified in "doit" (see below)
90- mount piece and determine geometry with millp
91  (from http://svn.openmoko.org/developers/werner/cncmap)
92- define conversion in "doit" script
93- coordinate transform and conversion to Roland's RML-1 (**)
94  ./doit >job
95- send job with cncmap/spool
96
97(*) In this case, the following parameters were used:
98
99    - 32 mil Carbide End Mill
100    - step over 0.2 mm (default)
101    - step down 2 mm
102    - start depth 0.5 mm
103    - final depth 6.5 mm
104    - rapid down to height 0.5 mm
105
106    These parameters depend on the mill, the tool, and the material.
107    Note that, in my setup, tool speed and the clearance height are
108    set by the "doit" script, and HeeksCAD's settings have no effect.
109
110    The CAD model uses only an approximation of machine coordinates.
111    The final transformation and alignment is also made by "doit".
112
113    Total machine time is about 7 hours for 2" pine, about 11 hours
114    for 3". Zig-Zag is quite inefficient and repeats some paths many
115    times. A better tool path could reduce machine time to about a
116    third.
117
118(**) HeeksCAD currently doesn't generate RML-1 output. I'm using a set
119     of utilities that process toolpaths in the gnuplot format and
120     generate RML-1 from that. Hence the detour.
121
122
123Gravity casting
124---------------
125
126Gravity casting is an efficient process for producing small numbers of
127counterweights. The mold is milled from a block of pinewood and has a
128life expectancy of about 20-40 cycles.
129

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