Re: Riki200 August 19, 2017 05:23AM |
Registered: 8 years ago Posts: 776 |
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o_lampe
Hmmm, I still don't get the argument for the Benson belt gear reduction?
You want to use it to reduce belt tension, but at the same time the belt is longer. Where do you see an advantage? Belt stretch would be the same, right?
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The additional idlers don't improve matters either.
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To increase steps/mm for better accuracy, you can simply use a pulley gear reduction between stepper and rod.
Re: Riki200 August 19, 2017 08:25AM |
Registered: 8 years ago Posts: 776 |
Re: Riki200 August 19, 2017 09:04AM |
Registered: 7 years ago Posts: 6 |
Sure. But I'm not sure why we're calculating belt acceleration. As long as the belt's mass is small compared to the mechanical parts, it doesn't matter much.Quote
lkcl
scenario 1: 1 belt, travel speed 100mm/sec, head acceleration target 3000 mm /sec.
belt acceleration is directly proportional, belt undergoes acceleration of 3000 mm / sec.
Agreed on belt speed, but I don't understand where you're coming up with acceleration; it seems like it should be double. But anyway, belt acceleration doesn't matter. Repeating the above...Quote
scenario 2: 1-pulley belt, same targets.
consider one belt. belt now does 200 mm / sec for the 100 mm / sec target. corresponding belt acceleration goes up FOUR fold but the mass is HALVED (because it's shared over 2 pulleys).
Re: Riki200 August 19, 2017 09:26AM |
Registered: 8 years ago Posts: 776 |
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benson
Sure. But I'm not sure why we're calculating belt acceleration. As long as the belt's mass is small compared to the mechanical parts, it doesn't matter much.
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Tension due to print-head and gantries:
Let's assign an effective mass of 1kg. We're looking at a worst case where the print head is all the way over to one side, so accelerating it falls entirely on one belt, while the two belts share the load of accelerating the gantry; thus this effective mass represents the print-head mass plus half the X (or Y) gantry mass; if you have actual numbers for this, multiply the result...
For a mass of 1kg and an acceleration of 3m/s/s, the total force on the carriage must be 3 N. Since there's only one belt segment taking the load, that segment sees a tension of 3 N, and the opposing segment has zero tension. (The top segment has either 3 N or 0, depending whether you're accelerating away from or towards the motor.)
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(In reality, you probably have the belt tensioned up a bit more, so it might be 5 N in the direction of acceleration, and 2 N opposing it, or some such.)
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Tension due to belt mass:
(Note: I'm consistently using "top" to refer to the long segment that goes the whole way across -- even though you actually have this on top for one axis, and on the bottom for the other.)
I'm having trouble finding a mass per unit length figure for 2mm pitch GT2 belts; I did find 8mm pitch, 12mm wide belts are 0.057 kg/m, so we could guess 1/8 of that (1/4 thickness, 1/2 width), or 0.015 kg/m. I'm gonna guess at some belt lengths here... call the "top" segment 0.5m (0.008 kg), and the sum of the "lower" segments 0.4m (0.006 kg); for the pulley case, the "middle" segments would also sum to 0.4m (0.006 kg).
Then since both the bottom segments of the belt are accelerating with the carriage, and the top segments are accelerating the same rate in the opposite direction, we have an additional 0.014 kg at 3m/s/s, for an extra 0.042 N of force -- see, negligible!
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Agreed on belt speed, but I don't understand where you're coming up with acceleration; it seems like it should be double. But anyway, belt acceleration doesn't matter. Repeating the above...Quote
scenario 2: 1-pulley belt, same targets.
consider one belt. belt now does 200 mm / sec for the 100 mm / sec target. corresponding belt acceleration goes up FOUR fold but the mass is HALVED (because it's shared over 2 pulleys).
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Tension due to print-head and gantries:
Effective mass of 1kg, again. For an acceleration of 3m/s/s, the total force on the carriage must still be 3 N. Since there's now two belt segments sharing the load, each segment now sees a tension of 1.5 N, and the opposing segments have zero tension. (Like before, the "top" segment has either 1.5N or 0, depending whether you're accelerating away from or towards the motor.)
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Tension due to belt mass:
We've now got three segments/pairs of segments. The "bottom" segments, being fixed to the frame, are not accelerating at all. The middle segments, as before, are accelerating in the same direction as the carriage, only now accelerating twice as fast; the "top" segment is accelerating in the opposite direction, and likewise twice the acceleration. So we have the same 0.014kg as before, now at 6 m/s/s, so 0.084 N -- again, this is negligible, about 5% of the 1.5 N. (And even if you're right about the 4x, still about 10%.) No need to get into that level of detail, and if we did, we'd also want to calculate the rotational inertia of the motor, shafts, pulleys -- too much work for too little difference in the result.
