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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