[Arm-netbook] screwed up the Riki200 plotter design

[Luke Kenneth Casson Leighton]

http://forums.reprap.org/read.php?177,767087,779977#msg-779977

ok so i screwed up.  i didn't do a full force-analysis of the plotter
design concept, and it was only when i tried turning each of the
motors that i realised that with a double pulley system the force on
one is double that of the other.  with the two pulleys being separated
by 320mm that means that just turning the motor *will* cause the
X/Y-ends to "shear".

this is... bad :)

fortunately someone posted a few days ago that they have a concept
which they've named "Edge-XY" - similar to CoreXY except the single
X-Gantry (the horizontal bar of an "H") is replaced with *dual* X/Y
(cross) rods.  now, what's particularly fascinating about Edge-XY is
that an important and previously under-appreciated design flaw of
CoreXY is completely sorted!

also very fortunately, the EdgeXY concept is very very similar to what
i was trying for the Riki200 plotter concept... so i can convert over
to EdgeXY with very little extra work.  about 16 hours of re-printing
parts but about... a week's worth of CAD design effort.  which isn't
too bad for correcting a major design cock-up :)

i will try to work in the double-pulley system into the redesign: due
to the fact that the belts are fixed onto the moving X/Y-ends of the
EdgeXY it would be a 3x force multiplier not a 4x one, but also the
forces (the pulleys) would be *on the same blocks* (separated by about
20mm) and would be force-balanced.

made that mistake once already.... not doing it again :)

l.

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