On Tue, Jul 25, 2017 at 2:28 PM, Luke Kenneth Casson Leighton <lkcl@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.
mass is also equal for x and y.
Which is also true for my proposal.
corexy has the weight of the x-gantry. moving in Y has more inertia that X.
Yeah, I'm not suggesting CoreXY -- I do get why that's quite unsuitable for what you're doing.
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.
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.
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. (The distribution of force between the red/blue belts (50/50 at center, progressively worse towards limits of travel) does help vs some other designs, but that benefit applies to the parallel-shafts system, too.) And given you seem to be building a square printer, you should be accelerating pretty much the same amount of mass around in X and Y -- so you have to design (choose belts and/or limit acceleration) based on the axis without mechanical advantage.
(It would be different if your build volume is way out of square, such that the yellow-block gantry masses twice the green-block gantry -- then you have mechanical advantage right where you need it, so the same acceleration in X or in Y give similar cable tension -- but AFAIK you're not doing that sort of build.)
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.
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? I wouldn't have thought you could get away with that! But if so, that does make a bunch of the savings/simplification I thought I was getting illusory.
-- 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
Actually 8, really (2 per shaft, 2 shafts per axis, 2 axes), whereas I thought you had 10 pulleys all told (two on the yellow blocks, 6 fixed, and 2 on the motors), based on where the belt wraps tooth-side in, whereas really you have... just the two on the motors?
Although, now that I think... hey, if you can wrap timing belts around bearings, so can I!
Keep the drive side (per axis) one long shaft + 2 pulleys on the drive side, but for the idler side use M5 screws/625 bearings just like you're talking about -- so it is just 4 pulleys after all. (Oh, and a couple flexible couplings, or yet more timing belt pulleys, to couple the motors to the drive shafts.)
and 4 sets of rods/rails
Again, you've already got all the rods and rails -- that stuff would really be _exactly_ like you have it. You're "only" redesigning the 4 green/yellow blocks to put the idler axes horizontal rather than vertical (pretty simple, I think), and completely redoing the motor/pulley mount brackets at the 4 corners of the frame (not really simple) -- point is, all the linear rails and rods stay in exactly the same places they are.
I don't think the shafts should be a big deal (Sure, not as cheap as M5 screws, but still can be pretty cheap... nothing fancy, just cold-drawn rods), but I see where the pulleys add up. The one thing that might make it worthwhile is that it dodges the skew problem completely -- each block is completely controlled by one belt.
Perhaps the best answer is to modify EtchXY to give you pulley reduction on both axes? That change is pretty straightforward. But I don't see what to do about the skew problem...
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?
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?
Benson Mitchell