[Arm-netbook] screwed up the Riki200 plotter design

[Luke Kenneth Casson Leighton]

On Tue, Jul 25, 2017 at 6:20 PM, Benson Mitchell
<benson.mitchell+arm-netbook at gmail.com> wrote:

> 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.)

 that would be superb

> But even if I'm wrong about the rigidity... why bother?

 pulley system - doubling.

 mass is also equal for x and y.

 corexy has the weight of the x-gantry.  moving in Y has more inertia that X.

 speed for this design i intend to try 500mm/sec and greater.  that's
TEN times what most people would ever conceive of running a 3D printer
at.  the ultimaker-2 is only rated for a maximum of 250-300mm/sec.

> 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.

 not "and guarantee rigidity and add a pulley doubling system and also
guarantee equal mass distribution" as well.

> 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.

 i know the sort of thing: i've seen it in use: it's used in the
ultimaker-2 and also in an open design pick-and-place amachine.   the
amount of force on the belt is considerable.  with the EtchXY design
the force on the belt is halved due to the pulley system.

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

 by the time it's all assembled, the 2 belts, 4 pulleys, 4 shafts,
then 4 sets of rails/rods, it really does add up very quick in terms
of ccomplexity.  then you have to CAD design it, make sure that
everything fits, that's a month's work right there...

>
> 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

 M5 16mm bolts, 2 M5x18mm washers.  $0.02 each.  625 bearings.  saves a lot.

> -- 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)

 plus 4 pulleys and 4 sets of rods/rails

> -- 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...

 pick-and-place, cnc, etc it's all the same principle.  except CNC
needs huge force and precision, whereas 3D filament deposition needs
speed and precision.  CNC typically uses backlash-free screw-drives,
low pitched, with planetary gearboxes to get the forcee and accuracy.

 i'm looking for a trade-off which maximises mm/sec/$ so a weird
combination of ways to increase speed without compromising rigidity,
using components that are considered "budget" but are not "cheap and
nasty", they're just "cheap" :)

 so adding pulleys to reduce the force on a standard "cheap" 6mm GT2
timing belt, that's important, because now you can try going twice as
fast but still use... cheap 6mm GT2 timing belt.  otherwise you would
need to use GT3 and 8 to 10mm, that's no longer "cheap".

 make sense?

l.