Re: Riki200 August 19, 2017 09:26AM |
Registered: 7 years ago Posts: 6 |
Re: Riki200 August 19, 2017 09:31AM |
Registered: 7 years ago Posts: 6 |
Pretty much -- I still think it's double, not quadruple (See post I wrote at the same time as yours...), but either way, yep, that effect only matters if you care about the belt's mass.Quote
lkcl
ohhh..... riiight, okaaay, so the belt's speed only comes into play if the BELT's mass is significant. THAT would be where the pulley system would have a quadruple effect, but we already concluded that the belt's mass is negligeable, so it's okay.
does that sound about right?
Re: Riki200 August 19, 2017 10:14AM |
Registered: 8 years ago Posts: 776 |
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benson
Pretty much -- I still think it's double, not quadruple (See post I wrote at the same time as yours...), but either way, yep, that effect only matters if you care about the belt's mass.Quote
lkcl
ohhh..... riiight, okaaay, so the belt's speed only comes into play if the BELT's mass is significant. THAT would be where the pulley system would have a quadruple effect, but we already concluded that the belt's mass is negligeable, so it's okay.
does that sound about right?
Re: Riki200 August 19, 2017 02:54PM |
Registered: 7 years ago Posts: 20 |
Re: Riki200 August 19, 2017 03:56PM |
Registered: 8 years ago Posts: 776 |
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xyze
lkcl, I don't believe your layout will work if you are really getting a mechanical advantage.
From the black and tackle wiki page Block and Tackle. Take a look at the "Mechanical Advantage" section.
"Ideal mechanical advantage correlates directly with velocity ratio. The velocity ratio of a tackle is the ratio between the velocity of the hauling line to that of the hauled load. A line with a mechanical advantage of 4 has a velocity ratio of 4:1. In other words, to raise a load at 1 metre per second, the hauling part of the rope must be pulled at 4 metres per second."
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and
"The increased force produced by a tackle is offset by both the increased length of rope needed and the friction in the system.
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In order to raise a block and tackle with a mechanical advantage of 6 a distance of 1 metre, it is necessary to pull 6 metres of rope through the blocks"
So if you really had a mechanical advantage in your layout, in order to move the carriage 10 mm you will have to move 20 mm of belt at the drive pulley (for a 2 to 1 mechanical advantage).
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Where would the extra 10 mm of belt go ?
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Since it wraps back to the idler on the opposite side of the carriage, your carriage would become unconstrained whenever you try to move it (with the drive pulley).
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You really will be much better off attaching the belts to the carriage and using a pulley gear reduction between stepper and drive rod like o_lampe suggested.
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Also, why use 4 rods to carry the carriage ? Four rods just adds friction, complexity, weight and cost. You only need one rod on each axis.
Re: Riki200 August 20, 2017 11:13AM |
Registered: 8 years ago Posts: 776 |
Re: Riki200 August 21, 2017 10:28PM |
Registered: 7 years ago Posts: 14 |
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o_lampe
Hmmm, I still don't get the argument for the Benson belt gear reduction?
You want to use it to reduce belt tension, but at the same time the belt is longer. Where do you see an advantage? Belt stretch would be the same, right?
m ... gantry mass (equal for both cases) a ... gantry acceleration (equal for both cases) k ... rate, spring constant or force constant of the belt Fg = m * a ... force on the gantry due to acceleration (equal for both cases) Fb ... force on the belt end (i.e. at the motor) Xb ... belt stretch due to Fb Xg ... gantry positioning error due to Xb
# No-pulley case Fb = Fg Xb = -Fb / k = -Fg / k Xg = -Fg / k # Pulley case: Fb = Fg / 2 ... force on the belt is halved due to pulley Xb = -2 * Fb / k ... twice the belt length -> 2 springs in series with the same force Xg = Xb / 2 ... the gantry moves only half the belt's amount Xg = -2 * Fb / (2 * k) = -Fb / k ... substituted Xb from above Xg = -Fg / (2 * k) ... substituted Fb from the top
Re: Riki200 August 22, 2017 03:57AM |
Registered: 8 years ago Posts: 776 |
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chrigel
This suggests the Xg (gantry positioning error due to belt stretch) is halved for the pulley case compared to the no-pulley case for the same gantry acceleration. Where have I gone wrong?
