[Arm-netbook] screwed up the Riki200 plotter design

[Luke Kenneth Casson Leighton]

---
crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68


On Tue, Jul 25, 2017 at 9:27 PM, Benson Mitchell
<benson.mitchell+arm-netbook at gmail.com> wrote:
> On Tue, Jul 25, 2017 at 2:28 PM, Luke Kenneth Casson Leighton <lkcl at lkcl.net
>> wrote:
>
>> > But even if I'm wrong about the rigidity... why bother?
>>
>>  pulley system - doubling.
>>
> Okay, but you aren't getting that with EtchXY (see below). And you can add
> it to my proposal just as easy.

 i keep getting *really* confused by the pulleys in the EtchXY :)
example: right motor turns one revolution, left-side-belt from motor
gear leading to yellow block is doubled-back... therefore it's a
pulley arrangement.  green block moves 1/2 speed.



>>
>>  mass is also equal for x and y.
>>
> Which is also true for my proposal.

 indeed.

>> 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.
>>
> The mass distribution seems fine; motors, shafts, and pulleys are all
> non-moving. The moving parts are the gantries and extruder platform, all
> just the same as you have them. Likewise rigidity seems pretty solid, with
> one belt per block.
>
> As for pulley doubling, it's simple to add. I just didn't go into details
> because I'd assumed you were willing to give it up, since you're talking
> about using EtchXY which lacks it.

 see above....

> Instead of anchoring the ends of the loop directly to the moving blocks,
> just put pulleys there, and bring the end back to the fixed chassis to
> anchor it. Going back to your original diagram for the Riki200, it's just
> like the bottom 20% of that diagram, but the belt wraps 180 instead of 90
> degrees around those chassis-mounted idlers. (And of course, it's rotated
> 90 degrees out of the page.)
>
> If that was unclear, say so -- I'll come up with a sketch.

 yes please - can we discuss it on the reprap forum?  it's easier as
they have the means to upload (and inline) images.  upload the file
then hit "create link in message" button, hit "preview" to check it...

>> 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.
>>
> Ah, good.
>
>> 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.
>>
> But the force is only halved for the Y-axis in EtchXY. Look closely -- the
> X-axis is anchored directly to the green blocks, so the accelerating force
> is just the sum of red and blue belt tension.

 nom, nom, nom, nom.... ah ha!  yes!  ok so now i know why i was so
totally confused: if i looked at one axis i would see a double-pulley
system, then looking at the other it wouldn't have one.

 ah ha!  yes - i know how to fix that: you'd need to put idlers on the
green blocks, and have the belt-mount points fixed in each corner (not
to the green blocks)

>  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...
>>
>
> Yeah, but you've got all the rails and rods, exactly the same -- it's
> really just 4 short belts vs. 2 long belts, 4 long shafts vs. many short
> shafts, and especially the pulleys. I think as a result of whatever
> misunderstanding has you thinking I'm adding extra rails, you're also
> overestimating the design work involved to redo it -- it should only be a
> little more radical than redesigning for EtchXY.

 i've come up overnight with an alternative, which i'll post on the forum.

>> 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.
>>
>
> Wait, 6 of those 8 are tooth-side-in -- and you're still wrapping them
> around bearings instead of timing belt pulleys?

 yyyup :)

> I wouldn't have thought you
> could get away with that!

 you can ... just about :)  surprisingly.  it starts to get pretty
noisy at high speed though.

 ... can we move this entirely to the forum? [snipped rest]


> Yeah, absolutely. I'm sure there are reasons not to gang two or three 6mm
> GT2 belts on an extended pulley to get more strength?

 cost and space.  seen it done - but on an all-aluminium design (CNC
machined) where the parts were strong enough to withstand the extra
height (double stack of pulleys).

l.