[Arm-netbook] screwed up the Riki200 plotter design

[Benson Mitchell]

On Tue, Jul 25, 2017 at 10:32 AM, Luke Kenneth Casson Leighton <
lkcl at lkcl.net> wrote:

> fortunately someone posted a few days ago that they have a concept
> which they've named "Edge-XY"

Actually, they called it EtchXY -- probably in reference to the
etch-a-sketch toy.

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!
>
I'm not convinced it's so much better -- think you've overlooked something
in your analysis. (I'm going to register and post a reply in that reprap
forum thread.)

But even if I'm wrong about the rigidity... why bother? The thing that made
CoreXY special is the combination of non-moving motors with a simple (thus
cheap and lightweight) gantry. Once you've committed to the more complex
(thus expensive/heavy) dual-gantry setup, as seen in both your Riki200
design and Etch-XY, I don't see any benefit to be had from long timing
belts wrapping around a half-dozen pulleys; there's a much simpler way to
drive each axis independently with non-moving motors.

For the X-axis, you put two shafts parallel to the Y-axis, at the left and
right sides. They each have two timing belt pulleys (at the top/bottom
ends), supporting one loop of timing belt to drive each green block. One
shaft is coupled to the motor, the other is an idler.

For the Y-axis, exactly the same thing, rotated 90 degrees.

Or perhaps you've considered this and aren't doing it because of
construction details. Etch-XY has 8 short shafts (idlers on the fixed
chassis -- not counting the motors or idlers on the moving gantries), all
with parallel axes (Z-axis), while the simple solution has 4 long shafts in
pairs (X-axis and Y-axis) -- it seems simpler and easier to me, but then
I'm a machinist by trade, so I'm not used to thinking in the constraints of
3d-printed and/or laser-cut construction...


Benson Mitchell