Re: Riki200 August 23, 2017 04:03PM |
Registered: 8 years ago Posts: 776 |
Re: Riki200 August 24, 2017 04:51AM |
Registered: 8 years ago Posts: 776 |
Masking tape as a flexible air-duct September 01, 2017 08:19AM |
Registered: 8 years ago Posts: 776 |
Re: Riki200 September 01, 2017 02:12PM |
Registered: 8 years ago Posts: 776 |
Re: Riki200 September 02, 2017 08:18AM |
Registered: 7 years ago Posts: 14 |
Re: Riki200 September 02, 2017 10:13AM |
Registered: 8 years ago Posts: 776 |
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chrigel
Awesome lkcl, thanks for getting the git link working. Cloning it was a breeze.
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Then the dreaded "make download" failed again of course, on the URL http://mutley3d.com/DLoads/FlexSTL.rar as mutley3d.com doesn't exist any more and there are no downloads at the new flex3drive.com page.
I asked Jason from Flex3Drive for the G2 extruder STL files but he emailed me G3 STLs instead (maybe because G2 isn't sold any more?).
class Flex3DriveG2Base(STLObject): def __init__(self): STLObject.__init__(self, "flexstl/g2/ExtruderBase.stl", BOM_3D, )
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So I thought, maybe you could just upload the extruder STL files to some site (e.g. hands.com) where your Makefile could find them, but Jason mentioned in an email that Flex3Drive will themselves make the STL files available shortly after figuring out the licensing And in the meanwhile he asked me not to distribute the files elsewhere...Sooo until that issue is resolved, I don't think anyone can re-create your work, at least without contacting Flex3Drive themselves and asking for the STLs.
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Now of course, I tried to "make" the Riki200 anyway and interestingly, the build failed *not* because of the missing STLs but instead at a missing "foursplit_spline" import... maybe a file you haven't added to the repo yet?
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One more question, on the G2 vs. G3 extruders: Which one will you be using for the Riki200?
Re: Riki200 September 02, 2017 07:07PM |
Registered: 8 years ago Posts: 776 |
if __name__ == '__main__': #c = Flex3DriveCarriagePlotCoolingFanHolder() c = Flex3DriveG2Base() u = c.obj() scad_render_to_file(u, "riki200_model.scad") #c.generate_stls()
Re: Riki200 September 05, 2017 10:45PM |
Registered: 7 years ago Posts: 14 |
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lkcl
make sure that there's at least... _some_ file there, even if it's blank, and if it's not the same just put in "self.visible(False)" to make it disappear temporarily ok?
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lkcl
no i will NOT be adding copies of library files to a git repo. you do not do that: it is extremely bad practice.
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lkcl
my *personal* one uses a G2 because that's what i have available. however i have a direct-drive adapter on order and will be making a direct-drive extruder to fit it. i will also be replacing the Flex3DriveG2ClampE3D part. probably. see class Flex3DriveG2Base in v3/flex3drive.py
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lkcl
ok so basic instructions, edit riki_model.py and look for show_zassembly = True and so on. this is such a CPU-intensive model that i had to make ways to cut certain portions. that's done with those booleans.
stow = False show_belts = True show_box = True show_zassembly = True show_plotxy = True show_bed = True show_panels = False...the first "stow" line had me confused for a second, as it was set to True
Re: Riki200 September 05, 2017 11:03PM |
Registered: 8 years ago Posts: 776 |
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chrigel
Thanks for the updates.
On the topic of missing STLs:
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lkcl
make sure that there's at least... _some_ file there, even if it's blank, and if it's not the same just put in "self.visible(False)" to make it disappear temporarily ok?
Whatever you did in the last commits, after the update and "make download", everything seems to be working - I haven't had the need for empty (or different) STL files so far, which is good.
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lkcl
no i will NOT be adding copies of library files to a git repo. you do not do that: it is extremely bad practice.
Sorry, I wasn't aware that "foursplit_spline.py" was a library. Anyway, it's not a problem any more.
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Regarding the G2 vs. G3 extruders:
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lkcl
my *personal* one uses a G2 because that's what i have available. however i have a direct-drive adapter on order and will be making a direct-drive extruder to fit it. i will also be replacing the Flex3DriveG2ClampE3D part. probably. see class Flex3DriveG2Base in v3/flex3drive.py
Interesting. I didn't realise until now that the G3 (not sure whether this applies to the G2 as well) probably isn't really suitable as it has the hot end fan hanging off the bottom - which looks to my untrained eye as it'd likely be interfering with your carriage. Now I also understand why you created that fancy fan duct...
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...anyway, that'll mean the G3 STL files I have aren't good either as they don't even include the Direct Drive Adapter models as far as I have seen. Since it looks rather like a very "short" NEMA motor with a driveshaft mount at its top, I guess it shouldn't be too hard to model
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lkcl
ok so basic instructions, edit riki_model.py and look for show_zassembly = True and so on. this is such a CPU-intensive model that i had to make ways to cut certain portions. that's done with those booleans.
