129 lines
4.9 KiB
Plaintext
129 lines
4.9 KiB
Plaintext
Counterweight
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=============
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This project defines a counterweight to prevent the Ben NanoNote from
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falling over, and describes the process of making a wooden mold for
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casting the counterweight using a lead alloy commonly used for
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soldering.
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Problem statement
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-----------------
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The weight distribution of the Ben NanoNote makes the device unstable
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and often fall over when the display is opened. This can be remedied by
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adding a counterweight near the front of the bottom shell.
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Experiments have shown that a torque of about 2.5 mNm is sufficient to
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balance the device with the display fully opened. A feeling of solid
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stability is reached around 6.5 mNm.
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The counterweight defined in this project is made of plumbing solder, a
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Pb67Sn33 alloy with a density of about 10.0 g/ccm. The counterweight
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has a nominal mass of 17.9 g and a nominal torque of 7.3 mNm. Due to
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mold compression (which one could compensate for) and process
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tolerances, the mass achieved in DIY casting ranges from about 14.5 g
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to 17 g. This mass is slightly increased by the addition of protective
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painting.
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In experiments, the torque produced by these counterweights proved to
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be sufficient to give the user the feeling that the Ben is solidly
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standing on its feet.
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Mechanical stacking
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-------------------
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From the bottom to the top, we have the following elements:
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- Ben case, bottom shell
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- a few drops of glue or silicone, to hold the counterweight in place
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- the counterweight, covered by protective paint
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- a few drops of glue or silicone, to keep the cover sheet in place
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- a cover sheet of thin hard plastic, e.g., the type of plastic film
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used to make transparencies
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- isolating tape, applied to tall components of the Ben's main PCB
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- the Ben's main PCB
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Dangers and protection
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----------------------
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The counterweight contains lead, which is toxic and also conducts
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electricity. While the health risk caused by handling the counterweight
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is very low compared to other lead sources, it's still a good idea to
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prevent accidental exposure. While there is normally an air gap between
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the PCB and the counterweight, they may touch if the countereight is
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improperly installed, if the PCB gets bent, or if the counterweight
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comes loose for some reason. Electrical contact can cause the Ben to
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malfunction and may even result in permanent damage.
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The counterweight is covered by one or more layers of paint, to prevent
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direct skin contact with the lead during handling. The paint may also
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offer some amount of protection against electrical contact.
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A layer of hard plastic is placed on top of the counterweight, to
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isolate it from electrical contact. The plastic also resists being
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punctured by pointy components or solder joints of the main PCB.
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Finally, all elements on the main PCB that are unusually tall are taped
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over, to further reduce the risk of them working their way into the
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counterweight. Right now, the only component where problems are
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considered likely is the buzzer.
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Workflow
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--------
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This is the workflow for generating the CAD model and making a mold for
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gravity casting with a Roland Modela MDX-15 CNC mill.
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- analyze geometry, e.g., by viewing ben-bottom-inside-500um
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- define CAD model in cw.py
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- generate in HeeksCAD with "import cw" (requires HeeksCNC and
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HeeksPython)
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- define Zig-Zag operation (*)
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- generate Python script and run it (takes a while, about 10-20
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minutes for the Python script on my Q6600, plus 20-100 minutes for
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HeeksCAD to read the data back)
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- save NC file, using the name specified in "doit" (see below)
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- mount piece and determine geometry with millp
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(from http://svn.openmoko.org/developers/werner/cncmap)
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- define conversion in "doit" script
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- coordinate transform and conversion to Roland's RML-1 (**)
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./doit >job
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- send job with cncmap/spool
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(*) In this case, the following parameters were used:
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- 32 mil Carbide End Mill
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- step over 0.2 mm (default)
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- step down 2 mm
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- start depth 0.5 mm
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- final depth 6.5 mm
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- rapid down to height 0.5 mm
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These parameters depend on the mill, the tool, and the material.
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Note that, in my setup, tool speed and the clearance height are
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set by the "doit" script, and HeeksCAD's settings have no effect.
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The CAD model uses only an approximation of machine coordinates.
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The final transformation and alignment is also made by "doit".
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Total machine time is about 7 hours for 2" pine, about 11 hours
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for 3". Zig-Zag is quite inefficient and repeats some paths many
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times. A better tool path could reduce machine time to about a
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third.
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(**) HeeksCAD currently doesn't generate RML-1 output. I'm using a set
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of utilities that process toolpaths in the gnuplot format and
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generate RML-1 from that. Hence the detour.
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Gravity casting
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---------------
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Gravity casting is an efficient process for producing small numbers of
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counterweights. The mold is milled from a block of pinewood and has a
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life expectancy of about 20-40 cycles.
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