[Robotgroup] Still working on XY frame

[Paul Atkinson]

I was going to mention the bead chain drive at Maker Faire, but you saw that
already. I did see someone making their own pulley for bead chain by using a
small V-belt pulley and some epoxy putty (thick knead-able stuff) to get the
bead-grabbing-shape.

Another variation is the string drive that was used in radio dials.
Basically they wrapped string around a couple of pulleys and had a spring to
keep it tensioned. I imagine that there was some slippage, but with a human
in the loop it didn't matter much.

Since there are a lot of cheap printers out there, you could use their
method - toothed belts and matching pulleys. Maybe harvest some parts from
dead printers.

Paul

On 10/25/07, Gray Mack <gray_mack at yahoo.com> wrote:
>
> This is to be a stepper driven 2ft x 2ft frame.
> The goal is to move a small object around inside the
> frame like a white board marker or robotic gripper and
> only needs enough torque to lift itself, this is not a
> machining tool.
>
> My first experiment was with pieces of an actual
> etch-a-sketch. I cut the corners off, mounted them in
> the corners of the 2ftx2ft frame and attached motors
> to the drive wheel and ran the mono-filament line. The
> line loops around 3 sides of the frame and then back
> again (about 12 feet total) and the cross bar attaches
> at two points for parallel travel.
> Results- the torque was a little much for my steppers,
> the wheels made screeching noises and there was
> occasional slippage at the drive wheel. Any slippage
> means I don't know where the XY plotting tool is and
> that's bad.
>
> Next I tried a screw drive. I wanted to mount the
> screw drives on the sides rather than the middle of
> the frames to keep it open and this increased the
> amount of side torque, but maybe that could be offset
> by having something better than one bolt connecting
> the drive hardware around the all-thread push rod.
> Results- the movement was 200 steps per rotation of
> the stepper motor times 20 threads per inch on the
> all-thread, thus 4000 steps per inch. So the speed of
> my stepper tops out at about 1 inch per minute or 24
> minutes to draw a line from end to end -unacceptable
> for this application.
>
> I am considering going back to some sort of
> etch-a-sketch like design that wont slip
> so I would probably need some type of ladder chain or
> toothed belt drive with at least 2-3 feet of drive
> belt spliced to the rest of the cord to make 12ft
> length.
>
> I have also been thinking about ball-chain or
> bead-chain drive (
> http://www.raymortool.com/Sprocket_Index.html ) Does
> anyone know anything about this type of stuff? I have
> seen metal bead-chain at Breed&co but have not seen
> bead-chain sprockets. Maybe they could be made somehow
> or found in old window blinds?
>
> I saw a ball-chain drive used at maker faire on a
> Hektor like plotter ( http://www.hektor.ch/ ) An upper
> right and upper left chain are adjusted and gravity
> triangulates the plotter tool position.
>
> I am looking for some low cost ideas I can experiment
> with in order to find what will work best.
>
> Another possibility is a belt drive horizontal
> positioning system that is vertically lifted
> mini-blind style?
>
> I am hoping for something relatively quiet.
>
> -Gray
>
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