Thanks, found it. For anyone wanting to look at the whole thing (except the side panels), this is what I used:
stow = False show_belts = True show_box = True show_zassembly = True show_plotxy = True show_bed = True show_panels = False...the first "stow" line had me confused for a second, as it was set to True... looks like a leftover from the Sandwich200.
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Also note that initially (or after resetting the view in OpenSCAD), the whole thing is upside-down, i.e. the z-axis is inverted or, in other words, higher values for z mean further down the Riki200 model.
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Also I was able to generate the STLs and BOM PDF, yay!
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And yes, it is definitely slow to look at......but what can you do, it just is quite a complex model!
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One thing I (as a complete noob) do miss about OpenSCAD is the ability to click on an element in the 3D view, highlight or hide it, tell me what it is and ideally where I can find it in the source... but that's probably better addressed to the OpenSCAD community.
Re: Riki200 September 06, 2017 03:59AM |
Registered: 7 years ago Posts: 14 |
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lkcl
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chrigel
...anyway, that'll mean the G3 STL files I have aren't good either as they don't even include the Direct Drive Adapter models as far as I have seen. Since it looks rather like a very "short" NEMA motor with a driveshaft mount at its top, I guess it shouldn't be too hard to model
there's a NEMA-17 already in the model *grumble* i seem to have added the STL file *grumble* that'll have to go... no the G3 is a totally different product and design from the direct-drive adapter so it's a bit unfair to say "the archive doesn't even include it".
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lkcl
... i will need to develop a direct-drive extruder which will sit on that carriage, then you would simply (optionally) drop in the Direct-Drive adapter and get some weight-reduction.
Re: Riki200 September 07, 2017 07:07PM |
Registered: 8 years ago Posts: 776 |
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chrigel
Sorry, again I wasn't clear enough: What I meant was the STL package Jason from Flex3Drive emailed me in response to my request for the G2 extruder STLs. So he sent the G3 STL package (the "archive" I was referring to because it was a RAR file),
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in which I couldn't find anything that looks like the Direct Drive Adapter they have on their web page. But since that adapter looks like a shortened NEMA-17 motor (or *should* look like one, as it is arguably designed to replace the same), hopefully modelling it won't be too hard if it can be based on the existing NEMA-17 model that you already have there. Ah yes, just saw that it's part of the git repo.
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I can try and create a patch to remove it from there and add it to the "make download" target instead if that's what you want...
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bear with me though, while I know Mercurial somewhat, git is still quite new to me... but I guess it's never too late to learn!
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lkcl
... i will need to develop a direct-drive extruder which will sit on that carriage, then you would simply (optionally) drop in the Direct-Drive adapter and get some weight-reduction.
Now I am a bit confused - do you mean that the standard Riki200 will have a NEMA-17 sitting on the carriage instead of the Direct-Drive adapter? To save those 65GBP?
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Or maybe I am wrong in my understanding of the available options here, which I *think* are:
1. NEMA-17 axis connected directly to your custom-developed direct-drive extruder, both on the carriage.
2. NEMA-17 mounted on the frame, its axis driving a flexible drive shaft which powers the Flex3Drive Direct-Drive adapter (40:1 worm gear) which is then connected to your custom-developed direct-drive extruder.
3. Same as 2, but using a ready-made, complete extruder (including the 40:1 gear) from Flex3Drive (such as the G3) instead of the Direct-Drive-plus-custom-extruder... but we've ruled that out already because of the fan (see above).
Currently, I am thinking option 2 would be preferable because of the weight, just as you said.
Re: Riki200 September 07, 2017 11:50PM |
Registered: 8 years ago Posts: 776 |
Re: Riki200 September 08, 2017 05:30AM |
Registered: 8 years ago Posts: 776 |
Riki200 v3 FIrst print! September 09, 2017 09:46PM |
Registered: 8 years ago Posts: 776 |
Re: Riki200 v3 FIrst print! September 10, 2017 02:22AM |
Registered: 9 years ago Posts: 5,232 |
Re: Riki200 v3 FIrst print! September 10, 2017 02:49AM |
Registered: 8 years ago Posts: 776 |
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o_lampe
You already run it with 24V?
I'm sure there is a golden spot in gear ratio. Maybe try 30:20 tooth ratio?
Re: Riki200 v3 FIrst print! September 10, 2017 05:59AM |
Registered: 9 years ago Posts: 5,232 |
Re: Riki200 v3 FIrst print! September 10, 2017 06:16AM |
Registered: 8 years ago Posts: 776 |
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o_lampe
You mentioned the belt tension would bend the rod.
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Maybe it's the right time to split the 5mm rod and use dual bearings and flexible couplings?
This way you could make it more maintenance friendly and stiffer the same time.